Purpose: Our objective was to evaluate the fate of Salmonella, E. coli O157:H7 and L. monocytogenes on pecans and peanuts stored at -20, 4 and 23°C for up to 12 months, and to characterize Salmonella isolates remaining after 18 months of storage.
Methods: Raw pecans and peanuts were inoculated with cocktails of nalidixic acid-resistant strains of Salmonella, E. coli O157:H7 and L. monocytogenes, dried for 72 h, and stored at -20, 4, 23°C for 18 months. Microbial populations were enumerated over 12 months by stomaching 10-g subsamples and plating onto specific and non-specific media supplemented with nalidixic acid. After 18 months, 100 Salmonella colonies isolated from each storage temperature were characterized to determine the prevalence of each cocktail strain by a combination of serogrouping, antibiotic resistance, and PCR for strain-specific genes.
Results: At 23ºC, Salmonella, E. coli O157:H7 and L. monocytogenes declined linearly at rates of -0.20, -0.33, -0.54 log CFU/g/month on peanuts and -0.16, -0.33, -1.05 log CFU/g/month on pecans, respectively. At 4°C, population of Salmonella declined on peanuts at -0.039 log CFU/g/month and remained relatively stable on pecans. E. coli O157:H7 and L. monocytogenes populations remained stable at 4ºC storage on both nut types. At -20ºC, populations of the three pathogens remained stable on both nuts with the exception of L. monocytogenes which declined at -0.06 log CFU/g/month on peanuts. Salmonella Tennessee predominated on peanuts and pecans -20, 4 and 23°C, except on peanuts stored at 23°C storage where Salmonella Enteritidis 9C was predominantly isolated.
Significance: Pathogens, if present, can survive for extended periods of time at a broad range of storage temperatures on raw pecans and peanuts. Storage under refrigerated or freezing temperatures does not result in pathogen reduction.