Occurrence and Levels of Salmonella, Enterohemorrhagic Escherichia coli, and Listeria in Raw Wheat

From wheat grain to flour: a review of potential sources of enteric pathogen contamination in wheat milled products

The number of food safety issues linked to wheat milled products have increased in the past decade. These incidents were mainly caused by the contamination of wheat-based products by enteric pathogens. This manuscript is the first of a two-part review on the status of the food safety of wheat-based products. This manuscript focused on reviewing the available information on the potential pre-harvest and post-harvest sources of microbial contamination, and potential foodborne pathogens present in wheat-based products. Potential pre-harvest sources of microbial contamination in wheat included animal activity, water, soil, and manure. Improper grain storage practices, pest activity, and improperly cleaned and sanitized equipment are potential sources of post-harvest microbial contamination for wheat-based foods. Raw wheat flour products and flour-based products are potentially contaminated with enteric pathogens such as Shiga toxin-producing E. coli (STECs), and Salmonella at low concentrations. Wheat grains and their derived products (i.e., flours) are potential vehicles for foodborne illness in humans due to the presence of enteric pathogens. A more holistic approach is needed for assuring the food safety of wheat-based products in the farm-to-table continuum. Future developments in the wheat supply chain should also be aimed at addressing this emerging food safety threat.

Acidic water tempering and heat treatment, a hurdle approach to reduce wheat Salmonella load during tempering and its effects on flour quality

The cultivation and processing of wheat render it susceptible to microbial contamination from varied sources. Hence, pathogens such as Salmonella can contaminate wheat grains, which poses a food safety risk in wheat-based products. This risk is displayed by the incidence of foodborne illness outbreaks linked to Salmonella-contaminated wheat flour and flour-based products. The purpose of this study was to assess the effectiveness of combining acidic water and heat treatment in reducing the Salmonella load of hard red spring (HRS) wheat grains during tempering. Effective treatments were then evaluated for their effects on wheat flour quality. Tempering with sodium bisulfate (SBS), lactic acid (LA), and citric acid (CA) at 15% w/v alone reduced (p < 0.001) wheat Salmonella load by 3.15, 3.23, and 2.91 log CFU/g, respectively. Heat treatment (55 °C) reduced (p < 0.001) wheat Salmonellaload by 4.1 log CFU/g after 24 h of tempering. Combining both tempering and heat treatments resulted in a greater reduction in Salmonella load as non-detectable levels (<2 log CFU/g) of Salmonella in the wheat grains were obtained after 12 h of tempering with LA (15%) + heat. A similar result were achieved for both SBS (15%) + heat and CA (15%) + heat treatments after 18 h of tempering. Applying the combined treatments in HRS wheat grains resulted in comparable wheat flour baking (volume, texture, and crumb structure) and physicochemical properties (rheology and composition) relative to the control (tempering with water alone). The results from this study has the potential to be utilized for developing more effective methods for improving the food safety of wheat flour against Salmonella contamination.

Foodborne disease outbreaks in flour and flour-based food products from microbial pathogens in the United States, and their health economic burden

The most comprehensive and inclusive estimates for the economic burden of foodborne illness yield values as high as $97.4 billion USD annually. However, broad incidence and cost estimates have limited use if they cannot be attributed to specific foods, for the purposes of food safety control. In this study, we estimated the economic burden of foodborne illnesses resulting from flour and flour-based food products in the United States from the years 2001 to 2021. The outbreak, illness burden, and health economic data are combined to generate these estimates. Our model combined outbreak data with published Centers for Disease Control and Prevention multipliers to estimate the annual number of illnesses associated with flour-borne pathogens. We then integrated illness severity data with an updated economic model that accounts for costs related to medical care, productivity loss, loss of life, along with the quality of life loss that entails pain and suffering. In total, 752 cases and 223 hospitalizations from flour-related illnesses were reported from 2001 to 2021, with an average of 37.6 cases of reported cases annually. However, the actual number of cases, accounting for underreporting and underdiagnosis, can be as high as 19,440 annually. Pathogens involved in these outbreaks are Salmonella, E. coli O157:H7, and E. coli O121. Our estimates suggest average annual economic losses, including healthy years of life lost, of $108 and $258 million using two alternative models.

