Effect of multi-treatment of freeze-thaw or low temperature on wheat starch gel

We investigated the effect of multiple treatments (10, 20 and 30 times, one cycle of treatment: 24 h) of freeze-thaw (- 30 deg C) or low temperature (4 deg C) on wheat starch gel for elucidation of the formation mechanism of Furunori (the traditional paste for decoration of Japanese painting). Furunori needs over ten years maturation for its formation. Hence, the development of a scientific method for making Furunori in a short term is very much in demand. Our report of the molecular characteristics of Furunori was previously published, and it was suggested that the main factor of Furunori formation might be temperature change with transition of season during maturation. The Furunori model by the above treatment was set for elucidation of this factor effect. The results are as follows : 1) The SEM image of the materials in the 4 deg C treatment shows a pebble-like mass and fairly closely resembling Furunori, but the SEM image the materials in the -30 deg C treatment shows a flat flaky mass. The size of both the 4 deg C treatment and -30 deg C treatment masses becomes smaller with each treatment. 2) The peak of x-ray diffraction pattern for the -30 deg C treatment is higher than that for the 4 deg C treatment and Furunori. 3) The gelatinization degree in the 4 deg C treatment is far larger than in the -30 deg C treatment measured by the BAP method, but that in the -30 deg C treatment is larger than in the 4 deg C, treatment measured by the glucoamylase method. 4) All GPC patterns of both the -30 deg C treatment and 4 deg C treatment on Toyopearl HW 75 F have an amylopectin peak which Furunori does not have. 5) DSC results show that the -30 deg C treatment has a clear endothermal peak which becomes larger with each treatment whereas the 4 deg C treatment scarcely has an peak. In conclusion, the -30 deg C treatment is more retrograded than the 4 deg C treatment, but the mechanism of retrogradation might be different. The formation mechanism of Furunori might resemble that of the 4 deg C treatment, but we can not explain the disagreement between the two GPC patterns. Probably, the special gelatinization process of Furunori (heating and stirring over 24 h) may hold key to the explanation of its GPC pattern.