Factors Affecting Bacterial Cross-contamination Using Salmonella and a Surrogate Organism during Almond Processing

Introduction:  Outbreaks and recalls associated with Salmonella in low water activity foods suggest a need for improved understanding of factors contributing to pathogen transfer and cross-contamination with these types of products, including water activity (a_w), bulk handling parameters, and mode of transfer.

Purpose:  The objective was to compare inoculation method and the use of Enterococcus faecium as a surrogate for Salmonella cross-contamination using almonds at different water activities and rotational speeds in a rotating drum.

Methods:  Almond kernels (200 g) were wet- and dry-inoculated with Salmonella Enteritidis PT30 or Enterococcus faecium.  The inoculated kernels (5 g, ~8 log CFU/g) and un-inoculated kernels (200 g) were conditioned to 0.2, 0.4, or 0.6 a_w and tumbled in a stainless steel drum (140 mm diameter, 64 mm depth) for the total number of rotations (TNR) of 5, 20, 40, and 80 rev (three rotational speed of 8, 16, and 24 rpm × durations (10-300 s)), in triplicate.  At each condition, samples (four kernels) were retrieved, stomached, plated on modified trypic soy agar, and incubated to enumerate transferred bacteria.

Results: Water activity significantly affected maximum transferred bacterial load (MTBL) for wet inoculation method (P < 0.001), but not for dry inoculation methodology.  For lower water activity levels (0.2 and 0.4 a_w), choice of inoculation methodology significantly affected MTBL (P < 0.001), but not at 0.6 a_w (P = 0.10). E. faecium may not be a surrogate for Salmonella at 0.4 a_w (P = 0.04), but needs further verification.  

Significance: Knowing how environmental/physical conditions influence bacterial transfer will enhance the accuracy of cross-contamination modeling, in terms of secondary models, and contribute to elucidating mechanism of cross-contamination in low-moisture foods.