Formation of advanced glycation end-products in silver carp (Hypophthalmichthys molitrix) surimi products during heat treatment as affected by freezing-thawing cycles

Lipid oxidation and protein degradation, along with the formation of advanced glycation end-products (AGEs), including N epsilon-carboxymethyl-lysine (CML), N epsilon-carboxyethyl-lysine (CEL) and fluorescent AGEs, in raw and subsequently heated surimi products were investigated during freezing-thawing cycles. Lipid oxidation, protein degradation, Schiff base, and AGEs formation constantly increased during freezing-thawing cycles and heat treatment (P < 0.05). Heat-induced increase of AGEs in surimi products was accelerated by freezing-thawing treatment. Formation of CML in one-stage heated (45 <degrees>C, 30 min) samples increased from 0.10 +/- 0.04 to 0.53 +/- 0.11 mg/kg and CEL increased from 0.03 +/- 0.32 to 0.92 +/- 0.74 mg/kg. In two-stage heated samples (45 degrees C, 30 min and 90 degrees C, 20 min), CML increased from 0.86 +/- 0.06 to 1.12 +/- 0.11 mg/kg and CEL from 1.00 +/- 0.34 to 2.11 +/- 1.86 mg/kg, during up to 6 freezing-thawing cycles. Formation of fluorescent AGEs derived from heating was also significantly accelerated by freezing-thawing treatment (P < 0.01). Correlation analysis suggested that the chemical synthesis of AGEs in surimi products was promoted by lipid oxidation and protein degradation during freezing-thawing cycles. AGEs formation through Schiff base oxidation likely occurred only under thermal treatment since no relationship was found between Schiff base and AGEs levels in raw surimi products.