Prevalence and levels of Shiga toxin-producing Escherichia coli and Salmonella in California raw almond kernels from the 2021 harvest compared with data from harvests between 2001-2013.

Abstract

The prevalence and levels of pathogens in foods are critical inputs for quantitative microbial risk assessment. This study aimed to determine the prevalence, levels, and distribution of Escherichia coli O157:H7, non-O157 Shiga toxin-producing (STEC), and Salmonella on raw almond kernels from the 2021 California harvest, and to compare the results with data from 2001-2013. During the 2021 harvest, 1-2 kg samples were collected at processing facilities from 601 incoming single-variety almond lots representing 15 varieties. Each lot was tested for presence of E. coli O157:H7 and STEC using 375-g subsamples and for Salmonella using 100-g subsamples. Aerobic plate count (APC) and levels of coliforms, Enterobacteriaceae, yeasts, and molds were determined for a subset of positive and negative lots. Salmonella levels in positive lots were estimated using a modified three-tube most-probable-number (MPN) method with additional 25- and 50-g subsamples processed through sample depletion. A subset of Salmonella-negative lots was also retested using multiple 100-g subsample enrichments. None of the 601 lots was positive for either E. coli O157:H7 or STEC. Salmonella was isolated from 18 lots (3.0% [95% CI, 1.8 to 4.7%]), a higher prevalence than the 9-year average from earlier surveys (0.98% [95% CI, 0.83 to 1.2%]). Estimated Salmonella levels in 2021 ranged from 0.0016 to 0.011 MPN/g (mean 0.0034 ± 0.0023 MPN/g), consistent with 2001-2013 levels (mean 0.012 ± 0.017 MPN/g; range, 0.00020-0.15 MPN/g). Whole-genome sequencing of 2021 isolates identified Salmonella enterica subspecies arizonae, diarizonae, and enterica, including nine Salmonella enterica subspecies enterica serovars. No significant differences (P > 0.05) in APC or levels of yeasts, molds, coliforms, or Enterobacteriaceae were observed between Salmonella-positive and -negative lots in 2021, consistent with findings from earlier surveys. Salmonella detection was not associated with almond variety.