Bradley P. Marks

Previous studies have shown that using a surface growth method for preparing inoculum impacted pathogen population stability and inactivation kinetics. Here, we quantified the desiccation survival and responses to tempering treatment of Salmonella and Shiga-toxin producing Escherichia coli (STEC) on wheat grain when grown using five different methods: broth, lawn-aerobic, lawn-anaerobic, lawn-acid-adapted, and lawn-low inoculum. Three strains of Salmonella and STEC each were individually inoculated onto wheat grain, conditioned to 0.45 a _w and stored up to 24 weeks. Pathogen survival curves on grain were different when inoculated with a lawn-grown culture compared to broth-grown. Acid adaptation of STEC led to increased ( P < 0.001) tolerance to desiccation, while no change in Salmonella desiccation tolerance was observed. While most survival curves were better described by the log-linear model, survival of pathogens from broth-grown inoculum was better characterized by the Weibull model. Five-strain cocktails of each species were inoculated onto wheat, stored for 1, 2, 7, 28, and 84 days, followed by tempering for 18 h with three chemical interventions: water (control), 800 ppm chlorine, 5% lactic acid (LA) + 26.6% NaCl and 5% sodium bisulfate (SBS). These treatments led to different ( P < 0.001) average reductions of 0.52, 1.04, 1.43, and 1.96 log CFU/g, respectively. The length of storage and inoculum growth method affected ( P < 0.001) pathogen survival during tempering. In general, pathogens inoculated on grain survived better during tempering when inoculum was prepared by acid-adapted ≥ lawn-aerobic ≥ low inoculum > lawn-anaerobic = broth. IMPORTANCE: Outbreaks linked to wheat flour increased interest in evaluating pathogen survival kinetics. With minimal information on how foodborne pathogens contaminate wheat grain, the “worst-case scenario” should be identified to characterize pathogen survival kinetics on grain and be used to assess the effectiveness of food safety interventions. Using an antimicrobial solution during wheat tempering, an existing unit operation where grain is exposed to water prior to milling into flour can be a cost-effective way to mitigate the risk of foodborne pathogens. The lack of consistent inoculum preparation methods makes it difficult to compare results across studies evaluating tempering treatments. We assessed five different inoculum growth methods to quantify pathogen survival during desiccation and long-term storage and pathogen inactivation efficacy of several existing tempering solutions. In addition, these data provide insights on statistically important parameters to consider for low-moisture food challenge study experimental design, such as inoculum growth, inoculation level, and pathogen adaptation.

This multi-institutional study assessed the efficacy of Enterococcus faecium NRRL B-2354 as a nonpathogenic Salmonella surrogate for thermal processing of nonfat dry milk powder, peanut butter, almond meal, wheat flour, ground black pepper, and date paste. Each product was analyzed by two laboratories (five independent laboratories total), with the lead laboratory inoculating (E. faecium or a five-strain Salmonella enterica serovar cocktail of Agona, Reading, Tennessee, Mbandaka, and Montevideo) and equilibrating the product to the target water activity before shipping. Both laboratories subjected samples to three isothermal treatments (between 65 and 100°C). A log-linear and Bigelow model was fit to survivor data via one-step regression. On the basis of D80°C values estimated from the combined model, E. faecium was more thermally resistant (P < 0.05) than Salmonella in nonfat dry milk powder (DEf-80°C, 100.2 ± 5.8 min; DSal-80°C, 28.9 ± 1.0 min), peanut butter (DEf-80°C, 133.5 ± 3.1 min; DSal-80°C, 57.6 ± 1.5 min), almond meal (DEf-80°C, 34.2 ± 0.4 min; DSal-80°C, 26.1 ± 0.2 min), ground black pepper (DEf-80°C, 3.2 ± 0.8 min; DSal-80°C, 1.5 ± 0.1 min), and date paste (DEf-80°C, 1.5 ± 0.0 min; DSal-80°C, 0.5 ± 0.0 min). Although the combined laboratory D80°C for E. faecium was lower (P < 0.05) than for Salmonella in wheat flour (DEf-80°C, 9.4 ± 0.1 min; DSal-80°C, 10.1 ± 0.2 min), the difference was ∼7%. The zT values for Salmonella in all products and for E. faecium in milk powder, almond meal, and date paste were not different (P > 0.05) between laboratories. Therefore, this study demonstrated the impact of standardized methodologies on repeatability of microbial inactivation results. Overall, E. faecium NRRL B-2354 was more thermally resistant than Salmonella, which provides support for utilizing E. faecium as a surrogate for validating thermal processing of multiple low-moisture products. However, product composition should always be considered before making that decision.HIGHLIGHTSE. faecium was more thermally resistant than Salmonella.Thermal resistance of E. faecium and Salmonella was impacted by product type.Inactivation results were statistically similar across five independent laboratories.Standardized methodologies yielded low cross-laboratory differences.

