Effect of ultra-high pressure on moisture status and protein structure of large yellow croaker surimi gel

Abstract: To compare the effects of water bath heating and ultra-high pressure processing on the gel properties of large yellow croaker (Larimichthys crocea) surimi gels, we integrated low field nuclear magnetic resonance (Low-field NMR) and Raman spectroscopic technique to analyze the changes of moisture status and protein structure. Meanwhile, we analyzed the differences in traditional characteristic indices of surimi gels such as texture properties, water holding capacity and whiteness to explore their correlation with the variation of moisture status and protein structure. The surimi gels were prepared by heating at 40 ℃ for 1 h and then at 90 ℃ for 30 min in water bath or by high pressure processing at different pressure of 300, 400 and 500 MPa for 30 min. The results showed that surimi gels under high pressure processing had higher water holding capacity, springiness and cohesiveness, but lower hardness compared to heat-induced gels (P<0.05). The cohesiveness, springiness, chewiness and a* of the surimi gels significantly decreased, while the whiteness, L* and b* significantly (P<0.05) increased, when the pressure varied from 300 to 500 MPa. In contrast, water holding capacity was insensitive to the variation of the pressure (P>0.05). High pressure could constrain the mobility of water, resulting in the firm gel network, which was evidenced by the undetectable free water (T23) in the pressure-induced surimi gel. However, high pressure significantly (P<0.05) increased the mobility of movable water and the content of bound water. The increased bound water content was conducive to the formation of gel network, and thereby improved the texture properties and water holding capacity. There was no significant difference (P>0.05) in the frequency of amide I band and the content of β-sheet between high pressure-induced and heat-induced surimi gels of large yellow croaker. However, high pressure significantly increased the α-helix content and decreased the content of random-coil and β-turns of the surimi gels (P<0.05). In addition, changes in the tertiary structures were observed between the pressure-induced and heated-induced surimi. The normalized intensity of the Raman band near 760 cm?1 in pressure-induced surimi gels of large yellow croaker was stronger than that of heat-induced gels. This means that the level of hydrophobic microenvironment exposure to the polar aqueous solvent is lower in pressure-induced surimi gels. These changes had significant correlations with the changes of texture properties and water holding capacity. The mobility of bound water was positively related to the springiness and cohesiveness of surimi gels. The content of movable water was negatively associated with cohesiveness. Consistently, the content of bound water and the mobility of movable water were in parallel with water holding capacity. Furthermore, the contents of α-helix and β-sheet were positively correlated with cohesiveness and water holding capacity, but negatively correlated with gumminess, chewiness and hardness. Collectively, high pressure altered the protein structure in flesh of large yellow croaker, resulting in the changes of surimi moisture status and texture quality. This study provides a theoretical reference for the application of Low-field NMR and Raman spectroscopic technique to evaluate and enhance the quality of surimi products.