Evaluation of visible-light-activated ZnO nanoparticles for sustainable postharvest protection of strawberries

Strawberries are highly valued for their taste and nutritional benefits; however, their postharvest shelf life is limited due to microbial spoilage and contamination by foodborne pathogens. Traditional preservation methods, such as fungicides and chemical sanitizers, face increasing restrictions due to concerns about their impact on public health and environmental sustainability. This study investigates the antimicrobial potential of visible-light-activated zinc oxide nanoparticles (ZnO NPs) against bacterial pathogens and natural microbiota on strawberries, assessing their impact on fruit shelf life and antioxidant capacity. In vitro antibacterial assays showed that ZnO NPs alone did not affect the survival of Enterococcus faecalis MSCL 302, Listeria monocytogenes ATC_L3C 7644, or Escherichia coli O157:H7. However, exposure to visible light at 405 nm led to significant bacterial inactivation, with L. monocytogenes demonstrating the highest susceptibility. There was a clear relationship between the antibacterial properties of ZnO NPs and the illumination dose. SEM analysis revealed NPs attachment to bacterial cells, suggesting possible membrane disruption and leakage of intracellular contents. DNA integrity assays further indicated oxidative stress in light-treated samples, although the exact antibacterial mechanism remains to be clarified. The application of photoactivated ZnO NPs on strawberries resulted in a 1.4-log reduction in yeasts and molds, significantly inhibiting their regrowth over a six-day period. Treated strawberries displayed a shelf-life increase of up to two days, with no adverse effects on their antioxidant activity. These findings advance current understanding by demonstrating the potential of visible-light-activated ZnO NPs as a safe, effective, and non-destructive approach to enhance microbial safety and extend the shelf life of strawberries.