Salmonella Survival Kinetics on Pecans, Hazelnuts, and Pine Nuts at Various Water Activities and Temperatures

Comparative growth kinetics of Listeria monocytogenes and Salmonella enterica on dehydrated enoki and wood ear mushrooms during rehydration and storage

Specialty mushrooms have been implicated in foodborne illness outbreaks in the U.S. in recent years. These mushrooms are available to consumers in both their fresh and dried states. Dehydrating mushrooms is a convenient way to increase shelf life. The dehydration process results in a lowered water activity (aw) of the commodity, creating an environment where both spoilage and pathogenic bacteria cannot proliferate. Prior to food preparation and consumption, these mushrooms are typically rehydrated and possibly stored for later use which could lead to increased levels of pathogens. This study examined the survival and growth of Listeria monocytogenes and Salmonella enterica on dehydrated enoki and wood ear mushrooms during rehydration and subsequent storage. Mushrooms were heat dehydrated, inoculated at 3 log CFU/g, and rehydrated at either 5 or 25°C for 2 h. Rehydrated mushrooms were stored at 5, 10, or 25°C for up to 14 d. L. monocytogenes and S. enterica survived on enoki and wood ear mushroom types during rehydration at 5 and 25°C, with populations often <2.39 log CFU/g. During subsequent storage, no growth was observed on wood ear mushrooms, regardless of the rehydration or storage temperature, with populations remaining <2.39 log CFU/g for both pathogens. When stored at 5°C, no growth was observed for either pathogen on enoki mushrooms. During storage at 10 and 25°C, pathogen growth rates and populations after 14 d were generally significantly higher on the enoki mushrooms rehydrated at 25°C; the highest growth rate (3.56 ± 0.75 log CFU/g/d) and population (9.48 ± 0.62 log CFU/g) after 14 d for either pathogen was observed by S. enterica at 25°C storage temperature. Results indicate a marked difference in pathogen survival and proliferation on the two specialty mushrooms examined in this study and highlight the need for individual product assessments. Data can be used to assist in informing guidelines for time and temperature control for the safety of rehydrated mushrooms.

Effect of dehydration on the inactivation of Listeria monocytogenes and Salmonella enterica on enoki and wood ear mushrooms

Foodborne illness outbreaks in the U.S. associated with consumption of both fresh and dried specialty mushrooms have recently occurred. Dried wood ear mushrooms were implicated in a salmonellosis outbreak in 2020, while fresh enoki mushrooms were associated with two listeriosis outbreaks in 2020 and 2023. These specialty mushrooms are commercially available in both their fresh and dried states. Due to the short shelf life of mushrooms, dehydration is a common method used in both industry and by consumers to extend the shelf life and preserve quality. Therefore, the aim of this study was to evaluate the use of dehydration on the inactivation kinetics of both Listeria monocytogenes and Salmonella enterica on enoki and wood ear mushrooms. Fresh mushrooms were inoculated with four strain cocktails of either L. monocytogenes or S. enterica and dried at ambient conditions for 10 min. Following drying of the inoculum, mushrooms were placed into food dehydrators preheated to 70, 80, or 90°C and treated for up to 24 h. At treatment intervals, mushrooms were removed from the dehydrators for pathogen enumeration. Inactivation kinetics for both pathogens were modeled using the Weibull, log-linear with tail, and log-linear with shoulder models. Pathogen reductions of >4 log CFU/g were achieved on both enoki and wood ear mushrooms during dehydration at 90°C after only 2-4 h. At 70 and 80°C, log reductions of >4 log CFU/g were observed on wood ear mushrooms after 4-8 h. On enoki mushrooms, a tailing effect was observed with residual populations (>2 log CFU/g) of L. monocytogenes and S. enterica remaining even after 24 h of treatment at both 70 and 80°C. This study emphasizes the need for an individualized dehydration strategy for each mushroom type to ensure the effectiveness of dehydration as a process to reduce pathogen populations. Results of this study will aid in informing proper time and temperature combinations for dehydration of specialty mushrooms to ensure product safety.

Survival kinetics of Listeria monocytogenes and Salmonella enterica on dehydrated enoki and wood ear mushrooms during long-term storage

Two specialty mushrooms have recently become novel vectors for foodborne outbreaks in the U.S.: fresh enoki and dried wood ear mushrooms were linked to a listeriosis and salmonellosis outbreak, respectively. The aim of this study was to evaluate the survival kinetics of Listeria monocytogenes and Salmonella enterica on dehydrated enoki and wood ear mushrooms during long-term storage. Following heat dehydration, mushrooms were inoculated with either L. monocytogenes or S. enterica, allowed to dry for 1 h, and then stored for up to 180 d at 25 °C and 33% relative humidity. Both pathogens were enumerated from the mushrooms at intervals during the storage period. Survival kinetics of both pathogens were modeled using both the Weibull and log-linear with tail models. After inoculation and 1 h drying, both pathogen populations decreased 2.26-2.49 log CFU/g on wood ear mushrooms; no decrease was observed on enoki. Both pathogens survived during storage on both mushroom types. On wood ear mushrooms, a 2-log decrease of both pathogens occurred during storage. On enoki mushrooms, 4-log decreases of both pathogens were modeled to occur after 127.50-156.60 d. The results of this study suggest that L. monocytogenes and S. enterica can persist on dehydrated specialty mushrooms during long-term storage.

