Sweet Potato: Post Harvest Aspects in Food, Feed and Industry (Food Science and Technology)

FOOD SCIENCE AND TECHNOLOGY SERIES

SWEET POTATO: POST HARVEST ASPECTS IN FOOD, FEED AND INDUSTRY

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FOOD SCIENCE AND TECHNOLOGY SERIES Food Science and Technology: New Research Lorenzo V. Greco and Marco N. Bruno (Editors) 2008. ISBN: 978-1-60456-715-1 Food Science and Technology: New Research Lorenzo V. Greco and Marco N. Bruno (Editors) 2008. ISBN: 978-1- 61668-106-7 (Online Book) The Price of Food Meredith N. Fisher (Editor) 2009. ISBN: 978-1-60692-440-2 Food Processing and Engineering Topics Maria Elena Sosa-Morales and Jorge F. Velez-Ruiz (Editors) 2009. ISBN: 978-1-60741-788-0 Traditional Chinese Foods: Production and Research Progress Li Zaigui and Tan Hongzhuo 2009. ISBN: 978-1-60692-902-5 Food Science Research and Technology Isaak Hülsen and Egon Ohnesorge (Editors) 2010. ISBN: 978-1-60741-848-1 Sweet Potato: Post Harvest Aspects in Food, Feed and Industry Ramesh C. Ray and K.I. Tomlins (Editors) 2010. ISBN: 978-1-60876-343-6

FOOD SCIENCE AND TECHNOLOGY SERIES

SWEET POTATO: POST HARVEST ASPECTS IN FOOD, FEED AND INDUSTRY

RAMESH C. RAY AND

K.I. TOMLINS EDITORS

Nova Science Publishers, Inc. New York

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Published by Nova Science Publishers, Inc.  New York

CONTENTS Preface

vii

About the Editors Chapter 1

Chapter 2

Sweet Potato Growth, Development, Production and Utilization: Overview Maniyam Nedunchezhiyan and Ramesh C. Ray Post Harvest Handling, Storage Methods, Pests and Diseases of Sweet Potato Ramesh C. Ray, V. Ravi, Vinayak Hegde, K. Rajasekhara Rao and Keith I. Tomlins

9 1

27

Chapter 3

Physiological Functions and Utilization of Sweet Potato Makoto Yoshimoto

59

Chapter 4

Sweet Potato Starch S. N. Moorthy and S. Shanavas

91

Chapter 5

Sweet Potato Purees and Dehydrated Powders for Functional Food Ingredients Van-Den Truong and Ramesh Y. Avula

Chapter 6

Bio-Processing of Sweet Potato into Food, Feed and Bio-Ethanol Ramesh C. Ray, Samir K. Naskar and Keith I. Tomlins

Chapter 7

Sweet Potato Utilization in Human Health, Industry and Animal Feed Systems Adelia C. Bovell-Benjamin

Chapter 8

Sweet Potato in Animal Nutrition Ibisime Etela

Chapter 9

Sweet Potato and Pigs: Traditional Relationships, Current Practices and Future Prospects Dai Peters

117 163

193 225

245

vi Chapter 10

Index

Contents Sweet Potato Utilization, Storage, Small-Scale Processing and Marketing in Africa Keith Tomlins, Debbie Rees, Claire Coote, Aurélie Bechoff, Julius Okwadi, Jaquelino Massingue, Ramesh Ray and Andrew Westby

271

295

PREFACE Sweet potato (Ipomoea batatas L.), the seventh most important food crop after wheat, rice, maize, potato, barley and cassava, is a staple food in many developing countries of the tropics and sub-tropics, and also serves as animal feed and raw material for several food and feed based industries. This New World crop has high biological efficiency of converting solar energy into edible energy (152 MJ/ha/day) in the form of tuberous (storage) roots and could be the food for the ever growing human population in future. Asia leads in area (60.75%) and production (86.89%) of sweet potato in the world. Sweet potato was originally a herbaceous perennial but was domesticated as an annual and grows best in moderately warm temperature of 21-26oC. It requires light textured soil with the optimum pH of 5.5 -6.5 for good growth of the crop. Current research has focused on development of high starch, high dry matter and coloured (ß-carotene and anthocyanin-rich) sweet potato varieties for industrial applications in addition to traditional usage as food and animal feed. The first chapter in this book by Maniyam Nedunchezhiyan and Ramesh Ray provide an overview on the growth, development, production and utilization of sweet potato. Sweet potato storage roots are subjected to several forms of post harvest losses during harvest, transportation from farmers‘ field to market and storage. These are due to mechanical injury, weight loss, sprouting, diseases and pests. Chapters 2 and 10 in this book deal with these aspects in detail. In Chapter 2, Ramesh Ray and colleagues have discussed the various methods (curing, fungicide treatment, bio-control by antagonistic yeasts, gamma irradiation and storage in controlled atmospheric conditions) that could reduce fungal rot and enhance shelf-life of sweet potato roots in storage. Keith Tomlins and his colleagues, in Chapter 10, have attributed the main challenges for the crop in Africa with respect to post-harvest issues which include the management of storage pests, particularly sweet potato weevils (Cylas spp.), increasing yields and improving the marketing systems along the value chain. Sweet potato contains various kinds of physiologically functional components such as polyphenolics, anthocyanins, fibres and carotenoids in roots and leaves. These physiological functions include the potential for anti-oxidation, anti-diabetics and anti-hypertension. The sweet potato roots or leaves with these functions are commercially used as materials of confectionary, noodles, alcoholic drinks and beverage. In Chapter 3, Makoto Yoshimoto, reviews the recent research work on this topic, mainly concerned with the technological concepts on value addition to ß-carotene and anthocyanin rich sweet potato varieties. Starch is one of the major biochemical components of root and tuber crops. In Chapter 4, Subramony Moorthy and Shanavas have discussed the different characteristics of sweet

Preface

viii

potato starch and its potential applications. Processing technologies have been developed in various parts of the World to convert sweet potatoes into purees and dehydrated forms that can be used as functional ingredients in numerous food products. In Chapter 5, Van Den Truong and Ramesh Avula, review the processing operations involved in these technologies and their effect on quality, storability, nutritional values and rheological properties of sweet potato purees and powders/flours. With high level of carbohydrate, ß-carotene (orangefleshed varieties) and anthocyanin (purple-fleshed varieties), sweet potato purees and dehydrated forms can be used as functional ingredients to impart desired textural properties and phytonutrient content in processed food products. Bio-processing (fermentation) of sweet potato offers novel opportunities to commercialize this crop by developing functional foods and beverages such as sour starch, lacto-pickle, soy sauce, acidophilus milk, etc. through either solid-state or submerged fermentation. Sochu, traditional Japanese distilled liquor with an alcohol content of 20-25% is made from sweet potato. Sweet potato flour and basassae are used as substrates for production of microbial enzymes, organic acids, sodium glutamate, etc. Ramesh Ray and colleagues have discussed these aspects in depth in Chapter 6. In Chapter 7, Adelia Bovell-Benjamin has reviewed the biochemical, bioactive and functional properties of sweet potato relevant to human health, industry and animal feed systems. Sweet potato starch has industrial applications such as sweeteners, citric acid, beverage, noodle production, industrial alcohol and derived products such as glucose and maltose syrups. Two chapters in this book have been exclusively devoted to the utilization of sweet potato as animal feed in traditional livestock system. Ibisime Etela, in Chapter 6, has given an in-depth description on the use of sweet potato roots and leaves as food for cattle, goat, pig and sheep, particularly in Africa, Asia and Latin America. Dai Peters, in Chapter 9, has described four case studies in four different countries (China, Vietnam, Indonesia and Uganda) on the utilization of sweet potato in pig feed systems. She emphasizes that though these four systems share the same characteristics of feeding sweet potato to pigs, the agronomic, ecological, marketing and even socio-cultural contexts vary greatly resulting in distinctly different production and marketing approaches. The subject, post harvest aspects of sweet potato in food, feed and industries, are a topic of current interest. This book attempts to highlight some of the more significant aspects of the subject within a framework of 10 chapters. We are very grateful to the author(s) of each chapter for clearly presenting recent developments and research perspectives. We also appreciate the promptness of the individual authors in providing and processing their manuscripts.

Ramesh C. Ray Keith I. Tomlins

ABOUT THE EDITORS Dr. Ramesh C. Ray, Principal Scientist (Microbiology) and Head, Regional Centre of Central Tuber Crops Research Institute, Bhubaneswar, India is well known for his research work in the field of post harvest technology and bioprocessing of tropical root and tuber crops into value-added products. He has published 95 original research papers and 8 reviews and concept papers in peer reviewed national and international journals, and 30 book chapters. He has developed several foods and industrial processes and is co-inventor of 3 patents. He has edited/co-edited 8 books and is a member of the editorial board of international journals like Annals of Tropical Research and Journal of Environmental Biology. He has been currently selected as American Society of Microbiology International Professor from India. Dr. Keith I. Tomlins is a Reader in Food Safety and Quality at the Natural Resources Institute of the University of Greenwich, UK (www.nri.org). He has experience in international project management, research and consultancy in food safety and quality management of food and drink products worldwide and is a member of the University of Greenwich Research Ethics Committee. He is also an external consultant for a London based marine investigation company. He is the author or co-author of 35 publications in international peer reviewed journals, 24 conference papers and three books. With reference to sweet potato, he is currently (as of 2009) the councilor for publications of the International Society for Tropical Root Crops (www.istrc.org). His expertise in sweet potato research involves the post-harvest aspects in sub-Saharan Africa. He is currently involved in the Harvestplus Reaching End Users project in Uganda and Mozambique which seeks to increase consumption of provitamin A sweet potato.

In: Sweet Potato: Post Harvest Aspects in Food Editors: R. C. Ray and K. I. Tomlins

ISBN 978-1-60876-343-6 © 2010 Nova Science Publishers, Inc.

Chapter 1

SWEET POTATO GROWTH, DEVELOPMENT, PRODUCTION AND UTILIZATION: OVERVIEW Maniyam Nedunchezhiyan and Ramesh C. Ray** Central Tuber Crops Research Institute (Regional Centre), Bhubaneswar - 751019, India

ABSTRACT Sweet potato, a staple food in many of the developing countries of tropics and subtropics also serves as animal feed and raw material for the industries. This New World crop has high biological efficiency of converting solar energy into edible energy. It has spread into Europe, Africa, India, and East Indies through the batatas line and to the Philippines from Central and South America through the kamote line. Asia leads in area (60.75%) and production (86.89%) of sweet potato in the world. Sweet potato, originally the herbaceous perennial is domesticated as an annual and grows best in moderately warm climate and temperature of 21-26° C. It requires light textured soil with the optimum pH of 5.5-6.5. The crop is grown on ridges, mounds and flat beds depending upon the soil and agro-climatic conditions. As sweet potato removes appreciable quantities of plant nutrients, incorporation of 5 tonnes/ha of organic manure and a moderate dose of inorganic fertilizers (50-75 kg N, 25-50 kg P2O5 and 75-100 kg K2O/ha) is recommended. Sweet potato requires one or two weeding followed by earthing up for easing storage root bulking. Dry season planting always produces higher storage root yield than wet season planting but it requires supplemental irrigation. Sweet potato weevil, which is causing losses in certain parts of the world, can be reduced by following integrated pest management techniques. Virus diseases can be avoided by selecting disease free quality planting materials. Development of high starch, dry matter and coloured sweet potato varieties has opened up new vistas in industrial applications apart from traditional usage as food and feed. Sweet potato starch is used in textiles, paper and food manufacturing industries, preparation of liquid glucose and adhesives. ß-carotene and anthocyanins are extracted from coloured sweet potatoes, which are used as food colorants and anti-oxidants. Coloured sweet potato flour is used in various bakery and noodles preparations. Enzymes like sporamin and ß-amyalse are also produced from sweet potato storage roots. Sweet potato leaves are rich in polyphenols (mainly 

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Maniyam Nedunchezhiyan and Ramesh C. Ray

2

chlorogenic acid and iso-chlorogenic acid) which have strong suppressive effects on food poisoning microorganisms. The leaves can be used for food, green drink like tea or medicine.

ABBREVIATIONS CIP CTCRI IPM IW CPE SPFMV USRDA

International Potato Centre; Central Tuber Crops research Institute; Integrated Pest Management; Irrigation water Cumulative Pan Evaporation sweet potato feathery mottle virus; United States Recommended Dietary Allowance

INTRODUCTION Sweet potato [Ipomoea batatas L. (Lam.)] is the seventh most important food crop and next to cassava among the root and tuber crops grown in the world (Ray and Ravi, 2005). It is cultivated through out the tropics, subtropics and warmer temperate regions. Sweet potato is a staple food in many of the developing countries. It is consumed both fresh and in the processed forms. It is also used as animal feed. It has great potential as a raw material for the manufacture of a wide range of industrial products such as starch, liquid glucose, citric acid, mono-sodium glutamate and ethanol (Woolfe, 1992). Sweet potato is expected to play a vital role in combating the food shortages and malnutrition that may increasingly occur as a result of population growth and pressure on land utilization (Naskar et al., 2008b). It can produce high amount of energy per unit area per unit time. On a world scale sweet potato provides significant amounts of energy and protein (Table 1). Its production efficiency of edible energy and protein is outstanding in the developing world. The average energy output/input ratios for rice and sweet potato on Fijian farms were 17:1 and 60:1, respectively (Norman et al., 1984). The protein content of sweet potato on a fresh weight basis is low, but the average protein production/ha from sweet potato in the tropics is same that of cereals, beans and chickpeas (Yamakawa, 1997). Sweet potato is also having additional advantages. It does not normally require high levels of inputs. Weeding by either cultivation practices or herbicides is minimum in sweet potato as the vines grow very rapidly and cover the ground within few weeks after planting. Application of insecticides and fungicides are often not necessary as with the exception of sweet potato weevil, most insect damage is negligible and the fungal diseases are not usually a problem in growing areas (Naskar, 2006). Total crop failure is very rare owing to biotic and abiotic stresses and many farmers plant sweet potato as an insurance crop against food emergency (Yamakawa, 1997). Sweet potato in spite of having many desirable traits, its use as a staple food has declined in many countries. The low status accorded to both roots and vines due to their image as a ‗subsistence‘ crop, a ―poor mans‘ food‖ or something to be eaten only at the times of dire

Sweet Potato Growth, Development, Production and Utilization: Overview

3

need such as famine or war may have been a limiting factor in their exploitation as foods of high nutritional quality (Vinning, 2003). Table 1. Protein yield of various food crops Crop Cassava Sweet potato Yams Bananas Soybean Groundnut Beans Chickpea Rice Maize Sorghum Millet

Average tropical yield (t/ha) 9 7 7 13 .>1 CA = ChA. Sweet potato leaves also revealed relatively much higher activity in 1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activity than the 43 vegetables mentioned above (Islam et al., 2003b, 2003c). There are significant positive correlations between radical-scavenging activity and the polyphenol concentrations of sweet potato leaves (Islam et al., 2003a). These data indicate that sweet potato leaves are a good supplementary resource of antioxidants.

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Antimutagenicity Cancers can occur through initiation, promotion, and progression in body cells. Initiation is a type of mutation that occurs in cancer and anticancer genes. Therefore, controlling the gene mutation, brought about by the carcinogens, leads to cancer prevention (Berenblum, 1941). The mutagens contained in food may include ingredients of vegetables, mold toxins, and pollutants in food. These mutagens are considered as factors involved in formation and occurrence of human cancers (McCann et al., 1975). Development of screening methods for environmental carcinogens by determining their mutagenicity has enabled various types of mutagens and carcinogens to be detected and identified in daily foods (Ames et al., 1975). It is now known that various types of inhibitors that act against mutagens and carcinogens are present in our daily food, and that they play an important role in reducing the risks of mutagenesis and carcinogenesis (Shinohara et al., 1988). CQA derivatives effectively inhibit the reverse mutations induced by Trp-P-1 on Salmonella typhimurium TA and the antimutagenicity of these derivatives in order of effectiveness is 3,4,5-triCQA > 3,4-diCQA = 3,5-diCQA = 4,5-diCQA > ChA (Yoshimoto et al., 2002b).

Anticarcinogenesis Growth suppression of three kinds of cancer cells, stomach cancer (Kato-III), colon cancer (DLD-1), and promyelocytic leukemia (HL-60) cells, by QA, CA, and CQA derivatives has been researched (Kurata et al., 2007). QA has no effect on the growth of each kind of cancer cells. 3,4,5-TriCQA, however, suppresses the growth of each kind of cancer cells. Promyelocytic leukemia cells (HL-60) are especially sensitive to the CQA derivatives compared with the others. CA and the three kinds of di-CQA derivatives (3,4-diCQA, 3,5diCQA, and 4,5-diCQA) suppress the growth of HL-60 cells. CA exceptionally suppresses the cell multiplication of HL-60. These results show the necessity of the caffeoyl group bound to QA as well as the differential sensitivity of tumor cells to these compounds. Nuclear granulation and DNA fragmentation in HL-60 cells treated with 3,4,5-triCQA suggest that the cellular death is due to apoptosis induction (Kurata et al., 2007).

Antidiabetes International Diabetes Federation reports that the diabetic mellitus population is increasing globally and it is estimated at 246 million persons around the world at 2007. . Insulin-secretion ability in the rat pancreas RIN-5F cells is promoted in order of 3,4,5-triCQA > 3,4-diCQA = 4,5-diCQA = 3,5-diCQA > ferulic acid (FA) > CA > QA = ChA. 3,4,5TriCQA especially shows a remarkable insulin-secretion promoting effect (Tsubata et al., 2004). Ferulic acid (FA) and its amide derivatives stimulate insulin secretion in the rat pancreas RIN-5F cells (Nomura et al., 2003). CQA derivatives except for QA, ChA, and CA reveal higher activity on insulin secretion than FA, indicating that sweet potato leaves containing CQA derivatives may be excellent sources for antidiabetes. However, more research is needed before a direct link is established.

