Gels of agar, starch, ovalbumin, gelatin and an industrial -lactoglobulin protein isolate, were frozen conventionally in a Ϫ30 ЊC freezer and by pressure-shift freezing at 200 MPa at Ϫ15 ЊC. Thawing was carried out conventionally at 20 ЊC and by the application of a pressure of 200 MPa. The microscopic structure and mechanical properties of the thawed gels were compared with those of the initial gels. Microscopic examination showed that pressure-shift freezing produces smaller and more uniform ice crystal damage than conventional freezing at Ϫ30 ЊC. The results also suggest that the freeze-thaw behaviour of food gels can be categorized into two general types: (1) gels which have a reduced gel strength as a result of mechanical damage to the gel microstructure caused by ice crystal formation, and (2) gels which have an enhanced gel strength, as a result of molecular structural changes that take place in the frozen state. Agar and gelatin were found to be typical of type (1) gels, whereas starch, -lactoglobulin protein isolate and ovalbumin were found to be typical of type (2) gels. In the case of starch, retrogradation during thawing was found to be the most important factor.
GelatinGelatin (Sigma, type A: porcine skin ~175 bloom) gels were prepared by initially dispersing in water and then stirring at 80 ЊC for 15 minutes. The solutions were poured into aluminium coated Pressure-shift freezing of model food gels M. T. Kalichevsky-Dong et al.