The effect of moderate-temperature (≤60 °C) dehydration of plant-based foods on pathogen inactivation is unknown. Here, we model the reduction of E. coli O157:H7 as a function of product-matrix, a_w, and temperature under isothermal conditions. Apple, kale, and tofu were each adjusted to a_w 0.90, 0.95, or 0.99 and inoculated with an E. coli O157:H7 cocktail, followed by isothermal treatment at 49, 54.5, or 60.0 °C. The decimal reduction time, or D-value, is the time required at a given temperature to achieve a 1 log reduction in the target microorganism. Modified Bigelow-type models were developed to determine D-values which varied by product type and a_w level, ranging from 3.0–6.7, 19.3–55.3, and 45.9–257.4 min. The relative impact of a_w was product dependent and appeared to have a non-linear impact on D-values. The root mean squared errors of the isothermal-based models ranged from 0.75 to 1.54 log CFU/g. Second, we performed dynamic drying experiments. While the isothermal results suggested significant microbial inactivation might be achieved, the dehydrator studies showed that the combination of low product temperature and decreasing a_w in the pilot-scale system provided minimal inactivation. Pilot-scale drying at 60 °C only achieved reductions of 3.1 ± 0.8 log in kale and 0.67 ± 0.66 log in apple after 8 h, and 0.69 ± 0.67 log in tofu after 24 h. This illustrates the potential limitations of dehydration at ≤60 °C as a microbial kill step.

Listeriosis, a foodborne illness caused by Listeria monocytogenes , has relatively low incidence, but a substantial mortality rate, particularly in immunocompromised populations. Because of the known risk of L. monocytogenes and other pathogens in produce, immunocompromised individuals are often placed on neutropenic diets that exclude fresh produce. Therefore, this study aimed to evaluate several kitchen-scale treatments as potential interventions to reduce L. monocytogenes in prepared produce. Cucumbers, apples, and celery were dip inoculated with a three-strain cocktail of L. monocytogenes and dried for 24 h. Inoculated products were subjected to the following treatments as applicable: commercial sanitizer soak (90 s, with agitation), tap water rinse (15 s), tap water soak (90 s, with agitation), surface blanching (25 s), tap water rinse (15 s) followed by peeling, and surface blanching (25 s) followed by peeling. Additionally, inoculum uptake in celery and the impact of two different types of peelers (mechanical crank and manual) were assessed. Treated samples were plated on differential media and incubated for 48 h at 37°C. L. monocytogenes populations were then enumerated and compared to the untreated control (log CFU/g). All treatments lacked efficacy for celery, with reductions significantly less ( P < 0.05) than in other products, likely due to inoculum internalization. The sanitizer soak, tap water rinse, and tap water soak did not differ in efficacy ( P > 0.05), which was low for cucumbers (< 1.5 log CFU/g), apples (< 1.3 log CFU/g), and celery (< 0.7 log CFU/g). The two types of apple peelers did not differ in efficacy ( P > 0.05). Surface blanching and surface blanching followed by peeling were the most effective treatments in both cucumbers and apples ( P < 0.05), with average reductions of 4.2 to 5.1 and 3.5 to 5.9 log CFU/g, respectively.