Inactivation of Salmonella and Shiga-toxin producing Escherichia coli on soft wheat kernels using vacuum steam pasteurization.

Wheat, the raw material for flour milling, can be contaminated with enteric pathogens, leading to outbreaks linked to flour. In previous lab-scale studies, vacuum steam treatment was able to reduce Salmonella Enteritidis PT30 and Shiga-toxin producing E. coli (STEC) O121 levels on soft wheat kernels while maintaining flour quality and gluten functionality. This study used a newly designed lab-scale vacuum steam pasteurizer (VSP) to evaluate its efficacy to inactivate multiple strains of Salmonella and STEC on soft wheat by modeling the non-isothermal time-temperature history during treatment and reduction of the microbial populations. The results demonstrated that vacuum steam treatment could effectively disinfect wheat grains inoculated with enteric pathogens. In this study, Salmonella strains were less thermally resistant than STEC strains. The D75°C of Salmonella strains were 2.8 and 3.2 min, and the D75°C of STEC ranged from 3.1 to 4.6 min. E. faecium had a D75°C of 3.3 min, which indicates that it could be used as surrogate for larger scale evaluation of vacuum steam pasteurization in the future but was not conservative compared to some of the STEC strains.

Survival of Escherichia coli O157:H7 during Moderate Temperature Dehydration of Plant-Based Foods.

The effect of moderate-temperature (≤60 °C) dehydration of plant-based foods on pathogen inactivation is unknown. Here, we model the reduction of E. coli O157:H7 as a function of product-matrix, aw, and temperature under isothermal conditions. Apple, kale, and tofu were each adjusted to aw 0.90, 0.95, or 0.99 and inoculated with an E. coli O157:H7 cocktail, followed by isothermal treatment at 49, 54.5, or 60.0 °C. The decimal reduction time, or D-value, is the time required at a given temperature to achieve a 1 log reduction in the target microorganism. Modified Bigelow-type models were developed to determine D-values which varied by product type and aw level, ranging from 3.0-6.7, 19.3-55.3, and 45.9-257.4 min. The relative impact of aw was product dependent and appeared to have a non-linear impact on D-values. The root mean squared errors of the isothermal-based models ranged from 0.75 to 1.54 log CFU/g. Second, we performed dynamic drying experiments. While the isothermal results suggested significant microbial inactivation might be achieved, the dehydrator studies showed that the combination of low product temperature and decreasing aw in the pilot-scale system provided minimal inactivation. Pilot-scale drying at 60 °C only achieved reductions of 3.1 ± 0.8 log in kale and 0.67 ± 0.66 log in apple after 8 h, and 0.69 ± 0.67 log in tofu after 24 h. This illustrates the potential limitations of dehydration at ≤60 °C as a microbial kill step.

Predictive Microbial Modeling of Enterococcus faecium NRRL B-2354 Inactivation during Baking of a Multicomponent Low-Moisture Food.

ABSTRACT: The use of baking ovens as a microbial kill step should be validated based on results of thermal inactivation models. Although traditional isothermal models may not be appropriate for these dynamic processes, they are being used by the food industry. Previous research indicates that the impact of additional process conditions, such as humidity, should be considered when validating thermal processes for the control of microbial hazards in low-moisture foods. In this study, the predictive performance of traditional and modified thermal inactivation kinetic models accounting for process humidity were assessed for predicting inactivation of Enterococcus faecium NRRL B-2354 in a multi-ingredient composite food during baking. Ingredients (milk powder, protein powder, peanut butter, and whole wheat flour) were individually inoculated to achieve ∼6 log CFU/g, equilibrated to a water activity of 0.25, and then mixed to form a cookie dough. An isothermal inactivation study was conducted for the dough to obtain traditional D- and z-values (n = 63). In a separate experiment, cookies were baked under four dynamic heating conditions: 135°C, high humidity; 135°C, low humidity; 150°C, high humidity; and 150°C, low humidity. Process humidity measurements; time-temperature profiles for the product core, surface, and bulk air; and microbial survivor ratios were collected for the four conditions at six residence times (n = 144). The traditional isothermal model had a high root mean square error (RMSE) of 856.51 log CFU/g, significantly overpredicting bacterial inactivation during the process. The modified model accounting for the dynamic time-temperature profile and process humidity data was a better predictor with an RMSE of 0.55 log CFU/g. These results indicate the importance of accounting for additional process parameters in baking inactivation models and that model performance can be improved by utilizing model parameters obtained directly from industrial-scale experimental data.

