Physicochemical properties and antibacterial mechanism of TP microcapsules/LZM-PVA gradual sustained-release composite coatings

Tea polyphenol (TP) has excellent preservation effect on the aquatic product, however, its long-term antibacterial and preservation property is poor due to its volatilization. Therefore, a novel sustained –release formulation of TP should be developed for improving the applicability. In this study, the TP microcapsules/LZM-PVA composite coatings with gradual slow release properties were prepared by tape-casting method. Its micro-morphology was characterized and its physicochemical properties were determined. Meanwhile, the antibacterial properties and mechanism of the composite coatings were studied with Pseudomonas fluorescens and Shewanella putrefaciens, the dominant spoilage bacteria in aquatic products, as the objects. The results show that the addition of TP and other substances affects the structure of PVA coatings, and changes the intermolecular force of PVA, the coatings is wrinkled and denser, which effectively improve the mechanical properties of the PVA coatings, and finally lead to the decrease of air permeability. Among them, the TP microcapsules/LZM-PVA composite coating has the best mechanical properties, the least amount of CO2 and water vapor penetration. The results of antibacterial experiments show that the PVA coating without preservatives has no antibacterial property. Compared with the bacterial cells treated by the corresponding composite coatings without LZM, the integrity of the cell membrane is poor and the permeability is higher after treated by the composite coating with LZM, which is due to the synergistic antibacterial effect of TP and LZM. More importantly, when TP is replaced by TP microcapsules, the antibacterial ability of the composite coatings is poor at the initial stage. However, TP is embedded in the space network structure formed by CS-sodium tripolyphosphate. TP in the coatings can be released slowly from microcapsules to coatings, and then from coatings to the culture medium, which prolong the antibacterial effect time and cause continuous damage to the bacteria. Moreover, the chitosan which is the wall material of TP microcapsules also has a certain antibacterial effect. So the TP microcapsules/LZM-PVA coating could be an ideal choice in the food preserving.