Survival of Salmonella during Production of Partially Sprouted Pumpkin, Sunflower, and Chia Seeds Dried for Direct Consumption

U.S. Consumer Practices of Homemade Nut-based Dairy Analogs and Soaked Nuts

Tree nuts, a low-moisture food, are typically perceived as being a low risk for foodborne illness. In the past five decades, the consumption of tree nuts (dry, soaked, or as nut-based dairy analogs [NBDA]) has increased along with corresponding foodborne illness outbreaks and recalls associated with these products. We developed an online survey to assess tree nut handling practices of U.S. consumers, and to select study participants who have soaked tree nuts and/or made NBDA at home. We distributed our initial survey questions in October 2021 to a convenience sample (n = 12) to test for clarity and comprehension. In January 2022, participants (n = 981) who met the criteria completed the survey. The most popular soaked tree nuts were almonds (54%), followed by cashews (36%), walnuts (32%), and pistachios (22%). Participants soaked tree nuts for direct consumption (67%) and during the preparation of NBDA (80%). Participants soaked tree nuts under refrigerated conditions for 1-24 h (22%), on the countertop at room temperature (est. 65-75°F [18-24°C]) for 1-5 h (21%), or at room temperature for 12 h or more (6%); 16% used a hot or boiling water, short time treatment. Some participants added acid (28%) or salt (25%) to the soaking water. Among those participants who dried their tree nuts after soaking (63%), 89% reported drying at a temperature lower than 46°C (115°F). Some participants (34%) used their tree nuts to make fermented dairy analogs (e.g., "cheese" or "yogurt") by adding "probiotics" (56-86%) or a yogurt starter culture (37-99%), respectively, and then, most frequently, holding at or below 20°C (68°F) for 12 h or less (29%). The safety of many of these practices has not been adequately investigated, but the findings of this study will inform future risk assessment and risk modeling studies on tree nut food safety in home kitchen settings.

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 subsequent 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°C for up to 24 h, and then dried at 64°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 walnut 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°C or after 12 h at 22°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°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°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 ambient temperatures.

Thermal Inactivation Kinetics of Salmonella and Enterococcus faecium NRRL-B2354 on Whole Chia Seeds (Salvia hispanica L.)

ABSTRACT

Intervention technologies for inactivating Salmonella on whole chia seeds are currently limited. Determination of the thermal inactivation kinetics of Salmonella on chia seeds and selection of an appropriate nonpathogenic surrogate will provide a foundation for selecting and optimizing thermal pasteurization processes for chia seeds. In this study, chia seed samples from three separate production lots were inoculated with a five-strain Salmonella cocktail or Enterococcus faecium NRRL-B2354 and equilibrated to a water activity of 0.53 at room temperature (25°C). After equilibration for at least 3 days, the inoculated seeds were subjected to isothermal treatments at 80, 85, or 90°C. Samples were removed at six time points, and surviving bacteria were enumerated. Whole chia seeds were diluted in a filter bag at 1:30 because bacterial recovery with this method was similar to that obtained from ground seeds. Survivor data were fitted to consolidated models: one primary model (log linear or Weibull) and one secondary model (Bigelow). E. faecium had higher thermal resistance than did Salmonella, suggesting that E. faecium may be a suitable conservative nonpathogenic surrogate for Salmonella. The Weibull model was a better fit for the survivor data than was the log-linear model for both bacteria based on the lower root mean square error and corrected Akaike's information criterion values. Lipid oxidation measurements and fatty acid concentrations were significantly different from those of the control samples, but the overall magnitude of the differences was relatively small. The thermal inactivation kinetics of Salmonella and E. faecium on chia seeds may be used as a basis for developing thermal pasteurization processes for chia seeds.

HIGHLIGHTS

Complete Genome Sequences of Five Salmonella enterica Strains Used in Inoculation Cocktails in Low-Moisture Food Storage Studies

Survival kinetics of Salmonella enterica have been previously studied using an inoculum cocktail composed of different strains that have been associated with low-moisture foods. Here, we report the closed genome sequences of five strains of Salmonella enterica that are commonly used in these storage studies.