Effectiveness of Pulsed Light Treatment on the Inactivation of Pathogenic and Spoilage Bacteria on Cheese Surface
Cheese products are susceptible to post-processing cross-contamination that can lead to both food safety issues and significant losses due to spoilage. Pulsed Light (PL) treatment, consisting of short, high-energy light pulses, could represent a solution to address this issue since it is a non-destructive technology that can effectively inactivate microorganisms on surfaces. This study examined the effectiveness of PL on the inactivation of the spoilage microorganism P. fluorescens and the pathogen surrogates E. coli ATCC 25922 and L. innocua. The effect of inoculum level, cheese surface topography, and the presence of clear polyethylene packaging were evaluated in a full factorial experimental design.
The challenge microorganisms were grown to stationary phase: P. fluorescens 1150 was grown at 30°C in tryptic soy broth (TSB) while E. coli ATCC 25922 and L. innocua FSL C2-008 were grown at 37°C in TSB and brain heart infusion (BHI), respectively. White cheddar and processed cheese, chosen for their different surface topography, were cut into 2.5 cm x 5 cm slices. The samples were then spot inoculated using ten droplets of 10 µL per slice, resulting in an initial concentration of either 5 or 7 log CFU/slice. Inoculated samples were dried overnight at 4°C. For treatments through packaging, sterile UV-transparent low-density polyethylene packaging was placed on top of the inoculated cheese samples immediately before the PL treatment. Cheese samples were then exposed to PL doses of 1.1 to 13.2 J/cm2. PL-treated samples were stomached for 2 min in Butterfield Phosphate Buffer, the extract then plated on selective media and survivors enumerated by standard plate counting (SPC). When survivor counts fell below the SPC detection limit, the most probable number was used. Experiments were performed in triplicate and data was analyzed using a general linear model.
PL was most effective against E. coli, achieving a maximum log reduction of 5.4 ± 0.3, at a dose of 13.2 J/cm2. For P. fluorescens, a maximum log reduction of 3.7 ± 0.9 and for L. innocua a maximum log reduction of 2.9 ± 0.8 at 13.2 J/cm2 were obtained. The process parameter effects tested showed varying statistical significance when used in different combinations, but PL treatments through packaging and without packaging consistently resulted in similar inactivation levels. This study suggests that PL has strong potential for decontamination of cheese surface.
Keywords: Pulsed Light, Cheese, Pathogenic and spoilage bacteria