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

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 (a) 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 a on bacterial resistance during steam treatments. Almond kernels were inoculated with ~8 log CFU/g of either E. faecium or L. innocua and equilibrated to a 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 log CFU/g reductions for L. innocua and 1.0-2.0 log CFU/g reductions for E. faecium when the surface temperature of almonds reached 100-130 °C, depending on the a of the almonds. The obtained D-values were 2.0-16.6 s, and D-values were 4.0-21.8 s, depending on the a of almonds. In general, elevating steam temperatures and almond a 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 a. The z-values were 36.6 °C and 35.7 °C, while z-values were 48.9 °C and 42.7 °C in almonds with a 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.