Preservation Effect of Zein-Tea Polyphenol-Apple Pectin Self-assembled Coating on Fresh Walnut Kernels

In this study, mixed films and self-assembled films consisting of zein, tea polyphenols, and apple pectin were prepared by the casting method. Fresh walnut kernels were immersed in the mixed coating solution, the self-assembled coating solution or sterile water as control and stored at 4 ℃ for 40 days to evaluate the preservation effects of the coatings on fresh walnut kernels. The results showed that the mixed film generated particles, exhibiting a relatively uneven surface distribution, while the self-assembled film exhibited a more uniform distribution and a relatively smoother surface when compared with the mixed film. After 40 days of storage, the L*, hardness, and moisture content of fresh walnut kernels in the self-assembled coating group were significantly higher than those in the mixed coating and control groups. Total bacterial count was significantly lower in the self-assembled coating group when compared with the mixed coating and control groups. The antioxidant activity of the self-assembled coating was notably superior to that of the mixed coating, as shown by a significant decrease in acid value, peroxide value, and malondialdehyde (MDA) content in the self-assembled coating group when compared with the mixed coating group during storage. In addition, the self-assembled coating demonstrated potential in preserving the flavor of fresh walnuts. After 40 days of storage, the self-assembled coating group exhibited the highest levels of sweet and umami amino acids, and the levels of acetic acid with the sour and pungent odor significantly lower than those of the mixed coating and control groups. This study showed the potential of the self-assembled coating in maintaining the quality of refrigerated fresh walnuts, providing a theoretical basis for the construction of multilayer self-assembled coatings for fresh walnut kernel preservation.