The Effects of beta-Amylase Activity and Starch Pasting Temperature on Maltose Generation in Steamed Storage Roots of Sweet Potato

The effects of beta-amylase activity and starch pasting temperature on maltose generation in steamed sweet potato were investigated using various cultivars and breeding lines. The maltose content per fresh weight of steamed roots increased to about 10% (w/w) with beta-amylase activity up to about 0.2m mole maltose/min/mg protein, and maltose content was maintained at that level with activity up to 0.55 m mole maltose/min/mg protein. The maltose concentration showed a slight positive correlation (r (sic) 0.38) with the starch content of storage roots and a practically negative correlation (r (sic) -0.53) with the starch pasting temperature, as measured by rapid visco analyzer (RVA). Neither the content nor the pasting temperature of starch showed any significant correlation with the beta-amylase activity in roots. The influence of starch pasting temperature on the maltose concentration was more apparent in the storage roots with highn beta-amylase activity (0.2 similar to 0.55 m mole maltose/min/mg protein) than in those with low activity (0 similar to 0.2m mole maltose/min/mg protein). The pasting temperature of starch isolated from the roots of "Beniazuma" and "Beniharuka" harvested in Hokkaido Prefecture was lower (about 6.9 degrees C and 5.0 degrees C, respectively) than in those harvested in Ibaraki Prefecture. The maltose concentration of steamed roots of the two varieties harvested in Hokkaido Prefecture was higher than those harvested in Ibaraki Prefecture; while the beta-amylase activity of the roots harvested in Hokkaido was not higher than in those from the southern part of Ibaraki. These results suggested the importance of starch pasting temperature for maltose generation by heating in sweet potato, particularly in cultivars with high beta-amylase activity.