Effect of ultrasonic time on gel properties and protein structure of Hypophthalmichthys molitrix surimi and correlation analysis

Hypophthalmichthys molitrix is the main freshwater aquaculture fish in China, which belongs to low-value fish due to the less content of fish meat and large amounts of bones. Therefore, it was necessary to produce deep processing food, such as surimi products,to realize high-value utilization of H. molitrix. However, the gelatinization of H. molitrix surimi was weak, which was not conducive to the formation of dense surimi gels. Currently, exogenous additives and physicochemical technology were usually used to improve the surimi gel properties. Ultrasonic technology is a new non-thermal physical technology to detect food quality and improve food characteristics. In addition, it is simple to operate and has many potential applications in food industry. Commonly, low-frequency and high-intensity ultrasound (16-100 kHz) was used in the modification of plant protein and animal protein which contributed to the cavitation effect.. More research focused on the mechanism of protein modification by ultrasonic technology, but few on its practical application. In order to improve the gel properties of H. molitrix surimi and realize the practical application of ultrasonic technology, this research explored the physical characteristics of surimi gel and the secondary structure of surimi protein by 0-50 min ultrasonic treatment, as well as the relationship between gel properties and secondary structure. Furthermore, the principle of improvement and deterioration of gel network by ultrasonic technology was explained. Thus, the effects of ultrasonic time on the gel strength, color, relaxation time and microstructure of surimi gels by low-frequency and high intensity ultrasonic technology were studied. Moreover, the changes of protein molecular weight and secondary structure in surimi after ultrasonic treatment were also investigated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Fourier transform infrared spectrometer (FTIR Spectrometer). The results showed that after ultrasonic pretreatment for 10 min, the gel strength of surimi increased significantly from (260.81±3.26) g·cm to (285.13±5.96) g·cm, and the tightness of surimi gel improved. And the gel matrix showed higher whiteness of 72.74±0.13, carried out by ultrasound in 10 min. Additionally, the proportion of immobile water was increased, and the capacity to bind water of gels was enhanced, which made the network structure more compact. However, with the increase of ultrasonic time, the gel strength decreased, and the immobile water was changing into free water. Therefore, the ratio of free water increased, and the internal structure of gels tended to be weak and loose. SDS-PAGE image proved that salt-soluble proteins (myosin, actin, tropomyosin) mainly consisted in surimi, and did not degrade or aggregate with the increase of ultrasonic time. While the relative content of α-helix decreased, β-sheet, random coil structure and β-turn increased after appropriate ultrasonic processing. Through Pearson correlation analysis, ultrasound pretreatment could promote the unfolding of salt soluble protein, which was more conducive to the formation of compact network structure, enhancing gel strength and water holding capacity in surimi processing. In conclusion, proper ultrasonic pretreatment could improve the gel properties of surimi by changing the three-dimensional structure of surimi protein, but excessive ultrasonic time was not beneficial to the formation of dense surimi gel. This research provided an experimental basis for improving the gel properties of surimi products, the practical application of ultrasonic technology and the innovative development of surimi products.