Desiccation and thermal resistance of Salmonella and Enterococcus faecium NRRL B-2354 in almond meal as impacted by water activity and storage temperature

Abstract

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.