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α -Glucosidase (EC 3.2.1.20), which is a membrane-bound enzyme located at the epitherium of the small intestine. It catalyzes the cleavage of glucose from disaccharides (Hauri et al., 1982). Thus, retardation of the action of this enzyme by any inhibitor may be one of the most effective approaches to control non-insulin-dependent diabetes (Toeller, 1994). Matsui et al. (2004a) reported that the maltase inhibitory effect (IC50) of 3,4-diCQA, 3,5-diCQA, and 4,5-diCQA are at levels of 1,910 μM, 1,890 μM, and 413 μM, respectively. Maltase inhibitory effect (IC50 = 24 μM) of 3,4,5-triCQA is much higher than the other three diCQAs and YGM-6 (one of anthocyanin pigments from sweet potato root with purplecolored flesh), and is about one fifty-sixth of acarbose (IC50: 0.43 μM). In Japan, acarbose as a therapeutic α-glucosidase inhibitor is widely used to delay glucose absorption from the small intestine (Goto et al., 1989; Odaka et al., 1992). Further, Matsui et al. (2004a) indicate that oral administration of 3,4,5-triCQA to diabetic model rats reduces significantly their blood glucose content. Oral administration of 1.0 g and 0.1 g fine powder of the dried sweet potato leaves/kg body weight/day to STZ-induced insulin-deficient diabetic rats for seven days significantly decreased their blood glucose and increased their blood insulin levels. Further, oral administration of sweet potato leaves significantly decreased blood-glucose content in oral starch loading human volunteers (Tsubata et al., 2004). At the later stage of non-insulin-dependent diabetes mellitus (NIDDM), which is the predominant type of human diabetes, symptoms result mainly from decreased secretion of insulin by pancreatic Langerhans cells. Prevention of the NIDDM and inhibition of the serious adverse effects of diabetes such as retinopathy, neuropathy, and cataracts, are important subjects for researchers. Therefore, food materials with antidiabetic effect are desired for diet therapy.

Antihypertension A single oral administration of 3,4-diCQA, 3,5-diCQA, and 3,4,5-triCQA each at a dose of 10 mg/kg in spontaneously hypertensive rats showed antihypertensive effects (Mishima et al., 2005). Spontaneously hypertensive rats, which were orally administered with sweet potato tops exhibited dose-dependent suppression in blood pressure increases in comparison with the control group. These results suggest that sweet potato tops have a hypertensive effect in spontaneously hepertensive rats (SHR), which is at least in part due to the angiotensin I converting enzyme(ACE) inhibitory activity of CQA (Ishiguro et al., 2007b).

Antibacterial Activity Removal of pathogenic fungi in food, extension of storage time by the control of putrefying bacteria, and eradication of parasites are important for the maintenance of human health. Currently, an orientation towards healthy and natural foods is strengthening among consumers, and there may come a situation when it will be difficult to use food preservatives and disinfectants. There is an indication of a worldwide prevalence of infection by Escherichia coli O-157, and surveillance and preventive measures are required for this emerging infectious disease (Itoh and Kai, 1997).

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Lyophilized leaf powder from the ‗Simon-1‘ sweet potato cultivar strongly suppresses the growth of E. coli O-157, and its effect is detectable even after autoclave treatment. Unlike leaves, petioles or stems promote markedly the growth of O-157, suggesting that the antibacterial components exist only in the leaf. The antibacterial extract reveals that the main components are polysaccharides (Islam and Jalaluddin, 2004). In the polysaccharide fraction, the relative quantities of neutral sugars are in the order of xylose > galactose > arabinose > glucose > rhamnose > mannose > fructose. Galacturonic acid accounts for 28.7%, which is the highest among the sugar components detected. These results suggest that the antibacterial component of sweet potato leaves may be pectin-like material (Islam and Jalaluddin, 2005). Furthermore, the water extract from the leaves suppresses effectively the growth of other food-poisoning bacteria such as Staphylococcus aureus and Bacillus cereus as well as pathogenic E. coli (Islam and Jalaluddin, 2005).

Other Physiological Functions Including Anti-HIV HIV infection in humans is one of the most terrible pandemics around the world. Suitable candidates for investigating the potential in counteracting the transmission of HIV infection have been positively screening from various kinds of plants (Mahmood et al., 1993; Lim et al., 1997; Kobayashi et al., 2000; Ma et al., 2000; Tamura et al., 2006). 3,4,5-TriCQA is suggested to depress the transmission of HIV infection by the inhibition of the virus integrase (Tamura et al., 2006) and specific binding to the virus glycoprotein, gp120, which prevents its interaction with CD4 on T-lymphocytes and thus inactivates virus infectivity (Mahmood et al., 1993). A pathogenic hallmark of Alzheimer‘s disease is the formation of senile plaques. βAmyloid peptide (Aβ) is a major component of these plaques. Aβ is shown to have the potential to induce oxidative stress and inflammation in the brain, which are postulated to play important roles in the pathogenesis of Alzheimer‘s disease. Aβ induces the production of hydrogen peroxide and lipid peroxide in neurons. In addition, Aβ has been reported to induce superoxide and proinflammatory cytokines in astrocytes as well as in microglial cells. Antioxidant such as α-tocopherol protect against cytotoxicity in vitro as well as learning and memory deficits induced by Aβ. Furthermore, α-tocopherol and anti-inflammatory agents such as indomethacin reportedly slow the progression of Alzheimer‘s disease (Sano et al., 1997; Rogers et al., 1993). Long-term administration of FA, a phenolic compound, with potent antioxidant and anti-inflammatory activities, induces resistance to Aβ1-42 toxicity in the brain (Yan et al., 2001). Administration of diCQA to Alzheimer-model rats protects against the aging, especially learning and memory deficits induced by Aβ (Isoda et al., 2006).

RELATIONSHIP OF FUNCTIONAL COMPONENT AND STRUCTURE The structural feature responsible for the antioxidative and free radical-scavenging activity of CA is the ortho-dihydroxyl functionality in the catechol (Mahmood et al., 1993). The cathecol structure also plays an important role in the strong antimutagenicity of anthocyanin pigments (Yoshimoto et al., 2001). Therefore, the physiological function of the

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CQA derivatives with plural caffeoyl groups is more effective than with a monocaffeoyl one. The radical scavenging activity and the antimutagenicity of these derivatives in order of efficacy is triCQA > diCQAs > monoCQA, suggesting that the number of caffeoyl groups bound to QA plays a role in the radical scavenging activity of the CQA derivatives. In other words, additional caffeoyl groups bound to QA are necessary for higher function. The 3,4,5triCQA exhibits a greater selective inhibition of HIV replication than other CQA derivatives (Mahmood et al., 1993; Tamura et al., 2006). Thus, although there is no direct association, the CQA derivatives have the potential to protect humans from various kinds of diseases. Especially 3,4,5-triCQA shows remarkable activities for various kinds of physiological functions (Yoshimoto et al., 2002b; Matsui et al., 2004a; Mishima et al., 2005). ChA and diCQA derivatives have been isolated from various plants including sweet potato (Walter et al., 1979; Shimozono et al., 1996), but there are very few reports on 3,4,5-triCQA. Several varieties of sweet potato contain a high content of 3,4,5-triCQA (Islam et al., 2002a; 2003a), suggesting that the sweet potato leaf is a source of not only mono- and diCQA derivatives but also 3,4,5-triCQA. A large scale purification of 3,4,5-triCQA from sweet potato leaves is established in KONARC (unpublished data). As reviewed previously, sweet potato anthocyanins have been reported to possess multifaceted action, including antioxidation, antimutagenicity, anti-inflammatory, and anticarinogenesis. Extensive structure-activity studies have shown that the number of sugar units and hydroxyl groups on aglycons is associated with biological activities of anthocyanins. The activities appear to increase with a decreasing number of sugar units, and with an increasing number of hydroxyl groups on aglycons (Yoshimoto et al., 2001; Hou et al., 2004). Oral intake of anthocyanins from purple sweet potato and red cabbage color suppresses rat colon carcinogenesis induced by 1,2-dimethylhydrazine (DMH) and 2-amino-1 methyl-6-phenylimidazo [4,5-b] pyridine (PhIP) (Hagiwara et al, 2002). Of the six anthocyanins tested, only those with an ortho-dihydroxyphenyl structure on the B-ring suppressed 12-O-tetradecanoylphorbl-13-acette (TPA)-induced cell transformation and activator protein-1 transactivation, suggesting that the ortho-dihydroxypehnyl may contribute to the inhibitory action (Hou et al., 2003). The structural feature responsible for the antioxidative and free radical scavenging activity of CA is the ortho-dihydroxyl functionality in the catechol (Son and Lewis., 2002). These activities might depend on the number of hydroxyl group in the structure (Yoshimoto et al., 2001; Hou, 2003). Cyanidin containing two hydroxyl groups shows stronger activity on antimutagenicity than that of peonidin, which has only one group (Yoshimoto et al., 1999b; 2001). Based on additional studies with enzyme activity, the cyanidins protect against the mutagenesis partly by direct reactions with enzymatically activated carcinogens (heterocyclic amines) rather than by the interaction with metabolic enzyme (Yoshimoto et al., 1999b).

SWEET POTATO USE FOR NON- FOOD Sweet potato root is used to make various processed food and food materials, such as juice, natural food colorant, confectionery, shochu, and starch. This process unavoidably discharged wastes and the cost of disposing of these wastes is a main cause of lowering profitability of food processors. In such a circumstance, it is an important question to find

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ways to use effectively the wastes from food processing. In Japan, studies are actively conducted clarify the characteristics of these wastes and to develop recycling technology. The subjects of these studies include the method of converting wastes from starch production into biodegradable plastics and recycling of waste liquids from shochu making not merely as feed and manure but also as biodegradable farming materials and foods. Treatment of sweet potato waste derived from shochu, starch, and leaves is important in the southern Kyushu area in Japan. Shochu waste is used as a raw material of a vinegar-like beverage (Yoshimoto et al., 2004) and bread (Sho et al., 2008) with high content of polyphenolics. Research of polyphenolic composition in shochu waste demonstrates the enzymatic hydrolysis of CQA derivatives to CA and QA in shochu fermentation process by koji, fungi for traditional fermented products in Japan (Yoshimoto et al., 2005a). CA is a raw material for environmentally degradable, high-performance thermoplastics (Kaneko et al., 2006). Furthermore ethyl caffeate isolated from sweet potato shochu distillery by-products inhibits weed seed germination and radical elongation, suggesting a potential as herbicide (Okuno et al., 2006). Sweet potato leaves also can be used as an animal feed for egg-laying hens (Takenoyama et al., 2007) and beef cattle (Takenoyama et al., 2008). Sweet potato leaves contains high content of polyphenolics (Islam et al., 2002a ) and the leaf extract is used in cosmetics. Starch waste fiber from sweet potato is industrially used for the material of environmentally degradable sheets for agriculture.

CONCLUSIONS Sweet potato root is a resource of anthocyanin pigments with thermo- and photostability. Furthermore, anthocyanin composition in sweet potato affects not only the quality of food colorants (Odake et al., 1994) and paste color (Yoshinaga et al., 1999) but also physiological activities (Islam et al., 2002b; Yoshimoto et al., 1999a, 2001). KONARC (Japan) is currently focusing on development of new varieties of sweet potato with different pigment composition and more thermo- and photostable pigments. Sweet potato leaves have been shown to contain higher levels of oxalic acid than leafy vegetables from temperate climate, highest being reported in spinach (Evensen and Standal 1984). Oxalate concentrations in food crops have long been a concern in human diet, because of the negative health effects associated with high intake of oxalate levels that can cause acute poisoning, resulting in hypocalcaemia. Furthermore, oxalic acid and soluble oxalates can bind calcium, reducing its bioavailability and humans poorly utilize calcium oxalate itself. The average content of oxalic acid of sweet potato variety ―Suioh‖ leaves is 280 mg/ 100 g fresh weight. This content is not high compared with the 930 mg/100 g fresh weight in spinach (Ishiguro et al., 2004b). Oxalic acid contents of other sweet potato varieties tested are also several times less than that of spinach (Yoshimoto et al., 2002a). Sweet potato contains various kinds of physiologically functional components in roots and leaves, which have the potential to maintain human health and mitigate the diseases. However, much of the evidence is based on research using rats and cell cultures and there is no evidence to directly support benefit to humans. Therefore, moderate consumption of these functional components through the intake of the products may be linked with the chemoprevention of the diseases, further epidemiological and efficacy studies on this aspect

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are required. In the worldwide food shortage and increasing food prices, sweet potato is a crop that can contribute to the effective use as not only foods with various kinds of physiological functions, but also the natural resources and the reduction in environmental load.

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Pace, R.D., Sibiya, T.E., Phills, B.R. and Dull, G.G. (1985). Ca, Fe and Zn content of ‗Jewel‘ sweet potato greens as affected by harvesting practices. J. Food Sci. 50: 940-941. Peluso, G., De Feo, V., De Simone, F., Bresciano, E. and Vuotto, M.L. (1995). Studies on the inhibitory effects of caffeoylquinic acids on monocyte migration and superoxide anion production. J. Nat. Prod. 58: 639-646. Picha, D.H. (1987). Chilling injury, respiration, and sugar changes in sweet potatoes stores at low temperature. J. Am. Soc. Hortic Sci. 112: 497-502. Prior, R.L., Cao, G., Martin, A., Sofic, E., McEwen, J., O‘Brien, C., Lischner, N., Ehlenfeldt, M., Kalt, W., Krewer, G. and Mainland, C.M. (1998). Antioxidant capacity as influenced by total phenolic and anthocyanin content, maturity and variety of Vaccinium species. J. Agric. Food Chem. 46: 2686-2693. Reshef, Y., Mizrachi, E., Maretzki, T., Silberstein, C., Loya, S., Hizi, A. and Carmeli, S. (1997). New acylated sulfoglycolipids and digalctolipids and related known glycolipids from cyanobqacteria with a potential to inhibit the reverse transcriptase of HIV-1. J. Nat. Prod. 60: 1251-1260. Rhodes, D., Handa, S., and Bressan, R.A. (1986). Metabolic changes associated with adaptation of plant cells to water stress. Plant Physiol. 82: 890-903. Rice-Evans, C.A. Miller, N.J., Bolwell, P.G., Bramley, P.M. and Pridham, J.B. (1995). The relative antioxidant activities of plant-derived polyphenolic flavonoids. Free Rad. Res. 22:375–83. Robards, K., Prenzler, P.D., Tucker, G., Swatsitang, P. and Glover, W. (1999). Phenolic compounds and their role in oxidative processes in fruits. Food Chem. 66: 401-436. Roberton, A.M., Ferguson, L.R., Hollands, H.J. and Harris, P.J. (1991) Adsorption of a hydrophobic mutagen to dietary fiber preparations. Mutat. Res. 262: 195-202. Rogers, J., Kirby, L.C., Hempelman, S.R., Berry, D.L., McGeer, P.L., Kaszniak, A.W., Zalinski, J., Cofield, M., Mansukhani, L. and Willson, P. (1993). Clinical trial of indomethacin in Alzheimer‘s disease. Neurology 43: 1609-1611. Salvador, L.D., Suganuma, T., Kitahara, K., Tanoue, H. and Ichiki, M. (2000). Monosaccharide composition of sweet potato fiber and cell wall polysaccharides from sweet potato, cassava, and potato analyzed by the high-performance anion exchange chromatography with pulsed amperometric detection method. J. Agric. Food Chem. 48: 3448-3454. Sano, M., Ernesto, C., Thomas, R.G., Klauber, M.R., Schafer, K., Grundman, M., Woodbury, P., Growdon, J., Cotman, C.W., Pfeiffer, E., Schneider, L.S. and Thal, L.J. (1997). A controlled trial selegiline, alpha-tocopherol, or both as treatment for Alzheimer‘s disease. The Alzheimer‘s disease cooperative study. N. Engl. J. Med. 336: 1216-1222. Scarpati, M.L. and Guiso, M. (1964). Structure of the three dicaffeoyl-quinic acids of coffee (isochlorogenic acid). Tetrahedron Lett. 39: 2851-2853. Shahrzed, S. and Bitsch, I. (1996). Determination of some pharmacologically active phenolic acids in juice by high-performance liquid chromatography. J. Chromatogr. A. 741: 223231. Shimozono, H., Kobori, M., Shinmoto, H. and Tsushida, T. (1996). Suppression of the melanogenesis of mouse melanoma B16 cells by sweet potato extract. Nippon Shokuhin Kagaku Kaishi 43: 313-317 [In Japanese with English summary]. Shinohara, K., Kurogi, M., Miwa, M., Kong, Z. and Hosoda, H. (1988). Antimutagenicity of dialyzates of vegetables and fruits. Agric. Biol. Chem. 52: 1369-1375.

