Fate of Salmonella and Listeria monocytogenes in different frozen produce commodities

Abstract

Outbreaks have been associated with the consumption of frozen produce, and produce that was purchased fresh and then frozen at home. Consumers may expose produce to repeated freezing and thawing, at different temperatures and times. This study aims to (i) model the survival of Salmonella and Listeria monocytogenes in frozen banana, cantaloupe, and sweet corn for up to 90 and 180 days, respectively, and (ii) to evaluate the single and synergistic effect of repeated freeze-thaw cycles on the survival of Salmonella. For objective 1, ten-gram samples were spot inoculated with rifampicin-resistant inoculum cocktails composed of five-strains of Salmonella or six-strains of Listeria monocytogenes at ~5.0 log10 CFU/g and stored at -18±2°C. Salmonella was enumerated at 0, 0.02, 1, 3, 6, 10, 15, 30, 60 and 90 days post inoculation (dpi), after storage at -18±2°C. Listeria monocytogenes monitoring followed the same structure, with an additional timepoint at 180 dpi. For objective 2, ten-gram samples of cut-cantaloupe were inoculated with a five-strain Salmonella cocktail at ~ 4 log10 CFU/g. Bacteria was enumerated throughout seven freeze-thaw cycles of freezing at -18±2°C for 18-20 hours and thawing at temperatures of 23°C or 35°C, for periods of 15 minutes, 2 hours or 4 hours. In objective 1, Salmonella changed by -0.04 log10 CFU/g per day (95% CI= -0.08, -0.01) from 0-2 dpi, and by -1x10-3 log10 CFU/g per day (95% CI=-1x10-3, -9x10-4) from 2-90 dpi. Listeria monocytogenes reduced by 2x10-3 log10 CFU/g per day (95% CI= 1x10-3, 2x10-3) on bananas. While on cantaloupe and corn, Listeria monocytogenes changed by -0.01 log10 CFU/g per day (95% CI= -8x10-3, -4x10-3) from 0-20 dpi, and by -2x10-4 log10 CFU/g per day (95% CI=-5x10-4, 4x10-5) from 20-180 dpi. In objective 2, Salmonella exposed to freeze-thaw cycles showed a decrease per cycle of -0.03 log10 CFU/g at 23°C and -0.02 log10 CFU/g at 35°C. Thawing at 35°C for 4 hours reported the highest impact on Salmonella levels. Despite stressful conditions, Salmonella and Listeria monocytogenes can survive for extended periods of time on different commodities. Frozen cantaloupe should not be thawed for more than 2 hours at any temperature to avoid the growth of Salmonella. Results of this study emphasize the importance of Good Agricultural and Good Manufacturing practices to avoid contamination of produce throughout the production line. As well as the need for consumer education on the risks of improper handling practices.