Thermal inactivation of Salmonella, Shiga toxin-producing Escherichia coli, Listeria monocytogenes, and a surrogate (Pediococcus acidilactici) on raisins, apricot halves, and macadamia nuts using vacuum-steam pasteurization.

Salmonella, Shiga toxin-producing Escherichia coli (STEC), and Listeria monocytogenes have been isolated from low water activity foods (LWAF), where they may survive for extended periods. The ready-to-eat nature of many LWAF, such as dried fruits and nuts, warrants effective post-harvest thermal treatment for the reduction of pathogens such as low-temperature, saturated steam, also known as vacuum-assisted steam pasteurization. The objective of this study was to determine reductions of Salmonella, STEC, L. monocytogenes, and a possible surrogate (Pediococcus acidilactici) on dried apricot halves, whole macadamia nuts, and raisins after treatment with vacuum-assisted steam at three temperatures (62 °C, 72 °C, or 82 °C) and multiple time intervals. Bacterial inactivation was variable between commodities, with higher temperatures and longer times necessary to achieve comparable reductions of pathogens on apricot halves and macadamia nuts compared to raisins. Reductions of the tested pathogens were comparable; therefore, one species was not more resistant than the others. Pathogens were reduced by 5-log CFU/g on apricot halves after 20 min at 72 °C and after 5 min at 82 °C. Longer treatment times were necessary to achieve reductions of each pathogen on macadamia nuts. Pathogens were reduced by nearly 5 log CFU/g on macadamia nuts after 38 min at 72 °C (4.6-6.5 log CFU/g) and after 12 min at 82 °C (4.9-5.7 log CFU/g). Reductions of pathogens on raisins were achieved at lower temperatures than necessary for the other foods. A 5-log reduction for each of the pathogens (CFU/g) on raisins occurred after 20 min at 62 °C and after 5 min at 72 °C. Overall, the reductions of the pathogens exceeded those of P. acidilactici on both the dried fruits and macadamia nuts. Statistically significant differences, indicating greater confidence as a conservative surrogate, were observed at lower treatment temperatures. Inactivation kinetics were modeled for each pathogen on each food type and temperature. Bacterial survival was best described by the Weibull model for raisins and macadamia nuts, while the Gompertz model best described reductions on apricot halves according to Akaike information criterion (AIC) and root-mean-square error (RMSE) evaluations. Water activity and moisture content were increased due to the treatments, which could be addressed through implementation of drying steps. Thermal inactivation kinetic models and 5-log reduction parameters can help food processors design and evaluate similar vacuum-assisted steam interventions to comply with FSMA regulations and preventive control plans. However, results or model predictions should not be extrapolated to assume the safety of other types of foods.

Fate of Listeria monocytogenes on Broccoli and Cauliflower at Different Storage Temperatures.

ABSTRACT: Understanding a food's ability to support the growth and/or survival of a pathogen throughout the supply chain is essential to minimizing large-scale contamination events. The purpose of this study was to examine the behavior (growth and/or survival) of Listeria monocytogenes on broccoli and cauliflower florets stored at different postharvest temperatures utilized along the supply chain. Broccoli and cauliflower samples were inoculated with L. monocytogenes at approximately 3 log CFU/g and stored at 23 ± 2, 12 ± 2, 4 ± 2, and -18 ± 2°C. Samples were evaluated for L. monocytogenes levels after 0, 0.167 (4 h), 1, 2, 3, and 4 days at 23 ± 2°C; 0, 0.167, 1, 2, 3, 4, 7, 10, and 14 days at 12 ± 2°C; 0, 0.167, 1, 2, 3, 4, 7, 10, 14, 21, and 28 days at 4 ± 2°C; and 0, 1, 7, 28, 56, 84, 112, 140, and 168 days at -18 ± 2°C. L. monocytogenes populations were determined by plating samples onto tryptic soy agar and modified Oxford agar supplemented with nalidixic acid. Broccoli and cauliflower supported the growth of L. monocytogenes at 23, 12, and 4°C, and higher growth rates were observed at higher temperatures. Populations of L. monocytogenes on broccoli and cauliflower samples significantly increased within 1 day at 23°C (by 1.6 and 2.0 log CFU/g, respectively) (P ≤ 0.05). At 12°C, populations of L. monocytogenes on broccoli and cauliflower samples significantly increased over 14 days by 1.4 and 1.9 log CFU/g, respectively (P ≤ 0.05). No significant difference over time was observed in L. monocytogenes populations on broccoli and cauliflower samples held under refrigeration until populations began to grow by day 10 in both commodities (P > 0.05). Under frozen storage (-18°C), populations of L. monocytogenes survived on broccoli and cauliflower at least up to 168 days. Storage of broccoli and cauliflower at lower temperatures can minimize L. monocytogenes growth potential; growth rates were lower at 4°C than at 12 and 23°C.

