BEHAVIOR OF WATER AND ICE IN FOOD UNDER HIGH- PRESSURE

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To determine the effect of high-pressure-freezing on the quality of frozen gels, gels composed of protein (tofu and egg custard gel) and gels of polysaccharides (agar, konjac, carrageenan-locust bean gum, curdlan and gellan gums) were pressurized at 100 〜686 MPa at ca. -18℃ 〜 -20℃. After reducing to atmospheric pressure, gels were stored at -30℃ then thawed at 20℃. Texture and structure (cryo-SEM observation) were then compared with gels frozen (-20℃, -30℃ or -80℃) at atmospheric pressure (0.1 MPa). In gels without sucrose, frozen at 0.1, 100, 600 and 686 MPa, texture and structure differed significantly from original gels. On the other hand, when all gels (except konjac) were frozen at 200 - 400 MPa, the quality of frozen gels improved. With an increase of sucrose, the appearance, texture and structure of all gels improved. To determine the phase transition of ices during pressurization, temperatures were determined. When gels were pressurized at 200 〜 400 MPa and -20℃, the gels did not freeze (supercooled), but after depressurization, the gels froze quickly. The freeze-thaw tolerance differed with the kinds of gels being examined. They were classified in three types : gels which became hard during freezing (tofu, konjac), the gels which became soft (agar, kappa-carrageenan, unsubstituted form-gellan gum gels), and the gels which did not change (curdlan, substituted form-gellan gum, iota-carrageenan gels). Finally, pressure-shift-freezing at 200 〜 400 MPa was found to be effective in improving the quality of all frozen gels.