Phage Biocontrol Effectively Reduces Contamination of Wheat with Shiga Toxin-producing Escherichia coli O121 and O26 Without Adverse Effects on Flour Quality

Contamination of wheat flours with Shiga toxin-producing E. coli (STEC) is a concern for the milling industry. Milling-specific interventions are needed to address this food safety hazard. The objectives for this study were to determine the efficacy of bacteriophage treatment in reducing wheat STEC contamination during tempering, and assess its effects on flour milling and baking quality. Bacteriophage solutions were prepared by mixing sterile water with the bacteriophage treatment at the following levels: 1 × 106 (0.1%), 2.5 × 106 (0.25%), 5 × 106 (0.5%), 1 × 107 (1.0%), and 1 × 108 (10%) PFU/g wheat dosage. Sterile water (0%) was used as the control. Predried wheat grains were inoculated with STEC (O121 and O26) at 5.0 and 6.0 log CFU/g to restore its original moisture content followed by resting for 24 h. Inoculated grains were then tempered (16% moisture, 24 h) using the prepared bacteriophage solutions. Grains were sampled at 0.5, 1, 2, 6, 12, 18, and 24 h during tempering to determine STEC concentration. The effects of the phage solutions on the flour milling and baking quality were also tested. Tempering time, bacteriophage dose, and their interaction had significant effects on phage efficacy (P < 0.05), with better reductions observed at longer tempering times and higher bacteriophage doses. The use of phage solutions reduced (P < 0.05) wheat STEC concentration after tempering, with the 10% treatment (3.2 logs) achieving ahigher reduction than the 1% (2.4 logs) treatment under similar phage preparation. Phage tempering (including at the highest concentration examined, i.e., 10%) produced wheat flours with comparable quality to the control. Phage-treated wheat flour resulted in breads with finer crumb structure, and comparable texture compared to the control. Phage tempering is a viable intervention for wheat milling as it reduced STEC loads of wheat with no detrimental effects to flour milling and baking quality.

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.

Occurrence of Antibiotic Resistant Bacteria in Flours and Different Plant Powders Used in Cuisine

In recent years, several alimentary diseases have been connected with the consumption or tasting of raw flour and dough. Microbiological quality concern is also raising due to increased consumer demand for plant powders, while some of them can be consumed without prior thermal processing. In this study, we have focused on the occurrence of antibiotic-resistant coliform bacteria and enterococci in flour, plant powder and dough from Slovak retail. Our results indicated the presence of both total and antibiotic-resistant coliform bacteria and enterococci in the flour and powder samples. Lower numbers of the total, as well as resistant bacteria, were detected in flours compared to plant powders. Coliform bacteria isolates were predominantly identified as Klebsiella spp. and Enterobacter spp. Ampicillin resistance appeared in 97% of isolates followed by chloramphenicol resistance (22%) and tetracycline resistance (17%). The presence of the bla_SHV gene was confirmed in 13% of isolates. The tetA and tetE genes were present in 25% of isolates of coliform bacteria. The presence of enterococci was detected only in plant powders. Antibiotic-resistant strains were identified as the following: Enterococcus casseliflavus, E. gallinarium and E. faecium. Despite the isolates showing resistance to vancomycin, the presence of the vanA gene was not detected. The majority of antibiotic-resistant isolates belonged to the group of medium biofilm producers. None of these isolates showed efflux pump overproduction. Antibiotic-resistant coliform bacteria and enterococci were not detected in the processed doughs.

End-Use Quality of Historical and Modern Winter Wheats Adapted to the Great Plains of the United States

Improving milling and baking properties is important during wheat breeding. To determine changes in milling and baking quality of hard winter wheat, 23 adapted cultivars released in the Great Plains between 1870 and 2013 were grown in triplicate in a single location (Mead, NE, USA) over two crop years (2018 and 2019). Grain yield and kernel hardness index increased by release year (p < 0.05). The observed increase in hardness index was accompanied by a decrease in percent soft kernels (p < 0.05). Diameter and weight decreased with release year in 2019 (p < 0.05), and their standard deviation increased with the release year (p < 0.05). Flour protein content decreased with release year (p < 0.05) and dough mixing quality increased (p < 0.05). No significant relationship was found for baking property variables, but bran water retention capacity (BWRC), which is correlated with whole wheat bread quality, increased with release year (p < 0.05). In conclusion, wheat kernels have become harder but more variable in shape over a century of breeding. Mixing quality showed significant improvements, and loaf volume and firmness remained constant, even in the presence of a decrease in protein concentration. Bran quality decreased across release year, which may have implications for whole grain baking quality and milling productivity.

