Effect of Starch Pasting Temperature on Remaining Total Ascorbic Acid Content in Steamed Sweet Potatoes

The ascorbic acid content of raw and steamed storage roots of 3 sweet potato cultivars with different starch pasting temperatures was measured. The cultivars were "Beniazuma", "Hoshikirari" and "Quick Sweet", and their pasting temperatures measured by RVA were approximately 75 degrees C, 65 degrees C and 55 degrees C, respectively. The total ascorbic acid contents per fresh weight of raw roots of "Beniazuma", "Hoshikirari" and "Quick Sweet" were 275.88 +/- 38.93 mu/g, 244.70 +/- 50.48 mu g/g and 218.26 +/- 50.20 mu g/g, respectively. After steaming, these values changed to 154.27 +/- 31.94 mu g/g, 170.20 +/- 26.20 mu g/g and 175.18 +/- 44.38 mu g/g. Therefore, about 57 %, 71 % and 80% of the ascorbic acid contents in raw roots of "Beniazuma", "Hoshikirari" and "Quick Sweet" were remained after steaming, respectively. The steaming process resulted in maltose generation at ca. 50 degrees C for "Quick Sweet", ca. 60 degrees C for "Hoshikirari" and ca. 80 degrees C for "Beniazuma", according to their starch pasting temperatures. The sweet potato cultivar "Okikogane" in which maltose is not generated upon heating due to the lack of beta-amylase activity remained less than 50 % of total ascorbic acid presented in raw roots after steaming. These results indicated that sweet potato cultivars with low starch pasting temperatures retained a larger percentage of total ascorbic acid content presented in their raw roots after steaming due to their early generation of maltose.