Background: Foodborne pathogen contamination in low water activity (aw) foods is a critical problem for both the food industry and public health. Low-moisture foods and food ingredients have been implicated in numerous Salmonella outbreaks and are increasingly involved in the recall of products with possible contamination by Listeria monocytogenes. L. monocytogenes is singled out as a true environmental species and a pathogenic bacterium of concern in ready-to-eat foods including low-moisture foods. L. monocytogenes can survive in a dry environment or in low-moisture foods for an extended period with increased stability at lower storage temperatures. Adaptation to low-moisture foods enables L. monocytogenes to exhibit enhanced thermal resistance. However, compared with Salmonella in low-moisture foods, less information is available about the fate of L. monocytogenes in low-moisture foods during thermal inactivation and the factors influencing its thermotolerance. Scope and approach: This review summarizes pertinent literature on the desiccation and thermal resistance of L. monocytogenes in low-moisture foods, discusses factors impacting the desiccation and thermal stability, and compares desiccation and thermal stability of L. monocytogenes with Salmonella in respective low-moisture foods. The possible mechanisms underlying the stability of L. monocytogenes in low-moisture foods are also discussed. Key findings and conclusions: L. monocytogenes can survive in low-moisture foods for an extended duration and is highly thermoresistant. Thermal resistance of L. monocytogenes has an inverse relationship with aw, and is dependent on the food matrix, and other factors.

Almond products have been implicated in multiple outbreaks associated with Salmonella. In this study, the survival of Salmonella in almond meal under different water activity (aw) was evaluated over one-year storage at 4 and 22 ?C. The impacts of aw during heat treatment on thermal tolerance of Salmonella and Enterococcus faecium NRRL B-2354 in almond meal were further assessed prior to and after one-year storage. Almond meal was inoculated with E. faecium or a three-strain Salmonella cocktail and equilibrated to aw 0.25 or aw 0.45 at 22 ?C. The inoculated almond meal samples were subjected to isothermal treatments before or after one-year storage. Salmonella remained stable in almond meal for 1 year at 4 ?C regardless of aw. Salmonella count in aw 0.25 and aw 0.45 almond meal declined by 0.8 or 1.5 log10 CFU/g in one-year storage at 22 ?C. Under all the test conditions using either thermal death time (TDT) or thermal water activity (TWA) cells, the inactivation kinetics of Salmonella and E. faecium in almond meal fitted the log-linear model well; thermal tolerance of both bacteria in almond meal was inversely related to aw of almond meal. The D-values of Salmonella in almond meal of aw 0.25 obtained using TDT cells were 49.6, 18.0, and 8.5 min at 80, 85 and 90 ?C, respectively, but were 27.5, 13.7, and 7.0 min in the sample with aw 0.45 under the same temperatures. The D-values of Salmonella at 85?95 ?C in aw 0.25 almond meal obtained using TWA cells in which aw was relatively stable during heating were 2.0?2.4 times of those determined using TDT cells where aw was subjected to change during heating. D-values of E. faecium in almond meal obtained by TDT and TWA cells were 1.3?1.5 and 1.6?1.8 times of Salmonella, respectively, indicating that E. faecium was a suitable surrogate for Salmonella during thermal pasteurization of almond meal under constant moisture or aw. Furthermore, the thermal resistance of Salmonella in almond meal was not affected by one-year storage. The thermal resistance data provide useful information for the food industry in designing thermal pasteurization processes for almond meal and ensuring the safety of almond products.

