Enhanced storability of fresh walnut kernels with zein/tea polyphenol/apple pectin coatings by layer-by-layer assembly technique

Abstract

The study utilized zein and apple pectin as matrices with tea polyphenols as functional components to develop three treatment groups for walnut kernels via immersion: a composite zein nanoparticles-pectin coating (composite coating ZNP-P group), multilayer assembled coating pectin-zein nanoparticles-pectin-zein nanoparticles coating (P-ZNP-P-ZNP group), and a sterile water control. After 40 d of cold storage at 4 °C, the P-ZNP-P-ZNP group demonstrated superior preservation effects. Specifically, this group maintained significantly higher moisture content (23.66 %), water activity (0.9750), firmness (3217.81 g), total sweet amino acids (0.83 mg/g), and polyunsaturated fatty acid proportion (53.18 %) compared to control (P < 0.05). Conversely, ΔE (3.77), total viable count (4.08 log CFU/g), lipid oxidation, the response increments of E-nose sensors to sulfide compounds and ammonia/amines, hexanal content (19.27 μg/100g), lipoxygenase activity (13.67 U/g), peroxidase activity (1.71 U/g), and polyphenol oxidase activity (5.11 U/g) were significantly lower than both the ZNP-P group and control. These results indicate that the P-ZNP-P-ZNP multilayer coating creates a gradient barrier that effectively delays physiological metabolism and quality deterioration in walnut kernels, offering a novel technological approach for cold chain preservation.