Behavior of Salmonella During Preparation of a Fermented Cashew Cheese Analog

Between 2013 and 2021, there were three reported salmonellosis outbreaks in North America linked to the consumption of cashew cheese analogs that were prepared from soaked and fermented cashews. The behavior of Salmonella was evaluated during fermentation of cashews to better understand the risks associated with plant-based fermentations. Single or seven-strain rifampin-resistant Salmonella-inoculated cashews (1-2 log CFU/g) were soaked 1:1 (w/v) in sterile ultrapure water at 4 °C for 24 ± 1 h, drained, and then blended with additional water. Salmonella-inoculated or uninoculated cashews with or without added commercial Lactococcus lactis starter culture (LAB), and with LAB and NaCl (0.8% and 1.6% w/w), citric acid (0.4% w/w), or a combination of NaCl and citric acid, were held at 24 ± 1 °C for up to 72 h. The pH, aerobic plate counts (M17 agar), and Salmonella populations (CHROMagar Salmonella with 50 µg/mL of rifampin) were measured at 0, 24, 48, and 72 h in replicate experiments. When LAB was present, aerobic plate counts increased from ∼8 log CFU/g to ∼9 log CFU/g after 24 h. The pH decreased from an initial pH ∼6 to pH 4.5-5.0 at 24 h in the presence of LAB or at 48 h in the absence of LAB. The presence of LAB significantly (P < 0.0001) impacted populations of Salmonella during the fermentation. There was no significant difference in Salmonella populations between the treatments with LAB alone and the treatments with LAB in combination with added NaCl (P = 0.3484) or citric acid (P = 0.8630). After 24 h, populations of Salmonella increased by 5.3-5.5 log in the absence of LAB and by 0.5-1.7 log in the presence of LAB, with or without added NaCl. These data demonstrate the need to consider a range of control measures for safe preparation of plant-based fermented products.

The effect of seed germination and Bacillus spp. on the ripening of plant cheese analogs

Consumer demand for plant cheeses is increasing, but challenges of improving both flavor and quality remain. This study investigated the microbiological and physicochemical impact of seed germination and fermentation with Bacillus velezensis and Bacillus amyloliquefaciens on the ripening of plant cheese analogs. Chlorine treatment or addition of Lactiplantibacillus plantarum and Lactococcus lactis controlled microbial growth during seed germination. Lp. plantarum and Lc. lactis also served as starter cultures for the acidification of soy and lupine milk and were subsequently present in the unripened plant cheese as dominant microbes. Acidification also inhibited the growth and metabolic activity of bacilli but Bacillus spores remained viable throughout ripening. During plant cheese ripening, Lc. lactis was inactivated before Lp. plantarum and the presence of bacilli during seed germination delayed Lc. lactis inactivation. Metagenomic sequencing of full-length 16S rRNA gene amplicons confirmed that the relative abundance of the inoculated strains in each ripened cheese sample exceeded 99%. Oligosaccharides including raffinose, stachyose, and verbascose were rapidly depleted in the initial stage of ripening. Both germination and the presence of bacilli during seed germination had impact on polysaccharide hydrolysis during ripening. Bacilli but not seed germination enhanced proteolysis of plant cheese during ripening. In conclusion, the use of germination with lactic acid bacteria in combination with Bacillus spp. exhibited the potential to improve the quality of ripened plant cheeses with a positive effect on the reduction of hygienic risks.

IMPORTANCE

The development of novel plant-based fermented food products for which no traditional templates exist requires the development of starter cultures. Although the principles of microbial flavor formation in plant-based analogs partially overlap with dairy fermentations, the composition of the raw materials and thus likely the selective pressure on the activity of starter cultures differs. Experiments that are described in this study explored the use of seed germination, the use of lactic acid bacteria, and the use of bacilli to reduce hygienic risks, to acidify plant milk, and to generate taste-active compounds through proteolysis and fermentative conversion of carbohydrates. The characterization of fermentation microbiota by culture-dependent and culture-independent methods also confirmed that the starter cultures used were able to control microbial communities throughout 90 d of ripening. Taken together, the results provide novel tools for the development of plant-based analogs of fermented dairy products.

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.

