Enhancing grape freshness and quality with nano zinc oxide coating: a study on post-harvest preservation and grape molasses

Grapes are important sources of phenolic compounds, anthocyanins, and ascorbic acid, which are recognized for their significant biological benefits. This study aimed to explore the potential of using a nano zinc oxide coating to preserve the freshness and quality of grapes, as well as the properties of grape molasses. Nano zinc oxide was applied at three different concentrations (0.0%, 0.5%, and 1% w/v) through a three-stage spraying process during the flowering, unripe, and ripening stages, 20 days before harvest. Grape samples were then stored at 4 °C for 35 days. Throughout the storage period, various parameters were measured, including pH, titratable acidity, weight loss, Brix°, total phenolic content, anthocyanin and ascorbic acid levels, antioxidant activity, color indexes (L*, a*, b*), and microbial counts. Also, the properties of molasses were evaluated. Over time, there was an increase in pH (3.58–3.75 to 4.19–4.62), weight loss (0 to 7.12–10.2%), Brix° (22.44–22.69 to 25.95–28.62), color index b* (20.44–22.18 to 23.01–25.77), total viable count of bacteria (3.24–3.34 to 3.52–3.83 Log CFU/g), and total mold and yeast count (3.06–3.35 to 3.48–3.74 Log CFU/g), while titratable acidity (0.78–0.82 to 0.58–0.67%), phenolic content (1.56–1.88 to 1.29–1.43 mg/g), anthocyanin (0.04–0.05 to 0.03–0.04 mg/g) and ascorbic acid levels (10.03–12.53 to 6.66 to 9.10 mg of vitamin C/100 mL), antioxidant activity (30.06–35.6 to 26.23–29.13%), and color indexes L* (42.91–43.74 to 37.50–39.18) and a* (− 2.10 to − 3.30 to − 3.80 to − 5.36) decreased, along with a decline in sensory scores. By increasing the concentration of zinc oxide nanoparticles from 0 to 1% w/v, total sugar (66.85 to 67.56%) and Brix° (76.93 to 77.5) increased while conductivity (1.80 to 1.53 mS/cm) and specific gravity (1.55 to 1.38 g/cm³) decreased. The findings indicate that using a 0.5% w/v nano zinc oxide coating is effective in enhancing the freshness and maintaining the quality of grapes during storage.