Insights into the endogenous cathepsins on modori of fermented carp ( Cyprinus carpio ) sausage gels in acid environment

Summary This research aims to monitor the modori of fermented fish sausage and determine the effect of endogenous cathepsins. Proteolysis, structure and physico‐chemical properties of carp sausage were determined using texture profile, rheological, SEM, FTIR and SDS‐PAGE, respectively. In the forming stage of gels, the rapid reduction in pH dropped below 5.16 within 36 h of fermentation. Meanwhile, the hardness (g), elasticity, Chewiness (mJ) and gumminess increased. The results of elasticity modulus (G′) exhibited strong correlation with TPA. Gels presented a three‐dimensional network structure at 36 h. In the degradation stage of gels, the increase of TVB‐N, TCA‐soluble peptides and FAAs indicated that muscle proteins were strongly degraded. SDS‐PAGE demonstrated that the degradation of insoluble protein was coincided with the modori of gels. During fermentation, cathepsin B (CPB) and cathepsin L (CPL) exhibited high activity, while cathepsin D (CPD) revealed low activity. β‐ sheet and α‐ helix increased to 1.07 and 1.05 times of the initial value ( P > 0.05), while random coil and β‐turn dropped to 10.27% and 21.98%, respectively. PCA suggested that acids played a major role in the formation of gels and that CPB and CPL were closely related to the modori of gels. Moreover, this study provided theoretical guidance for controlling the formation and inhibiting the modori of fish sausage gel.