Listeria monocytogenes can survive in dry conditions for long periods. Despite an increasing research studying Salmonella inactivation in low-moisture foods, there is a general lack of knowledge related to L. monocytogenes inactivation in low-moisture foods during thermal processing and the factors impacting their survival in these products. Cocoa powder is an essential and widely incorporated ingredient in many desserts and drinks that do not need thermal processing. This study evaluated the thermal resistance of L monocytogenes in cocoa powder and investigated the impact of water activity (a(w)) on its survival in cocoa powder. Natural unsweetened cocoa powder was inoculated with a 3-strain L. monocytogenes cocktail (similar to 9.0 Log(10) CFU/g), equilibrated to a(w) 0.30, 0.45 or 0.60 at 22 degrees C and subjected to isothermal treatments. Survivors were enumerated to obtain thermal inactivation parameters. L. monocytogenes population was stable in cocoa powder (a(w) 0.30) over the first month of storage, then decreased gradually but remained detectable after 12-month storage at 22 degrees C. Thermal inactivation of L. monocytogenes in cocoa powder at target a(w) and different temperatures showed a log-linear trend. Heat resistance of L. monocytogenes is a(w)-dependent with the highest resistance at a(w) 0.30. The range of D-values (in min) at 70, 75 and 80 degrees C at a(w) 0.30. and 0.45, respectively, were: 21.9-5.0 and 7.3-1.8. The range of D-values (in min) at 65, 70 and 75 degrees C at a(w) 0.60 was 9.1-2.0. The z-value at a 0.30, 0.45, and 0.60 was 15.5, 15.9, and 14.9 degrees C, respectively. In summary, L. monocytogenes can survive in cocoa powder stored at 22 degrees C for an extended time. Thermal resistance of L. monocytogenes adapted to low a(w) cocoa was conversely related to a(w). This study provides valuable information for the food industry to develop thermal inactivation strategies to control L. monocytogenes in cocoa powder.

This study evaluated the fate of Listeria innocua, a non-pathogenic species closely related to Listeria monocytogenes, on Fuji apple fruit surfaces during commercial cold storage with and without continuous low doses of gaseous ozone. Unwaxed Fuji apples of commercially acceptable maturity were inoculated with 6.0-7.0 Log(10) CFU L. innocua/apple, and subjected to refrigerated air (RA, 33 degrees F), controlled atmosphere (CA, 33 degrees F, 2% O-2, 1% CO2), or CA with low doses of ozone gas (50.0-87.0 ppb) storage in a commercial facility for 30 weeks. A set of uninoculated apples was simultaneously subjected to the above storage conditions for total plate count and yeasts and molds enumeration. L. innocua survival under RA and CA storage was similar, which led to 2.5-3.0 Log(10) CFU/apple reduction during storage. Continuous gaseous ozone application decreased L. innocua population on Fuji apples to similar to 1.0 Log(10) CFU/apple after 30-week storage, and suppressed apple native flora. CA storage delayed apple fruit ripening through reduction of apple firmness and titratable acidity loss, and low dose gaseous ozone application had no negative influence on apple visual quality, including both external and internal disorders. In summary, L. innocua decreased on Fuji apple surfaces during commercial long-term RA and CA storage. Ozone gas has the potential to be used as a supplemental intervention method to control Listeria spp. and to ensure fresh apple safety. (c) 2018 Elsevier Ltd. All rights reserved.

Foodborne pathogens including Salmonella have been implicated in recent recalls of low-water activity (a(W)) foods, such as peanut butter, almond flour, wheat, flour and dry milk powder, and are primary concerns for the microbiological safety of dry food products. Although there are an increasing number of studies on Salmonella thermal resistance conducted in low-moisture foods, little information is available on Listeria monocytogenes thermal resistance in those products. This study evaluated the survival of L monocytogenes in wheat flour during long-term storage as well as its thermal resistance in wheat flour equilibrated to a(W) 0.30, 0.45, and 0.60. L monocytogenes survived in wheat flour at both a(W) 0.31 and 0.56 during 6 months of storage at room temperature, with populations decreasing about 2.52 and 6.27 logs at a, 0.31 and 0.56, respectively. Equilibration in low-a(W) flour enabled L monocytogenes to become more resistant to thermal treatment. At treatment temperature between 70 and 80 degrees C, D-values increased with decreasing a(W). For a(W) 0.30. 0.45, and 0.60 (measured at room temperature), respectively, D-value (in min) ranges for 70-80 degrees C were 37.10-7.08,17.44-3.13, and 16.85-1.59. The z-values were 12.9,14.2, and 9.9 degrees C for a, 0.30, 0.45, and 0.60, respectively. These data highlight the need for vigilance when processing dry foods, and provide valuable information for the industry to validate thermal processing for control of L monocytogenes in low-moisture foods. This study also offers insight into the development of thermal inactivation strategies to control L monocytogenes and other foodborne pathogens in foods with similar matrices. (C) 2018 Elsevier Ltd. All rights reserved.