High-pressure shift freezing versus high-pressure assisted freezing: Effects on the microstructure of a food model

Gelatin gel samples (10% gelatin, w/w) were frozen by high-pressure shift freezing (HPSF) and by high-pressure assisted freezing (HPAF) with the phase transition at identical pressure conditions (0.1, 50, and 100 MPa) so as to allow valid comparisons. The corresponding temperature/pressure profiles were recorded in order to characterize the processes. Also, the ice crystal distributions were analyzed to estimate the effects of both freezing methods on the food microstructure. Results clearly showed that high-pressure shift freezing is the more advantageous method. The supercooling attained after the expansion and the consequent instantaneous freezing of water, together with the temperature drop in the pressure medium, induced short phase transition times (5.9, 8.6, and 13.7 min in HPSF versus 14.8, 14.1, and 23.1 min in HPAF at 0.1, 50, and 100 MPa, respectively) and a homogeneous distribution of small ice crystals throughout the sample.