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Takenoyama, S., Kawahara, S., Ahhmed, A., Mori, H., Tarumizu, K., Ishiguro, K., Yoshimoto, M. and Muguruma, M. (2008). Effects of sweet potato litters as supplemental feed on beef cattle. Bull. Facul. Agric., Univ. of Miyazaki 54: 77-84 [In Japanese with English summary]. Takenoyama, S., Kawahara, S., Hayashi, H., Imoto, T., Nagase, T., Ando, T., Ishiguro, K., Yoshimoto, M. and Muguruma, M. (2007). Effects of sweet potato litters as supplemental feed on egg-laying hens. Bull. Facul. Agric., Univ. of Miyazaki 53: 93-99 [In Japanese with English summary]. Takeyama, E., Yokokawa, N., and Tanimura, A. (1996) Changes in polysaccharide components and metal adsorption ability of soybean dietary fiber on heating. Nippon Shokuhin Kagaku Kogaku Kaishi 43: 231-237 [In Japanese with English summary]. Tamura, H., Akioka, T., Ueno, K., Chujyo, T., Okazaki, K., King, P.J. and Robinson, W. Jr. (2006). Anti-human immunodeficiency virus activity of 3,4,5-tricaffeoylquinic acid in cultured cells of lettuce leaves. Mol. Nutr. Food Res. 50: 396-400. Terahara, N., Konczak, I., Ono, H., Yoshimoto, M. and Yamakawa, O. (2004). Characterization of acylated anthocyanins in callus induced from storage root of purplefleshed sweet potato, Ipomoea batatas L. J. Biomed. Biotechnol. 5: 279-286. Terahara, N., Matsui, T., Fukui, K., Matsugano, K., Sugita, K. and Matsumoto, K. (2003). Caffeoylsophorose in red vinegar produced through fermentation with purple sweet potato. J. Agric. Food Chem. 51: 2539-2543. Terahara, N., Shimizu, T., Kato, Y., Nakamura, M., Maitani, T., Yamaguchi, M. and Goda, Y. (1999). Six diacylated anthocyanins from the storage roots of purple sweet potato, Ipomoea batatas. Biosci. Biotechnol. Biochem. 63: 1420-1424. Terahara, N. and Sugita, K. (2000). High antioxidative activity of red vinegar produced by fermentation with purple sweet potato. International Congress and Symposium on Natural Colorants 2000 INF/COL, San Diego, CA, The Hereld organization Papers, 2000, p. 354-364. Toeller, M. (1994). α-Glucosidase inhibitors in diabetes: efficacy in NIDDM subjects. Eur. J. Clin. Invest. 24 (Suppl. 3): 31-35. Tsubata, M., Takagaki, K., Yoshimoto, M., Kurata, R. and Yamakawa, O. (2004). Effects of young sweet potato leaf powder on blood glucose. Food Processing and Ingredients 39: 57-58 [In Japanese]. Tsukui, A., Suzuki, A., Komaki, K., Terahara, N., Yamakawa, O. and Hayashi, K. (1999). Stability and composition ratio of anthocyanin pigments from Ipomoea batatas Poir. Nippon Shokuhin Kagaku Kogaku Kaishi 46: 148-154 [In Japanese with English summary]. Tsushida, T., Suzuki, M. and Kurogi, M. (1994). Evaluation of antioxidant activity of vegetable extracts and determination of some active compounds. Nippon Shokuhin Kogyo Gakkaishi 41: 611-618 [In Japanese with English summary]. Villareal, R.L., Tsou, S.C.S., Lin, S.K. and Chiu, S.C. (1979). Use of sweet potato (Ipomoea batatas) leaf tips as vegetables. II. Evaluation of yield and nutritive quality. Exptl. Agric. 15: 117-122. Walter, W.M.Jr., Purcell, A.E. and McCollum, G.K. (1979). Use of high-pressure liquid chromatography for analysis of sweet potato phenolics. J. Agric. Food Chem. 27: 938941.

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Woolfe, J. A. (1992). Sweet potato: an Untapped Food Resource. Cambridge University Press, Cambridge, UK, p. 1-621. Wrick, K.L. (1983). The influence of dietary fiber source on human intestinal transit and stool output. J. Nutr. 113: 1464-1479. Yamada, S. (1994). Satsumaimo-Denrai to Bunka. Shuneido Publication, Kagoshima [In Japanese]. Yamakawa, O., Yoshinaga, M., Hidaka, M., Kumagai, T. and Komaki, K. (1997). Ayamurasaki: a new sweet potato cultivar. Bull. Kyushu Natl. Agric. Exp. Stn. 31: 1-22 [In Japanese]. Yan, J.-J., Cho, J.-Y., Kim, H.-S., Kim, K.-L., Jung, J.-S., Huh, S.-O., Suh, H.-W., Kim, Y.H. with and Song, D.-K. (2001). Protection against amyloid peptide toxicity in vivo long-term administration of ferulic acid. Brit. J. Pharmacol. 133: 89-96. Yoshimoto, M. (2001). New trends of processing and use of sweet potato in Japan. Farming Japan 35: 22-28. Yoshimoto, M., Kurata-A., R., Fujii, M., Hou, D.-X., Ikeda, K., Yoshidome, T. and Osako, M. (2004). Phenolic composition and radical scavenging activity of sweet potato-derived shochu distillery by-products treated with koji. Biosci. Biotechnol. Biochem. 68: 24772483. Yoshimoto, M., Kurata-A., R., Fujii, M., Hou, D.-X., Ikeda, K., Yoshidome, T. and Osako, M. (2005a). Enzymatic production of caffeic acid by koji from plant resources containing caffeoylquinic acid derivatives. Biosci. Biotechnol. Biochem. 69: 1777-1781. Yoshimoto, M., Kurata, R., Okuno, S., Ishiguro, K., Yamakawa, O., Tsubata, M., Mori, S., and Takagaki, K. (2006). Nutritional value and physiological functions of sweet potato leaves. Acta Hort. 703: 107-115. Yoshimoto, M., Okuno, S., Kumagai, T., Yoshinaga, M., and Yamakawa, O. (1999a). Distribution of antimutagenic components in colored sweet potato. Jpn Agric. Res. Quarterly 33: 143-148. Yoshimoto, M., Okuno, S., Yoshinaga, M., Yamakawa, O., Yamaguchi, M. and Yamada, J. (1999b). Antimutagenicity of sweet potato (Ipomoea batatas) roots. Biosci. Biotechnol. Biochem. 63: 537-541. Yoshimoto, M., Okuno, S., Suwa, K., Sugawara, T. and Yamakawa, O. (2002a). Effect of harvest time on nutrient content of sweet potato leaves. Proc. 12th Symp. of the International Society for Tropical Root Crops, pp. 319-323. Yoshimoto, M., Okuno, S., Yamaguchi, M., and Yamakawa, O. (2001). Antimutagenicity of deacylated anthocyanins in purple-fleshed sweet potato. Biosci. Biotechnol. Biochem. 65: 1652-1655. Yoshimoto, M., Yahara, S., Okuno, S., Islam, M.S., Ishiguro, K. and Yamakawa, O. (2002b). Antimutagenicity of mono-, di-, and tricaffeoylquinic acid derivatives isolated from sweet potato (Ipomoea batatas L.) leaf. Biosci. Biotechnol. Biochem. 66: 2336-2441. Yoshimoto, M., Yamakawa, O., and Tanoue, H. (2005b). Potential chemopreventive properties and varietal difference of dietary fiber from sweet potato (Ipomoea batatas L.) roots. Jpn Agric. Res. Quarterly 39: 37-43. Yoshimoto, M., Yamakawa, O. and Tomita, Y. (2003). Sweet potato and carotenoid. Proc. 16th Annual Meeting on Carotenoid Research. 6: 2-4. Yoshimoto, M., Yamakawa, O. and Suda, I. (1998). Physiological function of purple colored flesh sweet potato. Food Processing and Ingredients 33: 15-17 [In Japanese].

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Yoshinaga, M., Yamakawa, O. and Yoshimoto, M. (1999). Genotypic diversity of anthocyanin content and composition in purple-fleshed sweet potato (Ipomoea batatas (L.) LAM). Breed Sci. 49: 43-4. Yu, B.P. (1994). Cellular defenses against damage from reactive oxygen species. Physiol. Rev. 76:139-162.

In: Sweet Potato: Post Harvest Aspects in Food Editors: R. C. Ray and K. I. Tomlins

ISBN 978-1-60876-343-6 © 2010 Nova Science Publishers, Inc.

Chapter 4

SWEET POTATO STARCH S. N. Moorthy and S. Shanavas Central Tuber Crops Research Institute, Sreekariyam, Thiruvanathapyram- 695 017, Kerala, India

ABSTRACT Sweet potato is an important food crop in the tropical countries and the roots are rich in starch. The starch has very desirable physicochemical and functional properties and therefore can have applications in food and industries. This chapter discusses the different characteristics of the starch in comparison with other root and tuber starches and the potential applications. The starch granule size ranges from 4-43 µm, with ‗A‘ type XRD pattern and an amylose content of around 20%. The swelling power and solubility are similar to other root starches. The viscosity and pasting temperature are almost in the same range as cassava. The enzyme digestibility, water binding capacity and rheological properties have also been described. The conditions for the liquefaction and saccharification of sweet potato starch for possible application in the production of ethyl alcohol are given in detail.

ABBREVIATIONS BU DP DSC HPAEC FTIR PV RVA SEM 

Branbender units; Degree of polymerization; Differential Scanning Colorimetry; High Performance Anion Exchange Chromatography; Fourier Transform Infra Red; Peak viscosity; Rapid Visco Analyzer; Scanning Electron Microscopy

Corresponding author: E-Mail: [email protected] Tel: 91-471-2598551; Fax: 91-471-2590063

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INTRODUCTION Starch is one of the major biochemical components in the plant kingdom, especially in the root and tuber crops. The starch content varies from 10-30% and the different starches have different functional properties. The application of the starch in food depends on the starch content and starch properties. Next to cassava, sweet potato has the highest starch content among the root crops and the extraction process is comparatively simple. In developing countries, sweet potatoes are processed into starch, noodles, candy, flour and desserts. In China for example, sweet potato starch production has become an important cottage industry. Moreover, China is the largest grower of sweet potatoes, providing about 80% of the worlds supply. There are more than 2000 varieties of sweet potatoes in China which can be roughly divided into ‗general type‘, ‗high starch type‘ and ‗food consumption type‘ (Liu, 2004). The major content of the dry matter in sweet potato is mainly starch which can be extracted from the roots. The uses of sweet potato starch are primarily determined by its physicochemical properties like starch granule shape and size, amylose content, molecular starch structure and pasting properties, retrogradation tendency, etc. A number of studies on the distinctive properties of sweet potato starch have been undertaken in different laboratories in the last decade. Crystalline structure, gelatinisation, pasting behaviour and retrogradation have been investigated (Takeda, 1986; Noda et al., 1992, 1996; Collado and Corke, 1997; Garcia and Walter, 1998; Katayama et al., 2002; Katayama et al., 2004). The objective of this chapter is to bring together the present knowledge on starch derived from this crop.

STARCH EXTRACTION The properties of sweet potato starch are very similar to cassava starch. However, though the extraction of starch from cassava is widely practiced, starch extraction from sweet potato is not so widely prevalent. The main reasons attributed are that the settling of starch is slow such that the longer residence time can lead to microbial growth and thereby lower the quality; this reduces the price of starch. For getting optimum yield and quality of starch, the correct time of maturity, methodology used for extraction and processing machinery are important. If roots are harvested late, the starch may get converted to sugar and fibre and thus affect yield and quality. Delays in processing sweet potatoes can result in the accumulation of sucrose and reducing sugars (Heinze and Appleman, 1943). Delays between shredding and starch extraction in sweet potato or the roots may lead to the synthesis of toxic compounds such as the alkaloid ipomeamarone, and the derived starch may become inedible and hazardous (Jain et al., 1951). The method of starch isolation (Figure 1) can affect both the physicochemical properties of the starch and the level of non-starch components, which in turn may also affect the physico-chemical properties of the starch indirectly (Lii and Chang, 1978; Takeda et al., 1986). The recovery of starch from sweet potato roots increased by more than 20% by using pectinase and cellulase enzymes. These enzymes act by breaking the pectin-cellulosic matrix of cell membranes resulting in the release of the starch granules. The treatment up to 0.05% concentration of enzyme gives higher yield without affecting its starch properties (Kallabinski and Balagopalan, 1991; Moorthy, 1999). Other methods used to

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improve yield of starch is use of lime (Radley, 1976a) and dilute acetic and lactic acid during extraction.

Figure 1. Course starch production by farming households (Liu, 2004).

BIOCHEMICAL CHARACTERISTICS Though the starch appears to be in the pure form free from other components, thorough investigation of extracted starch have revealed that it is invariably contaminated by various other components (Table 1), i.e., fibre, lipids, proteins and minerals depending on a number of factors such as method of extraction, age of the crop, environmental conditions, etc. Some of these impart desirable qualities to the starch, while others have a detrimental effect on quality. Table 1. Proximate composition of isolated starch Parameters (g/kg) Moisture

Range 139-150

Reference Lii and Chang, 1978

Ash

0.8-1 2.6-5.1 0.7-1.8 0.5-1 4.8-5.4 1.3-2 0.6-6 0.19 980-988

Delpeuch et al., 1978,1979 Lii and Chang, 1978 Delpeuch et al.1978,1979 Lii and Chang, 1978 Delpeuch et al. 1978,1979 Lii and Chang, 1978 Delpeuch et al. 1978,1979 Lii and Chang, 1978 Delpeuch et al., 1978,1979

Fibre Crude protein Crude lipid Phosphorus Starch

The starch content in the extracted starch is nearly more than 95% but this depends on maturity. The moisture content suggested for safe storage of starch is 13% (ISI, 1970; Radley, 1976a), but among tuber and root starches large variation have been found (Kay, 1987; Takeda et al., 1986; Soni et al., 1990; Melo et al., 1994). The root starches contain much

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smaller quantities of native lipids in them, hence the addition of lipids or surfactants was found to enhance the properties of starch quality and it was found that there is no hindrance for the root starches to complex with surfactants or lipids added externally. The phosphorous content in sweet potato is similar to that in cassava starch (Rickard et al., 1991) but both of these are less than that in Irish potato (Hizukuri, 1969). Phosphate is believed to be an important factor in determining the granular strength by forming cross linkages.

GRANULAR CHARACTERISTICS Size and Shape The granule size varies from less than 1µm to more than 100µm. Sweet potato starch granules have been reported as round, oval an polygonal shapes with size ranging between 3 and 28µm(Chen et al., 2003).The size and shape of starch granules from sweet potato are given in Table 2. Madamba et al. (1975) found significant differences among all sweet potato varieties studied (Figure 2). Sweet potato granules are of a similar size to those of cassava and maize but are smaller than those of potato which also have a larger range of granular size (Dreher and Berry, 1983). Starch grains are of variable shape (oval, round, faceted round and polygonal) and are normally non-aggregated. Granule size ranges from 4-43µ, depending on the cultivar. The mean size of the granule ranges between 12.3-21.5µ. The granule size is reported to affect some functional properties like swelling, solubility and digestibility. Bowkamp (1985) reported negative correlation between particle size and susceptibility to amylase and acid degradation for sweet potato cultivars. According to Rasper (1971), particle size including size distribution, is one of the characteristics that most markedly affects the functional properties of starch granules. Smaller granules are reported to have both high solubility and water absorption capacity (Georing and Dehaas, 1972). Earlier studies revealed that sweet potato starch is polygonal or nearly round in shape (Tian et al. 1991; Woolfe, 1992; Shin and Ahn, 1983; Bouwkamp, 1985) and has a centric distinct hilum. Polarisation crosses are comparatively less distinct. Table 2. Size, shape and X-ray diffraction pattern of sweet potato starches Sl.no 1 2 3 4 5

Size (μm) 14-34 3-42 4-40 10-14 4-43

6

9-38

Shape Round polygonal Round polygonal Polygonal oval round non-aggregated Non-aggregated, oval polygonal

X-ray pattern Ca Ca -

Reference Shin and Ahn, 1983 Seog et al., 1987 Delpeuch et al., 1978 Lii and Chang, 1978 Bouwkamp, 1985

-

Madamba et al., 1975

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Figure 2. Frequency curves of the granules sizes of starch of six sweet potato varieties grown in Philippines (Madamba et al., 1975).

Crystalline Structure The Crystalline nature of a starch granule can be defined by the position of the X-ray diffraction peaks (Zoebel, 1988). Table 2 and Figure 3 represent the X-ray pattern of some sweet potato starches. Hizukuri (1969) demonstrated that mixtures of A- and B- type starches produced intermediate pattern (C-type). Sweet potato starch has a variable X-ray pattern between C and A, in contrast to cereal starches such as wheat and corn which have A-type and potato which has B-type pattern (Zoebel, 1988). Sweet potato starch also has ‗A‘ (Takeda et al., 1986; Szylit et al., 1978; Gallant et al., 1982), ‗C‘ (Shin and Ahn, 1983; Zoebel, 1988; Chiang and Chen, 1988) or intermediate pattern (Tian et al., 1991). Takeda et al. (1986) observed ‗A‘ pattern for two varieties while it was ‗CA‘ for another variety with absolute crystallinity of 38%.

Molecular Weight Studies on sweet potato starch has revealed that amylopectin to have peaks at DP (degree of polymerisation) =12 and DP=8. The concentrations of the peaks at DP=6 and DP=7 were 7.1-7.5% and 6.7-7.0%, respectively. Takeda et al. (1986) reported trimodal pattern for the sweet potato amylopectin and Hizukuri (1969) a bimodal distribution. They concluded that sweet potato has a higher proportion of ‗A‘ chains and short ‗B‘ chains compared to potato.

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Figure 3. An X-ray diffraction pattern of sweet potato starch (Chiang et al., 1988).

