Sustainable Bio-Based Chitosan/Shellac–Spinel Nanocomposite Coatings for Postharvest Preservation of Tomatoes and Cucumbers

Sustainable coatings based on biodegradable biopolymers can reduce postharvest losses and extend the shelf life of fresh produce. This study evaluated chitosan/shellac coatings reinforced with CoAl2O4 or CuAl2O4 spinel nanoparticles as laboratory-scale postharvest coatings for tomatoes and cucumbers. The coating system was based on previously characterized chitosan/shellac–spinel composite films and was further evaluated at the fruit-application level. Under the experimental ambient storage conditions, the optimized spinel-containing coatings extended the observed acceptable storage period of tomatoes to more than 33 days, whereas uncoated tomatoes deteriorated within 10–14 days. Weight loss remained below 5% for coated tomatoes under the reported conditions, while coated cucumbers showed a more modest preservation response. The coatings were associated with reduced visible microbial deterioration, improved pH retention, and lower chlorophyll degradation in cucumber samples. The observed preservation performance is attributed to complementary contributions from (i) chitosan film formation and antimicrobial activity, (ii) shellac-induced hydrophobicity and moisture-barrier functionality, and (iii) reinforcing and antimicrobial effects from the spinel nanoparticles. A short-term non-choice feeding experiment in female albino rats provided preliminary information on food acceptance and changes in food intake and body weight; however, the study was not designed to establish comprehensive toxicological or food-contact safety.