Effect of sample volume on the variation of freezing characteristics, mechanical properties, and microstructure of gelatin gel with depth after liquid nitrogen spray freezing.

Liquid nitrogen spray freezing (LNSF, -20, -60, and - 100 °C) was adopted to evaluate the effect of four cubic sample (with side lengths of 25, 50, 75, and 100 mm) volumes on the freezing curves, microstructure, texture, and gel strength of gelatin gels (16 % with 4 % sucrose, w/w), serving as a food model, along the depth in gels. The total freezing time significantly increased (P < 0.05) with increasing freezing temperature and depth, leading to larger ice crystals and more severe damage to the gel network. The gel strength significantly decreased from 3480 g at a depth of 5 mm to 2570 g at 25 mm for 503 mm3 gels frozen at -100 °C. An increase in sample volume resulted in prolonged freezing times at all depths, thereby increasing the cross-sectional area of ice crystals at the center from 851 to 1800 μm2 as the cubic gel volume increased from 503 to 1003 mm3. Scanning electron microscopy (SEM) observations revealed densely packed irregular pores with thin membranes at the centers of larger samples as compared to small and regular pores at the surface, indicating more severe structural damage. Rapid LNSF for the smallest sample might lead to a high internal stress, causing an unexpected fracture of the gel network. Results showed that the overall strength of gels was reduced by the increased sample volume. It was recommended to appropriately decrease the freezing rate for the product surface during the early freezing stage to prevent the formation of a consolidated inelastic layer and reduce the internal stress.