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Retrogradation Mechanism of Rice Starch¹

¹Presented at 10th European Carbohydrate Symposium, Calway, Ireland, July 11–16, 1999. ²Associate professor, Department of Bioscience and Biotechnology, University of the Ryukyus, Nishihara, Okinawa 903-0123, Japan. ³Professor emeritus, Department of Biological Science and Technology, Kagoshima University, Kagoshima 890-0065, Japan.

The non-Newtonian behavior and dynamic viscoelasticity of rice starch (Akihikari, 18.8% amylose content) solutions after storage at 25 and 4°C for 24 hr were measured with a rheogoniometer. The flow curves, at 25°C, of Akihikari starch showed plastic behavior >3.0% (w/v) after heating at 100°C for 30 min. The dynamic viscoelasticity of the starch increased after storage at 25 and 4°C for 24 hr and stayed at a constant value with increasing temperature. A small dynamic modulus of rice starch was observed upon addition of urea (4.0M) at low temperature (0°C), but it produced a sigmoid curve when plotted against increasing temperature. A small dynamic modulus was also observed in 0.05M NaOH solution. However, it increased rapidly after the temperature reached 70°C. Possible models of retrogradation mechanism of rice starch were proposed.