Chain length has been found to vary in some varieties based on the low viscosity and high reducing values (Woolfe, 1992). Sweet potato amylose appears to have more branches per amylose molecule than that from cassava, potato, wheat or maize, and have a higher molecular weight than maize, wheat and cassava but less than potato. Takeda et al. (1986) suggested this was the reason for the low retrogradation tendency of sweet potato amylose. The degree of polymerization and branching has been reported to have a substantial effect on the physicochemical properties of amylose and amylopectin (Zobel, 1988).

PHYSICOCHEMICAL PROPERTIES Amylose Content Sweet potato can have amylose contents slightly higher than that of cassava but less than that of wheat, maize or potato (Rikard et al., 1991). The amylose content of sweet potato is considered to be one of the most important factors influencing the cooking and textural qualities of storage roots and sweet potato starch based products (Collado et al., 1999). Sweet potato starch amylose content has been reported between 8.5 and 35% (Table.3). Madamba et al. (1975) reported amylose contents of sweet potato starches to be from 29.4 to 32.2 % for the six cultivars. They found that six varieties of sweet potatoes from the Philippines had amylose content that were lower than that of other root crops including cassava. Uehara (1983) found an amylose content of 21.6 % in sweet potato starch. Watanabe et al. (1982) reported that the amylose content of sweet potato starch was 20.9 %. Garcia and Walter (1998) obtained values ranging from 20-25% (by potentiometric titration) for some Peruvian cultivars. Curing had only a minor effect on amylose content (Bertoniere et al., 1966) or a slight increase (Hammet and Barrentin, 1961). In general, sweet potato can have amylose content slightly higher than that of cassava but less than that of wheat, maize or potato

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(Rickard et al., 1991). Delpeuch et al. (1978) concluded that the amylose content in sweet potato was not affected by the manner of cultivation or the year of harvest. Table 3. Physicochemical properties of sweet potato starches Properties

Amylose content (%)

Water binding capacity (%) Swelling volume (ml/g)

Solubility (%)

Digestibility (%) Acid resistance to 16 % sulphuric acid at 50°C

Range 23.2-26.3 29.6-32.4 16.1-24.4 17.5-18.3 13.4-22.5 22 23.6-27.6 8.5-17.3 14.8 17.2-19 25-28 19.4-22.8 27-38 21.5-22 22-25 178.9-185.5 66.3-211.6 46 27.5-33.3 (95°C) 24.5-27.4(85°C) 63-95 (95°C) 32-46 (80°C) 18 13.2-14.4(95°C) 11.4-12.9(85°C) 60-79(95°C) 30-50(80°C) 14.9- 43.3 20.8 43.7% acid -resistant portion, 49.6 % low acid- resistant portion

Reference Hammett and Berrentin, 1961 Madamba et al., 1975 Madamba and San Pedro, 1976 Delpeuch et al., 1979 Shen and Sterling, 1981 Watanabe et al., 1982 Shin and Ahn, 1982 Liu and Liang, 1983 Liu et al., 1985 Takeda et al., 1986a Seog et al .1987 Chiang and Chen, 1988 Martin and Deshpande, 1985 Kitada et al. 1988 Shiotahi et al., 1991 Shin and Ahn, 1983 Seog et al., 1987 Woolfe,1992 Chiang and Chen, 1988 Seog et al., 1987 Seog et al., 1987 Seog et al., 1987 Woolfe, 1992 Chiang and Chen, 1988 Seog et al., 1987 Seog et al., 1987 Seog et al., 1987 Fuwa et al., 1977 Ueda and Jaha, 1983 Nara et al., 1983

Alkali Number The alkali number is a measure of the number of reducing end groups and is related to the molecular weight (Schoch, 1964 a). Seog et al. (1987) reported that the alkali number values of six Korean sweet potato starches ranged between 7.66 and 12.13.

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Swelling and Solubility Swelling power and solubility of starch is an important physicochemical property determining the use of starch in different applications. When starch is heated in the presence of water, the individual granules swell and a portion of the starch dissolves in the surrounding aqueous medium. The degree of swelling and the amount of solubilisation depend on the extent of chemical cross-bonding within the granules (Schoch, 1964b). The pattern of progressive swelling and solubilisation of various starches have been compared over a range of temperatures to provide information about the relative strengths of bonding within granules (Rasper, 1969). Swelling power and solubility indicates the strength of non-covalent bonding between starch molecules and depend on factors that include the amylose-amylopectin ratio, chain length and molecular weight distribution, degree of branching and conformation (Rickard et al., 1991). The swelling and solubility of starch permits comparison of relative bond strength at specific temperatures (Leach et al., 1959). The presence of non-carbohydrate substances in starch such as lipid or phosphate may affect swelling (Leach et al., 1959; Moorthy and Ramanujan, 1986). Swelling power of sweet potato starch varies considerably not only among varieties, but also at different temperatures. Delpeuch and Favier (1980) have reported a two stage swelling but Madamba et al. (1975) found a single stage swelling for the same starch (Figure 4). The lower swelling volume of sweet potato starch has been attributed to a higher degree of intermolecular association compared to cassava or potato starch.

Figure 4. Swelling and solubility patterns of sweet potato starches grown in Philippines (Madamba et al., 1975).

Solubility of starch is influenced by a number of factors that include the source, interassociative forces, swelling power and presence of other components like lipids, surfactants, salts, sugars, etc. The high swelling volume of the sweet potato starch is reflected in its

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solubility, which is similar to cassava starch. It was presumed that the bonding forces might be tenuous but comparatively extensive, immobilising the starch within the granules even at high levels of swelling. Reported solubilities of sweet potato starch ranged from approximately 10-18 % (Madamba et al., 1975). The relatively high swelling of sweet potato is not accompanied by high solubilities. This characteristic was also observed by Leach et al. (1959) in potato. As reviewed by Tian et al. (1991), sweet potato amylose appears to be more branched than that from cassava. Comparative experiments have shown that the swelling and solubility of sweet potato starch (Table 3) are less than those of potato and cassava, but generally more than those of maize (Rasper, 1969; Delpeuch and Favier, 1980). It has therefore been suggested that sweet potato starch has a higher degree of intermolecular association in its starch granules than has potato or cassava starch (Madamba et al., 1975).

Water Binding Capacity The water – binding capacity of starch gels has been commonly determined by the method of Medcalf and Gilles (1965). The values for sweet potato range from 66.3 to 211.6% as shown in Table 3. In general, root and tuberous starches have higher water –binding capacities than those of cereal origin (Banks and Greenwood, 1975), and the majority of workers have demonstrated that sweet potato starch has higher water –binding capacity than potato (93%) (Dreher and Berry, 1983) and cassava starches (72-92%) (Rickard et al., 1991).

DIGESTIBILITY Starch digestibility by enzymes is of importance for evaluating nutritive value and in industrial applications. For raw starches, digestibility of cassava, sweet potato, Colocasia, Xanthosoma and Amorphophallus starches is quite high (65-75%), comparable to corn starch (76%). Sweet potato starch was found to be very susceptible to degradation by -amylase and glycoamylase (Cerning-Beroard and Le Dividich, 1976). Digestibility of raw starch of eight sweet potato varieties by glycoamylases was compared by Noda et al. (1992). Gallant et al. (1982) found that ‗A‘ type starches showed high susceptibility to - amylase. They found that pelletisation increased the raw starch digestibility with bacterial - amylase from 17% to 45%. Scanning electron microscopy (SEM) studies indicated that enzymatic corrosion occurs mainly at the surface of the granules. The susceptibility of sweet potato starch to - amylase after 1-day incubation was found to range from 35.7-65.5 % weight loss among the six cultivars tested.

Degradation by Acid Dilute acids can be used to elucidate the architecture of the starch granule (Banks and Greenwood, 1975). There is an initial attack on the amorphous regions which enhances crystallinity and increases thermal stability (Biliaderis et al., 1981). The solubility on heating

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increases with acid degradation and the viscosity is lowered but granular integrity can be maintained even when 25% of the starch has been hydrolysed (Banks and Greenwood, 1975). The susceptibility of sweet potato starches to acid corrosion showed highly significant differences among cultivars (Rasper, 1969). Nara et al. (1983) investigated the kinetics of acid degradation and found that it could be described by two exponential hydrolysis rates, a fast hydrolysis of the amorphous regions and a slow hydrolysis of the crystalline regions (Table 3). Sweet potato and maize both had a large amount of acid-resistant starch, but the acid-resistant component of sweet potato starch was hydrolysed at a faster rate than that of other starches.

Degradation by Enzymes Enzymatic degradation can be evaluated by quantitative determination of the products from digestion or by measuring the decrease in hot paste viscosity (Rasper, 1969). SEM can also be used to examine the starch granules after attack (Hizukuri, 1969). Characteristics of αamylase action on sweet potato starch granules have been the subject of numerous investigations and reports (Noda et al., 1992). Theses studies have shown that starches vary in their resistance to the action of α-amylase. Starch susceptible to enzyme attack is influenced by several factors such as amylose and amylopectin content, crystalline structure, particle size and the presence of enzyme inhibitors. Among theses granular structure is believed to be most important. Both amylose and amylopectin are attacked by β-amylase in a step wise manner from the non reducing ends, until cleavage reaches, on average, apposition two residues from the branch points. β- amylase can be used to determine external chain lengths and to estimate the number of branch points (Hokama et al., 1980; Lii et al., 1987; Manners, 1989). Lii et al. (1987) reported a β-amylase limit for the amylase of sweet potato of 87.9%, substantially greater than the results of Takeda et al. (1969). In contrast, α-amylase is able to attack the polymers randomly at any α-1, 4-linkage that is sterically accessible. Varietal differences among sweet potato starches in susceptibility to attack by α-amylase have been reported to be highly significant (Madamba et al., 1975).

Retrogradation On cooling, dispersions of gelatinized starch granules in water acquire the consistency of gels. Above a critical concentration the swollen granules become entangled in amylose chains which have diffused out of the starch granules. The resultant composite is in essence an amylose gel with the swollen starch granules as a filter (Gidley, 1989). The above situation may be further complicated where the starch granules are ruptured by shearing or other methods of thermal or mechanical damage (Mestres et al., 1988). Further changes occur on storage, involving recrystallisation (or retrogradation) of the polymer chains. Retrogradation is affected by the amylose and amylopectin concentrations, the presence of other molecules such as sugars, salts and emulsifiers, molecular size, temperature, pH and other non-starch components (Del Rosario and Pontiveros, 1983).

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Takeda et al. (1986a) examined the retrogradation of sweet potato amylose which appeared to retrograde at the same rate as that of cassava but more slowly than that of Irish potato amylose. In contrast, Rasper (1969a) reported that sweet potato amylose retrograded slower rate than that of cassava and also that sweet potato amylopectin retrograded at a greater rate than that of cassava amylopectin. Del Rosario and Pontiveros (1983) found that sweet potato starch retrograded more slowly than wheat, corn and cassava starches and suggested that this was the reason for the observation that bread containing sweet potato flour as a substituent staled at a slower rate than other breads. Retrogradation is usually accompanied by gel hardening and by leakage of water from starch gel during storage. Retrogradation properties of tuber and root starches have been investigated by Differential Scanning Colorimetry (DSC), rheological measurements, FTIR (Fourier Transform Infra Red), Raman spectroscopy and X-ray diffraction. However, most of the information available are on potato and cassava starches (Hoover, 2001).

Sol Stability Sol stability or paste stability reflects the retrogradation tendency of starch pastes. Cassava and sweet potato starches have low retrogradation tendency and therefore exhibits high paste stability.

THERMAL CHARACTERISTICS DSC is an important tool to investigate starch gelatinisation (Biliaderis, 1983, 1990; John and Shastri, 1998; Eliasson, 1994). Most of these DSC studies have been carried out on cereal starches and to some extent on potato and cassava starches (Moorthy et al., 1996; Defloor et al., 1998; Farhat et al., 1999; Stevens and Elton, 1971; Wootton and Bamunuarachchi, 1979; Asaoka et al., 1992), whereas information on the DSC of the other root starches is comparatively limited. As starch grains are heated in aqueous suspension, they take up water. There are thought to be at least three main stages, hydration, swelling and melting of the crystallites (Blanshard, 1979). The gelatinization properties of starch are related to variety of factor including the size, proportion and kind of crystalline organization and ultra-structure of the starch granules (Singh et al., 2005) Gelatinisation temperature is indicative of the temperature at which the starch granules starch gelatinising. The gelatinisation temperature is controlled not only by the water content but also by the presence of salts, sugars and other small molecules. Average gelatinisation temperature for starch from six cultivars of sweet potatoes was found to range from 63.6-70.7 °C by Madamba et al. (1975). A significant positive correlation was found between average gelatinisation temperature and amylose content of the starches. A typical DSC pattern of sweet potato starch is given in Figure 5. Gelatinisation occurred over a range of 12-17°C of temperature change. Barham et al. (1946) found five cultivars had average gelatinisation temperatures from 69-75.5 °C and that the average gelatinisation temperature was reduced after curing the roots. Rasper (1969 b) reported that sweet potato starch began to gelatinise at 77 °C and continued the increase in viscosity until a temperature of 85°C was attained.

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Figure 5. DSC pattern of sweet potato starch (Moorthy, 2002).

Collado et al. (1999) obtained considerable variation in all the DSC parameters of 44 sweet potato varieties. The mean Tonset was 64.6°C and range 61.3-70° C, mean Tpeak 73.9°C (range 70.2-77°C) and mean Tend 84.6°C, range being 80.7-88.5°C and the mean gelatinisation range was 20.1° with a range of 16.1 to 23°C. Garcia and Walter (1998) found the range to be between 58-64°C for Tonset, 63-74°C for Tpeak and 78-83°C for Tend for the two varieties cultivated at different locations. While selection index did not affect the values, location influenced the parameters (Tian et al., 1991). Kitada et al. (1988) found that the gelatinisation temperatures were affected by the region in the sweet potatoes had been grown. Noda et al. (1998) reported that To, Tp, ∆H of 51 sweet potato starches differing in variety or cultivation condition ranged between 55.7-73.1°C, 61.3-77.6 °C and 12.7-16.8 J/g. Noda et al. (1995, 1998) reported that only small variations in chain length distributions (DP6-17) of amylopectin determined by HPAEC (High Performance Anion Exchange Chromatography) were observed in 31 varieties and 51 samples of sweet potato. Noda et al. (1996) did not find effect of fertilisation on the DSC characteristics of two sweet potato varieties. During the growth period, the Tonset was the lowest at the latest stage of development. Table 4 gives the thermal characteristics of some of the common sweet potato starches. They reported that increase in short outer chains of amylopectin reduced the packing efficiency of double helices within the crystalline region, resulting in lower gelatinisation temperature and enthalpy. Valetudie et al (1995) have compared the gelatinisation temperatures of starch from fresh roots and freeze dried roots of sweet potato (Table 5). A major factor controlling swelling is the strength of the internal structure of the granule being the size, amylose content, molecular weight, crystallinity and the internal granular organization (Banks and Greenwood, 1975; Takeda and Hizukuri, 1974; Madamba et al., 1975). Starch gelatinisation may be described either in structural terms as a loss of

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macromolecular organization and order or as a swelling process (which also has major rheological effects). Table 4. Thermal properties of sweet potato starch No 1 2 3 4 5

T onset (°C) 67-75 65.6-68.2 61.3 58-64 67.3

T peak(°C) 73-79 72.8- 74.3 70.2-77 63-74 72.7

T endset (°C) 81.4-84.8 84.6-86.8 80.7-88.5 78-83 79.6

∆H (J/g) 10-12.3 15.1-16.3 15.1-16.3 14.8-18.6 13.6

Reference Chiang and Chen, 1988 Kitada et al, 1988 Collado et al., 1999 Wankhede and Sajjan, 1981 Valetudie et al., 1995

Gelatinisation enthalpy depends on a number of factors like crystallinity, intermolecular bonding, etc. For sweet potato starch, the value for gelatinisation enthalpy lies between 10.018.6 J/ g (Tian et al., 1991; Garcia and Walter 1998; Collado et al., 1999). The effect of variety and environmental conditions was also evident (Garcia and Walter, 1998; Noda et al., 1996). During growth period, the H was lowest at the earliest stage of development in two sweet potato cultivars and the enthalpy ranged between 11.8-13.4 J/ g (Noda et al., 1992).

RHEOLOGICAL PROPERTIES The intrinsic viscosity is related to the ability of polymer molecules to increase the viscosity of the solvent, in the absence of any intermolecular interactions (Young, 1981). Intrinsic viscosity is directly related to molecular size and hence to the degree of polymerisation (Daniels, 1966). The intrinsic viscosity of starches from six cultivars was found to be 120-155ml/g. This indicated that sweet potato starches are not as highly polymerised as potato starch. Rasper (1969) found a maximum viscosity of sweet potato starch of 590 BU (Branbender units), slightly viscous than gelatinised cassava starch but more viscous than corn starch. The use of a lower concentration of starch would result in a general lowering of the paste viscosities and the softening of peak viscosities and breakdown because of reduced friction due to a lesser number of swollen granules (Figure 6) (Collado et al., 1999). Several studies found that sweet potato starch does not show a peak viscosity at 4-6 % (w/v) concentration (Tian et al., 1991). However Lii and Chang (1978) reported a moderate peak and a high set back on cooling with at a starch concentration of 7%. Varietal differences in viscosity have been reported as significant (Madamba et al., 1975; Liu et al., 1985). Sweet potato amylose has a limiting viscosity higher than that of wheat but lower than that of cassava or Irish potato amylose (Takeda et al., 1984, 1986). Similarly, sweet potato amylopectin has a lower limiting viscosity number than Irish potato amylopectin, suggesting smaller or more spherical molecules (Suzuki et al., 1985; Takeda et al., 1986). Since the peak viscosity value indicates how readily the starch granules are disintegrated, cohesive forces within the granules having higher values are stronger than those having lower values. The consistency of the paste after holding at 93°Cfor 15 min, i.e. breakdown viscosity provides an estimate of the resistance of the paste to disintegration in response to heating and stirring. Setback defined as the difference between the breakdown

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viscosity and the viscosity at 50 °C has been directly related to the amount of amylose leached from the granule (Greenwood, 1979).