Impact of Chlorinated Water on Pathogen Inactivation during Wheat Tempering and Resulting Flour Quality

ABSTRACT

Outbreaks of enteric pathogens linked to wheat flour have led the wheat milling industry to seek solutions addressing this food safety concern. Chlorinated water at 400 to 700 ppm has been used in the flour milling industry as a tempering aid to control growth of yeast and mold in tempering bins. However, the effectiveness of chlorinated water for inactivating enteric pathogens on wheat kernels was unknown. Five strains of Shiga toxin-producing Escherichia coli and two strains of Salmonella were inoculated onto hard red spring wheat at 7 log CFU/g and stored at room temperature for 1 month. Inoculated wheat was tempered with four concentrations (0, 400, 800, and 1,200 ppm) of chlorinated water (pH 6.5). The reduction due to chlorine was determined by calculating change in microbial loads at each chlorine level by using the response at 0 ppm as a reference. Uninoculated wheat tempered with chlorinated water was used to measure flour quality parameters. Changes in pathogen population over 18 h ranged from -2.35 to -0.30 log CFU/g with 800 ppm of chlorinated water and were not significantly different from changes at 400 and 1,200 ppm. Significant (P < 0.05) differences in the extent of reduction were observed among strains. However, the effect of chlorinated water at reducing native microbes on wheat kernels was minimal, with an average reduction of 0.39 log CFU/g for all concentrations. No significant (P > 0.05) changes occurred in flour quality and gluten functionality or during bread making for grains tempered at 400 and 800 ppm of chlorinated water. There were small but significant (P < 0.05) changes in flour protein content, final viscosity, and water absorption when tempered with 1,200 ppm of chlorinated water. The data showed that the level of chlorinated water currently used in industry for tempering could reduce enteric pathogen numbers by 1.22 log CFU/g for Shiga toxin-producing Escherichia coli and 2.29 log CFU/g for Salmonella, with no significant effects on flour quality and gluten functionality.

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Monitoring and Early Warning Analysis of the Epidemic Situation of Escherichia coli Based on Big Data Technology and Cloud Computing

The purpose of this study is to analyze the molecular epidemiological characteristics and resistance mechanisms of Escherichia coli. The study established a big data cloud computing prediction model for the epidemic mechanism of the pathogen. The study establishes the early warning, control parameters, and mathematical model of Escherichia coli infectious disease and monitors the molecular sequence of the pathogen based on discrete indicators. A nonlinear mathematical model equation was used to establish the epidemic trend model of Escherichia coli. The study shows that the use of the model can control the relative error at about 5%. The experiment proves the effectiveness of the combined model.

Online Media Attention Devoted to Flour and Flour-Related Food Safety in 2017 to 2020

ABSTRACT

In light of extended stay-at-home periods during the coronavirus disease 2019 pandemic, recent societal trends have revealed an increased use of online media to remain connected. Simultaneously, interests in at-home cooking and baking, particularly of "comfort foods" have increased. Because flour is a crucial component in many of these products, we analyze how the U.S. public, in social and online media spaces, references "flour" and its use. We also quantify the share of media mentions about flour that are devoted to flour-related food safety risks and/or risk mitigation. It was found that the volume of mentions about flour and its use fluctuate seasonally, often increasing ahead of the winter holiday season (November to December). Further, the volume of interest rapidly increased in March 2020 when stay-at-home orders were issued. The share of media devoted to flour-related food safety risks or associated illness was extremely small but generally corresponded with flour recall announcements or other public risk communications. Overall, the interest in flour and its use remains seasonal and predictably related to societal trends, such as increased baking at home during the holidays or 2020 stay-at-home orders. However, awareness of flour-related food safety risks seems largely absent on the basis of online media data collection and analysis, except in immediate reactions to flour recalls. This study suggests that more flour safety education programs may be desired to support consumers' informed decision making.

