P3-81 Survival of Salmonella, Escherichia coli O157:H7 and Listeria monocytogenes on Inoculated Inshell Chandler Variety Walnuts

Wednesday, July 31, 2013
Exhibit Hall (Charlotte Convention Center)
John Frelka, University of California-Davis, Davis, CA
Linda Harris, University of California-Davis, Davis, CA
Introduction: After harvest, hulling and dehydration, walnuts are stored in-the-shell for up to 1 year prior to cracking.

Purpose: Our objective was to determine the survival of Salmonella, Escherichia coli O157:H7 and Listeria monocytogenes on inoculated inshell walnuts under conditions that mimic commercial storage. 

Methods: Inshell walnuts were collected from a commercial huller prior to drying and inoculated with five-strain cocktails of rifampicin-resistant Salmonella, E. coli O157:H7 or L. monocytogenes suspended in walnut-rinse water. Walnuts (400 g) were separately inoculated at 8 log CFU/nut by mixing with 25 ml of each cocktail. Inoculated nuts were dried at 43°C and 35% relative humidity (RH) to mimic commercial practices, and stored using previously-determined commercial storage conditions (10°C, 65% RH). Samples from each of two separate trials (n=6) were processed at monthly intervals. Appropriate dilutions were plated onto tryptic soy (TSA) and either bismuth sulfite (Salmonella), sorbitol MacConkey (SMAC) (E. coli O157:H7) or modified Oxford agars (MOX) (L. monocytogenes), all supplemented with rifampicin (Rif). All media was incubated at 37°C for 24 h (Salmonella and E. coli O157:H7) or 48 h (L. monocytogenes)

Results: Population densities of all pathogens decreased by 3 log CFU/nut during the 10- to 12-h drying period. During storage, Salmonella declined by a consistent 1 log CFU/nut/month. In contrast, E. coli and L. monocytogenes populations decreased rapidly within the first 8 and 27 days of storage, to 3 and 2 log CFU/nut, respectively; thereafter, population densities declined very slowly as measured on TSARif. Counts on SMACRif and MOXRif were significantly lower indicating the presence of injured cells. 

Significance: Significant decreases in foodborne pathogens are observed during drying and initial storage of inshell walnuts but low-level long-term persistence can occur.