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 changes of cod muscle proteins were investigated. The activities of α-glucosidase and β-N-acetyl-glucosaminidase increased as the the number of freeze-thaw cycles increased. A loss of Ca²⁺-ATPase and Mg²⁺-Ca²⁺-ATPase activities was observed, while Mg²⁺-EGTA-ATPase activity increased, especially after one cycle of freezing-thawing, with a concomitant decrease in Ca²⁺ sensitivity. The surface sulphhydryl group content decreased with increasing freeze-thaw cycles, while no changes in total sulphhydryl group content were found. The surface hydrophobicity of actomyosin did not change significantly with the number of freeze-thaw cycles. The loss of protein solubility increased with increasing freeze-thaw cycles. However, no cross-linked proteins induced by formaldehyde were found. The results revealed that freeze-thaw cycles directly affected the physicochemical and enzymatic properties of cod muscle proteins. (C) 2000 Society of Chemical Industry.