Synergistic Photodynamic Effect of Chitosan and Curcumin on the Inactivation of Foodborne Pathogens on Cherry Tomatoes

Foodborne pathogens are a major threat to human health. The aim of this study was to investigate the inactivating effect of chitosan (Chi) combined with photodynamic treatment (Chi-PDT) mediated by curcumin (Cur) on pathogens. After determining minimal inhibitory concentration of Chi, we compared the bactericidal effects of PDT and Chi-PDT, and evaluated the influence of curcumin concentration and illumination time on the inactivating effect of Chi-PDT against Staphylococcus aureus, Bacillus cereus, Escherichia coli and Salmonella Typhimurium. The mechanism for the synergistic bactericidal effect of Chi and PDT, the inactivating effect of Chi-PDT on pathogen cocktails and Salmonella on the surface of cherry tomatoes and the impact of Chi-PDT on the quality of cherry tomatoes were assessed. The results showed that chitosan could augment the inactivating effect of PDT by enhancing the interaction between bacteria and Cur. The sterilization effect of Chi-PDT was dependent on Cur concentration and illumination time. Moreover, 20 min illumination after treatment with 0.05% chitosan combined with 15 μmol/L curcumin reduced the counts of S. aureus and B. cereus by 7.90 and 7.60 (lg (CFU/mL)), respectively. However, for E. coli and Salmonella, similar effects required treatment with 0.05% Chi plus 100 μmol/L Cur and 0.05% Chi plus 25 μmol/L Cur, respectively. Chi-PDT reduced the total count of the quaternary bacterial mixture by 6.00 (lg (CFU/mL)), which was 5.09 (lg (CFU/mL)) higher than that with PDT alone. These results showed that Chi-PDT had a better bactericidal effect against both individual and mixed bacteria than PDT. The count of Salmonella on the surface of cherry tomatoes was reduced by 1.62 (lg (CFU/fruit)) by Chi-PDT. Chi-Cur coating combined with PDT treatment inhibited bacterial growth on cherry tomatoes during storage without affecting the color, firmness or mass loss. Thus, Chi-Cur coating combined with PDT treatment has great potential for application in the fields of food sterilization and fruit and vegetable preservation.