Method of Inoculation Influences the Survival of Salmonella enterica on Retail and Orchard Peaches

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.

Dynamics of Listeriamonocytogenes and Salmonella enterica on Cooked Vegetables During Storage

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.

Survival of Listeria monocytogenes on Frozen Vegetables during Long-term Storage at -18 and -10°C

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.

Effect of dehydration on the inactivation of Listeria monocytogenes and Salmonella enterica on enoki and wood ear mushrooms

Foodborne illness outbreaks in the U.S. associated with consumption of both fresh and dried specialty mushrooms have recently occurred. Dried wood ear mushrooms were implicated in a salmonellosis outbreak in 2020, while fresh enoki mushrooms were associated with two listeriosis outbreaks in 2020 and 2023. These specialty mushrooms are commercially available in both their fresh and dried states. Due to the short shelf life of mushrooms, dehydration is a common method used in both industry and by consumers to extend the shelf life and preserve quality. Therefore, the aim of this study was to evaluate the use of dehydration on the inactivation kinetics of both Listeria monocytogenes and Salmonella enterica on enoki and wood ear mushrooms. Fresh mushrooms were inoculated with four strain cocktails of either L. monocytogenes or S. enterica and dried at ambient conditions for 10 min. Following drying of the inoculum, mushrooms were placed into food dehydrators preheated to 70, 80, or 90°C and treated for up to 24 h. At treatment intervals, mushrooms were removed from the dehydrators for pathogen enumeration. Inactivation kinetics for both pathogens were modeled using the Weibull, log-linear with tail, and log-linear with shoulder models. Pathogen reductions of >4 log CFU/g were achieved on both enoki and wood ear mushrooms during dehydration at 90°C after only 2-4 h. At 70 and 80°C, log reductions of >4 log CFU/g were observed on wood ear mushrooms after 4-8 h. On enoki mushrooms, a tailing effect was observed with residual populations (>2 log CFU/g) of L. monocytogenes and S. enterica remaining even after 24 h of treatment at both 70 and 80°C. This study emphasizes the need for an individualized dehydration strategy for each mushroom type to ensure the effectiveness of dehydration as a process to reduce pathogen populations. Results of this study will aid in informing proper time and temperature combinations for dehydration of specialty mushrooms to ensure product safety.

Survival kinetics of Listeria monocytogenes and Salmonella enterica on dehydrated enoki and wood ear mushrooms during long-term storage

Two specialty mushrooms have recently become novel vectors for foodborne outbreaks in the U.S.: fresh enoki and dried wood ear mushrooms were linked to a listeriosis and salmonellosis outbreak, respectively. The aim of this study was to evaluate the survival kinetics of Listeria monocytogenes and Salmonella enterica on dehydrated enoki and wood ear mushrooms during long-term storage. Following heat dehydration, mushrooms were inoculated with either L. monocytogenes or S. enterica, allowed to dry for 1 h, and then stored for up to 180 d at 25 °C and 33% relative humidity. Both pathogens were enumerated from the mushrooms at intervals during the storage period. Survival kinetics of both pathogens were modeled using both the Weibull and log-linear with tail models. After inoculation and 1 h drying, both pathogen populations decreased 2.26-2.49 log CFU/g on wood ear mushrooms; no decrease was observed on enoki. Both pathogens survived during storage on both mushroom types. On wood ear mushrooms, a 2-log decrease of both pathogens occurred during storage. On enoki mushrooms, 4-log decreases of both pathogens were modeled to occur after 127.50-156.60 d. The results of this study suggest that L. monocytogenes and S. enterica can persist on dehydrated specialty mushrooms during long-term storage.

Efficacy of Power Ultrasound-Based Hurdle Technology on the Reduction of Bacterial Pathogens on Fresh Produce

Minimally processed produce is frequently contaminated with foodborne bacterial pathogens. Power ultrasound is a non-thermal and cost-effective technology that can be combined with other chemical sanitization methods. This study investigated the reduction of Listeria monocytogenes and Salmonella Newport on grape tomato, romaine lettuce, and spinach washed with water, chlorine, or peroxyacetic acid alone or in combination with 25 or 40 kHz power ultrasound for 1, 2, or 5 min. Produce items were inoculated with either pathogen at 10 log CFU/g, dried for 2 h, and treated. Combined treatment of ultrasound and sanitizers resulted in 1.44-3.99 log CFU/g reduction of L. monocytogenes and 1.35-3.62 log CFU/g reduction of S. Newport, with significantly higher reductions observed on grape tomato. Synergistic effects were achieved with the hurdle treatment of power ultrasound coupled with the chemical sanitizers when compared to the single treatments; an additional 0.48-1.40 log CFU/g reduction of S. Newport was obtained with the addition of power ultrasound on grape tomato. In general, no significant differences were observed in pathogen reductions between the ultrasound frequencies, the sanitizers, or the treatment lengths. Results from this study suggest that incorporation of power ultrasound into the current washing procedure may be beneficial for the reduction, but not elimination, of bacterial pathogens on certain produce items, including tomatoes.

