Purpose: The inactivation of three potential surrogate bacteria was compared to that of Salmonella, after subjection to propylene oxide processing, in accordance with the United States Environmental Protection Agency label instructions.
Methods: Macadamia nuts and cashews were inoculated with a cocktail of TSA-grown Salmonella enterica strains and one of three potential surrogates; Enterococcus faecium, Pediococcus acidilactici, or Staphylococcus carnosus. Samples were dried, packaged in polywoven polypropylene bags, and placed within uninoculated nuts. PPO treatments were performed by Cosmed, using their proprietary process. Samples were returned for enumeration by plating onto appropriate selective media and TSA overlaid with XLT-4. Processing was completed three times, with six samples per process. The mean log CFU/g reductions of Salmonella and each potential surrogate, within a sample and amongst all trials, were compared using a paired T-test; P<0.05 was considered significant.
Results: PPO processing resulted in significant differences in the mean reduction log CFU/g of S. enterica (7.3±0.19) and E. faecium (6.4±0.31) and P. acidilacti (6.3±0.33) on whole macadamia nuts. On cashews the mean reduction log CFU/g of S. enterica (5.2±0.22) was not significantly different from that of E. faecium (5.2±0.25) after PPO processing, however reduction of P. acidilacti (4.1±0.25) was significantly different on cashews. Reduction of S. carnosus was greater than that of S.enterica.
Significance: Reductions of the target bacteria were significantly different for the two nut types, despite processing at the same time under the same parameters. This indicates the importance of comparing the efficacy of surrogate bacteria for the food of interest.