Transfer of generic Escherichia coli and attenuated Salmonella enterica Typhimurium from the soil to the surface of in-shell pecans during harvest

During harvest pecan nuts are at risk of contamination with foodborne pathogens from extended contact with the ground. The objective of this study was to determine the potential transfer of Escherichia coli and Salmonella from the ground to in-shell pecans during the harvesting process. Plots (2 m2) were sprayed with 1 L of a rifampicin (rif) resistant strain of either E. coli TVS 353 or an attenuated Salmonella Typhimurium inoculum at a low (∼4 log CFU/ml), mid (∼6 log CFU/ml) or high (∼8 log CFU/ml) concentrations. The following day, nuts were mechanically harvested and samples from each plot were collected at 1 min, 4 h, and 24 h. Samples were enumerated for Salmonella and E. coli on tryptic soy agar supplemented with rif. The Salmonella levels in the soil from the inoculated plots were 2.0 ± 0.3, 4.1 ± 0.1, and 6.4 ± 0.2 log CFU/g for the low, mid, and high inocula, respectively. The E. coli levels in the soil from the inoculated plots were 1.5 ± 0.4, 3.7 ± 0.3, and 5.8 ± 0.1 log CFU/g for the low, mid, and high inocula, respectively. There was a significant difference in the average daily rainfall among the three trials. Trial 3 received 23.8 ± 9.2 cm, while trials 1 and 2 received much less (0.1 ± 0.1 0.0 ± 0.0 cm, respectively). Inoculation concentration and trial were significant (P<0.05) factors that influenced the transfer of E. coli and Salmonella to pecans. For the high inoculum treatment, bacterial transfer to pecans ranged from 0.7 ± 0.3 to 4.1 ± 0.2 for E. coli and 1.3 ± 0.7 to 4.3 ± 0.4 log CFU/g for Salmonella. For the medium inoculum treatment, transfer ranged from <0.3 to 1.5 ± 0.1 for E. coli and <0.3 to 1.9 ± 0.2 log CFU/g for Salmonella. For the low treatment, transfer ranged from <0.3 to 0.4 ± 0.2 and <0.3 to 0.5 ± 0.1 log CFU/g for E. coli and Salmonella, respectively. These results show the need for implementing agricultural practices that prevent potential transfer of foodborne pathogens onto the surface of in-shell pecans during harvest.

The influence of almond's water activity and storage temperature on Salmonella survival and thermal resistance

This study investigated the effects of inoculation method, water activity (a_w), packaging method, and storage temperature and duration on the survival of Salmonella on almonds as well as their resistance to subsequent thermal treatments. Whole almond kernels were inoculated with a broth-based or agar-based growth Salmonella cocktail and conditioned to a_w of 0.52, 0.43 or 0.27. Inoculated almonds with a_w of 0.43 were treated with a previously validated treatment (4 h of dry heat at 73 °C) to determine the potential differences in heat resistance resulted from the two inoculation methods. The inoculation method did not significantly (P > 0.05) impact the thermal resistance of Salmonella. Inoculated almonds at a_w of 0.52 and 0.27 were either vacuum packaged in moisture-impermeable mylar bags or non-vacuum packaged in moisture-permeable polyethylene bags before stored at 35, 22, 4, or -18 °C for up to 28 days. At selected storage intervals, almonds were measured for a_w, analyzed for Salmonella population level, and subjected to dry heat treatment at 75 °C. Over the month-long storage of almonds, Salmonella populations remained almost unchanged (<0.2 log CFU/g) at 4 °C and -18 °C and declined slightly (<0.8 log CFU/g) at 22 °C and more substantially (1.6-2.0 log CFU/g) at 35 °C regardless of the inoculation method, packaging method, and almond a_w. When stored at 35 °C, almonds with initial a_w of 0.52 had significantly higher (P < 0.05) Salmonella reductions than those with initial a_w of 0.27. Prior storage of almonds vacuum packaged in mylar bags at temperatures between -18 °C and 35 °C for 28 days affected their a_w levels but did not significantly (P > 0.05) affect the subsequent thermal resistance of Salmonella at 75 °C regardless of almond a_w and storage duration. Salmonella on almonds with higher a_w was more sensitive to heat treatment than those with lower a_w. To achieve >5 log CFU/g reductions of Salmonella, a dry heat treatment at 75 °C for 4 and 6 h was needed for almonds with initial a_w of 0.52 and 0.27, respectively. When applying the dry heating technology for almond decontamination, the processing time needs to be determined based on initial a_w of almonds regardless of storage condition or age of almonds within the current design frame.

Practice and Progress: Updates on Outbreaks, Advances in Research, and Processing Technologies for Low-moisture Food Safety

Large, renowned outbreaks associated with low-moisture foods (LMFs) bring to light some of the potential, inherent risks that accompany foods with long shelf lives if pathogen contamination occurs. Subsequently, in 2013, Beuchat et al. (2013) noted the increased concern regarding these foods, specifically noting examples of persistence and resistance of pathogens in low-water activity foods (LWAFs), prevalence of pathogens in LWAF processing environments, and sources of and preventive measures for contamination of LWAFs. For the last decade, the body of knowledge related to LMF safety has exponentially expanded. This growing field and interest in LMF safety have led researchers to delve into survival and persistence studies, revealing that some foodborne pathogens can survive in LWAFs for months to years. Research has also uncovered many complications of working with foodborne pathogens in desiccated states, such as inoculation methods and molecular mechanisms that can impact pathogen survival and persistence. Moreover, outbreaks, recalls, and developments in LMF safety research have created a cascading feedback loop of pushing the field forward, which has also led to increased attention on how industry can improve LMF safety and raise safety standards. Scientists across academia, government agencies, and industry have partnered to develop and evaluate innovate thermal and nonthermal technologies to use on LMFs, which are described in the presented review. The objective of this review was to describe aspects of the extensive progress made by researchers and industry members in LMF safety, including lessons-learned about outbreaks and recalls, expansion of knowledge base about pathogens that contaminate LMFs, and mitigation strategies currently employed or in development to reduce food safety risks associated with LMFs.

