Physicochemical and enzymatic changes of cod muscle proteins subjected to different freeze-thaw cycles

The effects of freeze-thaw cycles on the physicochemical and enzymatic chan ges of cod muscle proteins were investigated. The activities of alpha-gluco sidase and beta-N-acetyl-glucosaminidase increased as the the number of fre eze-thaw cycles increased. A loss of Ca2+-ATPase and Mg2+ -Ca2+- ATPase act ivities was observed, while Mg2+-EGTA-ATPase activity increased, especially after one cycle of freezing-thawing, with a concomitant decrease in Ca2+ s ensitivity. The surface sulphhydryl group content decreased with increasing freeze-thaw cycles, while no changes in total sulphhydryl group content we re found. The surface hydrophobicity of actomyosin did not change significa ntly with the number of freeze-thaw cycles. The loss of protein solubility increased with increasing freeze-thaw cycles. However, no cross-linked prot eins induced by formaldehyde were found. The results revealed that freeze-t haw cycles directly affected the physicochemical and enzymatic properties o f cod muscle proteins. (C) 2000 Society of Chemical industry.