Silver carp (Hypophthalmichthys molitrix) surimi was heated after freeze-thaw cycles at different freezing temperatures, and the changes of nonanal, 1-octene-3-alcohol, endogenous fluorescent substances, protein subunits, lysine, α-dicarbonyl compounds (glyoxal, GO and methylglyoxal, MGO), N^ε-carboxylmethyl-lysine (CML) as an advanced glycation end product (AGE) and fluorescent AGEs in surimi during freeze-thaw and heating treatment were determined. The results showed that freezing temperature had no significant effect on any of the parameters measured (P > 0.05). As the number of freeze-thaw cycles increased, the levels of nonanal, 1-octene-3-ol, endogenous fluorescent substances, CML and fluorescent AGEs in surimi increased significantly, the content of lysine increased, and the levels of GO and MGO decreased significantly (P < 0.05). The levels of 1-octene-3-ol, GO, MGO, CML and fluorescent AGEs increased significantly during heat processing, the content of lysine increased at first and then decreased, the level of endogenous fluorescent substances decreased significantly (P < 0.05), and the myosin heavy chain was gradually degraded into smaller proteins. The above results showed that freeze-thaw treatment provided a large number of precursors for the formation of AGEs in surimi. Thermal processing promoted fat oxidation and protein degradation, stimulating the formation of AGEs in surimi through various pathways, which could in turn affect the safety of surimi.

In this study, the effects of soybean oil addition (1%, 3%, 5% and 7%) on the structure of surimi gels during freeze-thaw were investigated, and the first-order Ogden model was used to describe the relationship between the true stress and the true strain of surimi gels with different mass fractions of soybean oil under large compression deformation (stretch ratio of 0.4–1). The results showed that the addition of soybean oil to fresh surimi reduced the gel strength and enlarged the pores of the gel network. It was found that the addition of 5% and 7% soybean oil had a protective effect on the gel network structure of surimi after six freeze-thaw cycles. The gel strength of surimi with 5% and 7% soybean oil (319.30 and 338.64 g · cm, respectively) was significantly higher (P < 0.05) than that (286.04 g · cm) of the control group (without soybean oil), and the gel network structure was more complete than that of the latter. The Ogden model was fitted well to the true stress-strain compression response curves of fresh and freeze-thawed surimi gels with the addition of soybean oil (R² = 0.916–0.998). The model fitting results showed that the initial shear modulus changes of fresh and freeze-thawed surimi gels were consistent with the gel strength results. This study suggested that the addition of soybean oil at 5% and 7% can reduce the damage caused to the surimi gel structure by freeze-thaw cycles. The Ogden model can well describe the effect of soybean oil addition on the large deformation behavior of fresh and frozen-thawed surimi gels. These findings may provide theoretical and technical support for the quality improvement of surimi products and the control of transportation and storage conditions.

In order to investigate the influence of antifreeze peptide properties on its protective effect on myofibrillar proteins in frozen surimi, the present study was conducted to characterize the properties of three antifreeze peptides (sericin peptides, collagen peptides and silver carp hydrolysates), and we analyzed the changing pattern of myofibrillar protein properties under different freezing conditions, and constructed a correlation model between myofibrillar protein properties and frozen storage parameters and antifreeze peptide properties using least absolute shrinkage and selection operator (Lasso) regression. The results showed that all antifreeze peptides used in the study were “hyperactive” antifreeze peptides with thermal hysteresis activity (THA) (> 0.60 ℃ ), low molecular mass (< 10000 Da), high hydrophilicity (θ < 30°), and polar amino acid content of more than 60%; the antifreeze peptides were able to significantly mitigate the negative effects of frozen storage on myofibrillar proteins. The mean content of myofibrillar proteins in frozen surimi with added antifreeze peptides was 50.44-54.47 mg/g, the mean Ca²⁺-ATPase activity was 1.61-1.67 U/mg, the mean surface hydrophobicity index increased only by 39.91%-42.40% when compared with that of fresh surimi, and the rate of myofibrillar protein denaturation was much lower than that of the control group without antifreeze peptides. The Lasso regression model explained the effects of antifreeze peptide properties on changes in the properties of myofibrillar protein in frozen surimi, demonstrating that antifreeze peptides with lower molecular mass and higher THA contributed to slowing down the decline in myofibrillar protein content, Ca²⁺-ATPase activity, surface hydrophobicity, and characteristic indexes. The differences in the hydrophilicity/hydrophobicity of antifreeze peptides resulted in differences in their interactions with ice crystals and proteins, and consequently caused different effects on surimi myofibrillar proteins. This study provides a theoretical basis for the application of antifreeze peptides and the quality regulation of surimi products and also provides a direction for the development of novel efficient antifreeze agents.