Thermal Death Kinetics of Three Representative Salmonella enterica Strains in Toasted Oats Cereal

Several reports have indicated that the thermal tolerance of Salmonella at low-water activity increases significantly, but information on the impact of diverse food matrices is still scarce. The goal of this research was to determine the kinetic parameters (decimal reduction time, D; time required for the first decimal reduction, δ) of thermal resistance of Salmonella in a previously cooked low water activity food. Commercial toasted oats cereal (TOC) was used as the food model, with or without sucrose (25%) addition. TOC samples were inoculated with 108 CFU/mL of a single strain of one of three Salmonella serovars (Agona, Tennessee, Typhimurium). TOC samples were ground and equilibrated to aw values of 0.11, 0.33 and 0.53, respectively. Ground TOC was heated at temperatures between 65 °C and 105 °C and viable counts were determined over time (depending on the temperature for up to 6 h). Death kinetic parameters were determined using linear and Weibull regression models. More than 70% of Weibull's adjusted regression coefficients (Radj2) and only 38% of the linear model's Radj2 had values greater than 0.8. For all serovars, both D and δ values increased consistently at a 0.11 aw compared to 0.33 and 0.53. At 0.33 aw, the δ values for Typhimurium, Tennessee and Agona were 0.55, 1.01 and 2.87, respectively, at 85 °C, but these values increased to 65, 105 and 64 min, respectively, at 0.11 aw. At 100 °C, δ values were 0.9, 5.5 and 2.3 min, respectively, at 0.11 aw. The addition of sucrose resulted in a consistent reduction of eight out of nine δ values determined at 0.11 aw at 85, 95 and 100 °C, but this trend was not consistent at 0.33 and 0.53 aw. The Z values (increase of temperature required to decrease δ-value one log) were determined with modified δ values for a fixed β (a fitting parameter that describes the shape of the curve), and ranged between 8.9 °C and 13.4 °C; they were not influenced by aw, strain or sugar content. These findings indicated that in TOC, high thermal tolerance was consistent among serovars and thermal tolerance was inversely dependent on aw.

Survival of Salmonella enterica in Military Low-Moisture Food Products during Long-Term Storage at 4, 25, and 40°C

ABSTRACT

Salmonella enterica has been increasingly implicated in foodborne outbreaks involving low-moisture foods (LMF) during the recent decade. This study aimed to investigate the potential for persistence of S. enterica in a range of LMF during storage at three temperatures. LMF products, boil-in-bag eggs (freeze-dried product), chocolate protein drink, cran-raspberry First Strike bars, mocha dessert bar, and peanut butter, were inoculated with a five-strain cocktail of S. enterica and stored at 4, 25, or 40°C for 36 months. Salmonella populations remained above 7 log CFU/g in all products stored at 4°C and above 6 log CFU/g in products stored at 25°C, excluding the cran-raspberry First Strike bars. Storage at 40°C resulted in Salmonella populations above 5.5 log CFU/g in boil-in-bag eggs after 36 months and demonstrated survivability for 12 months or less in the other five products. Additionally, a mocha bar production temperature profile study identified rapid cooling of bars in which the temperatures reached would have no measurable impact on Salmonella populations. The results indicate the ability of Salmonella to survive in a variety of LMF category foods, even under adverse storage conditions and identifies how the food matrix may affect Salmonella survivability. The data indicate the importance of establishing food processing procedures that adequately mitigate the presence of Salmonella throughout food processing systems, while also increasing comprehensive understanding of Salmonella survivability mechanisms.

HIGHLIGHTS

Desiccation Survival of Salmonella enterica,Escherichia coli, and Enterococcus faecium Related to Initial Cell Level and Cellular Components

ABSTRACT

Salmonella enterica is well known for its ability to survive and persist in low-moisture environments. Previous studies have indicated a link between the initial cell level and the population of Salmonella that survives after desiccation and subsequent storage; however, how the initial cell concentration affects survival is unknown. This study was conducted to examine this phenomenon and to determine whether it occurs in other microorganisms, specifically Shiga toxigenic Escherichia coli (STEC) and Enterococcus faecium. Salmonella, STEC, and E. faecium were grown as sessile cells on Trypticase soy agar with yeast extract (TSAYE) and harvested in buffered peptone water (BPW). To determine recovery at different initial cell levels, cultures were diluted to 9, 7, and 5 log CFU/mL and applied to filters. Filters were dried for 24 h and then stored for 28 days at 25°C and 33% relative humidity. During storage, cells were recovered from filters with BPW and cultivated on TSAYE. Recovery of both Salmonella and E. coli, but not E. faecium, was nonproportional. Lower initial populations were less viable after 24 h of desiccation; ≥10 log CFU/mL was recovered when 11 log CFU/mL was desiccated, but <3 log CFU/mL was recovered when 5 log CFU/mL was desiccated. Once dried, persistence did not appear affected by initial cell concentration. When inactivated (heat-treated) cells were added to the diluent, recovery of Salmonella was proportional with respect to the initial cell level. To further examine the response to desiccation, Salmonella was diluted in BPW containing 1 of 11 test cell components related to quorum sensing or known to affect desiccation resistance to assess recovery and persistence. Of the 11 additions, only cell debris fractions, cell-free extract, and peptidoglycan improved recovery of Salmonella. Desiccation survival appears related to cell wall components; however, the exact mechanism affecting survival remains unknown.

