Dry-inoculation method for thermal inactivation studies in wheat flour using freeze-dried Enterococcus faecium NRRL B-2354

Salmonella and Listeria monocytogenes survival on Field Packed Cantaloupe Contact Surfaces

Field-packing of cantaloupes involves numerous food contact surfaces that can contamination melons with foodborne pathogens; the soil on these surfaces increases throughout the harvest day. Data is lacking on the cross-contamination risk from contaminated food contact surfaces under the dry conditions typical of cantaloupe field-packing operations. This study sought to evaluate the survival of Salmonella and Listeria monocytogenes on cantaloupe field-pack food contact surfaces using both a wet and dry inoculum to provide insights into managing foodborne pathogen contamination risks. Five clean or fouled materials (cotton gloves, nitrile gloves, rubber gloves, cotton rags, and stainless steel) were inoculated with a cocktail of either Salmonella or L. monocytogenes. A wet inoculum was spot inoculated (100 µL) onto coupons. A dry inoculum was prepared by mixing wet inoculum with 100 g of sterile sand, and shaking the coupons with the inoculated sand for 2min. Coupons were held at 35°C (35% RH) and enumerated at 0, 2, 4, 6 and 8 h. Significant differences in pathogen concentrations over time were calculated and the GInaFiT add-in tool for Excel was used to build Log-linear, Weibull, and Biphasic die-off models. Depending on the material type, coupon condition, and inoculum type, Salmonella and L. monocytogenes reductions over 8 h ranged from 0.3-3.3 and -0.4-4.2 log10 CFU/coupon, respectively. For all material types, Salmonella reductions were highest on wet-inoculated clean coupons; L. monocytogenes varied by material type. Weibull and biphasic models were a better fit of respective pathogen die-off curves than linear models. Overall, faster die-off rates were seen for wet inoculated and clean materials. Since pathogen populations remained viable over the study duration and both inoculum type and coupon condition impacted survival, frequent sanitation or replacement of food contact surfaces during the operational day is needed to reduce the risk of cross-contamination.

Enhanced heat tolerance of freeze-dried Enterococcus faecium NRRL B-2354 as valid Salmonella surrogate in low-moisture foods

In microbial studies of low-moisture foods (LMFs, water activity less than 0.85), freeze-dried bacteria benefit us to inoculate LMFs without introducing extra water or altering food physiochemical properties. However, the freeze-drying process would bring unavoidable damage to bacterial cells and results in less-resistant inoculum that are unlikely to be qualified in microbial studies. Herein, we enhanced bacterial heat tolerance by subjecting the cells to mild heat (42-50 °C) to counteract the reduced heat tolerance and survivability of freeze-dried bacteria. Enterococcus faecium NRRL B-2354 (E. faecium), a Salmonella surrogate in LMFs, was used as the target microorganism because it was widely accepted in microbial validation of thermal pasteurizing LMFs. Three types of LMFs (peanut powder, protein powder, and onion powder) were used as LMFs models to validate the freeze-dried E. faecium in comparison with Salmonella enterica Enteritidis PT 30 (S. Enteritidis) prepared by the traditional aqueous method. The heat tolerance (D65℃ value) of E. faecium increased at all treatments and peaked (+31.48 ± 0.13%) at temperature-time combinations of 45 °C-60 min and 50 °C-5 min. Survivability of freeze-dried inoculum and its heat tolerance retained well within 50 d storage. The freeze-dried E. faecium was prepared in this study brought equal or higher heat tolerance (D85℃ or D75℃) than S. Enteritidis in tested LMFs models. For instance, the D85℃ of freeze-dried E. faecium (heat-treated at 50 °C for 5 min) and S. Enteritidis in whole egg powder are 35.56 ± 1.52 min and 28.41 ± 0.41 min, respectively. The freeze-dried E. faecium with enhanced heat tolerance appears to be a suitable Salmonella surrogate for dry-inoculating LMFs. Our protocol also enables industry-scale production of freeze-dried inoculum by broth-cultivation method combined with mild-heat treatment.

Practice and Progress: Updates on Outbreaks, Advances in Research, and Processing Technologies for Low-moisture Food Safety

