Evaluation of a Coinoculation Method for Salmonella enterica and Enterococcus faecium NRRL B-2354 on Whole and Broken Almonds Exposed to Dry Heat

Traditional laboratory-based pathogen-surrogate comparison studies often involve inoculating each organism onto separate samples, which can subsequently introduce variability due to differences in food microenvironments and processing conditions. Coinoculating pathogens and surrogates onto the same sample minimizes this variability by subjecting both microorganisms to identical conditions. This study evaluated a coinoculation method for comparing the thermal resistance of Salmonella enterica and Enterococcus faecium on whole and broken almonds exposed to 168 °C for up to 12 min. Selective media were validated for accurate differentiation and enumeration of coinoculated wild-type or rifampin-resistant Salmonella and E. faecium on almonds. The media reliably distinguished and quantified each organism, with no cross-recovery observed. Reductions in Salmonella were not significantly different between wild-type and rifampin-resistant strains, nor between CHROMagar Salmonella and tryptic soy agar supplemented with rifampin at 50 μg/mL (P > 0.05). E. faecium demonstrated similar or greater thermal resistance compared to coinoculated Salmonella, supporting its use as a surrogate during dry heat treatment of almonds. While no significant difference in Salmonella reduction was observed between whole and broken almonds, greater reductions of E. faecium were observed on broken almonds, suggesting that almond structure may influence microbial thermal responses in a species-dependent manner. The findings support the utility of coinoculation for laboratory-based pathogen-surrogate comparison studies.