Abstract
The static linear viscoelastic behavior (creep compliance) of starch gels at the earlier retrogradation stage has been investigated. Wheat starch, corn starch, and non-glutinous rice starch were used. Starch gels (starch content 15.2-35.0%) were stored at 2°C up to 36 hr. Creep compliance curves of wheat starch gels and corn starch gels could be represented by a Voigt-type four-element model, which was combined of a elastic element, a Voigt element, and a viscous element in series array. In case of rice starch gels, creep behavior was represented by a six-element mechanical model combining a Voigt-type four-element model and a Voigt element in series array. Changes in creep compliance (J(300)), which were measured under uniaxial compression for 300 s, of wheat starch gels with storage period showed four regions of gel-hardening: 1) in region I, from 0 to several hours, J(300) decreased exponentially with increasing storage period, 2) in region II, the compliance increased with increasing storage time for several hours, 3) in region III, the slope of J(300) versus the storage period curve continued to decrease exponentially from the initial point until an inflection point, and 4) in region IV, the compliance continued with decreasing slowly. The occurence of region II was first confirmed by the present study. The gel-hardening process of corn starch gels also separated into the four regions. On the other hand, the process of retrogradation of rice starch was monomorphic. The values of initial creep compliance, J0, of 35.0% starch gels without storage or with shorter storage period, increased in the following order: rice starch>corn starch>wheat starch. The values of the hardening rate constant in region I, k1, of wheat starch gel and corn starch gel were increased about 2 times and 7 times higher than that of rice starch gel, respectively.
