Gel properties changes of myofibrillar protein of silver carp (Hypophthalmichthys molitrix) in the process of TG catalysis

In recent years, with the improvement of production and living needs, surimi products as a product of intensive processing, production and consumption are gradually increasing. Silver carp (Hypophthalmichthys molitrix) is low in price and large in output. But because of the existence of endogenous protease, it will lead to low gel strength and poor texture characteristics, and therefore the utilization rate of fish is not high. Improving production value and promoting its high value utilization has become a research hotspot. Transglutaminase (TG) is involved in the catalytic protein cross-linking reaction that improves the breaking force, hardness and water holding capacity of surimi products. However, with the increase of cross-linking degree of the product, the digestibility showed a downward trend, which was difficult to digest. In order to explore the changes of gel properties of silver carp myofibrillar protein during TG catalysis, further enhance the characteristics and nutritional value of silver carp surimi products, in this paper, myofibrillar protein (MP) of silver carp was catalyzed by TG for different time periods (0, 15, 30 and 60 min). The cross-linking extent, gel strength, whiteness, water-holding capacity, microstructure and digestive characteristic changes were analyzed to investigate the effect of TG on its gel properties and digestibility. The experiment results showed that the cross-linking extent of MP gel significantly increased after TG-induced protein gelling (P<0.05). At 30 min, the crosslinking degree was the highest, reaching 61.21%. The gel strength of protein increased by 186.68% compared with that of non catalyzed group. In the process of TG catalysis, the whiteness and water-holding capacity increased with the prolonged crosslinking time. At 60 min, the L^* and whiteness value increased by 5.37% and 5.30% respectively. Scanning electron microscope (SEM) analysis showed that the pore size of the gel network structure became smaller and denser. Under the catalytic time of 30 min, the cross-linking extent, whiteness and water-holding capacity significantly increased (P<0.05), and the relaxation time T₂₂ significantly decreased (P<0.05). The TG-induced MP gel had a low degree of hydrolysis in the initial stage of digestion, because the cross-linking extent increased and the protein network structure became denser and denser, which reduced the accessibility of pepsin to its hydrolysis site. After the digestion time reached 90 min, the protein network was destroyed and the hydrolysis rate increased significantly. These results suggested that appropriate crosslinking time can help to improve the gel properties of protein gels. This study provides an experimental basis for improving the quality and digestive characteristics of fish products by TG.