Pathogens in low-moisture foods are an emerging food safety concern due to increased survival and thermotolerance in matrices with low water activity. However, limited data is publicly available for the thermotolerance of Listeria monocytogenes, Salmonella spp., and Enterococcus faecium NRRL B-2354 (a Salmonella surrogate). The aims of this study were to identify differences in thermal inactivation rates between these organisms in three different low-moisture foods. Three model low-moisture foods (peanut butter, powder infant formula, and wheat flour) were inoculated with either E. faecium, a Salmonella spp. cocktail, or a L. monocytogenes cocktail using a dry inoculation method for a total of 9 treatments. Samples were heat treated in a hot water bath at predetermined temperatures, and bacterial survival was detected via direct plating on tryptic soy agar with 0.6% yeast extract. In peanut butter and most of the powder infant formula treatments, Salmonella spp. had significantly higher D-values than L. monocytogenes using comparable temperatures (p < 0.05). However, D-values between Salmonella spp. and L. monocytogenes were comparable in wheat flour and one of the treatment temperatures in powder infant formula (p > 0.05). For all but one of the treatments at the same temperature, E. faecium had significantly higher D-values than L. monocytogenes and Salmonella spp. in each food matrix (p < 0.05). The observed matrix effect on thermotolerance for each of the bacteria was reported in descending order as powder infant formula > peanut butter > wheat flour in the majority of the comparable D-values. While Salmonella continues to be the pathogen of concern in low-moisture foods due to survival and outbreaks, these results indicate L. monocytogenes can exhibit similar thermotolerances in relevant model low-moisture foods matrices.

Vegetative, non-sporeforming foodborne pathogens show notable survival and uncanny thermotolerance in low water activity (aw) foods. Controlled studies on Listeria monocytogenes, Salmonella spp., and Enterococcus faecium NRRL B-2354 (a Salmonella surrogate) in a variety of food matrices support thermal process validation studies required to achieve global food safety objectives. In this study, we determined and compared thermal inactivation rates using independent six-strain cocktails of pathogens in three plant-based butters. Direct determinations of decimal reduction times (D-values) for L. monocytogenes, Salmonella, and E. faecium, in corresponding butters were inoculated using peanut oil, almond oil, or sunflower oil. Thermal Death Time (TDT) studies for the organisms were conducted in triplicate. Uniform bagged plant- based butter samples of Salmonella spp. or L. monocytogenes, or E. faecium alone were sandwiched in copper plates immobilized with recessed magnets. Samples underwent rapid heat treatments via water immersion under isothermal conditions ranging from 70°C to 85°C. Bacterial destruction in peanut butter (46% fat, 0.20 aw @ 25°C), almond butter, (50% fat, 0.32 aw @ 25°C), or sunflower butter (56% fat, 0.15 aw @ 25°C) was determined by direct plating. The TDT studies showed Salmonella spp. had consistently higher D-values than L. monocytogenes in all treatments, but pair-wise comparisons found no statistical difference when assessing the thermotolerance of the two pathogens in the individual plant-based butters tested (p > 0.005). These data support Salmonella as the primary pathogen of concern in low water activity foods and show the heat resistance of L. monocytogenes can approximate destruction kinetics observed for Salmonella spp. in low aw matrices. E. faecium exhibited the highest thermotolerance. This further supports the utility of this surrogate for Salmonella spp. and L. monocytogenes in high fat, low-moisture foods similar to the plant-based butters tested. Thermotolerance differences between a dry talc vs. peanut oil-based inoculation procedures in peanut butter were also evaluated. Surprisingly, the oil-based inoculations resulted in lower D- values (p > 0.01) for Salmonella spp. and the surrogate when compared to the dry inoculum.

Limited data are currently available characterizing the thermal resistance of foodborne pathogens in semolina flour and intermediate pasta matrices representative of commercial conditions during mixing, extrusion, and drying. These data are essential to pasta producers seeking to be compliant with federal regulations since Salmonella spp. and Escherichia coli demonstrate survival in wheat flour and dried pasta products. This study investigated the heat resistance of Salmonella, pathogenic E. coli, and E. faecium NRRL B-2354 in raw semolina flour and partially dried pasta intermediates via thermal death time (TDT) studies. This study also assessed the appropriateness of E. faecium NRRL B-2354 as a surrogate in semolina flour and pasta matrices. Inoculated pasta matrices equilibrated to target water activities of 0.85, 0.88, and 0.91 (measured at 25°C) underwent isothermal inactivation treatments at 65°C, 70°C, and 75°C. Serial dilution and direct plating methods allowed for estimation of bacterial survival at each treatment. In representative pasta matrices, the D-values for each microorganism increased as water activity decreased from 0.91 to 0.85. Surprisingly, Salmonella and E. coli did not exhibit significantly different thermal resistance in pasta. The greatest heat resistance was seen in semolina flour (aw 0.45). E. faecium was significantly more thermal resistant than both pathogens in all treatments when the temperature was ≤ 70°C. The results show that E. faecium strain NRRL B-2354 is not an ideal proxy for Salmonella and E. coli in semolina and pasta matrices. Analysis of the TDT data also found that a long-goods pasta drying process can achieve ≥7-log reductions of Salmonella and E. coli when following Good Manufacturing Practices.