Levels and distribution of Salmonella in naturally contaminated cashews

Raw materials associated with foodborne illness outbreaks are rarely available for evaluation. The levels and distribution of Salmonella were determined in naturally contaminated raw cashews linked to a salmonellosis outbreak associated with a fermented cashew cheese analog. Two unopened 22.7-kg boxes from a single lot of cashew kernel pieces were each divided into seven approximately equal units, 14 in total. Three 10-g subsamples per unit (n = 21) were evaluated for aerobic plate count (APC), coliform counts, and Escherichia coli counts, and 10 50-g subsamples per unit (n = 70) were enriched for the presence of Salmonella. Presumptive Salmonella-positive colonies were confirmed using CHROMagar Salmonella and real-time PCR (InvA) and then serotyped using antigenic methods and genome sequencing prediction tools. APC and coliform counts ranged from 1.81-5.47 (mean 2.44 ± 0.63) log CFU/g and 0.60-5.20 (mean 1.74 ± 0.80) log CFU/g, respectively. Salmonella was recovered from four units in Box 1 and all seven units in Box 2. One of 10 subsamples was positive in all but four of the positive units; one (Box 1) and three (Box 2) units had two positive subsamples. The level of Salmonella in the two boxes combined was 0.0023 most probable number/g (95% confidence interval [0.0014, 0.0038]). Salmonella Urbana was isolated from three of five positive subsamples in Box 1 and eight of 10 positive subsamples in Box 2. Salmonella Fresno and Vinohrady were unique to single subsamples from Box 1, and Salmonella Nima was isolated from two subsamples from Box 2. Of the four serovars recovered, Salmonella Urbana and Salmonella Vinohrady were in common with outbreak-associated clinical or product isolates. Understanding the distribution and concentration of Salmonella in naturally contaminated cashews provides important information for hazard analysis and risk assessments for soaked and fermented cashew products.

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.

Growth and Survival of Foodborne Pathogens during Soaking and Drying of Almond (Prunus dulcis) Kernels

ABSTRACT

The practice of soaking almonds prior to consumption is popular both commercially and at home. The food safety implications of soaking almonds was investigated through analysis of blogs and videos (n = 85 recipes) to identify both the reasons for soaking almonds and the common practices employed. Among the recipes analyzed, the most common reasons for soaking almonds (94.1%) were perceived benefits such as improved digestion and nutrient uptake. Most recipes (34.1%) suggested soaking at "room temperature" for times that ranged from 4 to 24 h or, more commonly, "overnight" (51.8%). Postsoaking drying instructions were provided in 40 recipes (47.1%). Among those providing a drying temperature (31.8%), 85% specified 66°C and lower. To evaluate the growth of foodborne pathogens during almond soaking, separate cocktails of Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella enterica were inoculated onto raw almonds or into the soak water (almonds-to-water ratios of 1:1 and 1:3 [w/v]). Populations were monitored during soaking at 15, 18, and 23°C for up to 24 h, and during postsoak drying at 66°C for 14 h (for Salmonella only). At 15°C and a 1:1 almond:water ratio, no significant population increase (P > 0.05) was observed between 0 and 24 h for any of the pathogens. At 18°C, increases of 0.63, 1.70, and 0.88 log CFU per sample were observed over 24 h for populations of E. coli O157:H7, L. monocytogenes, and Salmonella, respectively. Populations of E. coli O157:H7, L. monocytogenes, and Salmonella increased by 3.48, 3.22, and 3.94 log CFU per sample, respectively, after 24 h at 23°C. When soaked almonds were dried for 14 h at 66°C, moisture and water activity decreased from 40 to ∼6% and 0.99 to 0.60, respectively, but no significant reduction in Salmonella populations was observed. Recommendations for using shorter times (<8 h) and cooler temperatures (≤15°C) should reduce the potential for foodborne pathogens, if present, to grow during soaking of almonds.

HIGHLIGHTS

A comprehensive review on recent developments of radio frequency treatment for pasteurizing agricultural products

Recent pathogen incidents have forced food industry to seek for alternative processes in postharvest pasteurization of agricultural commodities. Radio frequency (RF) heating has been used as one alternative treatment to replace chemical fumigation and other conventional thermal methods since it is relatively easy to apply and leaves no chemical residues. RF technology transfers electromagnetic energy into large bulk volume of the products to provide a fast and volumetric heating. There are two types of RF technology commonly applied in lab and industry to generate the heat energy: free running oscillator and 50-Ω systems. Several reviews have been published to introduce the application of RF heating in food processing. However, few reviews have a comprehensive summary of RF treatment for pasteurizing agricultural products. The objective of this review was to introduce the developments in the RF pasteurization of agricultural commodities and to present future directions of the RF heating applications. While the recent developments in the RF pasteurization were presented, thermal death kinetics of targeted pathogens as influenced by water activity, pathogen species and heating rates, non-thermal effects of RF heating, combining RF heating with other technologies for pasteurization, RF heating uniformity improvements using computer simulation and development of practical RF pasteurization processes were also focused. This review is expected to provide a comprehensive understanding of RF pasteurization for agricultural products and promote the industrial-scale applications of RF technology with possible process protocol optimization purposes.