Root-Derived Flammulina velutipes Polysaccharides Improve Myofibrillar Protein Stability and Maintain Catfish Surimi Quality During Freeze–Thaw Cycling

Frozen surimi, a commonly used raw material in processed aquatic products, is vulnerable to repeated freeze–thaw fluctuations that accelerate protein denaturation and quality loss. In this study, root-derived Flammulina velutipes polysaccharides (FVPs) were extracted from the root-like portion of enoki mushroom, and surimi supplemented with 2% FVP and a blank control (CK) were stored at −18 °C and subjected to a total of five freeze–thaw cycles. The effects of FVP on myofibrillar protein (MP) characteristics and the storage quality of catfish surimi during the freeze–thaw cycles were analyzed. Compared with CK, FVP markedly alleviated the deterioration of water-holding capacity, gel strength, and MP solubility throughout freeze–thaw cycling. It also effectively inhibited the increase in thiobarbituric acid reactive substance (TBARS) values and MP aggregation and delayed the rate of decrease in the storage modulus (G′) and loss modulus (G″) of surimi. Additionally, low-field nuclear magnetic resonance (LF-NMR) further showed that FVP limited the conversion of immobilized water to free water, indicating enhanced water retention under repeated freeze–thaw stress. Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) analyses revealed that FVP stabilized the secondary structure of MPs, making the microstructure of surimi more uniform and compact. The results of this study indicate that FVP exhibited significant cryoprotective effects during freeze–thaw cycles of surimi relative to the untreated control group, providing a theoretical basis for its potential application in aquatic product storage.