Figure 6. RVA pasting curve of sweet potato starch at 7% and 11% starch concentration (Collado et al., 1999).

The rheological properties of sweet potato starch have been examined in detail. During heating, the storage modulus (G‘) and loss modulus (G‖) increased while phase angle decreased indicating change from sol to gel. The initial increase in G‘ and G‖ has been attributed to progressive swelling of starch granules leading to close packing. When the starch granules became very soft, deformable and compressible, decrease has been observed. The rheological properties of various root starches have been compared using the Bohlin rheometer and wide variability in the values of G‘ and G‘‘ was observed. During heating the G‘ and G‖ increased and the phase angle from sol to gel was occurring (Figure 7). Both moduli reached a maximum during heating after their values decreased (Garcia and Walter, 1998). Most of the reported values for G‘ and G‘‘ refer to starch pastes that have been held at room temperature for several hours after heating. G; for 6% corn and potato starch solutions at 60°C have been reported to be 132 Pa and 124 Pa, respectively (Evans and Haisman, 1979). However, all the different root starches exhibited uniformity in their elastic behaviour predominating over viscous nature. Sweet potato starch behaves in a similar way to cassava starch in all its viscosity characters. In a study of 44 different sweet potato genotypes using Rapid Visco Analyser (RVA), the correlations among the RVA parameters were reported (Collado et al., 1999). They observed wide variation not only in the PV (peak viscosity) but broadness of peak. The rheological properties of sweet potato starch extracted using an enzymatic process did not vary among the different concentrations of enzyme (Moorthy and Balagopalan, 1999).

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Figure 7. Storage modulus, Loss modulus and Phase angle for Peruvian sweet potato starch (Garcia et al., 1998).

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Pasting Temperature The consistency of the paste, the properties of the gel and the latter‘s viscosity during the pasting cycle are important for many industrial applications because they influence important quality characteristics (Leelavathi et al., 1987). The Brabender amylograph provides a good method for defining these characteristics. The pasting temperature of sweet potato starch obtained using Brabender Viscograph varied between 66.0 and 86.3°C while microscopic determination gave values between 57-70°C to 70-90°C. Noda et al. (1996) observed the pasting temperatures of starch from two sweet potato cultivars grown at different fertiliser levels to be 70.8 –73.9 °C. Starch pasting properties influence sweet potato eating quality and noodle quality and also are directly responsible for starch industrial uses (Collado et al., 1999). The pasting temperature of sweet potato starch varies between 62-86°C (Tian et al., 1991, Kitahara et al. 1999, Collado and Corke, 1997). In the study conducted by (Katayama et al., 2002) also showed pasting temperatures similar to previous reports.

USE OF ENZYMATIC TECHNIQUES FOR STARCH SEPARATION As the recovery of sweet potato starch is very low and therefore expensive, the enzymatic modification has been employed. With the increased availability of commercial enzymes which break down cellulose and pectin, attempts have been made to use them to improve the extractability of starch. Kallabanski and Balagopalan (1991) studied the effect of cellulolytic and pectinolytic enzymes on the extraction of starch from sweet potato roots, the yield showed a substantial increase.

Properties of Enzymatically Separated Sweet Potato Starch The extract from sweet potato with different concentrations of enzyme (up to 0.2%) contained 90-93% of starch (Table 6) and this indicates that only a small quantity of fibrous material is being extracted. The absence of large amounts of fibre in the enzymatically separated starch from sweet potato indicates that the non-starchy polysaccharides are completely broken down and do not contaminate the starch. The reducing values of the starch from enzyme treatments were small (Table 6) and as expected since the enzymes are pectinolytic and cellulolytic and has not affected the starch granules. The viscosity data of starches from enzymatic and conventional extraction recorded using Brabender Viscograph is given in Table.7. The peak viscosity varied depending on the concentration of starch used. With 5 and 6 % pastes, the peak viscosity increased for the extracts obtained with increasing amounts of the enzymes, up to 0.025 or 0.05 % and with 0.25 (%) enzyme it dropped appreciably. With the 7% paste, there was a fairly steady drop in peak viscosity as the enzyme concentration increased. This is probably due to a weakening on the associate forces rather than to a breakdown of starch granules. The breakdown viscosity was also increased with the higher levels of the enzymes. The pasting temperature did not show any definite pattern, but generally there was a shift to lower temperatures with increasing concentrations of the enzyme.

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Table 5. DSC characteristics of the hydrothermic transition of purified starches and fresh and freeze dried* Sweet potato Starch Fresh tubers Freeze-dried tubers Small starch granules

T onset (°C) 67.3 67.4 67.8 75.6

T peak(°C) 72.7 73.5 73.2 82.6

T endset (°C) 79.6 80.1 81.5 88.3

∆H (J/g) 13.6 6.8 9.3 15.3

*S.N. Moorthy, unpublished results.

Table 6. Properties of enzymatically separated sweet potato starch* Starch conc (%)

5

6

7

Enzyme conc (%) 0.000 0.010 0.025 0.050 0.100 0.0200 0.000 0.010 0.025 0.050 0.100 0.0200 0.000 0.010 0.025 0.050 0.100 0.0200

*S.N. Moorthy, unpublished results.

Peak viscosity (BU) 260 280 300 280 280 220 440 460 500 500 460 400 780 760 740 760 680 600

Breakdown viscosity (BU) 0 0 40 40 100 40 20 40 40 100 120 60 0 60 100 160 200 120

Pasting temperature (°C) 87-95 86-95 85-95 84-95 82-89 82-90 88-95 87-94 86-95 84-92 82-92 82-88 8895 88-95 84-95 84-95 82-90 82-89

Table 7. Viscosity properties of enzymatically separated sweet potato starch (S.N. Moorthy, unpublished results) Enzyme conc (%) 0.000

Starch content (%) 90.33

Reducing value 1.37

Swelling volume (%) 19.50

Solubility (%)

0.010

91.00

1.35

17.50

19.5

0.025

92.05

1.50

20.50

20.2

0.050 0.100

90.92 91.25

1.85 2.25

17.85 17.75

22.3 37.5

0.200

90.85

1.95

18.25

39.5

22.5

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STUDIES ON THE LIQUEFACTION AND SACCHARIFICATION OF SWEET POTATO STARCH Liquefaction and Saccharification Liquefaction studies were repeated for sweet potato (variety: S10) using different concentrations of Termamyl 60 L and one concentration of starch, i.e., 20%. Sweet potato starch was prepared from mature, undamaged roots as in the case of cassava starch. Liquefaction was done at 90C at pH 6.5 and the time course production of reducing sugars was monitored up to 1h at 15 min intervals. The results presented in Table 8 indicate that as in the case of cassava, rapid hydrolysis of 1→ 4 linkages within 15–30 min itself occurred when higher concentration of enzyme was present in the system. After 1 h, around 26-28% starch conversion to dextrins and sugars was observed with 240-480 mg Termamyl. With 30 mg Termamyl, only 11% conversion was observed. As in the case of cassava starch, the low conversion of starch to reducing groups by 30 mg Termamyl did not influence the subsequent saccharification step by 0.05 ml Amyloglucosidase (Table 9). This reconfirmed the finding that a low amount of Termamyl at the liquefaction stage can economise the reaction by reducing the enzyme cost. Saccharification also appears to be almost completed by 48h and hence continuing up to 72h was not necessary, as this will only lead to increase in the operational expenses. Table 8. Enzyme hydrolysis of sweet potato (variety S10) starch using  - amylase – Effect of enzyme concentration on the rate of glucose production* amylase conc. (mg/100 ml slurry)

After 15 minutes Amt. of Percent reducing conversugar sion to formed reducing (g) sugar

After 30 minutes Amt. of Percent reducing conversugar sion to formed reducing (g) sugar

After 45 minutes Amt. of Percent reducing conversugar sion to formed reducing (g) sugar

After 60 minutes Amt. of Percent reducing conversugar sion to formed reducing (g) sugar

30 60 120 480

1.57 2.24 2.52 3.86

1.66 2.61 3.08 4.15

1.98 2.75 3.28 4.69

2.09 3.07 3.67 5.15

8.84 12.57 14.16 21.69

9.34 14.67 17.33 23.36

11.15 15.44 18.42 26.38

11.79 17.25 20.65 28.98

*S.N. Moorthy, unpublished results.

Viscosity Profile of the Liquefaction Reaction The viscosity changes during the liquefaction of sweet potato starch were monitored using the RVA using similar systems as in the case of cassava starch. Tremendous reduction in viscosity was observed when 30 -90 mg Termamyl 60L was added to the 1:10 starch slurry (Table 10). Sweet potato starch required a higher enzyme concentration of 30 mg to reach this stage in RVA. This indicates the possible differences in the initial susceptibility to α-amylase attack of the starches i.e., cassava and sweet potato starch.

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Post-harvest treatments such as the method of starch isolation, as well as genetic factors, may have a profound effect on the properties of sweet potato starch. The environmental conditions during growth of a plant, especially the temperature, constitute one of the important factors affecting the physicochemical characteristics of starch granules, besides genetic and endogenous factors (Hizukuri, 1969). Table 9. Percentage conversion of starch after saccharification by AMG on sweet potato starch slurry (liquefied using two concentrations of Termamyl* Liquefaction

Saccharification

Starch conc. (g)

 - amylase (mg/100ml slurry)

Percent conversion (after 1 h)

AMG conc. (ml/100ml slurry)

Percentage conversion After 24 h

48 h

72 h

20

30

11.79

0.05

85.08

82.73

86.83

20

240

26.43

0.05

80.40

84.69

82.33

*S.N. Moorthy, unpublished results.

Table 10. Viscosity reduction of sweet potato (variety-S10) starch by adding  - amylase as measured using Rapid Visco Analyser* Sl. No.

-amylase conc. (mg)

Peak 1 (cP)

Trough 1 (cP)

Breakdown (cP)

1. 2. 3. 4. 5. 6. 7. 8. 9.

Control 1.2 2.4 3.75 7.5 15 30 60 90

4020.00 1081.00 757.00 496.00 122.00 63.00 17.00 10.00 5.00

2254.00 16.00 10.00 13.00 2.00 -1.00 -1.00 -3.00 -2.00

1766.00 1065.00 747.00 483.00 120.00 64.00 18.00 13.00 7.00

Final Viscosity (cP) 3056.00 23.00 14.00 13.00 5.00 9.00 -1.00 0.00 -2.00

Set back (cP) 802.00 7.00 4.00 0.00 3.00 10.00 0.00 3.00 0.00

Peak time (Sec) 4.07 3.47 3.47 3.40 3.40 3.33 3.33 3.73 3.80

Pasting (C) 77.40 75.80 75.80 75.80 76.75 76.00 Error Error Error

*S.N. Moorthy, unpublished results.

Starch content and dry matter content are the main properties of raw material for starch production. A new sweet potato breeding line, Kanto 116 was developed, featuring low gelatinisation temperature and an altered starch fine structure and having pasting temperature of 20 V (viscosity) lower than those of ordinary cultivars (Katayama et al., 2002). Starch granule from Kanto 116 showed an abnormal morphology characterized by cracking into granules. A number of studies on the distinctive properties of sweet potato starch have been undertaken in the last two decades (Tian et al., 1991; Moorthy, 2002). Kitahara et al. (1996, 1999) reported a new line with low amylose content and two lines having approximately 10°C lower pasting temperatures than ordinary cultivars. Some new sweet potato lines were developed from progenies of a new cultivar, Quick Sweet, having a low pasting temperature (Katayama et al., 2004). The results indicated that this ‗Quick Sweet‘ is a useful breeding material for improving pasting and retrogradation properties in sweet potato starch. The

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potential chemopreventive properties of dietary fiber prepared from sweet potato roots were examined to promote the demand of this residue from the starch industry (Yoshimoto et al., 2005; Yoshimoto, chapter 3 in this Volume). Sweet potato starch has some unique characteristics and is mostly used by the food industry as an ingredient in products such as cakes, breads, biscuits, cookies and noodles (Zang and Oates, 1999). Noodles, bread, boiled rice and pasta have played an important role in the human diet, especially in the Asian countries (Japan, China, Taiwan, Korea, Vietnam and Thailand). Based on the raw materials, various types of noodles are produced throughout the world. In terms of food products, four quality attributes are important being nutritional, phytosanitary, shelf-life and organoleptic. These qualities depend on flour and starch quality. Thus starch properties largely influence noodle quality. Starch with high amylose content and with C-type pasting profile characterized by the absence of a peak viscosity and a constant or increased viscosity during continuous heating and shearing, i.e., good hot paste stability is reported to be suitable for noodle processing (Collado and Corke, 1999).They reported that the textural attributes of sweet potato noodles show high positive correlation with some starch paste properties. It is also reported that the smaller particle size of the granule improves the strength of uncooked noodles without affecting the firmness of cooked noodle (Oh et al., 1985).

CONCLUSION Sweet potato is therefore one of the worlds most important starch producing crops, with 95 % of all roots produced in Asia and Africa. Sweet potato is used as direct food, processed foods, industrial starch and animal feed. The utility of sweet potato is primarily determined by its physicochemical properties, being the amylose/amylopectin ratio, the molecular structure, granule size and inorganic constituents. Pasting properties influence the quality of food processing materials and industrial products. Being a nontraditional source of starch, the characterization of genetic variation and interrelationships of sweet potato starch physical properties that can guide utilisation is therefore essential. An awareness of their potential uses can help in large scale cultivation of these crops and extraction of starch from them. It is also possible to modify the starch properties by simple physical methods such as hydro- thermal or steam-pressure treatments. The latest developments in biotechnology are also being evaluated for their potential to modify the starches. These include fermentation of starch by the use of selective organism or enzymatic modification, which can bring about specific substitutions (Sair, 1967; Raja, 1990; Moorthy, 1999). Current research is seeking to produce a new cultivar and breeding materials with distinctive amylose content and pasting properties. The role of dietary fibre in human nutrition has attracted growing interest in recent years. Most of the research programmes carried out on sweet potato are attempting to reduce the content of crude fibre for improved eating quality. Furthermore, the production of ethanol from biomasses is a growing industry in this continuously developing society. The sweet potato having high starch yield and low gelatinisation temperature may be effective in reducing these production costs. The various improvements in starch properties are useful for providing consumers with starch products and spreading the demand for sweet potato starch.

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Katayama, K, Komae, K, Kohyama, K, Kato, T., Tamiya, S and Komaki, K. (2002) . New sweet potato line having low gelatinisation temperature and altered starch structure. Starch/Stärke 54 :51-57. Kay, D.E. (1987). TDRI Crop and Product Digest No.2, TDRI, London, pp.166-173. Kitada, Y., Sasaki,M., Yamazoe,Y. and Nakazawa, H. (1988). Measurements on the thermal behaviour and amylose content of kuzu and sweet potato starches. Nippon Shikokuhu Kogyo Gakkaish.. 35 (3):135-140. Kitahara, K., Mizukami, S., Suganuma, T., Nagahama, T., Yoshinaga, M., Kumagai, T. and Yamakawa, O. (1996). A new line of sweet potato with low amylose content. J. Appl. Glycosci. 43: 551-554. Kitahara, K., Ueno,J., Suganuma, T., Ishiguro, K. and Yamakawa. O. (1999). Physicochemical properties of root starches from new types of sweet potato. J. Appl. Glycosci. 46: 391-397. Leach, H.W, McCoven, L.D, Schoch, T.J. (1959). Structure on the starch granule. Cereal Chem. 36: 534-544. Leelavathi, K., Indrani, D., and Sidhu, J.S. (1987). Amylograph pasting behaviour of cereal and tuber starches. Starch/Stärke 39: 378-381. Lii, C.Y. and Chang, S.M. (1978). Studies on the starches in Taiwan, sweet potato, cassava, yam and arrowroot starches. Proc. Natl. Sci. Council ROC Part A. Phys. Sci. Eng. 2: 146423. Lii, C.Y., Chiou, R.W. and Chu, Y.L. (1987). The degree of branching in amylose from tuber and legume starches. Proc Natl. Sci. Council. ROC, Part A. Phys. Sci. Eng. 11(4):341345. Liu, S. Y. and Liang,, C.L. (1983). Studies in the efficiency of alcohol production in sweet potato, cassava and potato. J. Agric.Res. China 32 111-121. Liu, S.Y., Liang , C. L and Li,. L. (1985). Studies on the physicochemical properties of the tuber of new sweet potato lines. Chung-hua Nung Yeh Yen Chiu 34 341-345. Liu, H. (2004). Post-harvest processing technology of sweet potatoes in Sichuan, China. http://unapcaem.org/Activities%20Files/A20/4%20China.pdf. Assesses on 15 January, 2009. Lorenz, K., and Kulp, K. (1982).Cereal and root starch modification by heat-moisture treatment, Starch/Stärke 34: 50-54. Madamaba, L.S.P., Bustrillos, A.R., and San Pedro, E.L. (1975). Sweet potato starch; Physicochemical properties of whole starch. Philipp. Agric. 58: 338-350. Madamba, L.S.P and San Pedro, E.L. (1976). Chemical composition of sweet potato flour. Philipp. Agric. 59: 350-355. Manners, D.J. (1989). Recent developments in our understanding of amylopectin structure. Carbohydr. Polym. 11: 87-112. Martin, F. W. and Desphande, S. N. (1985) Sugar and starches in non sweet potato compared to those of conventional cultivars. J. Agric.Univ.Puerto Rico 69 169-176. Melo, E.de A., Krieger, N. and Stamford, T.L.M. (1994). Physicochemical properties of Jacatupe (Pacchyrrhizus erosus L. Urban) starch. Starch/Stärke 46: 245-247. Mestres, C., Colonna, P. and Bulean, A. (1988). Gelation and crystallization of maize starch after pasting, drum drying and extrusion cooking. J. Cereal Sci. 7(2):123-134. Medcalf, D.G. and Giles, K.A. (1965). Wheat starches: Comparison of physicochemical properties. Cereal Chem. 42: 558-568.

