P3-55 Evaluating the Recovery of Salmonella from Enriched Inshell Walnuts

Wednesday, July 12, 2017
Exhibit Hall (Tampa Convention Center)
Adam Cobert , University of California-Davis , Davis , CA
Linda J. Harris , University of California-Davis , Davis , CA
Introduction: The prevalence of Salmonella on in shell walnuts, as determined by enrichment in buffered peptone water (BPW) and detection with the mini-VIDAS system, was significantly lower than reported for other tree nuts.

Purpose: The objective of the study was to evaluate the impact of in shell walnuts in the recovery of inoculated Salmonella using enrichment and the mini-VIDAS detection method.

Methods: Inshell walnuts were collected from three collaborating hullers shortly after harvest in two separate years (n=6 lots). Approximately 50 CFU of rifampicin-resistant Salmonella Enteritidis PT 30 was inoculated into 450 ml of BPW with and without three inshell walnuts (~45 g), and was incubated at 42°C for 18 or 24 h. Samples were spiral-plated onto selective agar to determine levels of Salmonella and, in some cases, processed with the mini-VIDAS system.

Results:  The limit of detection (LOD) for Salmonella by the mini-VIDAS was five or six log CFU/ml in the absence or presence of walnuts, respectively. After incubation, background population levels of uninoculated walnuts consistently reached greater than eight log CFU/ml. Levels of Salmonella in BPW controls without walnuts were consistently greater than seven log CFU/ml. When walnuts were present, levels of Salmonella ranged from below the LOD of plating (0.6 log CFU/ml) to >7 log CFU/ml after 18 to 24 h at 42°C. Significant sample, huller, and year effects were observed. Four of 87 samples were below the LOD by plating and an additional 12 of the 87 samples were below the LOD for the mini-VIDAS (<6 log CFU/ml).

Significance: The presence of inshell walnuts can inhibit the growth of Salmonella during enrichment, which could lead to an inability to detect Salmonella by methods that rely on high concentrations of the target pathogen.