Large, renowned outbreaks associated with low-moisture foods (LMFs) bring to light some of the potential, inherent risks that accompany foods with long shelf lives if pathogen contamination occurs. Subsequently, in 2013, Beuchat et al. (2013) noted the increased concern regarding these foods, specifically noting examples of persistence and resistance of pathogens in low-water activity foods (LWAFs), prevalence of pathogens in LWAF processing environments, and sources of and preventive measures for contamination of LWAFs. For the last decade, the body of knowledge related to LMF safety has exponentially expanded. This growing field and interest in LMF safety have led researchers to delve into survival and persistence studies, revealing that some foodborne pathogens can survive in LWAFs for months to years. Research has also uncovered many complications of working with foodborne pathogens in desiccated states, such as inoculation methods and molecular mechanisms that can impact pathogen survival and persistence. Moreover, outbreaks, recalls, and developments in LMF safety research have created a cascading feedback loop of pushing the field forward, which has also led to increased attention on how industry can improve LMF safety and raise safety standards. Scientists across academia, government agencies, and industry have partnered to develop and evaluate innovate thermal and nonthermal technologies to use on LMFs, which are described in the presented review. The objective of this review was to describe aspects of the extensive progress made by researchers and industry members in LMF safety, including lessons-learned about outbreaks and recalls, expansion of knowledge base about pathogens that contaminate LMFs, and mitigation strategies currently employed or in development to reduce food safety risks associated with LMFs.

Wet versus Dry Inoculation Methods Have a Significant Effect of Listeria monocytogenes Growth on Many Types of Whole Intact Fresh Produce

ABSTRACT

Listeria monocytogenes causes relatively few outbreaks linked to whole fresh produce but triggers recalls each year in the United States. There are limited data on the influence of wet versus dry inoculation methods on pathogen growth on whole produce. A cocktail of five L. monocytogenes strains that included clinical, food, and environmental isolates associated with foodborne outbreaks and recalls was used. Cultures were combined to target a final wet inoculum concentration of 4 to 5 log CFU/mL. The dry inoculum was prepared by mixing wet inoculum with 100 g of sterile sand and drying for 24 h. Produce investigated belonged to major commodity families: Ericaceae (blackberry, raspberry, and blueberry), Rutaceae (lemon and mandarin orange), Rosaceae (sweet cherry), Solanaceae (tomato), Brassaceae (cauliflower and broccoli), and Apiaceae (carrot). Whole intact, inoculated fruit and vegetable commodities were incubated at 2, 12, 22, and 35 ± 2°C. Commodities were sampled for up to 28 days, and the experiment was replicated six times. The average maximum growth increase was obtained by measuring the maximum absolute increase for each replicate within a specific commodity, temperature, and inoculation method. Data for each commodity, replicate, and temperature were used to create primary growth or survival models describing the lag phase and growth or shoulder and decline as a function of time. Use of a liquid inoculum (versus dry inoculum) resulted in a markedly increased L. monocytogenes growth rate and growth magnitude on whole produce surfaces. Temperature highly influenced this difference: a greater effect seen with more commodities at higher temperatures (22 and 35°C) versus lower temperatures (2 and 12°C). These findings need to be explored for other commodities and pathogens. The degree to which wet or dry inoculation techniques more realistically mimic contamination conditions throughout the supply chain (e.g., production, harvest, postharvest, transportation, or retail) should be investigated.

HIGHLIGHTS

Control of Salmonella in low-moisture foods: Enterococcus faecium NRRL B-2354 as a surrogate for thermal and non-thermal validation

Salmonella has been implicated in multiple foodborne outbreaks and recalls associated with low water activity foods (La_wF). To verify the effectiveness of a process against Salmonella in La_wF, validation using a nonpathogenic surrogate strain is essential. Enterococcus faecium NRRL B-2354 strain has been used as a potential surrogate of Salmonella in different processing of La_wF. However, the survival of Salmonella and E. faecium in La_wF during food processing is a dynamic function of a_w, food composition and structure, processing techniques, and other factors. This review assessed pertinent literature on the thermal and non-thermal inactivation of Salmonella and its presumable surrogate E. faecium in various La_wF and provided an overview of its suitibility in different La_wF. Overall, based on the D-values, survival/reduction, temperature/time to obtain 4 or 5-log reductions, most studies concluded that E. faecium is a suitable surrogate of Salmonella during La_wF processing as its magnitude of resistance was slightly greater or equal (i.e., statistical similar) as compared to Salmonella. Studies also showed its unsuitability which either does not provide a proper margin of safety or being overly resistant and may compromise the quality and organoleptic properties of food. This review provides useful information and guidance for future validation studies of La_wF.

