Heat Resistance of Salmonella spp., L. monocytogenes, E. coli O157:H7 and E. faecium on Almonds, Peanuts, Cashews, and Macadamia Nuts

Introduction: Process validation aimed at addressing Salmonella spp. in nuts is an essential part of any food safety plan. Generally, surrogate microorganisms are used during in-plant validations.  Enterococcus faecium has been determined to be a good surrogate for Salmonella spp. on almonds. However, very little data is available to demonstrate that E. faecium is an appropriate surrogate for Salmonella spp. on other types of nuts.  Additionally, it has not been thoroughly investigated if E. faecium is an appropriate surrogate for Listeria monocytogenes and Escherichia coli O157:H7.

Purpose: The purpose of this study was to compare heat resistance of Salmonella spp., L. monocytogenes, E. coli O157:H7, and E. faecium on multiple nut types.

Methods: Nuts were evaluated for heat resistance according to the Almond Board of California’s “Guidelines for using Enterococcus faecium NRRL B-2354 as a Surrogate Microorganism in Almond Process Validation.”

Results: Heat resistance of Salmonella spp. on almonds, peanuts, cashews, and macadamia nuts showed a 2.88±0.12, 2.57±0.11, 2.75±0.13, and 1.18±0.08 log reduction, respectively. Heat resistance of E. coli O157:H7 on almonds, peanuts, cashews, and macadamia nuts showed a 6.76±0.80, 7.40±0.00, 7.77±0.52, and 5.05±0.17 log reduction, respectively. Heat resistance of L. monocytogenes on almonds, peanuts, cashews, and macadamia nuts showed a 7.13±0.24, 6.01±0.17, 6.85±0.34, and 6.77±0.07 log reduction, respectively. Heat resistance of E. faecium on almonds, peanuts, cashews, and macadamia nuts showed a 1.58±0.28, 3.00±0.25, 1.76±0.09, and 0.68±0.05 log reduction, respectively. 

Significance: Heat resistance of pathogens is not uniform across different nut types, and the amount of kill delivered by a lethality process cannot be applied to all nut types.  E. faecium was shown to be a good surrogate for Salmonella spp. on almonds, peanuts, cashews, and macadamia nuts.  L. monocytogenes and E. coli O157:H7 showed significantly lower heat resistance than Salmonella spp. and E. faecium.