Purpose: The objective of this study was to obtain dry inocula of Salmonella Tennessee and Enterococcus faecium and compare their stability over time in terms of survival and thermal resistance.
Methods: Two methods of cell growth were used: (1) cells harvested from lawns on tryptic soy agar (TSA-cells), and (2) cells from tryptic soy broth (TSB-cells), for dry inoculum preparation. Concentrated cultures of each organism were inoculated onto talc powder, followed by incubation at 35°C for 24h, and held for additional 24h at room temperature (RT ~25°C) for complete dryness before sieving. The recovery of dry inoculum was monitored on tryptic soy agar. Heat resistance at 85°C of the dry inoculum on talc inoculated in a model peanut paste (50% fat and aw= 0.6) was determined for up to 30 days.
Results: The dry inoculum on talc for both cell-growth methods was stable in terms of survival and thermal resistance over 30 days. In general, the dry inoculum on talc resulted in a higher cell concentration when TSA grown-cells were used, compared to TSB. However, the TSB grown cells had a significantly (P ˂ 0.05) greater heat resistance than TSA grown-cells (e.g., E. faecium TSB-cells D85°C = 3.43 min versus 2.68 for TSA-cells). The surrogate had consistently greater heat resistance than Salmonella Tennessee (e.g., TSB-cells D85°C = 1.05 min), regardless of what cell-type was used for dry inoculum preparation.
Significance: A dry inoculum of the surrogate for Salmonella appears to be more relevant and easier to use for process validation in low-moisture foods.