P2-113 Growth of Foodborne Pathogens on Inoculated Pistachios during Postharvest Handling

Tuesday, August 2, 2016
America's Center - St. Louis
Mahta Moussavi, University of California-Davis, Davis, CA
Vanessa Lieberman, University of California-Davis, Davis, CA
Chris Theofel, University of California-Davis, Davis, CA
Linda J. Harris, University of California-Davis, Davis, CA
Introduction: During harvest, pistachios are hulled and separated into streams based, in part, on nut density (sinker) and adhering hull (floater). Factors contributing to the observed higher prevalence of Salmonella in floater pistachios are unclear.

Purpose: To examine the behavior of Salmonella, Escherichia coli O157:H7, and Listeria monocytogenes on pistachios during simulated postharvest conditions.

Methods: In-hull and floater and sinker pistachios were collected from commercial processors. Samples were inoculated at 3 log CFU/g with cocktails of Salmonella, and in some cases E. coli O157:H7 or L. monocytogenes and incubated for up to 30 h under commercially-relevant conditions (37°C and 90% RH). Populations were measured by plating onto tryptic soy and selective agars.

Results: A 4- or 5-h lag and maximum populations of 7.2 (12 h) or 6.6 (8 h) log CFU/g were observed for Salmonella on early or late harvest in-hull pistachios, respectively. For hulled floaters and sinkers, no significant (P < 0.05) growth was observed in the first 3 h after inoculation. Thereafter, both the rate of growth and maximum populations differed between late and early harvest and among samples collected from different processors. Growth rates of Salmonella (0.38 and 0.24 log CFU/g/h, respectively) were significantly (P < 0.05) greater on floater compared to corresponding sinker pistachios and on floater pistachios with hull adhering to >25% of the shell surface. Maximum Salmonella populations were 2 log CFU/g greater on floaters than on sinkers. The growth of E. coli O157:H7 and Salmonella on hulled pistachios were similar but a longer lag (approx. 12 h) and significantly (P < 0.05) lower maximum population (4.3 log CFU/g) was observed for L. monocytogenes.

Significance: The growth of foodborne pathogens on pistachios during harvest can be managed by reducing the time between harvesting and hulling and from hulling to drying.