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Structural and Retrogradation Properties of Rice Endosperm Starch Affect Enzyme Digestibility of Steamed Milled-Rice Grains Used in Sake Production

¹National Research Institute of Brewing, 3-7-1, Kagamiyama, Higashihiroshima 739-0046, Japan. ²Corresponding author. Mailing address: National Research Institute of Brewing, 3-7-1, Kagamiyama, Higashihiroshima 739-0046, Japan. Phone: +81-82-420-0812. Fax: +81-82-420-0803. E-mail: okuda@nrib.go.jp ³Department of Applied Biological Science, Faculty of Life Science and Biotechnology, Fukuyama University, 1, Sanzo, Gakuen-cho, Fukuyama 729-0292, Japan.

Structural and physicochemical characteristics of endosperm starch from milled rice grains of seven Japanese cultivars used in sake production were examined. Amylose content was 15.2–20.2%, number-average degree of polymerization (DP_n) of amylose was 900–1,400, and the ratio of short-to-long chain amylopectin was 2.7–3.5, respectively. The degree of retrogradation of purified starch stored for seven days at 4°C after gelatinization was 20–31%. The degree of retrogradation correlated negatively with the ratio of short-to-long chain amylopectin. The effect of holding time after steaming on enzyme digestibility and starch retrogradation of steamed rice grains was investigated. The longer the holding time after steaming, the greater the extent of retrogradation, and the less the degree of enzymatic digestibility. The decreased rate of enzyme digestibility correlated with amylopectin chain length distribution. Samples with short-chain amylopectin exhibited a slow decrease in enzyme digestibility. It was determined that the structure and retrogradation properties of endosperm starch in Japanese rice cultivars affect the decreasing rate of enzyme digestibility of the steamed, milled rice grains.