Received: 1 November, 2011. Accepted: 1 November, 2012. Invited Review Fruit, Vegetable and Cereal Science and Biotechnology ©2012 Global Science Books Sweet Potato Starch: Physico-Chemical, Functional, Thermal and Rheological Characteristics Subramony N. Moorthy * • Moothandassery S. Sajeev • Salim Shanavas Central Tuber Crops Research Institute, Thiruvananthapuram – 695 017, India Corresponding author: * snmoorthy48@gmail.com ABSTRACT Among the tropical tuber crops, sweet potato comes next to cassava in starch content. The article reviews the extraction of starch from sweet potato tubers, and the physicochemical, functional, thermal and rheological characteristics of sweet potato flour and starch. Wide variation in biochemical constituents is observed depending on the origin of the tubers and processing conditions. The lipid and phosphorus contents are low. The starch granules are round, 3-45 μm in size and have distinct XRD patterns. The amylose content, swelling, solubility and digestibility characteristics depend on a number of factors like age of crop, method of extraction and varietal differences. The viscosity and rheology properties are also quite diverse among the varieties. DSC analysis indicates pasting temperature to be between 60 and 88°C while enthalpy of gelatinization is 10-18 J/g. Heat moisture treatment alters some of the properties. _____________________________________________________________________________________________________________ Keywords: enzymatic extraction, heat-moisture treatments, mobile starch extraction plant, physico-chemical, thermal, rheological and functional properties, starch, sweet potato flour Abbreviations: BU, Brabender units; cP, centi Poise; DP, degree of polymerization; DSC, differential scanning calorimetry; FTIR, fourier transform infrared; G’, storage modulus; G”, loss modulus; PV, peak viscosity; RVA, rapid visco analyser; SEM, scanning electron microscopy; XRD, X-ray diffraction CONTENTS INTRODUCTION...................................................................................................................................................................................... 124 STARCH EXTRACTION.......................................................................................................................................................................... 125 BIOCHEMICAL CONSTITUENTS.......................................................................................................................................................... 126 Size and shape ....................................................................................................................................................................................... 126 Crystalline structure............................................................................................................................................................................... 126 Molecular weight ................................................................................................................................................................................... 126 Amylose content .................................................................................................................................................................................... 127 Swelling and solubility .......................................................................................................................................................................... 127 Water binding capacity .......................................................................................................................................................................... 128 Digestibility ........................................................................................................................................................................................... 128 Degradation by acid ............................................................................................................................................................................... 128 Degradation by enzymes ....................................................................................................................................................................... 128 Retrogradation ....................................................................................................................................................................................... 128 Sol stability............................................................................................................................................................................................ 128 Thermal characteristics .......................................................................................................................................................................... 128 Rheological properties ........................................................................................................................................................................... 130 Pasting temperature ............................................................................................................................................................................... 131 MODIFICATION OF SWEET POTATO STARCH BY HEAT MOISTURE TREATMENT .................................................................... 131 CONCLUSION .......................................................................................................................................................................................... 131 REFERENCES........................................................................................................................................................................................... 131 _____________________________________________________________________________________________________________ INTRODUCTION Starch occupies an important position as a major bioche- mical component in the plant kingdom, especially in the tuber crops. The starch content varies from 10-30% on fresh weight basis in these crops and the starches extracted from tuber crops have diverse functional properties. The applica- tion of the starch in food and industry depends primarily on the starch content and also on the starch properties. Sweet potato has the highest starch content next to cassava among the tropical tuber crops and the extraction process is relatively easy compared to cereals. In third world countries, sweet potatoes are processed into starch, noodles, candy, flour and desserts. Sweet potato starch production has assumed the status as an important cottage industry in many parts of China. China has become the largest grower of sweet potatoes, providing about 80% of the world’s supply. More than 2000 varieties of sweet potatoes are available in China which can be roughly divided into ‘general type’, ‘high starch type’ and ‘food consumption type’ (Liu 2004). The major biochemical component of the dry matter in sweet potato is starch and a large proportion of the starch can be extracted from the tubers. The uses of sweet potato starch are primarily determined by its physicochemical pro- perties like granule shape and size, amylose content, mole- cular structure, pasting properties, retrogradation, enzyme ®