Fate of Salmonella and Enterococcus faecium during Pilot-Scale Spray Drying of Soy Protein Isolate.

ABSTRACT: Outbreaks and recalls associated with microbial contamination of powdered foods have raised concern for the safety of the spray-drying process and its products. However, little research on the fate of bacteria during the spray-drying process has been done, leaving much unknown about the risks of contamination in spray dryers. Therefore, quantifying the contamination levels of Salmonella and Enterococcus faecium (as a surrogate) in various locations within a pilot-scale spray dryer can help illustrate the distribution of bacterial contamination, including in the final product. A 10% (w/w) dispersion of water and soy protein isolate was mixed with tryptic soy broth containing yeast extract inoculated with Salmonella enterica serovar Enteritidis phage type 30 (PT30) or E. faecium strain NRRL B-2354. This dispersion was spray dried using a pilot-scale tall-form cocurrent spray dryer at an inlet air temperature of 180, 200, or 220°C. After drying, samples of powder from eight locations within the system were collected or surface swabbed, plated, and enumerated. Spray drying achieved 2.40 to 4.15 and 2.33 to 2.83 log reductions in the concentrations of Salmonella and E. faecium, respectively, in the final powder product accumulated in the dryer's collectors. Salmonella and E. faecium were found in various concentrations in all locations within the spray dryer after a complete drying cycle. Differences in inlet air temperature between 180 and 220°C had no significant effect on the inactivation levels. As a surrogate, E. faecium was more resistant to spray drying than Salmonella. Overall, spray drying is capable of significant bacterial reduction in the final powder product, which can be combined with other hurdle technologies. However, adequate cleaning and sanitization procedures must be taken into consideration to prevent cross-contamination.

Effect of Ph On Survival of Escherichia coli O157, Escherichia coli O121, and Salmonella enterica During Desiccation and Short-term Storage.

ABSTRACT: One intrinsic characteristic of low-moisture foods that is frequently overlooked is pH. Although pH affects the survival of microorganisms in high-moisture foods, its influence in low-moisture foods with less available moisture has not been examined. Escherichia coli O157:H7, E. coli O121, Salmonella enterica Anatum, and S. enterica Agona were grown on solid media with and without added glucose, harvested, and then suspended in buffer at pH 4, 5, and 7 for 10 min. All cultures were spotted individually onto cellulose filters and dried in a biohazard cabinet (23 ± 2°C) overnight (24 ± 2 h) and then stored in a 25°C incubator at 33% relative humidity. Populations were examined at regular intervals up to 26 (E. coli) or 29 (Salmonella) days. Additional controls for pH consisted of cultures held in buffer at pH 4, 5, and 7 at 25°C for the same time periods as the desiccated cells. For all strains tested, pH had an effect on survival whether stored dried or in liquid buffer (P < 0.05). However, when grown on solid media, acid adaptation (grown with glucose) before acid treatment did not appear beneficial to Salmonella during desiccation. Instead, both acid-adapted Salmonella serovars appeared less resistant during drying than did non-acid-adapted cells. Once dried, the rates of decline for Salmonella were not significantly different for acid-adapted and nonadapted cells (P > 0.05), indicating similar persistence following desiccation. A reverse trend was observed for E. coli O121; acid adaptation on solid media improved survival during desiccation and subsequent storage at low pH (P < 0.05). E. coli O157:H7 survival was significantly lower than that of either Salmonella or E. coli O121 under all conditions tested. Results indicate that the response to desiccation and pH stress differs between the microorganisms and under different growth conditions.

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.

Desiccation and Thermal Resistance of Escherichia coli O121 in Wheat Flour.

Non-O157 Shiga toxin-producing Escherichia coli infections have recently been associated with wheat flour on two separate accounts in the United States and Canada. However, there is little information regarding the thermal resistance and longevity of non-O157 Shiga toxin-producing Escherichia coli during storage in low-moisture environments. The objectives of this study were to determine the thermal inactivation kinetics of E. coli O121 in wheat flour and to compare the thermal inactivation rates with those of other pathogens. Wheat flour, inoculated with E. coli O121, was equilibrated at 25°C to a water activity of 0.45 in a humidity-controlled conditioning chamber. Inoculated samples were treated isothermally at 70, 75, and 80°C, and posttreatment population survivor ratios were determined by plate counting. D- and z-values calculated with a log-linear model, were compared with those obtained in other studies. At 70, 75, and 80°C, the D-values for E. coli O121 were 18.16 ± 0.96, 6.47 ± 0.50, and 4.58 ± 0.40 min, respectively, and the z-value was 14.57 ± 2.21°C. Overall, E. coli O121 was observed to be slightly less thermally resistant than what has been previously reported for Salmonella Enteritidis PT30 in wheat flour as measured under the same conditions with the same methods.

