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Challenge studies associated with fruits and vegetables generally utilize wet bacterial inoculation methods. However, a recent salmonellosis outbreak in the U.S. was linked to peaches plausibly contaminated via fugitive dust from a nearby animal operation. This outbreak has highlighted the need for a suitable inert carrier which can be used for the dry transfer of Salmonella enterica to produce. The purpose of this study was 1) to examine the population stability of S. enterica and its surrogate, Enterococcus faecium, in different dry matrices during extended storage to identify suitable carriers and 2) to evaluate the survival of S. enterica on peaches based on the mode of contamination (i.e., wet vs. dry). S. enterica and E. faecium were cultivated on tryptic soy agar (TSA) and inoculated into corn-cob small animal litter, sand, or silica at 10–11 log CFU/g. Matrices were mixed by hand and stored at 25°C and 33% relative humidity for up to 120 d. S. enterica remained relatively stable in the silica and litter, with no significant decrease in population after 14 and 28 d, respectively. E. faecium significantly reduced in all matrices, with the greatest reduction observed in silica (2.86 log CFU/g after 120 d). Additional carriers would need to be assessed for E. faecium which could maintain its population stability. Silica was ultimately selected for the dry carrier of S. enterica. Peaches available at retail or from orchards were inoculated with S. enterica using the silica carrier or by spot or dip inoculation methods at 5 log CFU/peach and stored at 5°C and 80% relative humidity for up to 28 d. The population of S. enterica significantly reduced on all peaches except for the dry inoculated orchard peaches, where the population remained stable (4.62 ± 0.35 log CFU/peach after 28 d). Results from this study determined that the mode of contamination influences the survival of S. enterica on peaches and that dry inoculation methods should be considered for produce in some instances.

Fresh vegetables have been linked to multiple foodborne outbreaks in the U.S., with Listeria monocytogenes and Salmonella enterica identified as leading causes. Beyond raw vegetables, cooked vegetables can also pose food safety concerns due to improper cooking temperature and time combinations or postcooking contamination. Cooked vegetables, having had their native microbiota reduced through heat inactivation, might provide an environment that favors the growth of pathogens due to diminished microbial competition. While the risks associated with raw vegetables are recognized, the survival and growth of pathogens on cooked vegetables remain inadequately studied. This study investigated the growth kinetics of both L. monocytogenes and S. enterica on various cooked vegetables (carrot, corn, onions, green bell pepper, and potato). Vegetables were cooked at 177°C until the internal temperature reached 90°C and then cooled to 5°C. Cooled vegetables were inoculated with a four-strain cocktail of either L. monocytogenes or S. enterica at 3 log CFU/g, then stored at different temperatures (5, 10, or 25°C) for up to 7 days. Both pathogens survived on all vegetables when stored at 5°C. At 10°C, both pathogens proliferated on all vegetables, with the exception of L. monocytogenes on pepper. At 25°C, the highest growth rates were observed by both pathogens on carrot (5.55 ± 0.22 and 6.42 ± 0.23 log CFU/g/d for L. monocytogenes and S. enterica, respectively). S. enterica displayed higher growth rates at 25°C compared to L. monocytogenes on all vegetables. Overall, these results bridge the knowledge gap concerning the growth kinetics of both S. enterica and L. monocytogenes on various cooked vegetables, offering insights to further enhance food safety.

Two recent listeriosis outbreaks have occurred in the United States and Europe due to contaminated individually quick-frozen (IQF) vegetables. While one of the outbreaks was due to frozen vegetables considered ready-to-eat (RTE), the other was linked to frozen corn whose packaging contained cooking instructions and was considered not-ready-to-eat (NRTE). However, consumers may thaw certain frozen vegetables and consume without cooking. Since no data is available on the survivability of L. monocytogenes on IQF vegetables during frozen storage, this study examined the population of six different strains (comprising lineages 1/2a, 1/2b, and 4b) on IQF vegetables during long-term storage. Individual strains were inoculated onto an IQF vegetable mix at 4 log CFU/g and stored at −18 or −10°C for 360 days. Although fluctuations in populations of all strains were observed on the vegetables during storage, no significant differences based on strain, lineages, or temperature were observed. Overall, L. monocytogenes populations were only reduced by up to 0.47 and 0.59 log CFU/g after 360 days at −18 and −10°C, respectively. Results from this study suggest that L. monocytogenes is able to persist on IQF vegetables for extended time periods with no significant reduction in population. Future studies could evaluate the survival and growth of L. monocytogenes on IQF vegetables during thawing and storage.