Evaluation of eating quality and starch properties of sweet potato produced in northern Japan

Sweet potato (Ipomoea batatas L.) is one of the major root vegetables in Japan. It is eaten as a baked or steamed product and has also been used as a raw material for liquor and starch production. Sweet potato is produced mainly in temperate regions, such as occur in the central or southern parts of Japan. Currently, however, the production area is distributed as far as Hokkaido in the subarctic region in northern Japan. There have been few reports on the taste and quality of sweet potatoes that are grown in the subarctic regions. Consequently, in this study, some characteristics related to food processing properties were compared between the crop grown in the Kanto or Kyushu region, in central or southern Japan, with that from Hokkaido. Sweet potatoes grown in Hokkaido had the following features compared with those grown in Kanto or Kyushu: 1) significantly lower dry matter ratio (2.0 to 7.0%) and starch content (2.1 to 5.2%) in the tuberous roots; 2) lower firmness (6.5 to 20.4 kgf cm^‑2) and higher Brix value (2.7 to 7.0 °Brix) in steamed roots; and 3) were moister and sweeter in a triangle sensory preference test. Furthermore, the pasting temperature of starch extracted from Hokkaido crops was significantly lower (2.5 to 8.7°C) than those from Kanto or Kyushu. Starch gel made from Hokkaido crops, with the lower pasting temperature, showed a significantly lower water separation ratio (8.3 to 9.4% after four weeks storage at 4°C) and fewer changes in firmness compared to those from Kanto and Kyushu. These results suggested that sweet potato starch from Hokkaido could be more resistant to retrogradation and more suitable for use in food processing.