TEXTURE AND STRUCTURE OF HIGH-PRESSURE-FROZEN FOOD GELS.

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To determine the effect of high-pressure-freezing on quality, gels composed of protein (tofu, egg custard and gelatin gel) and gels of polysaccharides (agar, konjac, carrageenan-locust bean gum, curdlan and gellan gums) were frozen at 100 MPa (ice I), 200 MPa (liquid phase), 340 MPa (ice III), 400, 500, 600 MPa (ice V) or 700 MIPa (ice VI) at ca. -20ﾟC.After reducing to atmospheric pressure, gels were stored at -30ﾟC then thawed at 20ﾟC.Texture and structure (cryo-SEM observation) were then compared with gels frozen (-20ﾟC, -30ﾟC or -80ﾟC) at atmospheric pressure (0.1 MPa). In gels without sucrose, frozen at 0.1, 100, 600 and 700 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 compared to gels frozen at 0.1 and 100 MPa (ice I). However, quality differed from unfrozen gels. With the increase of sucrose, appearance, texture and structure of all gels improved. To determine phase transition of ices during depressurization, high-pressure-frozen gels were thawed at the same high pressure. Aferwards, pressure was reduced. Texture of gels frozen at 200 - 500 MPa was the same as original gels. This suggested that phase transitions (from ice VI to iceV to ice III to liquid to ice I) occurred during reduction of pressure (-20ﾟC), or during storage in a freezer. However, when gels were pressurized at 200-400 MPa and -20ﾟC, the gels did not freeze (supercooling), while after depressurization, the gels froze quickly. Thus, high-pressure-freezing at 200-400 MPa was effective in improving the quality of all frozen gels.