HIGHLIGHTS

Modeling inactivation kinetics for Enterococcus faecium on the surface of peanuts during convective dry roasting

Dry roasting can reduce Salmonella contamination on peanuts. While previous studies evaluated impact of product temperature, process humidity, product moisture, and/or product water activity on Salmonella lethality, no published study has tested multiple primary and secondary models on data collected in a real-world processing environment. We tested multiple primary and secondary models to quantify Salmonella surrogate, Enterococcus faecium, inactivation on peanuts. Shelled runner-type peanuts inoculated with E. faecium were treated at various air temperatures (121, 149, and 177 °C) and air velocities (1.0 and 1.3 m/s) for treatment times from 1 to 63 min. Peanut surface temperature was measured during treatment. Water activity and moisture content were measured, and E. faecium were enumerated after treatment. Microbial inactivation was modeled as a function of time, product temperature, and product moisture. Parameters (D_ref, z_T, z_aw, z_MC, and/or n) were compared between model fits. The log-linear primary model combined with either the modified Bigelow-type secondary model accounting for a_w or moisture content showed improved fit over the log-linear primary model combined with the traditional Bigelow-type secondary model. The Weibull primary model combined with the traditional Bigelow-type secondary model had the best fit. All parameter relative errors were less than 15%, and RMSE values ranged from 0.379 to 0.674 log CFU/g. Incorporating either a_w or moisture content in the inactivation models did not make a practical difference within the range of conditions and model forms evaluated, and air velocity did not have a significant impact on inactivation. The models developed can aid processors in developing and validating pathogen reduction during peanut roasting.

Survival of Salmonella Enteritidis Phage Type 30 on Brazil Nut Kernels and Pumpkin Seeds Stored at 8, 23, and 37°C

ABSTRACT

Experiments were performed to assess the survival of Salmonella on whole Brazil nut kernels and pumpkin seeds stored at 8, 23, and 37°C. Brazil nut kernels and pumpkin seeds were inoculated with bacterial inoculum containing 10.4 log CFU/mL Salmonella Enteritidis phage type 30 and aseptically dried at room temperature for 24 h. After the drying step, levels of Salmonella recovered from Brazil nut kernels and pumpkin seeds were 8.67 ± 0.01 and 9.27 ± 0.03 log CFU/g, respectively. The survival of Salmonella and change in water activity was assessed over 413 days. Although Salmonella survived throughout the storage period, significant differences were recorded between the storage temperatures. Results showed that the survival of Salmonella Enteritidis phage type 30 was more enhanced at 8°C compared with storage at 23 and 37°C. Comparing the survival of Salmonella on the two products at different storage temperatures, there was no significant difference between the means of Salmonella counts for the two products. Results show that Salmonella survived longer on pumpkin seeds stored at 8°C (P = 0.53, compared with Brazil nut kernels), and at 23 and 37°C, Salmonella survived longer on Brazil nut kernels (P = 0.12, compared with pumpkin seeds). The highest and lowest survival of Salmonella was observed on pumpkin seeds with decay rates of -0.003 ± 0.001 and -0.015 ± 0.001 log CFU/g/day for pumpkin seeds stored at 8 and 37°C, respectively. The water activity values recorded on days 2 and 413 for Brazil nut kernels stored at 8, 23, and 37°C were 0.424 and 0.434, 0.383 and 0.385, and 0.372 and 0.256, respectively. For pumpkin seeds stored at 8, 23, and 37°C, water activity values recorded on days 2 and 413 were 0.754 and 0.412, 0.627 and 0.350, and 0.787 and 0.205, respectively. The data obtained in this study provide useful insight on the influence of temperature on the survival of Salmonella on the surface of Brazil nut kernels and pumpkin seeds.

HIGHLIGHTS

Genomic evidence of environmental and resident Salmonella Senftenberg and Montevideo contamination in the pistachio supply-chain

Pistachios have been implicated in two salmonellosis outbreaks and multiple recalls in the U.S. This study performed an in-depth retrospective data analysis of Salmonella associated with pistachios as well as a storage study to evaluate the survivability of Salmonella on inoculated inshell pistachios to further understand the genetics and microbiological dynamics of this commodity-pathogen pair. The retrospective data analysis on isolates associated with pistachios was performed utilizing short-read and long-read sequencing technologies. The sequence data were analyzed using two methods: the FDA's Center for Food Safety and Applied Nutrition Single Nucleotide Polymorphism (SNP) analysis and Whole Genome Multilocus Sequence Typing (wgMLST). The year-long storage study evaluated the survival of five strains of Salmonella on pistachios stored at 25 °C at 35% and 54% relative humidity (RH). Our results demonstrate: i) evidence of persistent Salmonella Senftenberg and Salmonella Montevideo strains in pistachio environments, some of which may be due to clonal resident strains and some of which may be due to preharvest contamination; ii) presence of the Copper Homeostasis and Silver Resistance Island (CHASRI) in Salmonella Senftenberg and Montevideo strains in the pistachio supply chain; and iii) the use of metagenomic analysis is a novel tool for determining the composition of serovar survival in a cocktail inoculated storage study.

