Dynamic oscillation rheometry in combination with repeating freeze-thaw (FT) cycles (DOR-FT) was applied to evaluate the rheological change of starch gel caused by long-term retrogradation. The gels prepared from corn, wheat, rice, potato, and sweet potato starch pastes (6%, w/w) were formed on a dynamic rheometer plate and the rheological changes induced by the FT cycles were measured while their gels remained on the rheometer plate. Comparing with the storage modulus (G 0 ) at 25°C of each starch gels without FT, G 0 of corn, wheat, rice, potato, and sweet potato starch gels after three FT cycles increased by 341, 474, 167, 1368, and 631%, respectively. The DOR-FT method enabled the evaluation of the rheological changes caused by the long-term retrogradation within 1 h, which is much shorter than with the conventional puncture test. Moreover, G 0 versus FT cycles could be expressed as an exponential function model, indicating that the function model would show the FT cycle dependence for the rheological change of starch gels.
The ginger rhizome has been known to contain proteolytic enzymes. In this study, we evaluated the protease activities in juice extracted from ginger rhizomes. The specific protease activity in juice from ginger ripened after harvesting was higher than that of ginger that was not ripened after harvesting. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis indicated that the ginger rhizome juice displaying high specific protease activity contained a 30 kDa protein. This juice was also observed to have milk gel-forming ability.
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