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Moorthy, S.N. (1999). Effect of steam pressure treatment on the physicochemical properties of Dioscorea starches. J. Agric. Food Chem. 47 : 1695-1699. Moorthy, S.N. and Ramanujam, T. (1986). Variation in properties of starch in cassava varieties in relation to age of the crop. Starch / Stärke 38: 58-61. Moorthy, S.N., Wenham, J.E. and Blanshard, J.M.V. (1996). Effect of solvent extraction on the gelatinisation properties of starch and flour of five cassava varieties. J. Sci. Food Agric. 72: 329-336. Moorthy, S.N.,and Balagopalan, C (1999) .Physicochemical properties of enzymatically separated starch from sweet potato. Trop. Sci. 39: 23-27. Moorthy. S.N. (2002). Physicochemical and functional properties of tropical tuber starches: A Review. Starch/Stärke 54 : 559-592. Nara, S., Sakakwa, M. and Komiya, T. (1983). The acid resistance of starch granules. Starch/Starke 35 (8) 266-270. Noda, T., Takahata, Y., and Nagata, T. (1992). Developmental changes in properties of sweet potato starch. Starch / Stärke 44: 405-409. Noda, T., Takahata, Y. and Nagata, T. (1992). Properties of sweet potato starches from different tissue zones. Starch/ Starke 44: 365-368. Noda, T., Takahata, T., Sato, T. (1995), Distribution of the amylopectin chain length of sweet potato differing in stages of development. tissue zone and variety. J Jpn Soc. Food Sci. Technol. 42: 200-206. Noda, T., Takahata, Y., Sato, T., Ikoma, H., and Mochida, H. (1996). Physicochemical properties of starch from purple and orange fleshed sweet potato roots at two levels of fertilizer. Starch / Stärke 48: 395-399. Noda,T., Takahata, Y. T., Suda, I., Morishita, T., Ishiguro, K., et al. (1998) Relationship between chain length distribution of amylopectin and gelatinization properties within the same botanical origin for sweet potato and buckwheat. Carbohydr. Polym. 37: 153-158. Oh, N.H, Seib, A. P.A., Ward,.A.B. and Deyoe, C.W. (1985). Noodles. IV. Influence of flour protein, extraction rate, particle size and starch damage on the quality characteristics of dry noodles. Cereal Chem. 62: 441-447. Radley, J.A. (1976 b). Examination and analysis of starch and its derivatives. Applied Science Publishers Ltd, London. Radley, J.A. (1976a) .Starch production technology. Applied Science Publishers Ltd, London) pp.189-229. Raja, K.C.M. (1990). Studies on physicochemical and textural qualities of cassava (Manihot esculanta Crantz), PhD Thesis, University of Mysore, India. Rasper, V. (1969). Investigations on starches from major starch crops grown in Ghana. II.Swelling and solubility patterns and amyloelastic susceptibility. J.Sci.Food. Agric. 20: 642-646. Rasper,V. (1971). Investigations on starches from major starch crops grown in Ghana.III. Particle size and size distribution. J. Sci. Food. Agric. 20: 572-580. Rickard, J.E., Asaoka, M. and Blanshard, J.M.V. (1991). The physicochemical properties of cassava starch‘ Trop. Sci. 31: 189-207. Schoch, T.J. (1964 ). Swelling power and solubility of granular starches, In: R.L. Whistler (Ed.), Methods in Carbohydrate Chemistry, Vol IV, Academic Press, New York, pp. 106-109. Seog, H.M., Park, Y.K., Nam, Y.J.. Shin, D.H. and Kim, J. P. (1987). Physicochemical properties of several sweet potato starches, Han’guk Nanghwa Hakhoechi 30 (2): 179-185.

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Shen,M.C. and Sterling, C. (1981). Changes in starch and other carbohydrates in baking Ipomoea batatas. Starch/Starke 33 (8): 261-268. Shin, M. and Ahn, S.Y. (1983). Physicochemical properties of several sweet potato starches, J. Korean Agric. Chem. Soc. 26 : 137-142. Singh, S., Raina, C.S, Bawa, A.S. and.Saxena, D. C. (2005). Effect of heat moisture treatment and acid modification on rheological, textural and differential scanning calorimetry characteristics of sweet potato starch. J. Food Sci. Physical Properties.70: 373-378. Soni, P.L., Sharma, H., Srivatsava, H.C.,and Gharia, M.M. (1990). Physicochemical properties of Canna edulis starch- comparison with maize starch. Starch/Stärke 42:460464. Stevens, D.J. and Elton, G.A.H. (1971). Thermal properties of Starch/Water systems, Starch/Starke 23: 8-11. Susuki, A., Takeda, Y. and Hizukuri, S. (1985). Relationship between the molecular structures and retrogradation of tapioca, potato and kuzu starches. J. Jap. Soc. Starch Sci. 32: 205-212. Szylit, O., Durand, M.,.Borgida, L.P, Atinkpahoun, H., Prieto, F. and Devort-Lavel, J. (1978). Raw and steam-pelleted cassava, sweet potato and yam cayensis as starch sources for ruminant and chicken diets. Anim. Feed Sci. Technol. 3: 73-87. Takeda, C. and Hizukuri, S. (1974). Characterization of the heat dependent pasting behavior of starches Nippon Nogeikagku Kaishi Vol. No. 663-669 Takeda, Y., Tokunaga, N., Takeda, C., and Hizukuri, S. (1986). Physicochemical properties of sweet potato starches. Starch/Stärke 38: 345-350. Tian, S.J., Rickard, J.E., and Blanshard, J.M.V. (1991). Physicochemical properties of sweet potato starch, J. Sci. Food Agric. 57: 459-491. Ueda, S. and Saha, B.C. (1983). Behaviour of Endomycopsis. Enz. Microbial. Technol. 5:196-198 Uehara. S. (1984).Amylose-amylopectin ratio of soluble and insoluble fraction of sweet potato granules treated with urea. J. Agric. Chem.Soc Japan 57(6): 529-533. Valetudie, J.C., Colonna, P., Bouchet, B. and Gallant, D.J. (1995). Gelatinisation of sweet potato, tannia and yam starches. Starch/Stärke 47: 298-306. Wankhede, D.B. and Sajjan, S. (1981). Isolation and physicochemical properties of starch extracted from yam, elephant (Amorphophallus compalunatus). Starch/ Stärke 33: 153157. Watanabe, T., Akiyama,Y., Takahashi, H., Adachi, T., Matsumoto, A. and Matsuda, K. (1982). Structural features and properties of Nageli amylodextrins. Carbohydr. Res. 109: 221-232. Woolfe, J.A. (1992). Sweet Potato: an Untapped Food Resource. Cambridge University Press, Cambridge, pp. 643. Wootton, M. and Bamunuarachchi, A. (1979). Application of differential scanning calorimetry to starch gelatinization. I. Commercial native and modified starches. Starch/Starke 31: 201-204. Yamakawa, O. (1996). A new line of sweet potato with a low amylose content. J. Appl. Glycosci.43: 551-554. Yoshimoto, M., Yamakawa, O. and Tanoue, .H. (2005). Potential chemo preventive properties and varietal difference of dietary fibre from sweet potato (Ipomoea batatas L.) root. JARQ 39 (1):37-43.

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Young, R.J. (1981). Introduction to Polymers. Chapman and Hall, New York. Zoebel, H. F. (1988). Molecules to granules- A comprehensive starch review, Starch/Stärke 40: 44-50. Zang.T. and Oates C.G. (1999). Relationship between α-amylase degradation and physicochemical properties of sweet potato starches. Food Chem. 65. 157-163.

In: Sweet Potato: Post Harvest Aspects in Food Editors: R. C. Ray and K. I. Tomlins

ISBN 978-1-60876-343-6 © 2010 Nova Science Publishers, Inc.

Chapter 5

SWEET POTATO PUREES AND DEHYDRATED POWDERS FOR FUNCTIONAL FOOD INGREDIENTS 1



Van-Den Truong1 and Ramesh Y. Avula2

USDA-ARS Food Science Research Unit, Department of Food, Bioprocessing and Nutrition Sciences, North Carolina State University, Raleigh, NC 27695-7624, USA 2 Department of Food Science and Technology, University of Georgia, Athens, GA 30602, USA

ABSTRACT Processing technologies have been developed in various parts of the world to convert sweet potatoes into purees and dehydrated forms that can be used as functional ingredients in numerous food products. This chapter reviews the processing operations involved in these technologies and their effects on quality, storability, nutritional values and rheological properties of sweet potato purees and powders/flours. For purees, the processing steps include peeling, cutting/grinding, and pre-cooking/finish-cooking with temperature-time program suitable for starch conversion by endogenous amylolytic enzymes to obtain the products with targeted maltose levels and viscosities. The purees can be subsequently preserved by refrigerated and frozen storage, canning, or aseptic packaging. However, poor product quality due to excessive thermal treatments in canning, high cost of investment associated with frozen products and limited package sizes of these preserved forms are the main hurdles for widespread applications of sweet potato purees in the food industry. These problems can be overcome by a new process using a continuous flow microwave system for rapid sterilization and aseptic packaging to produce shelf-stable purees with consistently high quality. Sweet potato purees can be further processed into drum- or spray-dried powders. In many countries, solar drying and 

Paper no. FSR08- … of the Journal Series of the Department of Food, Bioprocessing and Nutrition Sciences, NC State University, Raleigh, NC 27695-7624. Mention of a trademark or proprietary product does not constitute a guarantee or warranty of the product by the U. S. Department of Agriculture or North Carolina Agricultural Research Service, nor does it imply approval to the exclusion of other products that may be suitable.  Corresponding author: Van- den Truong at (919) 513-7781; fax (919) 513-0180; E-mail: [email protected]

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mechanical drying in cabinets and tunnels are common in producing sweet potato dried chips which are pulverized into flours. Extrusion technology and chemical treatments are also applied to produce sweet potato powders for specific functionality. With high levels of carbohydrates, ß-carotene (orange-fleshed varieties) and anthocyanins (purple-fleshed varieties), sweet potato purees and dehydrated forms can be used as functional ingredients to impart desired textural properties and phytonutrient content in processed food products.

ABBREVIATIONS CPV DPPH DS DSC DRI GI HPV NASA ORAC PER SAPP PV RTE

cold paste viscosity; 2, 2-diphenyl-1-picrylhydrazyl; degree of substitution; Differential Scanning Colorimeter; dietary reference intake; glycemic index; hot paste viscosity; National Aeronautics and Space Administration; oxygen radical absorbance capacity; protein efficiency ratio; sodium acid pyrophosphate; peak viscosity; ready- to- eat.

INTRODUCTION Sweet potato ranks the seventh most important food crop in the world and fourth in tropical countries (FAOSTAT, 2004). In comparison to other major staple food crops, sweet potato has the following positive attributes: wide production geography, adaptability to marginal condition, short production cycle, high nutritional value and sensory versatility in terms of flesh colors, taste and texture. Depending on the flesh color, sweet potatoes are rich in ß -carotene, anthocyanins, total phenolics, dietary fiber, ascorbic acid, folic acid and minerals (Woolfe, 1992; Bovell-Benjamin, 2007; ILSI, 2008). Therefore, sweet potato has an exciting potential for contributing to the human diets around the world. However, the world trends in sweet potato production and consumption do not support the position of this highly nutritious vegetable. In the United States, the annual per capita consumption of sweet potato was declined in the last decades from 12 kg to 2 kg while the potato consumption was increased to over 60 kg (USDA, 2002). The situation can be attributable to the inadequacy in sweet potato manufacturing technologies for processed products, and the increased demand of consumers for convenient products. Research efforts have demonstrated that sweet potatoes can be made into liquid and semi-solid food products such as beverages, soups, baby foods, ice cream, baked products, restructured fries, breakfast cereals, and various snack and dessert items (Collins and Walter, 1992; Dansby and Bovell-Benjamin, 2003a; Truong, 1992; Truong et al., 1995; Walter et al., 2001, Woolfe, 1992). Puree and dehydrated forms processed from

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sweet potatoes are the main ingredients that provide the functionality required in these processed products. For the food processing industry, the unavailability of puree and dehydrated forms for diverse functionalities is a limiting factor in the utilization of sweet potatoes in processed foods. Several excellent reviews dealing with processing and quality aspects of sweet potato purees, flakes and powders have been published over the past 20 years (Collins and Walter, 1992; Kays, 1985; van Hal, 2000; Woolfe, 1992). This chapter updates these reviews with recent developments in processing technologies to convert sweet potatoes into purees and powders that can be readily used by the food industry as functional ingredients in processed foods.

SWEET POTATO PUREES AS FUNCTIONAL INGREDIENTS The use of sweet potatoes in the food industry often involves processing of the roots into purees that can be subsequently frozen, canned or packaged in aseptic conditions to produce shelf-stable products for year-round availability of the produce. For pureeing, roots of all sizes and shapes can be processed to make acceptable puree and therefore, the entire harvested crop is utilized including the 30-40% off-grade from the fresh root markets. Purees from the orange-fleshed sweet potatoes have been commercially produced in cans or in frozen form in the U. S (Kays, 1985; Walter and Schwartz, 1993). In Japan, both white- or orangefleshed cultivars are utilized for processing into paste for incorporation into bread and ice cream (Woolfe, 1992). The challenges in puree processing industry are: (1) the difficulty in adjusting the process to account for differences in cultivar types; root handling, curing, and storage; and other parameters in order to produce consistent, and high puree quality, and (2) the preservation technology that could produce shelf-stable product for convenient incorporation in processed foods. A wide range of dry matter (18 – 45%) and starch content (8 – 33.5%, fresh weight basis) exists among sweet potato genotypes (Brabet et al., 1998; Yencho et al., 2008) which have significant impact on processing operations and quality of the purees. Post-harvest handling of sweet potatoes can have significant effect on the purees made from them. Metabolic changes may affect the appearance, texture, flavor and nutrient composition of the purees. Curing by subjecting sweet potatoes to 30C, 85% to 90% relative humidity for 4-7 days as commercially practiced in the U. S. can result in an increase in sugars, and a decrease in starch and alcohol-insoluble solids (Boyette et al., 1997). Several investigators reported that changes in carbohydrate components and enzyme activities (α-amylase, β-amylase, invertases and sucrose synthase) during curing and storage of sweet potatoes are genotype dependent (Huang et al., 1999; Picha, 1986; Takahata et al., 1995; Walter, 1987). In general, amylase activities in sweet potato roots are increased by curing and storage especially during the first few months, then remain fairly constant or decrease to the levels at harvest (Shen and Sterling, 1981; Walter et al., 1976; Zhang et al., 2002). On the other hand, there are genotypes, e.g. Kyukei 123, with relatively constant levels of starch, sucrose and amylase activity throughout storage (Takahata et al., 1995). The activities of α-amylase and β-amylase in raw sweet potatoes affect the processing operations and quality of the purees. When the sweet potatoes are heated to starch gelatinization temperature (60 to 78C), αamylase rapidly degrades the starch to lower molecular weight dextrins which are

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concurrently hydrolyzed into maltose by β-amylase. The degree of starch degradation and maltose formation is dependent on the activities of amylases and heating program in the process of pureeing. Therefore, for a given sweet potato variety, it is expected that cured and stored roots with increasing amylase activities will produce purees which are sweeter and less starchy than those of the just-harvested (green) roots. However, there is a genotype difference in the amounts of maltose produced in the cooked sweet potatoes. Takahata et al. (1994) classified sweet potato varieties into high, moderate and low maltose formation after steaming. The genotypes with high maltose formation in cooked roots tended to have early gelatinization of starch granules (< 70C) and ß -amylase with high heat stability up to 78 82ºC. A new sweet potato breeding line, Kanto 116, was developed in Japan; this genotype has starch with pasting temperature of 51.4 – 52.6ºC, approximately 20ºC lower than those in the common sweet potato cultivars (Katayama et al., 2002).