Heat Inactivation of Listeria monocytogenes on Pecans, Macadamia Nuts, and Sunflower Seeds

This project was undertaken to determine the kinetic parameters of thermal inactivation of Listeria monocytogenes on pecans, macadamia nuts, and sunflower seeds subjected to heat treatments simulating industry processes. Five strains were grown in nonselective medium, mixed, and resuspended before inoculating macadamia nuts, pecans, and sunflower seeds (6 to 9 Log CFU/g). Redried inoculated pecans and macadamia nuts were heated in an oven at a temperature range of 90 to 140°C. Unshelled sunflower seeds were heated in sunflower seed oil. The thermal inactivation was determined by measuring viable cell counts using standard microbiological methods. Average count data were fit to the log-linear model, and thermal-death kinetics were calculated. On pecans, the viable Listeria counts were reduced by 3 and 3.5 Log CFU/g after 40 min at 110°C and 8 min at 140°C, respectively. On macadamia nuts, the L. monocytogenes population was reduced by 5 Log CFU/g after 20 min at 120°C. Unshelled sunflower seeds were subjected to heat treatment via a hot-oil bath. On sunflower seeds, >7 Log CFU/g reductions were observed after 15 min at 120°C. The thermal resistance (D value) for inactivation on pecans at 140°C was 3.1 min and on macadamia nuts at 120°C was 4.4 min. The inactivation of L. monocytogenes was influenced by the kind of nut or seed. These results suggest that L. monocytogenes has a relatively high thermal tolerance. The findings from this study will contribute to the assessment of the effectiveness of heat treatment for control of this pathogen on nuts and seeds. IMPORTANCE Listeria monocytogenes is a major concern for the food industry in ready-to-eat (RTE) foods. In recent years, large-scale recalls have occurred with contaminated sunflower seeds and macadamia nuts that triggered product withdrawals. These events stress the importance of understanding Listeria's ability to survive heat treatments in these low-water activity foods. Nuts and seeds are subjected to a variety of thermal treatments typically referred as roasting. To date, no listeriosis outbreak has been linked to nuts and seeds, but the recent recognition that this pathogen can be detected in commercial products stresses the need for research on thermal treatments. The characterization of heat inactivation kinetics at temperatures typically used during roasting processes will be very beneficial for validation studies. This manuscript reports inactivation rates of L. monocytogenes strains inoculated onto macadamia nuts, sunflower seeds, and pecan halves subjected to temperatures between 90 and 140°C.

Validation of process technologies for enhancing the safety of low-moisture foods: A review

The outbreaks linked to foodborne illnesses in low-moisture foods are frequently reported due to the occurrence of pathogenic microorganisms such as Salmonella Spp. Bacillus cereus, Clostridium spp., Cronobacter sakazakii, Escherichia coli, and Staphylococcus aureus. The ability of the pathogens to withstand the dry conditions and to develop resistance to heat is regarded as the major concern for the food industry dealing with low-moisture foods. In this regard, the present review is aimed to discuss the importance and the use of novel thermal and nonthermal technologies such as radiofrequency, steam pasteurization, plasma, and gaseous technologies for decontamination of foodborne pathogens in low-moisture foods and their microbial inactivation mechanisms. The review also summarizes the various sources of contamination and the factors influencing the survival and thermal resistance of pathogenic microorganisms in low-moisture foods. The literature survey indicated that the nonthermal techniques such as CO₂ , high-pressure processing, and so on, may not offer effective microbial inactivation in low-moisture foods due to their insufficient moisture content. On the other hand, gases can penetrate deep inside the commodities and pores due to their higher diffusion properties and are regarded to have an advantage over thermal and other nonthermal processes. Further research is required to evaluate newer intervention strategies and combination treatments to enhance the microbial inactivation in low-moisture foods without significantly altering their organoleptic and nutritional quality.

Oregano Oil, Epsilon-Polylysine and Citric Acid Assisted Inactivation of Salmonella in Two Kinds of Tahini during Thermal Treatment and Storage

Tahini and tahini-based products are popular with consumers due to their special flavor and high nutritional values, but often have been linked to Salmonella outbreaks. The objective of this study was to compare effects of different kinds of natural antimicrobials on Salmonella inactivation in undiluted and diluted tahini during thermal treatment and storage. Results showed that the Weibull model was more suitable to describe the thermal inactivation behavior of S. montevideo CICC21588 in two kinds of tahini than the first-order model. Inactivation curves were concave-upward in undiluted tahini but concave-downward in diluted tahini. During storage of undiluted tahini, 3% oregano oil caused extra 1.44 or 0.80 log CFU/g reductions after 7 days at 25 °C or 4 °C compared to the control and 0.5% citric acid caused an extra reduction of 0.75 log CFU/g after 7 d at 4 °C. For diluted tahini, 2–3% oregano oil and 0.4–0.5% ε-polylysine reduced more populations compared to undiluted tahini. These antimicrobials all inhibited the growth of S. montevideo during 24 h at 25 °C and ε-polylysine had the best effect. Furthermore, these antimicrobials enhanced the Salmonella inactivation in diluted tahini during thermal treatment, and there was less of a synergistic effect of thermal and antimicrobials in undiluted tahini due to less sublethal injured cells caused by heat. This study may provide useful information for Salmonella inactivation in tahini.

