Thermal characterization and ice crystal analysis in pressure shift freezing of different muscle (shrimp and porcine liver) versus conventional freezing method

Abstract

Abstract In order to investigate the application of pressure shift freezing (PSF) on different meat products, fresh shrimp and porcine liver were frozen by PSF at 100 MPa (− 8.4 °C), 150 MPa (− 14 °C) and 200 MPa (− 20 °C), as well as by conventional air freezing (CAF) at − 20 °C and liquid immersion freezing (LIF) at − 20 °C. Temperature profile of test samples were recorded during different freezing processes and phase transformations were calculated. Microstructures of the formed ice crystals were analyzed for cross-section area, equivalent diameter, roundness and elongation. Size, shape and distribution of ice crystals were estimated and compared in different freezing processes. Results clearly showed that PSF is the more advantageous method. The phase transition times (2.97, 1.53 and 1.1 min in shrimp, and 2.47, 1.22 and 0.83 min in porcine liver, when subjecting to 100, 150 and 200 MPa PSF treatments, respectively) induced from the super-cooling attained in PSF treatments was much shorter than that in CAF (148 min in shrimp and 85 min in porcine liver) and LIF (5.9 and 5.5 min in shrimp and porcine liver, respectively). Small, regular and homogeneously distributed ice crystals were obtained in PSF treatments, especially at a higher pressure. The mean cross-section area of ice crystals formed in frozen shrimp was 2852, 1364 and 597 μm2 for PSF treatments at 100, 150, and 200 MPa, respectively, as compared with 151, 100 and 92 μm2 for frozen samples of porcine liver, respectively. The roundness of ice crystals in shrimp formed in PSFs was 0.87, 0.90 and 0.85, respectively, while the roundness of ice crystals in porcine liver was 0.87, 0.90 and 0.89, respectively. The size of ice crystals formed in PSF is pressure dependent, mainly due to a large super-cooling were created in a higher pressure of PSF. However, pressure does not play a major role in the shape of ice crystals. CAF created larger and irregular ice crystals due to the slow freezing rate, and LIF generated relatively smaller but irregular ice crystals, both of which resulted in irreversible damage to muscle tissue and caused degradation of product value. The results achieved in this research provides a better understanding of PSF process of shrimp and porcine liver, and PSF was proved as a promising freezing method for producing frozen products. Industrial relevance This paper compares different freezing methods and their effect on thermal behavior and microstructure of meat muscle. Pressure shift freezing proved to be superior to conventional freezing regarding high degree of super-cooling, short phase transition time, and small, regular ice crystals. Different meat muscles, including shrimp and porcine liver, were also compared to investigate the effect of PSF on thermal behavior and microstructure. Interestingly, the ice crystals in porcine liver were much smaller than those in shrimp. Results obtained in the paper provide a better understanding of the PSF process and suggest that PSF is a superior method for freezing shrimp and porcine liver.