Effect of Food Structure, Water Activity, and Long-Term Storage on X-Ray Irradiation for Inactivating Salmonella Enteritidis PT30 in Low-Moisture Foods.

Recent outbreaks and recalls of low-moisture foods contaminated with Salmonella have been recognized as a major public health risk that demands the development of new Salmonella mitigation strategies and technologies. This study aimed to assess the efficacy of X-ray irradiation for inactivating Salmonella on or in almonds (kernels, meal, butter), dates (whole fruit, paste), and wheat (kernels, flour) at various water activities (aw) and storage periods. The raw materials were inoculated with Salmonella Enteritidis PT30, conditioned to 0.25, 0.45, and 0.65 aw in a humidity-controlled chamber, processed to various fabricated products, and reconditioned to the desired aw before treatment. In a storage study, inoculated almond kernels were stored in sealed tin cans for 7, 15, 27, and 103 weeks, irradiated with X ray (0.5 to 11 kGy, targeting up to a ∼2.5-log reduction) at the end of each storage period, and plated for Salmonella survivors to determine the efficacy of irradiation in terms of D10-value (dose required to reduce 90% of the population). Salmonella was least resistant (D10-value = 0.378 kGy) on the surface of almond kernels at 0.25 aw and most resistant (D10-value = 2.34 kGy) on the surface of dates at 0.45 aw. The Salmonella D10-value was 61% lower in date paste than on whole date fruit. Storage of almonds generally had no effect on the irradiation resistance of Salmonella over 103 weeks. Overall, these results indicate that product structure (whole, meals, powder, or paste), water activity (0.25 to 0.65 aw), and storage period (0 to 103 weeks) should be considered when determining the efficacy of X-ray irradiation for inactivating Salmonella in various low-water-activity foods.

Erratum for Haendiges et al., "Complete Genome Sequences of Five Salmonella enterica Strains Used in Inoculation Cocktails in Low-Moisture Food Storage Studies".

[This corrects the article DOI: 10.1128/MRA.01588-18.].

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.

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

Ready-to-eat foods based on dried partially sprouted seeds have been associated with foodborne salmonellosis. Whereas research has focused on the potential for Salmonella initially present in or on seeds to grow and survive during fresh sprout production, little is known about the potential for growth and survival of Salmonella associated with seeds that have been partially sprouted and dried. The goal of this study was to determine the growth of Salmonella during soaking for partial germination of pumpkin, sunflower, and chia seeds and subsequent survival during drying and storage. Pumpkin, sunflower, and chia seeds were inoculated with a four-serotype Salmonella cocktail by the dry transfer method and were soaked in sterile water at 25 or 37°C for 24 h. During the soaking period, Salmonella exhibited growth rates of 0.37 ± 0.26, 0.27 ± 0.12, and 0.45 ± 0.19 log CFU/h at 25°C and 0.94 ± 0.44, 1.04 ± 0.84, and 0.73 ± 0.36 log CFU/h at 37°C for chia, pumpkin, and sunflower seeds, respectively. Soaked seeds were drained and dried at 25, 51, and 60°C. Drying resulted in >5 log CFU/g loss at both 51 and 60°C and ∼3 log CFU/g loss at 25°C on partially sprouted pumpkin and sunflower seeds. There was no decrease in Salmonella during drying of chia seeds at 25°C, and only drying at 60°C provided losses >5 log CFU/g. Dried seeds were stored at 37 and 45°C at 15 and 76% relative humidity (RH) levels. The combination of temperature and RH exerted a stronger effect than either factor alone, such that rates at which Salmonella decreased generally followed this order: 37°C at 15% RH < 45°C at 15% RH < 37°C at 76% RH < 45°C at 76% RH for all seeds tested. Rates differed based on seed type, with chia seeds and chia seed powder having the smallest rate of Salmonella decrease, followed by sunflower and pumpkin seeds. Drying at higher temperatures (50 and 61°C) or storing at elevated temperature and humidity (45°C and 76% RH) resulted in significantly different rates of Salmonella decrease.