Models for factors influencing pathogen survival in low water activity foods from literature data are highly significant but show large unexplained variance

Factors that control pathogen survival in low water activity foods are not well understood and vary greatly from food to food. A literature search was performed to locate data on the survival of foodborne pathogens in low-water activity (<0.70) foods held at temperatures <37 °C. Data were extracted from 67 publications and simple linear regression models were fit to each data set to estimate log linear rates of change. Multiple linear stepwise regression models for factors influencing survival rate were developed. Subset regression modeling gave relatively low adjusted R² values of 0.33, 0.37, and 0.48 for Salmonella, E. coli and L. monocytogenes respectively, but all subset models were highly significant (p < 1.0e-9). Subset regression models showed that Salmonella survival was significantly (p < 0.05) influenced by temperature, serovar and strain type, water activity, inoculum preparation method, and inoculation method. E. coli survival was significantly influenced by temperature, water activity, and inoculum preparation. L. monocytogenes survival was significantly influenced by temperature, serovar and strain type, and inoculum preparation method. While many factors were highly significant (p < 0.001), the high degrees of variability show that there is still much to learn about the factors which govern pathogen survival in low water activity foods.

Transfer of Salmonella from Inert Food Contact Surfaces to Wheat Flour, Cornmeal, and NaCl

ABSTRACT

Salmonella contamination in a dry processing facility frequently requires removal methods that are nonaqueous. Removal of pathogens from food processing systems with a purge of uncontaminated dry food materials has been proposed; however, little is known with respect to efficacy. In this study, survival of Salmonella on inert contact surfaces and transfer of Salmonella from inert contact surfaces to low-moisture foods were evaluated. Six stainless steel and polymeric food contact material types, in bead form, were contaminated at 11 log CFU/mL and then stored at two temperatures, 25 and 4°C, for 6 months. Simultaneously, three dry food materials or ingredients were used to remove Salmonella from contaminated beads. Wheat flour, cornmeal, and NaCl (1 g each) were mechanically mixed with 3 beads of each material type. The rate of microbial transfer from contaminated beads to food materials was measured. Further experimentation using multiple transfers was applied on two representative beads types, 316 stainless steel and polypropylene, representing common surface contact materials used in processing equipment. Survival of Salmonella on beads depended on storage temperature, with longer survival (P < 0.05) at 4°C than at 25°C, but survival was not influenced by type of bead material. Transfer of Salmonella from stainless steel beads to flour was significantly greater (P < 0.05) than from plastic. Transfer rates from stainless steel to wheat flour, cornmeal, and NaCl were measured as -0.5713, -0.2592, and -1.4221 log CFU of Salmonella removed per cm2 per g of clean material used. Transfer rates from polypropylene to whole wheat flour, cornmeal, and NaCl were more than 10-fold lower at -0.0156, -0.0148, and -0.0129 log CFU of Salmonella removed per cm2 per g of clean material used. These results indicate that although material type may not influence Salmonella survival during storage, Salmonella is more easily removed from stainless steel than polypropylene.

HIGHLIGHTS

Survey of Salmonella in raw tree nuts at retail in the United States

Abstract

The objective of this survey was to estimate the prevalence, contamination level, and genetic diversity of Salmonella in selected raw, shelled tree nuts (Brazil nuts, cashews, hazelnuts, macadamia nuts, pecans, pine nuts, pistachios, and walnuts) at retail markets in the United States. A total of 3,374 samples of eight tree nuts were collected from different types of retail stores and markets nationwide between September 2015 and March 2017. These samples (375 g) were analyzed using a modified FDA's BAM Salmonella culture method. Of the 3,374 samples, 15 (0.44%) (95% confidence interval [CI] [0.25, 0.73]) were culturally confirmed as containing Salmonella; 17 isolates were obtained. Among these isolates, there were 11 serotypes. Salmonella was not detected in Brazil nuts (296), hazelnuts (487), pecans (510), pine nuts (500), and walnuts (498). Salmonella prevalence estimates in cashews (510), macadamia (278), and pistachios (295) were 0.20% (95% CI [<0.01, 1.09]), 2.52% (95% CI [1.02, 5.12]), and 2.37% (95% CI [0.96, 4.83]), respectively. The rates of Salmonella isolation from major/big‐chain supermarkets (1381), small‐chain supermarkets (328), discount/variety/drug stores (1329), and online (336) were 0.29% (95% CI [0.08, 0.74]), 0.30% (95% CI [0.01, 1.69]), 0.45% (95% CI [0.17, 0.98]), and 1.19% (95% CI [0.33, 3.02]), respectively. Salmonella prevalence in organic (530) and conventional (2,844) nuts was not different statistically (P = 0.0601). Of the enumerated samples (15), 80% had Salmonella levels ≤0.0092 most probable number (MPN)/g. The highest contamination level observed was 0.75 MPN/g. The prevalence and contamination levels of Salmonella in the tree nuts analyzed were generally comparable to previous reports. Pulsed‐field gel electrophoresis, serotype, and sequencing data all demonstrated that Salmonella population in nuts is very diverse genetically.

