Ice-crystal formation in gelatin gel during pressure shift versus conventional freezing

Cylindrical specimens (7.8 mm diameter, 35 mm length) of gelatin gel (2% gelatin, w/w) were frozen by pressure shift freezing (PSF) at 100 MPa (−8.4 °C), 150 MPa (−14 °C) and 200 MPa (−20 °C) as well as conventional air freezing (CAF) and liquid immersion freezing (LIF) at −20 °C. Pressure and temperature profiles of gel samples were gathered during the freezing process. The ovoid microstructure of ice crystals in frozen gelatin gels after freeze-drying was evaluated for area, equivalent diameter, roundness and elongation. Equivalent diameter (mean ± s.d.) of the ice crystals was 145 ± 66, 84 ± 26, 91 ± 30, 73 ± 29, and 44 ± 16 μm for test samples subjected to CAF, LIF and PSF at 100, 150 and 200 MPa, respectively. Roundness and elongation were somewhat variable and did not show a clear trend with the different freezing techniques. Results from this study with a model food (gelatin gel) confirm that the PSF process promotes the production of larger number of smaller ice crystal in the frozen sample that help to retain a better texture in the product.