Impacts of water activity on survival of Listeria innocua and Enterococcus faecium NRRL B-2354 in almonds during steam treatments.

ABSTRACT

Raw almonds have been associated with Salmonella outbreaks and multiple recalls related to Listeria monocytogenes contamination. While steam treatment has been approved for pasteurizing both conventional and organic whole almonds, there is limited understanding of how water activity (aw) influences the effectiveness of steam treatments in decontaminating almonds. Hence, this study aimed to assess and compare the efficacy of steam treatments against Listeria innocua and Enterococcus faecium NRRL B-2354, the known non-pathogenic surrogates, on almonds. It also sought to investigate the impact of almond's aw on bacterial resistance during steam treatments. Almond kernels were inoculated with ~8 log10 CFU/g of either E. faecium or L. innocua and equilibrated to aw 0.25 or 0.45 before being subjected to steam treatments at temperatures of 100-135 °C. Our results revealed that L. innocua exhibited lower resistance to steam compared to E. faecium, with 1.2-2.6 log10 CFU/g reductions for L. innocua and 1.0-2.0 log10 CFU/g reductions for E. faecium when the surface temperature of almonds reached 100-130 °C, depending on the aw of the almonds. The obtained DL. innocua, 100-130°C-values were 2.0-16.6 s, and DE. faecium, 100-130°C-values were 4.0-21.8 s, depending on the aw of almonds. In general, elevating steam temperatures and almond aw decreased the tolerance of L. innocua and E. faecium during steam inactivation. In addition, the z-values indicated that E. faecium on almonds was less sensitive to change in steam temperature compared to L. innocua, especially at lower aw. The zL. innocua-values were 36.6 °C and 35.7 °C, while zE. faecium-values were 48.9 °C and 42.7 °C in almonds with aw 0.25 and 0.45, respectively. Results from this study suggest that steam treatments serve as effective interventions for controlling pathogen contaminations in raw almonds.