Practical Application

The prevalence, contamination level, and genetic diversity of Salmonella in eight types of tree nuts (3,374 samples collected nationwide) revealed in this survey could help the development of mitigation strategies to reduce public health risks associated with consumption of these nuts.

Microbiological Profile, Incidence, and Behavior of Salmonella on Seeds Traded in Mexican Markets

ABSTRACT

Consumption of seeds has increased in recent years due to their high nutrient content. However, Salmonella outbreaks associated with the consumption of low-water-activity food items have also increased, although these food items do not support microbial growth. The main goal of this study was to quantify microbial indicators and to determine the prevalence and content of Salmonella in chia, amaranth, and sesame seeds obtained from Mexican retail outlets. In addition, the behavior of this pathogen on seeds was evaluated. One hundred samples of each product (chia, amaranth, and sesame seeds) were collected from Queretaro City markets. Aerobic plate count, coliforms, and Escherichia coli bacteria were quantified, and the presence and number of Salmonella pathogens were also determined. Chia, amaranth, and sesame seeds (1 kg each) were inoculated with a cocktail of five Salmonella strains (∼6 log CFU mL-1) and stored at ambient temperature, and then populations of Salmonella were quantified. The median aerobic plate count contents in chia, amaranth, and sesame seeds were 2.1, 2.4, and 3.8 log CFU g-1, respectively, and the content of coliforms on the seeds ranged from 0.48 to 0.56 log most probable number (MPN) per g. E. coli was present at low concentrations in the three types of seeds. Salmonella was detected in chia (31%), amaranth (15%), and sesame (12%) seeds, and the population ranged from 0.48 to 0.56 log MPN g-1. Salmonella levels decreased through 240 days of storage, showing inactivation rates of 0.017, 0.011, and 0.016 log CFU h-1 in chia, amaranth, and sesame seeds, respectively. The high prevalence of Salmonella in the seeds highlights potential risks for consumers, particularly given that seeds are generally consumed without treatments guaranteeing pathogen inactivation.

HIGHLIGHTS

Thermal inactivation of Salmonella, Shiga toxin-producing Escherichia coli, Listeria monocytogenes, and a surrogate (Pediococcus acidilactici) on raisins, apricot halves, and macadamia nuts using vacuum-steam pasteurization

Salmonella, Shiga toxin-producing Escherichia coli (STEC), and Listeria monocytogenes have been isolated from low water activity foods (LWAF), where they may survive for extended periods. The ready-to-eat nature of many LWAF, such as dried fruits and nuts, warrants effective post-harvest thermal treatment for the reduction of pathogens such as low-temperature, saturated steam, also known as vacuum-assisted steam pasteurization. The objective of this study was to determine reductions of Salmonella, STEC, L. monocytogenes, and a possible surrogate (Pediococcus acidilactici) on dried apricot halves, whole macadamia nuts, and raisins after treatment with vacuum-assisted steam at three temperatures (62 °C, 72 °C, or 82 °C) and multiple time intervals. Bacterial inactivation was variable between commodities, with higher temperatures and longer times necessary to achieve comparable reductions of pathogens on apricot halves and macadamia nuts compared to raisins. Reductions of the tested pathogens were comparable; therefore, one species was not more resistant than the others. Pathogens were reduced by 5-log CFU/g on apricot halves after 20 min at 72 °C and after 5 min at 82 °C. Longer treatment times were necessary to achieve reductions of each pathogen on macadamia nuts. Pathogens were reduced by nearly 5 log CFU/g on macadamia nuts after 38 min at 72 °C (4.6-6.5 log CFU/g) and after 12 min at 82 °C (4.9-5.7 log CFU/g). Reductions of pathogens on raisins were achieved at lower temperatures than necessary for the other foods. A 5-log reduction for each of the pathogens (CFU/g) on raisins occurred after 20 min at 62 °C and after 5 min at 72 °C. Overall, the reductions of the pathogens exceeded those of P. acidilactici on both the dried fruits and macadamia nuts. Statistically significant differences, indicating greater confidence as a conservative surrogate, were observed at lower treatment temperatures. Inactivation kinetics were modeled for each pathogen on each food type and temperature. Bacterial survival was best described by the Weibull model for raisins and macadamia nuts, while the Gompertz model best described reductions on apricot halves according to Akaike information criterion (AIC) and root-mean-square error (RMSE) evaluations. Water activity and moisture content were increased due to the treatments, which could be addressed through implementation of drying steps. Thermal inactivation kinetic models and 5-log reduction parameters can help food processors design and evaluate similar vacuum-assisted steam interventions to comply with FSMA regulations and preventive control plans. However, results or model predictions should not be extrapolated to assume the safety of other types of foods.

Survival and thermal resistance among four Salmonella serovars inoculated onto flaxseeds

Thermal resistance among Salmonella serovars has been shown to vary, however, such data are minimal for Salmonella inoculated onto low moisture foods. We evaluated survival and subsequent thermal resistance for 32 strains of Salmonella from four serovars (Agona, Enteritidis, Montevideo, and Tennessee) on flaxseed over 24 weeks. After inoculation, flaxseeds were adjusted to a_w = 0.5 and stored at 22 °C. After 24 weeks at 22 °C, strains of serovar Agona had a significantly slower rate of reduction compared to those of Enteritidis and Montevideo (adj. p < 0.05). Inoculated flaxseeds were processed at 71 °C with vacuum steam pasteurization at 4 time points during storage. Average initial D_71°C values ranging from 1.0 to 1.5 min were similar across serovars. Over 24 weeks, D_71°C varied in a serovar-dependent manner. D_71°C at 8, 16, and 24 weeks did not change significantly for Enteritidis and Montevideo but did for Tennessee and Agona. While significant, the differences in D_71°C over time were less than 1 min, indicating that storage time prior to heat treatment would have a minimal effect on the processing time required to inactivate Salmonella on flaxseed.

