Uncovering the freezing energy release of salted grass carp (Ctenopharyngodon idella) flesh: Effects of water state and protein structure on the thermal properties

The effect of NaCl content on the protein structures, water status and thermal properties of grass carp flesh, along with its relationship with energy required for freezing was investigated to improve the quality of frozen flesh and to reduce energy consumption for freezing. Adding salt prompted the shifting of the secondary structure of α-helix to β-sheet, β-turn and random coil. The interaction between water molecules and hydrophilic groups in the unfolded protein structure increased nonfreezing water content, which decreased specific heat capacity, thermal conductivity of sample during freezing. The lowest energy required (235.69 kJ/kg) for freezing was found in 7% NaCl salted sample, indicating a 38.39% reduction compared to the unsalted sample. Few pores between muscle fibers were observed in the 3% and 5% NaCl salted sample. Therefore, an optimal salt concentration improved quality of frozen flesh and reduced the energy required for freezing, promoting energy-efficient freezing of aquatic products.