Development of a Dry Inoculation Method for Thermal Challenge Studies in Low-Moisture Foods by Using Talc as a Carrier for Salmonella and a Surrogate (Enterococcus faecium)

Journal of Food Protection. 2015;78(6):1106-1112

Abstract: The objective of this study was to obtain dry inocula of Salmonella Tennessee and Enterococcus faecium, a surrogate for thermal inactivation of Salmonella in low-moisture foods, and to compare their thermal resistance and stability over time in terms of survival. Two methods of cell growth were compared: cells harvested from a lawn on tryptic soy agar (TSA-cells) and from tryptic soy broth (TSB-cells). Concentrated cultures of each organism were inoculated onto talc powder, incubated at 35uC for 24 h, and dried for additional 24 h at room temperature (23 ¡ 2uC) to achieve a final water activity of #0.55 before sieving. Cell reductions of Salmonella and E. faecium during the drying process were between 0.14 and 0.96 log CFU/g, depending on growth method used. There was no difference between microbial counts at days 1 and 30. Heat resistance of the dry inoculum on talc inoculated into a model peanut paste (50% fat and 0.6 water activity) was determined after 1 and 30 days of preparation, using thermal death time tests conducted at 85uC. For Salmonella, there was no significant difference between the thermal resistance (D85uC) for the TSB-cells and TSA-cells (e.g. day 1 cells D85uC ~ 1.05 and 1.07 min, respectively), and there was no significant difference in D85uC between dry inocula on talc used either 1 or 30 days after preparation (P . 0.05). However, the use the dry inocula of E. faecium yielded different results: the TSB-grown cells had a significantly (P , 0.05) greater heat resistance than TSA-grown cells (e.g. D85uC for TSB-cells ~ 3.42 min versus 2.60 min for TSA-cells). E. faecium had significantly (P , 0.05) greater heat resistance than Salmonella Tennessee regardless what cell type was used for dry inoculum preparation; therefore, it proved to be a conservative but appropriate surrogate for thermal inactivation of Salmonella in low-moisture food matrices under the tested conditions.

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This work was supported by the ILSI North America Committee on Food Microbiology.