Evaluation of Enterococcus faecium NRRL B-2354 as a surrogate for Salmonella in ground black pepper at different water activities

Thermal inactivation kinetics of Salmonella in low moisture foods are necessary for developing proper thermal processing parameters for pasteurization. The effect of water activity on thermal inactivation kinetics of Salmonella and Enterococcus faecium NRRL B-2354 in ground black pepper has not been studied previously. Identification of a suitable surrogate assists in conducting in-plant process validations. Ground black pepper was inoculated with a 5-serotype Salmonella cocktail or E. faecium NRRL B-2354, equilibrated to water activities of 0.25, 0.45 or 0.65 in a humidity-controlled chamber, and isothermally treated at different temperatures. The survivor data were used for fitting the log-linear models to obtain the D and z-values of Salmonella and E. faecium in ground black pepper. Modified Bigelow models were developed to evaluate the effects of temperature and water activity on the thermal inactivation kinetics of Salmonella and E. faecium. Water activity and temperature showed significant negative effects on the thermal resistance of Salmonella and E. faecium in ground black pepper. For example, significantly higher D values of Salmonella were observed at water activity of 0.45 (D_70°C = 20.5 min and D_75°C = 7.8 min) compared to water activity of 0.65 (D_70°C = 3.9 min and D_75°C = 2.0 min). D-values of E. faecium were significantly higher than those of Salmonella at all three water activities, indicating that E. faecium is a suitable surrogate for Salmonella in thermal processing validation.

Moisture Content of Bacterial Cells Determines Thermal Resistance of Salmonella enterica Serotype Enteritidis PT 30

Salmonella spp. are resilient bacterial pathogens in low-moisture foods. There has been a general lack of understanding of critical factors contributing to the enhanced thermal tolerance of Salmonella spp. in dry environments. In this study, we hypothesized that the moisture content (X_W ) of bacterial cells is a critical intrinsic factor influencing the resistance of Salmonella spp. to thermal inactivation. We selected Salmonella enterica serotype Enteritidis PT 30 to test this hypothesis. We first produced viable freeze-dried S. Enteritidis PT 30, conditioned the bacterial cells to different X_W s (7.7, 9.2, 12.4, and 15.7 g water/100 g dry solids), and determined the thermal inactivation kinetics of those cells at 80°C. The results show that the D-value (the time required to achieve a 1-log reduction) decreased exponentially with increasing X_W We further measured the water activities (a_w) of the freeze-dried S. Enteritidis PT 30 as influenced by temperatures between 20 and 80°C. By using those data, we estimated the X_W of S. Enteritidis PT 30 from the published papers that related the D-values of the same bacterial strain at 80°C with the a_w of five different food and silicon dioxide matrices. We discovered that the logarithmic D-values of S. Enteritidis PT 30 in all those matrices also decreased linearly with increasing X_W of the bacterial cells. The findings suggest that the amount of moisture in S. Enteritidis PT 30 is a determining factor of its ability to resist thermal inactivation. Our results may help future research into fundamental mechanisms for thermal inactivation of bacterial pathogens in dry environments.IMPORTANCE This study established a logarithmic relationship between the thermal death time (D-value) of S. Enteritidis PT 30 and the moisture content (X_W ) of the bacterial cells by conducting thermal inactivation tests on freeze-dried S Enteritidis PT 30. We further verified this relationship using literature data for S. Enteritidis PT 30 in five low-moisture matrices. The findings suggest that the X_W of S. Enteritidis PT 30, which is rapidly adjusted by microenvironmental a_w, or relative humidity, during heat treatments, is the key intrinsic factor determining the thermal resistance of the bacterium. The quantitative relationships reported in this study may help guide future designs of industrial thermal processes for the control of S. Enteritidis PT 30 or other Salmonella strains in low-moisture foods. Our findings highlight a need for further fundamental investigation into the role of water in protein denaturation and the accumulation of compatible solutes during thermal inactivation of bacterial pathogens in dry environments.

Influence of low water activity on the thermal resistance of Salmonella Enteritidis PT30 and Enterococcus faecium as its surrogate in egg powders

Egg powders are increasingly popular ingredients, due to their functionality and compactness, in industrial food production and preparation at homes. However, there is a lack of studies that evaluate the thermal resistance of Salmonella Enteritidis PT30 and its potential surrogate Enterococcus faecium NRRL B-2354 in egg powders. This study examined the log-linear relationship between the thermal resistance of Salmonella Enteritidis (D-value) and the water activity (aw) of egg powders. The changes of aw in the egg powders with temperature were measured using a Vapor Sorption Analyzer and a high-temperature cell. The D80 ℃-value of S. Enteritidis PT30 and E. faecium inoculated in the egg powders preconditioned to three aw levels (0.3, 0.45, and 0.6) at 20 ℃ were determined using aluminum thermal death test cells. The aw values increased (P < 0.05) in all three egg powders when the temperature of the samples was raised from room temperature to 80 ℃. The D80 ℃-values ranged from 5.3 ± 0.1 to 25.9 ± 0.2 min for S. Enteritidis while 10.4 ± 0.4 to 43.8 ± 0.4 for E. faecium in samples of the three different aw levels. S. Enteritidis PT30 showed a log-linear relationship between D80 ℃-values and aw80 ℃ for the egg powders. This study contributes to our understanding of the impact of aw on the development of thermal treatments for low-moisture foods.