Survival kinetics of Listeria monocytogenes on chickpeas, sesame seeds, pine nuts, and black pepper as affected by relative humidity storage conditions

Nuts and seeds have been increasingly associated with recalls due to contamination with Listeria monocytogenes. Storage of these food commodities occurs at various relative humidity (RH) conditions for months or years. The objective of this study was to assess L. monocytogenes survival on four commodities representing dried legumes, seeds, and spices categories: chickpeas, sesame seeds, pine nuts, and black pepper kernels. Inoculated products at 10 log CFU/g were stored for 180 days (6 months) at 25°C and different relative humidity (RH) levels: 25% (low), 45% (ambient), and 75% (high). After 180 days at 25% RH, L. monocytogenes populations decreased to 2.67-6.59 log CFU/g; the highest survival of the pathogen was observed on pine nuts and sesame seeds with decay rates of -0.014± 0.001 log CFU/g per d. Significantly greater population reductions on all products were observed during storage at 45 and 75% RH. At 45% RH, L. monocytogenes levels decreased to 1.90-6.36 log CFU/g. On chickpeas and black pepper stored at 75% RH, the pathogen population decreased to below the limit of enumeration (1 log CFU/g) yet were still detected via enrichments. The lowest survival of L. monocytogenes occurred at 75% RH on black pepper with a decay rate of -0.058±0.003 log CFU/g per d. Overall, regardless of RH level, the ability of the products to support survival of the pathogen may be expressed in the following order: pine nuts > sesame seeds > chickpeas > black pepper. The results of this study can aid in understanding how L. monocytogenes survives on dried legumes, seeds, and spices, and the data can contribute to the risk assessment of this pathogen.

Modeling the Risk of Salmonellosis from Consumption of Peanuts in the United States

Peanut products were the target of the largest food recall in United States history from 2008 to 2009, with more than 3,200 products implicated, economic losses estimated at $1 billion, and more than 700 reported illnesses and 9 deaths. Predictive modeling tools such as quantitative microbial risk assessment can be used to aid processors in making risk management decisions that may reduce the chances of foodborne illness, but published risk assessment for peanuts is not currently available. A quantitative microbial risk assessment was performed to quantify salmonellosis risk from consumption of peanuts in the United States. Prevalence and concentration data for Salmonella on raw, shelled peanuts were used in combination with probability distributions of simulated log reductions achieved during production steps before consumption. Data for time-temperature combinations used in each step were obtained from published literature, industry surveys, or expert opinion, and survival data were obtained from the literature. A beta-Poisson dose-response model was used to predict probability of illness from ingestion of Salmonella cells. The model predicted 14.2 (arithmetic mean) or 0.0123 (geometric mean) illnesses per year. Sensitivity analysis showed that thermal inactivation log reductions applied had the biggest impact on predicted salmonellosis risk, followed by consumer storage time, Salmonella starting concentration, Salmonella starting prevalence, and number of originally contaminated 25-g servings per originally positive 375-g sample. Scenario analysis showed that increasing log reduction variability increased mean salmonellosis risk. Removing the effect of storage on Salmonella survival increased the arithmetic and geometric means to 153 and 0.598 illnesses per year, respectively. This study indicated that the risk of salmonellosis from consumption of peanuts can be lowered by reducing field contamination, control of storage steps, and monitoring of appropriate critical limits in peanut roasting.

Survival and Thermal Resistance of Salmonella Enteritidis PT 30 on Almonds after Long-Term Storage

Salmonella survival and thermal resistance on the surface of almond kernels were evaluated after periods of storage. Almond kernels were inoculated with Salmonella Enteritidis PT 30 and equilibrated to 0.45 water activity. Samples were separated into two groups (I and II) and stored in sealed metal cans at room temperature. Group I samples (stored 7, 15, 27, and 68 weeks) were re-equilibrated in controlled humidity chambers to 0.45 water activity before performing the thermal treatments after each storage period, but group II samples (stored 70 and 103 weeks) were thermally treated immediately after the cans were opened. For thermal treatments, individual almond kernels were vacuum sealed in thin plastic bags, heated isothermally in a water bath (80°C) for nine intervals, immediately cooled in an ice bath, and assayed for surviving Salmonella. Log-linear and Weibull models were fit to the inactivation data. Salmonella population decreased ( P < 0.05) more than 2 log CFU/g during the long-term storage. Salmonella survival in group II at 70 weeks (7.3 log CFU/g) was higher ( P < 0.05) than in group I (which had been re-equilibrated multiple times) at 68 weeks (6.2 log CFU/g). However, the thermal resistance of Salmonella Enteritidis PT 30 did not decrease ( P > 0.05) for up to 68 weeks of storage, and the log-linear model best described the thermal inactivation data. Overall, the results suggest that re-equilibrating almonds (group I) multiple times may have increased the rate of reduction of Salmonella populations during long-term storage. However, Salmonella thermal resistance on almonds appears to be essentially unaffected by long-term storage, which is important information for designing and conducting validation studies for pathogen control processes.

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.