Effects of frozen temperature and multiple freeze-thaw cycles on gel structure, protein and lipid oxidation and formation of advanced glycation end-products in unwashed silver carp surimi

The effects of frozen temperature (-18 or -60 degrees C) and multiple freeze-thaw (FT) cycles on the gel structure, protein denaturation and oxidation, lipid oxidation and formation of protein-bound N-epsilon-carboxymethyllysine (CML) and N-epsilon-carboxyethyllysine (CEL) in unwashed silver carp surimi were investigated. The levels of CML and CEL in raw and heat-treated surimi increased due to multiple FT treatment, which were positively correlated with the thiobarbituric acid-reactive substances (TBARS) and protein carbonyl content that also increased during FT cycles, and negatively correlated with the total sulfhydryl content and Ca2+-ATPase activity that decreased during FT cycles. Besides, the gel structure of unwashed surimi was improved by 3 FT cycles and frozen at -60 degrees C, which was significantly helpful to inhibit the formation of protein carbonyl and CEL in raw surimi, but showed no significant influence on the TBARS and CML levels as compared to that at -18 degrees C.