Abstract
Steamed and cured slices of sweetpotato, "hoshi-imo", is a traditional processed agricultural product in eastern area of Japan, especially in Ibaraki and Shizuoka prefectures. The products often suffer from a quality-damage called "shirota" which is a white-opaque defect, after a hot summer with little rainfall. We studied the mechanism of the occurrence of "shirota" defect mainly from the viewpoints of the contents of starch and water in storage roots in several sweetpotato cultivars. The defect occurred frequently in the cultivars with a high starch content such as "Hi-starch" and "Tamayutaka", and scarcely in "Okinawa 100" with a low starch content and "Quick Sweet" having a low starch-pasting temperature. The "shirota" part of the root tissues had a lower water content than the normal part of the same tissue, shown by MRI and other investigations. In "Tamayutaka", however, there was no significant difference in the contents of starch and soluble sugar and beta-amylase activity between the normal and "shirota" parts. On the other hand, the "shirota" part of "Hi-starch" had a lower starch content than the normal part of the roots. It was also confirmed that the "shirota" defect in "Tamayutaka" became serious with the decrease in water contents of the storage roots when the cultivating soil moisture decreased. The decrease in water content of the storage roots was considered to inhibit the starch pasting reaction during the steaming process, which resulted in the formation of "shirota" in "Tamayutaka".
