Survival of Listeria monocytogenes, Bacillus cereus and Salmonella Typhimurium on sliced mushrooms during drying in a household food dehydrator

The historic view on low-moisture foods (LMFs) as safe due to the lack of microbial growth in these foods is challenged by an increasing number of reports of outbreaks and recalls caused by LMFs contaminated with foodborne pathogens. The objective of this study was to determine the survival of Salmonella Typhimurium, Bacillus cereus and Listeria monocytogenes on sliced Portobello mushrooms (Agaricus bisporus variant Portobello) during hot-air drying (mushroom internal temperature below 45 °C) for 8 h (h) in a small household food dehydrator (250 W) and subsequent storage of the vacuum-packed dried product for 2 months at room temperature. Hot-air drying reduced the water activity (a_w) of the mushrooms to 0.17 ± 0.03 well below the limit for microbial growth. S. Typhimurium and L. monocytogenes displayed total log CFU reductions of 2.5 ± 0.4 and 2.6 ± 0.8, respectively, while B. cereus exhibited significantly (p < 0.05) lower log reductions of 1.2 ± 0.1. Storage of vacuum-packed dried mushrooms further reduced L. monocytogenes by 2 log CFU, while numbers of viable S. Typhimurium and B. cereus were not further reduced. The higher stability of S. Typhimurium and B. cereus were reflected in the number of reports in the European Rapid Alert System for Food and Feed system of the presence of these organisms in dried mushrooms. All three organisms regrew to high concentrations when dried mushrooms were soaked overnight at room temperature, simulating a scenario where mushrooms are improperly rehydrated. Combining results from hot-air drying and subsequent storage underlines that hot-air drying and prolonged storage at low aw cannot be relied on alone to reduce the microbial and pathogen load on Portobello mushroom.