Purpose: The objective of this study was to investigate the effect of substrate related factors and light-substrate interactions on the efficacy of PL inactivation.
Methods: Liquid substrates ranged from transparent Butterfield’s phosphate buffer (BPB), to apple juice, cider and milk of different composition. As challenge organisms, Listeria monocytogenes and its surrogate L. innocua, as well as E. coli O157:H7 and a nonpathogenic surrogate were used, at 107 to 109 CFU/mL. A thin layer of inoculated liquid sample, contained in either a glass chamber or a UV reflective aluminum dish, was treated with PL doses ranging from 0.4 to 14.9 J/cm2. Survivors were recovered, and inactivation was quantified by standard plate counting. The effect of temperature on inactivation was also evaluated. Treatments were replicated and inactivation data was evaluated statistically.
Results: PL inactivation was diminished by sample turbidity and color, due to scattering and absorption of light by the sample. A 9-log reduction of E. coli was achieved in BPB, but only 2-log reduction in apple juice and cider, under similar conditions. Inactivation in apple juice was improved significantly by turbulence, and more than 5-log reduction was achieved under turbulent conditions. Inactivation was also improved by using an UV reflective sample holder. A statistically significant difference (P<0.05) was achieved between inactivation in non-UV reflective (6.41±0.32 log) vs. UV-reflective (7.83±0.41 log) containers. The temperature of the sample during treatment also affected inactivation.
Significance: This data demonstrates the potential of PL to inactivate bacterial pathogens in foods, while highlighting the importance of considering food properties and environmental conditions when designing PL applications.