Growth and Survival of Listeria monocytogenes on Intact Fruit and Vegetable Surfaces during Postharvest Handling: A Systematic Literature Review.

Listeria monocytogenes may be present in produce-associated environments (e.g., fields, packing houses); thus, understanding its growth and survival on intact, whole produce is of critical importance. The goal of this study was to identify and characterize published data on the growth and/or survival of L. monocytogenes on intact fruit and vegetable surfaces. Relevant studies were identified by searching seven electronic databases: AGRICOLA, CAB Abstracts, Center for Produce Safety funded research project final reports, FST Abstracts, Google Scholar, PubMed, and Web of Science. Searches were conducted using the following terms: Listeria monocytogenes, produce, growth, and survival. Search terms were also modified and "exploded" to find all related subheadings. Included studies had to be prospective, describe methodology (e.g., inoculation method), outline experimental parameters, and provide quantitative growth and/or survival data. Studies were not included if methods were unclear or inappropriate, or if produce was cut, processed, or otherwise treated. Of 3,459 identified citations, 88 were reviewed in full and 29 studies met the inclusion criteria. Included studies represented 21 commodities, with the majority of studies focusing on melons, leafy greens, berries, or sprouts. Synthesis of the reviewed studies suggests L. monocytogenes growth and survival on intact produce surfaces differ substantially by commodity. Parameters such as temperature and produce surface characteristics had a considerable effect on L. monocytogenes growth and survival dynamics. This review provides an inventory of the current data on L. monocytogenes growth and/or survival on intact produce surfaces. Identification of which intact produce commodities support L. monocytogenes growth and/or survival at various conditions observed along the supply chain will assist the industry in managing L. monocytogenes contamination risk.

The efficacy of zero valent iron-sand filtration on the reduction of Escherichia coli and Listeria monocytogenes in surface water for use in irrigation.

The use of surface and recycled water sources for irrigation can reduce demand on critical groundwater resources. Treatment or mitigation may be necessary for the use of these alternative water sources in order to reduce risk associated with microbial pathogens present in the water. In this study, the efficacy of a zero-valent iron (ZVI) sand filter was assessed for the reduction of Listeria monocytogenes and Escherichia coli in surface water. Water recovered from an agricultural pond was inoculated with E. coli TVS353 and an environmental L. monocytogenes isolate at 7 Log10 CFU/mL and horizontally filtered over a six-month period through a PVC pipe filter, filled with 35%:65% (volume:volume) ZVI:sand or sand alone. Filtered water was used to irrigate lettuce and bacterial persistence on lettuce leaves was determined for 7 days post-irrigation. Both ZVI:sand-filtered water and sand-filtered water contained significantly (p < 0.005) lower levels of E. coli and L. monocytogenes compared to initial unfiltered inoculated water. Population reductions of E. coli and L. monocytogenes were comparable after sand filtration. However, ZVI:sand filtration resulted in significantly greater population reductions of L. monocytogenes (P < 0.05) compared to E. coli. Populations of E. coli on leaves of lettuce plants irrigated with ZVI:sand-filtered water were not significantly lower than populations on plants irrigated with sand-filtered irrigation water over the 7-day period. However, populations of L. monocytogenes on lettuce leaves irrigated with ZVI-treated water were significantly lower than counts on plants irrigated with sand-filtered irrigation water on days 3 and 4 post irrigation (p = 0.052 and p = 0.042 for days 3 and 4, respectively. The differences observed in reductions of L. monocytogenes and E. coli by ZVI filtration is due to the differing effect that ZVI disruption has on Gram-positive and Gram-negative cell walls and membranes. ZVI- sand filters show promising results as an inexpensive on-farm technology for the mitigation of enteric foodborne bacterial populations in pond water over a six-month period.