Processing of Sweet Potato Purees Over the years, techniques have been developed for puree processing in order to produce purees with consistent quality, as mentioned above, despite the variations due to cultivar differences and post-harvest practices. Several methods for sweet potato puree processing were developed since 1960‘s, and the subject was reviewed by Collins and Walter (1992), Kays (1985) and Woolfe (1992). Process operations for pureeing of sweet potatoes (Figure 1) involve washing, peeling, hand-trimming, cutting, steamed blanching or cooking, and grinding into purees which can be subjected to canning or freezing for preservation. Washing: Sweet potatoes are stored without removing the dirt for prolonging storability. In the United States, prior to delivery to the fresh root markets, stored sweet potatoes are passed through the packing line for washing, treating with fungicide and sizing. The roots are generally unloaded from the pallet bins into a tank of water, conveyed to high-pressure spray washers wherein water at 250 psi is directly sprayed at the surface of sweet potatoes as they tumble over rotating brushes. The washed roots are then sorted by size using pitch roller sizers or electronic sensors (Boyette et al., 1997). The size number 1 roots are selected and packed in carton boxes for table stock markets. The misshapen, undersized or jumbo-sized roots, about 30% of the crop, are considered as the rejects and offered to the processing companies. In places where the whole harvest is delivered to the processing factories, sweet potatoes can be washed with revolving drum washer. Truong et al. (1990) described a lowcost washer made of an empty drum with rotating frame holding brushes and having a capacity of 300 kg roots/hr. Peeling and Rewashing: Prior to peeling, the cleaned roots can be preheated in hot water for a short time to provide some benefits including reduction of peeling time and enzymatic discoloration by polyphenolic oxidase (Bouwkamp, 1985). However, several investigators reported that preheating treatment of the unpeeled roots is not necessary (Edmond and Ammerman, 1971). Sweet potato peel can be removed by abrasive rollers, lye solutions, a combination of lye and steam peeling or high pressure steam. In lye peeling, cleaned roots are conveyed into 10-22% lye solution at 104ºC for 3 to 6 min and then transferred to a rotary washer with high-pressure water spray to remove the lye residue, loosened peel and adhere softened tissue. Peeling losses range from 20-40% of the raw material depending on the lye concentration, residence time and root sizes (Scott et al., 1970). Due to the issues on

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equipment corrosion and waste disposal, lye peeling is no longer a common method in the industry. Abrasive peelers with capacity of few hundreds to over a thousand kg roots/hr can be used in peeling sweet potatoes (Kays, 1985; Taylor, 1982; van Hal, 2000). High-pressure steam peeling developed by Harris and Smith (1985, 1986) is being used by many sweet potato processing companies. The technology is referred as a thermal blast process in which the roots are enclosed for a short time (20 to 90 sec) in a chamber pressurized with heated steam, followed by an instantaneous release of pressure. As the pressure suddenly release, the super-heated liquid water beneath the skin surface immediately flashed into vapor, and blasted the peel off the roots. This process can be automated, result in less peeling loss than lye peeling, and produce a product with less enzymatic discoloration (Smith et al., 1980). Studies on the effect of lye peeling on amylase activities, starch hydrolysis, phenolic degradation and carotene loss on the surface of sweet potato roots were conducted by Walter and Schadel (1982), Walter and Giesbrecht (1982). Hagenimana et al. (1992) has shown that α-amylase is strongly localized in the periderm, the vascular cambium and the anomalous cambium of sweet potato roots while β-amylase is abundant and well distributed throughout the root. During lye peeling, heat and alkali gelatinize starch in the root outer layers where thermostable α-amylase results in starch conversion into maltose and dextrins. However, there is limited understanding in these aspects of the steam flash peeling on the surface of sweet potato roots. Trimming and Cutting: Peeled sweet potatoes are next conveyed along a trimming and inspecting line for trimming the surface blemishes and fibrous ends and removing the diseased roots. The materials are then fed to size reduction machine for cutting into slices, strips and cubes or grinding into fine particles using a hammer mill or pulp finisher. Cutting and grinding machines with capacity up to over 1000 kg/hr are being used for this operation. Pureeing Processes: The techniques that have been developed for processing sweet potato into purees are illustrated in Figure 1. The purees can be simply produced by steam cooking of the peeled roots, chunks, slices, strips, cubes or ground particles, and passing the cooked materials through a pulp finisher. However, the aforementioned challenges became an issue in getting the product with consistent quality. Addition of α- and β-amylases can be applied to obtain the desired amount of starch conversion (Hoover, 1966; Szyperski et al., 1986). This method, however, introduces food additives to the process that are usually disliked by consumers. Another approach employs the enzyme activation technique using the endogenous amylolytic enzymes for starch hydrolysis (Hoover and Harmon, 1967), and this process is now commonly used in the food industry. As shown in Figure 1, the peeled sweet potatoes can be either cut into cubes of 2 cm, strips of 2 x 2 x 6 cm and slices of 0.5 - 0.95 cm thick (Walter and Schwartz, 1993; Truong et al., 1994) or mashed using a hammer mill with rotating blades to chop and push the materials through a 1.5 – 2.3 mm mesh screen (Szyperski et al., 1986). Next, the materials are steamed blanched at 65 to 75°C which activates the amylases and gelatinizes the starch for hydrolysis. For the process with slices, strips and cubes, comminuting the blanched materials into puree is carried out at this point using the hammer mill. The blanched puree is pumped into a surge tank and hold at 65 - 75°C for further starch hydrolysis depending on the targeted maltose levels. Raw sweet potato mash as a source of amylases can be optionally added at this stage to increase starch conversion. Alpha- and ß- amylases hydrolyze the starch producing maltose, maltotriose, glucose and dextrins.

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Figure 1. Different processes for sweet potato puree production.

The majority of maltose production is likely completed in the first few minutes of the starch conversion process. Hoover and Harmon (1967) found maltose is the only sugar produced and the majority of maltose was produced in the first 10 minutes of cooking at temperatures of 70 to 80°C. McArdle and Bouwkamp (1986) also reported that rapid heating of raw sweet potato slurries to 80ºC may be optimal for starch conversion. However, further decreases in the molecular size of starch and dextrins occur for up to 60 minutes resulting in the purees with high maltose content and low apparent viscosity (Walter et al., 1976; 1999). In order to control the process to produce a consistent product, the length of conversion time can be adjusted from a few minutes to 1 hour depending on the starch content and amylase activity in the raw materials. After starch conversion, the temperature is raised to 100 - 110°C in a heat exchanger to inactivate the enzymes, and a final grinding step will be carried out with the use of a pulp finisher to obtain the smooth puree. The temperature and time program in the described pre-cook/finish cook process has significant effects on the puree quality. A very fast heating procedure tended to result in puree with low levels of maltose and high viscosity, and a temperature and time program that allows sufficient amylase-hydrolysis on gelatinized starch would produce sweet and more flowable purees (Walter and Schwartz, 1993; Ridley et al., 2005). The developed technologies for puree processing were based mainly on the orangefleshed sweet potato cultivars with high ß-carotene, low dry matter (18-21%) and low starch content [8-10% on fresh weight basis (fwb)] (Walter and Schwartz, 1993; Yencho et al., 2008). This sweet potato type has moist texture after cooking, produces purees that are viscous, but flowable, and can be handled in various processing operations (Truong et al., 1995; Coronel et al., 2005). On the other hand, sweet potatoes with white, yellow and purple

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flesh colors have higher levels of dry matter (25-38%) with potentially different starch properties (Walter et al., 2000), which may present challenges for the commercial production of flowable purees from these materials. Therefore, the processing hurdle in pureeing these sweet potato types could be overcome by either addition of water to decrease the solid levels of the material to 18-21%, amylase hydrolysis of starch components, or a combination of the two treatments. For cost-effective reasons, water addition can be adapted as a simple approach in processing of purple-fleshed sweet potato purees that have flowability similar to the purees from the orange-fleshed sweet potatoes (Steed and Truong, 2008).

Packaging and Preservation of Sweet Potato Purees Canning and Freezing: The finish-cooked puree can be packaged in cans and retorted to produce shelf-stable product. The puree can also be filled in plastic containers for refrigerated or frozen storage (Collins and Walter, 1992; Kays, 1985; Pérez-Díaz et al., 2008; Walter and Wilson, 1992). Ice et al. (1980) and Creamer et al. (1983) reported that pH adjustment of sweet potato puree to 1.5, 4.5 and 11.5 prior to filling in jars followed by pasteurizing at 90ºC could prolong the shelf-life of the product up to 9 months at room temperature. Preservation by canning for low acid food such as sweet potato purees (pH, 5.8 – 6.3) usually involves excessive thermal treatment of the product because heat transfer in the puree is mainly by conduction. Excessive thermal treatment of the product also results in severe degradation of color, flavor, texture, and nutrients. An example is the institutional-size can size 607x 700 which is required to retort for over 165 minutes at 121 °C (Lopez, 1987). The slow- rate of heat transfer from the wall to the center of the can to attain commercial sterilization of the product limits the maximum can size of number 10 for canned sweet potato purees. This size limitation is another obstruction for the wider uses of sweet potato purees as a food ingredient in the food industry. Other issues associated with canning include the difficulty in handling, opening and dispensing of the product, and disposal of emptied cans. Nevertheless, canning does not have the need for special storage, lower capital investment and unit of production is less when comparing to refrigerated and frozen puree. Frozen puree is an established method for preservation which provides the lower degradation on nutritional and sensory quality as compared to can processing. However, preservation by freezing requires considerable investment in frozen distribution and storage as well as space, energy, time-consuming, and poorly controlled defrosting treatment before use. Currently, only limited amount of canned and frozen sweet potato purees are commercially produced by a few companies in the U. S. and Japan. Microwave-assisted Sterilization and Aseptic Packaging: Aseptic processing is considered as a potential alternative to overcome the stated problems associated with canning and low temperature preservation. As opposed to conventional canning, the use of high temperature for a short period of time in aseptic processing can produce a higher quality product with equal or better level of microbiological safety as that in a conventional canning system. Smith et al. (1982) described an improved canning process for sweet potato purees which involved flash sterilization and followed by aseptic filling, that resulted in a shelfstable and high quality product. However, scaling-up of the technology for achieving beyond the cans and process validation were not carried out for commercial development. Since then, further application of aseptic processing and packaging technology of food products in

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flexible containers, has not been successfully carried out for purees from sweet potatoes and other vegetables. Recently, a process for rapid sterilization and aseptic packaging of the orange-fleshed sweet potato purees using a continuous flow microwave system operated at 915 MHz has been successfully developed (Coronel et al.., 2005). This process has the advantage of avoiding long retort processing schedules, maintaining high quality retention, and producing shelf-stable products. The resulting product packed in flexible plastic containers had the color and viscosity comparable to the non-sterilized puree and was shelf-stable for at least 12 months. Purple-fleshed sweet potato purees were also successfully processed into high quality aseptic product using the continuous flow microwave system (Steed et al.., 2008). With this technology, shelf-stable purees with consistently high quality can be packaged into virtually unlimited container sizes (up to 300 gallons) for conveniently utilizing as food ingredients in the food processing industry. This technology can be extended to highly viscous biomaterials and purees from other fruits and vegetables. In this new process, sweet potato puree is loaded into the hopper, and pumped through the system. Microwaves are generated from a 60 kW, 915 MHz microwave generator and delivered to the puree by a waveguide of rectangular cross-section which is split into two sections and led to two specially designed cylindrical applicators. The puree is preheated to 100ºC in the lower applicator, then to sterilizing temperatures of 130 - 135ºC in the upper applicator, stayed in the holding tube for 30 sec, rapidly cooled in a tubular heat exchanger, and then aseptically packaged in aluminumpolyethylene laminated bags (Coronel et al.., 2005; Simunovic et al., 2006). In microwave processing, dielectric properties have a major role in determining the interaction between puree and the electromagnetic energy. Matching the dielectric properties of the material and the required microwave energy for adequate thermal treatment is very important to avoid over- or under- heating in aseptic processing of sweet potato puree (Coronel et al., 2005; Fasina et al., 2003a). The variation in chemical composition of the sweet potato purees is due to cultivars and post-harvest handling of raw materials, as described above, which may affect the microwave heating behavior of the purees. Brinley et al. (2008) developed predictive equations for dielectric constant and dielectric loss factor as a function of processing variables and puree composition such as temperature, moisture, sugar, and starch in the purees. The predictive equations are helpful in scaling up a continuous microwave heating system as well as determining the microwave heating patterns of purees from sweet potatoes with varying flesh colors for commercial operations. A technique for microbial validation of the process using biological indicators containing spores of thermal resistant bacteria (Geobacillus stearothermophilus and Bacillus subtilis) was also developed (Brinley et al., 2007). Other technical aspects associated with the scale-up of this technology such as the application of static mixing devices to improve the uniformity of temperature distribution and process control parameters for extended operating times have been evaluated (Kumar et al., 2008). The first commercial venture on aseptically packaged sweet potato puree using this microwave-assisted sterilization technology has been carried out by a new company in North Carolina, USA. This development opens up a new market opportunity for the sweet potato industry, and potentially increases the utilization of sweet potato purees as functional ingredients in various food products.

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Quality of Sweet Potato Purees Color and Flavor: Discoloration of the peeled and cooked sweet potatoes can affect the puree color. Enzymatic discoloration is characterized by a brown, dark gray or black color. It occurs when polyphenol oxidase catalyzes the oxidative polymerization of phenolic acid during peeling and size reduction of sweet potatoes. This type of discoloration can be minimized or prevented by heat inactivation of the enzymes, lowering the pH with acidulants, or using inhibitors such as sulfite and ascorbic acid (Walter and Wilson, 1992). The nonenzymatic discoloration shows the gray, black or green color upon exposing the cooked sweet potatoes to air. This ―after-cooking darkening‖ is caused by phenolics complexing with metals especially ferrous iron. Sodium acid pyrophosphate (SAPP) which has a strong affinity to metal ions is effective in preventing the non-enzymatic discoloration (Hoover, 1964). SAPP at concentration of about 0.5% has been widely used in the blanching medium or added directly to the material to enhance the color of sweet potato purees. Citric acid added to the puree at 0.2% can preserve the bright orange color of the product (Bouwkamp, 1985). Among the preservation methods, the puree color is greatly degraded by excessive heat treatment during canning caused by the Maillard browning reaction between sugars and amino acids. Frozen storage has minor color changes over 6 months at -17ºC (Collins et al., 1995). For microwave processing and aseptic packaging, high color retention of purees from both orange- and purple-fleshed sweet potatoes has been reported (Coronel et al., 2005; Smith et al., 1982; Steed et al., 2008). The flavor of purees, as in baked sweet potatoes, is dependent on cultivars, curing, storage and cooking methods (Hamann et al., 1980; Wang and Kays, 2001). Starch hydrolysis and maltose formation during cooking is important in the flavor quality of cooked sweet potatoes (Koehler and Kays, 1991; Sun et al., 1994; Walter et al., 1975). Walter and Schwartz (1993) reported that approximately 52 - 82% of starch in Jewel sweet potatoes was hydrolyzed, depending on the heat treatment. Maltose is the predominant sugar in the purees from various cultivars (Table 1) followed by sucrose, glucose, and fructose (Brinley et al., 2008; Ridley et al., 2005). Wider ranges of these sugars and sweetness in cooked sweet potatoes were reported by other investigators (Chattopadhyay et al., 2006; Kays et al., 2005; Truong et al., 1986). Aside from the sugars, the release of bound compounds (e.g. from glycosides) and a group of terpenoids such as linalool, geraniol and -copaene contribute to the aroma of baked and microwaved sweet potatoes, but they were absent in the boiled samples (Wang and Kays, 2001). Thirty volatile compounds have been identified in baked sweet potatoes (Purcell et al., 1980). Several compounds such as 2, 3-pentanedione, 2-furyl methyl ketone, 5-methyl-2-furaldehyde and linalool were correlated with the good sweet potato flavor (Tiu et al., 1985). Rheological Properties: The rheological behavior is an important property of purees processed from fruits and vegetables and it has been studied by numerous researchers. Krokida et al. (2001) compiled data of several fruit and vegetable products and listed values for consistency coefficient and flow behavior index along with the corresponding ranges of temperature and concentration. In the presence of starch, sweet potato purees are naturally viscous and thicker than other processed purees from other commodities such as carrots and tomatoes. Sweet potato purees display shear thinning behavior with a yield stress, as most of fruit and vegetable purees.

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Table 1. Sugar Content (% fresh weight) of the sweet potato purees Cultivar

Glucose

Fructose

Sucrose

Maltose

Beauregard

2.2

1.8

3.3

7.2

Bon 99-447

0.7

0.5

2.2

7.2

Covington

1.5

1.1

3.7

6.1

FTA 94

0.3

0.2

1.1

3.8

Hernandez

2.8

2.3

3.0

7.4

NC 415

1.6

1.2

1.7

9.3

Norton

1.5

1.8

2.3

7.3

O‘Henry

1.9

1.7

1.7

7.6

Okinawa

0.5

0.3

1.3

3.7

Picadito

0.6

0.4

1.14

4.1

Porto Rico

1.3

1.1

3.3

8.8

Pur 01-192

0.3

0.2

1.1

3.6

Suwon 122

0.2

0.1

1.3

3.9

Source: Brinley et al., 2008.

