Effect of organic acids on the photodynamic inactivation of selected foodborne pathogens using 461Â nm LEDs

- The addition of organic acids enhances the antibacterial effect of LEDs.
- Lactic acid was the most effective in aiding the photodynamic inactivation.
- Efficacy of the treatment was not influenced by the Gram nature of pathogens.
- Blue LEDs in combination with organic acids can be used for food preservation.

Light emitting diodes (LEDs) are known to inactivate foodborne pathogens. However, the effect of food acidulants on the antibacterial effect of LEDs is unknown. In this study, the influence of organic acids on the photodynamic inactivation of four foodborne pathogens (Escherichia coli O157:H7, Salmonella Typhimurium, Listeria monocytogenes and Staphylococcus aureus) by blue LEDs was investigated. Each pathogen was illuminated by 10-W 461 nm LEDs (22.1 mW/cm2) at 15 °C for 7.5 h in tryptone soya broth of pH 4.5 adjusted with citric, lactic or malic acids. The total energy dosage imparted by the LEDs over the course of the illumination was 596.7 J/cm2. Our results showed that at the same pH, the nature of the acids significantly influenced the bacterial inactivation due to the LEDs, irrespective of the bacterial strain. Lactic acid was found to be the most effective in aiding the photodynamic inactivation of the pathogens, followed by citric and malic acids. The reparameterized Gompertz equation was fitted to the inactivation curves to determine the lag phase duration (λ) and the maximum inactivation rate (μmax) under each condition. The λ values for E. coli O157:H7, S. Typhimurium, L. monocytogenes and S. aureus with lactic acid were 3.68, 3.29, 2.25 and 2.27 h, respectively. The corresponding μmax values were 1.46, 1.31, 1.07 and 1.21 log CFU/h. The combination of organic acids and LEDs also caused significant sublethal injury to the bacterial cultures with trends similar to those of the inactivation tests. This study showed that food acidulants greatly enhanced the antibacterial effect of LEDs, suggesting the potential of LEDs in preserving acidic foods.