Purpose: The objective of this study was to investigate the inactivation of E. coli, L. monocytogenes and Salmonellaspp. on strawberries using PL.
Methods: Surfaces of fresh strawberries (25 g) were spot-inoculated with cocktails of E. coli, L. monocytogenes and Salmonella spp. Samples were treated with PL using Xenon Steripulse XL-3000TM system at a rate of 1.8, 3, and 100 Hz for 30, 60, 90, and 120 s at a distance of 10.2, 12.8, and 15.3 cm from the sample surface to the UV strobe. After treatment, samples were serially diluted with buffered peptone water, plated onto tryptic soy agar with 0.6% yeast extract (TSAYE), and overlaid with violet red bile agar, modified oxford agar, and xylose lysine desoxycholate for E. coli. L. monocytogenes and Salmonella spp., respectively.
Results: The initial inoculum level was approximately seven log CFU/g. For different time durations ranging from 30 to 120 s, various degrees of inactivation, from 1.0 ± 0.2 to 3.1 ± 0.2 log CFU/g, were observed for the three bacterial pathogens studied. For example, a 60s treatment at 1.8, 3, and 100 Hz achieved approximately 2 log reduction of bacteria on strawberries. Overall, no significant difference in the inactivation was observed at measured distances from the UV strobe to the sample surface (P<0.05).
Significance: The present study demonstrated that PL treatment was an effective intervention method to inactivate bacterial pathogens on strawberries. However, processing conditions need to be optimized to minimize the impact on the quality of berries.