In studying the relationship between rheological characteristics and mouthfeel of sweet potato purees, Rao et al. (1975a) found sweet potatoes to exhibit non-Newtonian, pseudoplastic behavior that fits the Herschel-Bulkley model. Yield stresses of the purees from eight different cultivars with cream, yellow and orange flesh color in their studies ranged from 230 to 663 dyne/cm2 (23 – 66.3 Pa) (Rao and Graham, 1982). Consistency coefficient values ranged from 17.9 to 248.1 dyne-s/cm2 (1.79-24.8 Pa-s) and flow behavior index values varied from 0.333 to 0.564. Apparent viscosity at 97.2 rpm in a coaxial cylinder viscometer ranged from 534 to 2893 centipoise (0.534 – 2.89Pa-s) among the puree samples of the tested cultivars over two months of root storage. Purple-fleshed sweet potato purees with solid content adjusted to 18% as that of the orange-fleshed sweet potato purees also exhibited pseudo-plastic behavior with the flow properties, apparent viscosity and yield stress within these ranges (Steed and Truong, 2008). Both apparent viscosity and yield stress significantly correlated with the mouthfeel attribute of sweet potato purees (Rao et al., , 1975b), and in general they appear to decrease with length of root storage (Rao et al., 1975a). Purees from cured roots were slightly, but not significantly, lower in apparent viscosity than those made from uncured roots (Ice et al., 1980; Hamann et al., 1980). Analysis of viscometric properties

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with the use of Bostwick consistometer and different types of rotational viscometers has been used to assess the quality of sweet potato purees. Apparent viscosity of sweet potato puree decreases with increasing shear rate and temperature (Figure 2). Kyereme et al. (1999) studied the effect of temperature from 15ºC to 90ºC on apparent viscosity of sweet potato puree with a shear rate sweep of 0.001 to 921/s. The flow behavior of sweet potato puree as affected by temperature was well represented by either the Herschel-Bulkley or Modified Casson models. The models can adequately predict the apparent viscosity of sweet potato puree at 50ºC but they did not perform well at higher temperatures. Ahmed and Ramaswamy (2006) observed a deviation in rheological behavior of sweet potato puree infant food at and above 65C that was possibly caused by gelatinization and possible formation of amylase-lipid complex of starch as confirmed by two distinct DSC (Differential Scanning Colorimeter) thermal transition peaks at 54ºC and 95.5ºC. Brinley et al. (2008) reported significant decrease in apparent viscosity of sweet potato puree at 130ºC at which the puree was sterilized in the microwave-assisted aseptic packaging. Sweet potato purees are usually thickened with temperature decreases that may lead to a difficulty in pumping during the processing operations but the phenomenon can be beneficial in providing the desired textural properties in processed food products (Steed et al., 2008). Amylose and amylopectin in the sweet potato puree form a gel network upon cooling. Aside from the steady-shear viscometry described above, the small-amplitude oscillatory tests have been used to characterize the viscoelastic behavior of sweet potato purees.

Figure 2. Apparent viscosity of orange-fleshed sweet potato puree cv. Beauregard at different temperatures (Truong, unpublished data).

Fasina et al. (2003b) and Ahmed and Ramaswamy (2006) reported that purees exhibit gel behavior illustrated by a larger storage modulus (G‘, the elastic component) than loss

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modulus (G‖, the viscous component) through oscillatory rheology. The parallel slopes of G‘ and G‖ with G‘ greater than G‖ throughout the frequency range define the solid-like behavior of a food material (Steffe, 1996). This gel network was further strengthened by the addition of alginate and calcium salts to form a firmer puree (Fasina et al., 2003b; Truong et al., 1995). The puree processing methods affect the viscoelastic properties and textural profiles of restructured products made from sweet potato purees (Walter et al., 1999). Nutritional Values: The nutrient content of sweet potato purees and pastes from varieties with different flesh color is shown in Table 2 (Brinely et al., 2008). It should be noted that the values of the paste samples (> 21% dry matter) would be lower since dilution needs to be carried out for having flowable purees during processing. Sweet potato purees have low protein and fat content, but they are high in calories, minerals such as potassium, phosphorus, magnesium and calcium, and a relatively good source of dietary fiber, 2.0 – 3.2 g/100g fresh weight basis (fwb) (Bovell-Benjamin, 2007; Woolfe, 1992; Yencho et al., 2008). The glycemic index (GI) of steamed, baked or microwaved sweet potatoes were about 63-66, as compared to 65-101 for potatoes cooked by these methods (Soh and Brand-Miller, 1999). Table 2. Nutritional value (% fresh weight) of purees from various sweet potato genotypes Cultivar

Dry matter

Starch

Total Sugar

Protein

Lipid

Ash

Beauregard

19.5

2.3

14.5

0.4

0.1

0.7

Bon 99-447*

24.7

10.6

10.6

1.9

0.1

1.0

Covington

19.3

1.9

12.4

0.4

0.1

0.8

FTA 94*

33.1

10.2

5.4

0.7

0.2

1.1

Hernandez

23.3

3.83

15.4

0.5

0.1

1.0

NC 415*

30.0

12.0

13.8

0.5

0.1

0.9

Norton*

25.9

6.6

12.7

0.4

0.1

0.8

O‘Henry

20.6

2.7

12.9

0.4

0.1

0.8

Okinawa*

32.0

3.2

5.8

0.6

0.1

0.9

Picadito*

31.3

12.5

6.2

0.3

0.1

0.8

Porto Rico*

26.9

2.8

14.5

0.5

0.1

0.8

Pur 01-192*

32.5

13.2

5.3

0.4

0.2

1.0

Suwon 122*

34.4

11.3

5.5

0.6

0.2

1.0

*Dry matter should be adjusted to < 21% for flowable purees Source: Brinley et al., 2008.

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Orange fleshed-sweet potato purees are rich in ß-carotene (Table 3). A wider range of βcarotene content in cooked orange-fleshed sweet potatoes, 6.7 – 16.0 mg/100g fwb, has been reported by different investigators (Huang et al., 1999; Namutebi et al., 2004; BovellBenjamin, 2007). The sweet potato carotenoids exist in an all trans configuration which exhibits the highest provitamin A activity among the carotenoids. van Jaarsveld et al. (2005) and Tanumihardjo (2008) advocate the increased consumption of orange-fleshed sweet potatoes as an effective approach to improve the vitamin A nutrition in the developing countries. Epidemiological studies indicated the beneficial effects of high carotene diets in reducing the risks of cancer, age-related macula degeneration and heart diseases (Kohlmeier and Hasting, 1995; Pandey and Shukla, 2002; van Poppel and Goldbohm, 1995). Carotenoids can be isomerized by heat, acid, air or light during puree processing and storage. When exposed to heat, the molecule may transform to a cis configuration typically at the 9, 13, and 15 carbon positions. The cis form reduces pro-vitamin activity but color remains mostly unaffected. Extremely high temperature processing will cause fragmentation products and release of volatile compounds. Chandler and Schwartz (1988) studied the changes in ßcarotene and its isomerization products as a result of blanching, canning, dehydrating, and cooking. The length and severity of the heat treatment increased ß-carotene loss and isomerization. Blanching, lye peeling, and pureeing actually showed an increase in ß-carotene content but this increase was attributed to enhanced extraction efficiency due to the heat treatment. However, other common sweet potato processing treatments showed significant reductions in ß -carotene content: steam injection – 8.0% loss, canning 19.7% loss, microwaving - 22.7% loss, and baking - 31.4% loss (Chandler and Schwartz, 1988). Lessin et al. (1997) quantified ß-carotene isomers after canning sweet potatoes. The total ß-carotene content increased by 22% from 256.5mg/g (db) in the fresh root to 312.3 mg/g (db) in the canned product which was attributed to increased extraction efficiency. Table 3. Phytonutrients in orange- and purple-fleshed sweet potatoes Varieties

Flesh color

Dry matter (g/100g)

-carotene (fwb) (mg/100g)

Antho cyanins¹

Total phenolic²

Beauregard

Orange

20.5

9.4

na

88.9

Covington

Orange

20.3

9.1

3.8

58.4

Stokes Purple

Dark purple

36.4*

na

80.2

401.6

NC 415

Dark purple

29.0*

na

69

652.5

Okinawa

Light purple

30.0*

na

21.1

458.3

*Dry matter needs to be adjusted to 18-20% for flowable purees; na = not analyzed. ¹mg cyanidin-3-glucoside/100g fw; ²mg chlorogenic acid/100g fw. Sources: Truong et al. (2007); Steed and Truong, 2008; Yencho et al., 2008.

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After canning however, some of the trans- form was transformed to 9-cis (25.3 mg/g), 13-cis (76.6mg/g), and 15-cis (19.4mg/g) forms while 191mg/g (db) remained in the trans configuration. A loss of less than 15% carotene content was observed by microwave sterilization and aseptic packaging of orange-fleshed sweet potato purees (Truong, unpublished data). Thus, the carotene-rich sweet potato purees can be a functional food ingredient which can help reduce the risk of chronic diseases and vitamin A deficiency in many parts of the world. The cooked paste of the purple-fleshed sweet potatoes has attractive reddish-purple color with high levels of anthocyanins and total phenolics (Table 3). The flowable purees with a solid content of 18% made from this material had total phenolic and anthocyanin contents of 314 mg chlorogenic acid equivalent/100g fwb and 58 mg cyaniding-3-glucosdie equivalent/100g fwb, respectively. The 2, 2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity was 47 µmol trolox equivalent/f fwb and oxygen radical absorbance capacity (ORAC) of 26 µmol trolox equivalent/f fwb (Steed and Truong, 2008). Therefore, the purple-fleshed sweet potato purees have polyphenolic content and antioxidant activities in a competitive level with other food commodities known to be a good source of antioxidants such as black bean, red onion, black berries, cultivated blueberries, sweet cherries and strawberries (Wu et al., 2006) 2006). Several clinical studies indicated that consumption of purple-fleshed sweet potatoes may have potential health benefits against oxidative stress associated with liver injury (Suda et al., 2008) and other chronic diseases (Suda et al., 2003).

Utilization of Sweet Potato Purees in Processed Foods Sweet potato purees has been used as an ingredient in numerous food products, including baby food, casseroles, puddings, pies, cakes, ice cream, yogurt, leather, bread, patties and soups (Collins and Walter, 1992; Collins et al., 1990; Collins and Washam-Hutsell, 1986; Hoover et al., 1983; Silva et al., 1988; Woolfe, 1992; Yasufumi and Shigeki, 2000). The most successful commercial application of sweet potato puree is for baby food. Recognizing the similarity in nutrient content of sweet potatoes and fruits, Truong (1987) conceptualized a novel strategy on value-added processing of sweet potatoes into products that have been traditionally produced from fruits. This novel approach was expanded to the development of a process for producing sweet potato puree-based beverages with sensory quality and nutrient content similar to fruit juices. Both orange- and purple-fleshed sweet potatoes were utilized, and the beverages were produced either in a concentrate form for reconstitution to a single strength of 100% sweet potato drinks, or in combination with other fruit juices and flavorings (Truong and Fementira, 1989, 1990). Such development raised interest among research institutions in several sweet potato producing countries including India, Japan, Malaysia, and the United States (Payton et al., 1992; KNAES, 1996; Sankari et al., 2002; Tan et al., 2004). Several patented processes on utilization of sweet potato purees in fruit and vegetable beverages were developed in Japan and the United States (Gladney, 2005; Payton et al., 1992), and currently there are several fruit and vegetable drinks with sweet potato purees as an ingredient being commercialized in these countries. Other commercial utilization of sweet potato puree includes jam and ketchup (Truong, 1994; Fawzia et al., 1999). Restructured products from sweet potato puree with the use of gelling agents such as carboxymethyl cellulose, hydroxymethyl cellulose and alginate-calcium system have also

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been developed. These products, simulated-baked sweet potatoes and restructured French fries have good sensory quality and textural properties (Truong and Walter, 1994; Truong et al., 1995; Utomo et al., 2005). Recently, sweet potato purees have been used in developing carotene-rich curd and fermented beverages with high antioxidant activity (Mohapatra et al., 2007; Saigusa et al., 2005). With the recent commercial development of the microwaveassisted processing and aseptic packaging of sweet potato purees (Coronel et al., 2005; Steed et al., 2008), it is expected that more processed food products from the puree will be developed. In the U. S., sweet potato puree has been used for dehydrating into flakes or powder for various food applications that is described in the following section.

DEHYDRATED POWDER / FLOUR AS FUNCTIONAL INGREDIENT Sweet potato roots can be processed into dehydrated forms such as dried chips and flour for storage and uses in food preparations (Peters and Wheatley, 1997). The flour can add natural sweetness, color, and flavor to processed food products. It can also serve as a source of energy and nutrients and minerals (Table 4), and contributes to the daily nutrient needs for β-carotene, thiamin, iron, vitamin C, and protein. Sweet potato flour provides 14% - 28% of the dietary reference intake (DRI) for magnesium and 20 - 39% for potassium (van Hal, 2000). For individuals diagnosed with celiac disease or with allergies to the gluten in wheat, sweet potato flour can serve as an alternative. Food allergies have become a public health issue in many countries (Maleiki, 2001). About 5% of the population has serious allergies to some foods, including the gluten in wheat and other cereals including rye, barley, triticale, and oats, (Mannie, 1999, Caperuto et al., 2000). In addition, the home production of a simple traditional processed sweet potato foods, as practiced by women and children in tropical countries could increase family income (Alcobar and Parrilla, 1987). Thus, sweet potato flour production for human feeding will aid in promoting year-round consumption, decreasing losses of food, increasing the economic value of the crop besides increasing the efficiency of the food delivery system. Table 4. Composition of sweet potato flour* Parameter Protein Fat Total dietary fiber Ash Phosphorous Total carbohydrates

Native Flour 6.6 1.0 17.5 1.0 0.1 73.0

Spray Dried** 3.18 0.61 5.85 2.7 85.23

Drum Dried 6.5 1.1 17.6 1.3 0.12 73.8

Hot air Dried (Cabinet dried) 6.3 1.1 17.2 1.1 0.11 73.6

*Dry weight basis. **contains added dextrins. Source: Avula et al. 2006; Grabowski et al., 2008.

Sweet potato flour is used as a raw material for processing into other products. A variety of products such as doughnuts, biscuits, muffin, cakes, cookies, extruded products, fried chips, ice cream, porridge, brownies, pies, breakfast foods, and weaning foods have been

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made from sweet potato flour (Greene et al., 2003; Lee, 2005; Toyokawa et al., 1989). In India, dried sweet potatoes grounded into flour are used to supplement flours in bakery products, chapathis, and puddings (Nair et al., 1987). Drying of root slices for sweet potato flour production is also practiced to certain extents in many countries in Asia including Bangladesh, China, Indonesia, Japan, Philippines and Vietnam. In Indonesia, fresh roots are sometimes soaked in 8-10% salt solution, a practice which is reported to inhibit microbial growth during drying (Winaro, 1982). In parts of East and West Africa, where there is a pronounced dry season, roots are peeled, sliced, and sun dried for storage. In Peru, sweet potato flour has been produced for decades, to prepare wheat/sweet potato bread (van Hal, 2000).

Processing of Sweet Potato Flour For sweet potato roots to produce good quality flour, they should be low in total free sugar content, reducing sugar content, ash content, amylase and polyphenol oxidase activities, and have high dry matter with white color (Bovell-Benjamin, 2007; Collado et al., 1997). Roots are still acceptable for processing if the reducing sugars do not exceed 2% on dry weight basis (van Hal, 2000). Generally, controlling the quality of a product is based on the acceptability of the users and food legislation (Bovell-Benjamin, 2007). Dehydration of sweet potato involves washing, peeling, slicing/shredding, blanching, soaking, pressing, and drying (van Hal, 2000; Woolfe, 1992). The losses during peeling and the ease of drying by slicing and shredding have been reported. In traditional practice, the roots, which may or may not be peeled and cooked but more often are directly cut up into pieces and spread out in the sun to dry. They yield dried chips or slices which can be ground in a mortar to flour, and then sieved. Mechanical driers such as cabinet, tunnel, drum, or spray drying as used in large commercial enterprises are highly technical processes using large amounts of energy, which add greatly to the cost of the final product (van Hal, 2000). Solar Drying: Solar drying is the cheapest technique since it uses free and non-polluting energy with a minimum investment in equipment. Drying of sweet potato root slices in direct sunlight or in a solar dryer is frequently carried out. Both white and colored varieties have been found suitable for solar drying. Drying times vary depending on climatic conditions from 4 h to 5 days. Slices were dried until they reached a moisture content of about 6-10% (Winaro, 1982). The use of dehumidified air increased the drying rates by about 6-8%. However, solar drying has a number of disadvantages, such as poor control of energy input and product quality, interruption of drying caused by cloud, rain, and nightfall and frequent contamination of food by microorganisms, dust, and insects (Woolfe, 1992). Mechanical Drying: Drying in a cabinet or tunnel dryer is based on the same principle as solar drying, with the difference being that the air is heated by fuel. In this type of dryer, the drying temperature, drying time and air velocity, and hence total dehydration conditions could be controlled. Slices/dices are also subjected to blanching to inactivate the enzyme responsible for browning reactions and soaked in solution containing sulphur dioxide to inhibit enzymatic and non enzymatic reactions for improving color and retaining quality during storage. Sweet potato slices are exposed to drying temperatures between 50°C - 80°C for 4 - 12 h (Avula et al., 2006; Collado et al.,. 1997; Hathorne et al.,. 2008). Special batch type cabinet dryers for drying sweet potato slices on small and industrial scales were also

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developed (Eusebio et al., 1996; Truong et al., 1990). Air velocity, slice thickness, and airdry bulb temperature were the main variables that affected drying rates of sweet potato slices (Diamante and Munro, 1991). The modified Page equation was found to be the best description for the drying curves of sweet potato slices dehydrated to a moisture content of 10%. Antonio et al. (2008) studied the influence of osmotic dehydration and high temperature short time drying process on dried sweet potato and found that 150°C for 10 min and 160 °C for 22 min were the best drying conditions for drying of sweet potato slices subjected to osmotic treatment and no osmotic treatment, respectively. Drum drying is also used for dehydration of sweet potato puree to produce flakes / powder. Walter et al (1983) and Valdez et al. (2001) dried the cooked and comminuted sweet potatoes in a double drum drier heated with steam at 80 psi. The flakes were milled into