Fate of foodborne pathogens during soaking and drying of walnuts

Walnuts are among the most popular tree nuts that are soaked at home. Recipes for preparing soaked walnut kernels from online blogs (n = 71) and YouTube videos (n = 29) were reviewed to identify typical consumer handling practices that were then used to determine the fate of foodborne pathogens during soaking and sub-sequent drying of walnut kernels. Individual five-strain cocktails of rifampin-resistant Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella, grown on agar plates and diluted in water, were inoculated onto walnuts and then dried. Inoculated walnuts were added to sterile water at a ratio of 1:4 (w/v), held at 15, 18, or 22 degrees C for up to 24 h, and then dried at 64 degrees C for up to 24 h (for Salmonella-inoculated walnuts). Pathogen populations during soaking and drying were enumerated on tryptic soy agar with rifampin and on CHROM agar. Initial wal-nut moisture was-4%, increased to-30% at 8 and 24 h of soaking and then decreased during drying to-4% at 6 h and <1% after 24 h. Initial E. coli, L. monocytogenes, and Salmonella populations were-1.0,-1.5, and 1.0-2.5 log CFU/g, respectively, after inoculation and drying. No significant (P > 0.05) increase in populations was observed after 24 h at 15 and 18 degrees C or after 12 h at 22 degrees C. Significant increases of 1.9-3.0, 1.2-2.1, and 1.8 log CFU/g for E. coli, L. monocytogenes, and Salmonella, respectively, were observed after 24 h of soaking at 22 degrees C. Growth rates of 0.19, 0.093, and 0.16 log CFU/sample per h, respectively, were observed. Lag times of 8.8 and 11 h at 22 degrees C were determined for E. coli and Salmonella, respectively. Populations of Salmonella declined by 1.04 log CFU/g over 12 h of drying; further significant (P < 0.05) decreases were not observed at 24 h. To limit food safety risks in soaked walnuts, educational materials should emphasize sourcing treated walnuts, kitchen sanitation, hygiene measures, and soaking at cooler temperatures or for shorter times at ambi-ent temperatures.

Automated summary

By reviewing recipes from online blogs and YouTube videos, typical consumer handling practices for soaked walnut kernels were identified. The fate of foodborne pathogens during soaking and subsequent drying of walnut kernels was determined. The study found that the populations of E. coli, Listeria monocytogenes, and Salmonella increased significantly after 24 hours of soaking at 22 degrees Celsius. To reduce food safety risks, consumer education should emphasize sourcing treated walnuts, kitchen sanitation, hygiene measures, and shorter soaking times or soaking at cooler temperatures.