Effect of enzymolysis alginate oligosaccharide on myofibrillar protein in simulated oral digestion

Processing silver carp (Hypophthalmichthys molitrix) into surimi products can improve consumers' acceptance and its economic value. Enzymolysis alginate oligosaccharide (EAO) can quickly react with myofibrillar protein (MP) extracted from silver carp to improve functional characteristics. Food will undergo physical and chemical changes after oral processing. To better understand the effects of oral digestion on the EAO-modified MP, we extracted the MP from fresh silver carp which was added 0.45 mg·mL^−­1 EAO to create a complex (EAO-MP). Then we investigated the variations of Fourier infrared spectroscopy, endogenous fluorescence spectroscopy, UV absorption spectroscopy, contents of sulfhydryl and active sulfhydryl, surface hydrophobicity, and hydrogen bonds in the simulated oral digestion of silver carp MP (Group M) and EAO-MP (Group A). The results show that, after the simulated oral digestion, the random coil structure and total sulfhydryl content increased, while the surface hydrophobicity decreased significantly in Group M (P<0.05), which demonstrates that the secondary and tertiary structures of MP had changed due to the effect of simulated oral digestive fluid. Besides, the random coil in Group A decreased by 2.97% after adding EAO. At three different digestion times, the α-helix increased by 7.29%, 2.73% and 5.55%, respectively, compared with Group M. Additionally, the hydrogen bond content increased significantly in Group A (P<0.05), revealing that MP and EAO were bonded to a covalent polymer by the hydrogen bond force. The significant increase in surface hydrophobicity and decrease in sulfhydryl content show that the addition of EAO promotes the expansion of the protein structure of MP in simulated oral digestive fluid. In conclusion, the addition of EAO can promote the molecular structure of silver carp MP to expand in the simulated oral digestive fluid, which may be conducive to the subsequent digestion and absorption of protein.