Effects of pressure-shift freezing and conventional freezing on model foodgels

Gels of agar, starch, ovalbumin, gelatin and an industrial beta-lactoglobul in protein isolate, were frozen conventionally in a -30 degrees C freezer a nd by pressure-shift freezing at 200 MPa at -15 degrees C. Thawing was carr ied out conventionally at 20 degrees C and by the application of a pressure of 200 MPa. The microscopic structure and mechanical properties of the tha wed gels were compared with those of the initial gels. Microscopic examinat ion showed that pressure-shift freezing produces smaller and more uniform. ice crystal damage than conventional freezing at -30 degrees C. The results also suggest that the freeze-thaw behaviour of food gels can be categorize d into two general types: (1) gels which have a reduced gel strength as a r esult 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, beta-la ctoglobulin 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.