Growth Kinetics of Listeria monocytogenes and Salmonella enterica on Dehydrated Vegetables during Rehydration and Subsequent Storage

Dehydrated vegetables have low water activities and do not support the proliferation of pathogenic bacteria. Once rehydrated, vegetables can be incorporated into other foods or held for later use. The aim of this study was to examine the survival and proliferation of Listeria monocytogenes and Salmonella enterica on dehydrated vegetables during rehydration and subsequent storage. Carrots, corn, onion, bell peppers, and potatoes were heat dehydrated, inoculated at 4 log CFU/g, and rehydrated at either 5 or 25 °C for 24 h. Following rehydration, vegetables were stored at 5, 10, or 25 °C for 7 d. Both L. monocytogenes and S. enterica survived on all vegetables under all conditions examined. After 24 h of rehydration at 5 °C, pathogen populations on the vegetables were generally <1.70 log CFU/g, whereas rehydration at 25 °C resulted in populations of 2.28 to 6.25 log CFU/g. The highest growth rates during storage were observed by L. monocytogenes on potatoes and S. enterica on carrots (2.37 ± 0.61 and 1.63 ± 0.18 log CFU/g/d, respectively) at 25 °C when rehydration occurred at 5 °C. Results indicate that pathogen proliferation on the vegetables is both rehydration temperature and matrix dependent and highlight the importance of holding rehydrated vegetables at refrigeration temperatures to hinder pathogen proliferation. Results from this study inform time and temperature controls for the safety of these food products.

Long-Term Survival of Listeria monocytogenes in Nut, Seed, and Legume Butters

Nut, seed, and legume butters have become increasingly popular with consumers. Listeria monocytogenes contamination of a variety of butters has resulted in several recalls, although no known outbreaks have been identified. L. monocytogenes has been shown to survive on a variety of seeds for up to 6 months, legumes and nuts for over 12 months, and in peanut butter and peanut-chocolate spreads for 21 to 60 weeks depending on formulation; however, long-term survival in other butters has not yet been characterized. In this study, the survival of L. monocytogenes in various nut, seed, legume, and chocolate-containing butters (n=10) based on inoculation level, storage temperature, and the pH, a_w, and nutrient contents of the butters, was examined. First, butters were inoculated with L. monocytogenes at 4 log CFU/g and stored at either 5 or 25°C with enumeration and/or enrichment at intervals over 12 months. L. monocytogenes survived in all butters examined with no significant change in population after storage at 5°C, whereas the population was reduced to <1.70 log CFU/g in as little as 3 months at 25°C; the only exception was for sunflower butter, where L. monocytogenes decreased approximately 1 log CFU/g. Subsequently, all butters were inoculated at 1 log CFU/g and stored at 25°C for 6 months with enrichment during storage. L. monocytogenes was detected in all butters, except pecan butter, after 6-month storage. Butters containing chocolate did not inhibit L. monocytogenes survival, regardless of the inoculation level. Results indicate there may be association between high fat and carbohydrate level and survivability of L. monocytogenes in various types of butters. This work highlights the need to mitigate the potential for cross-contamination of L. monocytogenes into nut, seed, and legume butters due to the potential for long-term survival. Running title: Listeria survival in butters.

Modeling the Fate of Listeria monocytogenes and Salmonella enterica on Fresh Whole and Chopped Wood Ear and Enoki Mushrooms

Two recent foodborne illness outbreaks linked to specialty mushrooms have occurred in the United States, both representing novel pathogen-commodity pairings. Listeria monocytogenes and Salmonella enterica were linked to enoki and wood ear mushrooms, respectively. The aim of this study was therefore to examine the survival of both L. monocytogenes and S. enterica on raw whole and chopped enoki and wood ear mushrooms during storage at different temperatures. Fresh mushrooms were either left whole or chopped and subsequently inoculated with a cocktail of either S. enterica or rifampicin-resistant L. monocytogenes, resulting in an initial inoculation level of 3 log CFU/g. Mushroom samples were stored at 5, 10, or 25°C for up to 7 d. During storage, the population levels of S. enterica or L. monocytogenes on the mushrooms were enumerated. The primary Baranyi model was used to estimate the growth rates of both pathogens and the secondary Ratkowsky square root model was used to model the relationship between growth rates and temperature. Both L. monocytogenes and S. enterica survived on both mushroom types and preparations at all temperatures. No proliferation of either pathogen was observed on mushrooms stored at 5°C. At 10°C, moderate growth was observed for both pathogens on enoki mushrooms and for L. monocytogenes on wood ear mushrooms; no growth was observed for S. enterica on wood ear mushrooms. At 25°C, both pathogens proliferated on both mushroom types with growth rates ranging from 0.43 to 3.27 log CFU/g/d, resulting in 1 log CFU/g increases in only 0.31 d (7.44 h) to 2.32 d. Secondary models were generated for L. monocytogenes on whole wood ear mushrooms and S. enterica on whole enoki mushrooms with goodness-of-fit parameters of r² = 0.9855/RMSE = 0.0479 and r² = 0.9882/RMSE = 0.1417, respectively. Results from this study can aid in understanding the dynamics of L. monocytogenes and S. enterica on two types of specialty mushrooms.