Oily, low water activity (OL aw) products including tahini (sesame seed paste), halva (tahini halva), peanut butter, and chocolate, have been recently linked to numerous foodborne illness outbreaks and recalls. This review discusses the ingredients used and processing of OL aw products with a view to provide greater understanding of the routes of their contamination with foodborne pathogens and factors influencing pathogen persistence in these foods. Adequate heat treatment during processing may eliminate bacterial pathogens from OL aw foods; however, post-processing contamination commonly occurs. Once these products are contaminated, their high fat and sugar content can enhance pathogen survival for long periods. The physiological basis and survival mechanisms used by pathogens in these products are comprehensively discussed here. Foodborne outbreaks and recalls linked to OL aw foods are summarized and it was observed that serotypes of Salmonella enterica were the predominant pathogens causing illnesses. Further, intervention strategies available to control foodborne pathogens such as thermal inactivation, use of natural antimicrobials, irradiation and hydrostatic pressure are assessed for their usefulness to achieve pathogen control and enhance the safety of OL aw foods. Sanitation, hygienic design of manufacturing facilities, good hygienic practices, and environmental monitoring of OL aw food industries were also discussed.

Salmonella contamination of low-water activity (aw) foods poses a serious concern worldwide. The present study was conducted to assess the effects of drying conditions, food composition, and water activity on the desiccation tolerance and thermal resistance of S. Enteritidis FUA1946, S. Senftenberg ATCC43845 and S. Typhimurium ATCC13311 in pet food, binder formulation, and skim milk powder. The samples were wet inoculated with the individual Salmonella strains and were equilibrated to aw 0.33 and 0.75, followed by an isothermal treatment at 70 °C. The thermal inactivation data was fitted to the Weibull model. Irrespective of the aw, food composition and physical structure of the selected foods, strain S. Enteritidis FUA1946 displayed the highest desiccation and thermal resistance, followed by S. Senftenberg ATCC43845 and S. Typhimurium ATCC13311. The food matrix and strain type significantly (p < 0.05) influenced the thermal resistance of microorganisms in foods along with aw change during thermal treatments. To further study the effect of food composition, an additional set of experiments using dry inoculation of the resistant Salmonella strain in the low-aw foods was designed. Significant (p < 0.05) matrix-dependent interaction on Salmonella reduction was observed. The water adsorption isotherms of selected low-aw foods were measured at 20 and 70 °C to relate the thermal inactivation kinetics with the change in the aw. The characterization of thermal resistance of the Salmonella serovars in low-aw products with different compositions and aw in this study may be used for the validation of thermal challenge studies.

A predictive model was made for the logarithm of the thermal decimal reduction time (logD) of Salmonella enterica (D = time to 90% reduction by inactivation). The model was fitted with multiple linear regression from 521 logD-values reported in literature for laboratory media and foods highly varying in water activity and pH. The single regression model with temperature as the only variable had a high residual standard error (RSE) of 0.883 logD and no predictive value (fraction of variance explained (R²) < 0.001). Adding water activity, sugar content and pH as predictors resulted in a model with a lower RSE of 0.458 logD and an adjusted R² of 0.73. The model was validated by comparing 985 predicted with observed logD for S. enterica from other publications. The model was subsequently validated with 1498 published logD-values for inactivation of vegetative cells of nine other pathogenic bacteria genera (mainly Listeria monocytogenes, Escherichia coli, Clostridium perfringens, Cronobacter spp., Staphylococcus aureus, Yersinia enterocolitica) in or on a variety of laboratory media, meat, fish, dairy, nuts, fruits and vegetables. Regression analyses for validation with the 985 logD of S. enterica and 2483 logD of all genera show deviations from the expected slope of 1 (both 0.81) and the expected intercept of 0 (0.04 and 0.19 logD respectively). However, only 0.7% and 2% respectively of the new logD (expected: 0.5%) were observed above the 99% prediction interval of the original S. enterica model based on 521 logD. The findings suggest that i) the variability of thermal resistance of strains within species is larger than between genera and species; ii) one generic predictive model, also accounting for variability, suffices for designing the thermal inactivation of a variety of vegetative pathogenic bacteria in many food types.

The objective of the study was to evaluate the combined effects of pasteurization intensity (no heat treatment and 10min at 70, 80 and 90°C), water activity (aw) (0.960–0.990), pH (5.5–7.0) and storage temperature (7 and 10°C) on the survival and outgrowth of psychrotolerant spores of Bacillus cereus FF119b and Bacillus pumilus FF128a. The experiments were performed in both artificial media and a validation was performed on real food products (cream, béchamel sauce and mixed vegetable soup). It was determined that in general, heat treatments of 10min at 70°C or 80°C activated the spores of both B. cereus FF119b and B. pumilus FF128a, resulting in faster outgrowth compared to native (non-heat treated) spores. A pasteurization treatment of 10min at 90°C generally resulted in the longest lag periods before outgrowth of both isolates. Some of the spores were inactivated by this heat treatment, with more inactivation being observed the lower the pH value of the heating medium. Despite this, it was also observed that under some conditions the remaining (surviving) spores were actually activated as their outgrowth took place after a shorter period of time compared to native non-heated spores. While the response of B. cereus FF119b to the pasteurization intensity in cream and béchamel sauce was similar to the trends observed in the artificial media at 10°C, in difference, outgrowth was only observed at 7°C in both products when the spores had been heated for 10min at 80°C. Moreover, no inactivation was observed in cream or béchamel sauce when the spores were heated for 10min at 90°C in these two products. This was attributed to the protective effect of fat in the cream and the ingredients in the béchamel sauce. The study provides some insight into the potential microbial (stability and safety) consequences of the current trend towards milder heat treatments which is being pursued in the food industry.