T1-01 Thermal Tolerance of Foodborne Pathogens on Inoculated Pistachios

Monday, July 10, 2017: 8:30 AM
Room 15 (Tampa Convention Center)
Mahta Moussavi , University of California-Davis , Davis , CA
Christopher Theofel , University of California-Davis , Davis , CA
Linda J. Harris , University of California-Davis , Davis , CA
Introduction:  Several recent outbreaks and recalls have been associated with pistachios contaminated with Salmonella. Thermal treatments are the main strategy used by the pistachio industry to control this organism.

Purpose: This study identified an appropriate target pathogen and surrogate organism for validation of thermal processes applied to pistachios.

Methods: Inshell pistachios were inoculated at nine log CFU/g with nonpathogenic strain Enterococcus faecium NRRL B-2354, Salmonella Enteritidis PT 30, or relevant individual strains (five each) of Salmonella spp., Escherichia coli O157:H7, or Listeria monocytogenes; pistachios were dried for 72 h, and equilibrated to a mean moisture content of 3.6±0.28% (water activity 0.37±0.03). Thermal tolerance of each strain was compared by exposing inoculated pistachios (n=3 to 9) to hot oil (121°C/1 min), hot water (80°C/1 min), or dry heat (138°C/15 min). Survivor curves in hot oil or hot water (0.5 to 6 min, n=6) were developed for the most resistant strains. Survivors were enumerated by plating on tryptic soy agar and appropriate selective media.

Results: In most cases, after exposure to heat treatments, reductions in populations of individual strains of Salmonella spp., E. coli O157:H7, or L. monocytogenes were significantly greater (P<0.05) or not significantly different than reductions of E. faecium and Salmonella Enteritidis. Survivor curves for the more resistant E. coli O157:H7 and L. monocytogenes strains (isolated from walnuts and onions, respectively) in hot water were not significantly different than those for E. faecium and Salmonella Enteritidis. In hot oil, significantly greater reductions of E. coli and L. monocytogenes were observed after three or two minutes of exposure, respectively.

Significance:  These data provided further evidence that Salmonella spp. and E. faecium NRRL B-2354 are the appropriate target pathogen and an appropriate surrogate, respectively, for validating pistachio thermal processes.