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Fate of Salmonella and Enterohemorrhagic Escherichia coli on Wheat Grain

ABSTRACT

Wheat flour has been connected to outbreaks of foodborne illnesses with increased frequency in recent years, specifically, outbreaks involving Salmonella enterica and enterohemorrhagic Escherichia coli (EHEC). However, there is little information regarding the survival of these pathogens on wheat grain during long-term storage in a low-moisture environment. This study aims to evaluate the long-term survival of these enteric pathogens on wheat grain over the course of a year. Hard red spring wheat was inoculated with strains of four serovars of Salmonella (Enteritidis, Agona, Tennessee, and Montevideo) and six serotypes of EHEC (O157:H7, O26:H11, O121:H19, O45:NM, O111:H8, and O103:H2) in triplicate, sealed in Mylar bags to maintain the water activity, and stored at room temperature (22 ± 1°C). The survival of each pathogen was evaluated by plating onto differential media. Viable counts of strains from all four serovars of Salmonella (Enteritidis, Agona, Tennessee, and Montevideo) were detected on wheat grain stored at room temperature (22 ± 1°C) for the duration of the study (52 weeks). Viable counts of strains from EHEC serotypes O45:NM, O111:H8, and O26:H11 were only detected for 44 weeks, and strains from serotypes O157:H7, O121:H19, and O103:H2 were only detected for 40 weeks until they passed below the limit of detection (2.0 log CFU/g). The D-values were found to be significantly different between Salmonella and EHEC (adjusted P ≤ 0.05) with Salmonella D-values ranging from 22.9 ± 2.2 weeks to 25.2 ± 1.0 weeks and EHEC D-values ranging from 11.4 ± 0.6 weeks to 13.1 ± 1.8 weeks. There were no significant differences among the four Salmonella strains or among the six EHEC strains (adjusted P > 0.05). These observations highlight the wide range of survival capabilities of enteric pathogens in a low-moisture environment and confirm these pathogens are a food safety concern when considering the long shelf life of wheat grain and its products.

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Shiga Toxin-Producing Escherichia coli in Wheat Grains: Detection and Isolation by Polymerase Chain Reaction and Culture Methods

Shiga toxin-producing Escherichia coli (STEC) are major foodborne pathogens and seven serogroups, O26, O45, O103, O111, O121, O145, and O157, often called top-7 STEC, account for the majority of the STEC-associated human illnesses in the United States. Two Shiga toxins, Shiga toxins 1 and 2, encoded by stx1 and stx2 genes, are major virulence factors that are involved in STEC infections. Foodborne STEC infections have been linked to a variety of foods of both animal and plant origin, including products derived from cereal grains. In recent years, a few STEC outbreaks have been linked to contaminated wheat flour. The microbiological quality of the wheat grains is a major contributor to the safety of wheat flour. The objective of the study was to utilize polymerase chain reaction (PCR)- and culture-based methods to detect and isolate STEC in wheat grains. Wheat grain samples (n = 625), collected from different regions of the United States, were enriched in modified buffered peptone water with pyruvate (mBPWp) or E. coli (EC) broth, and they were then subjected to PCR- and culture-based methods to detect and isolate STEC. Wheat grains enriched in EC broth yielded more samples positive for stx genes (1.6% vs. 0.32%) and STEC serogroups (5.8% vs. 2.4%) than mBPWp. The four serogroups of top-7 detected and isolated were O26, O45, O103, and O157 and none of the isolates was positive for the Shiga toxin genes. A total of five isolates that carried the stx2 gene were isolated and identified as serogroups O8 (0.6%) and O130 (0.2%). The EC broth was a better medium to enrich wheat grains than mBPWp for the detection and isolation of STEC. The overall prevalence of virulence genes and STEC serogroups in wheat grains was low. The stx2-positive serogroups isolated, O8 and O130, are not major STEC pathogens and have only been implicated in sporadic infections in animals and humans.

Validation of brownie baking step for controlling Salmonella and Listeria monocytogenes

Pathogens, such as Salmonella and Listeria monocytogenes, can survive under the dry environment of flour for extended periods of time and could multiply when flour is hydrated to prepare batter or dough. Therefore, inactivation of these pathogens during the cooking/baking step is vital to ensure the microbiological safety of bakery products such as brownies. The aim of this research was to validate a simulated commercial baking process as a kill-step for controlling Salmonella and L. monocytogenes in brownies and to determine thermal inactivation parameters of these pathogens in brownie batter. Independent studies were conducted in a completely randomized design for each pathogen. All-purpose flour was inoculated with a 5-serovar Salmonella and 3-strain L. monocytogenes cocktails. For baking validation, brownie batters were prepared from inoculated flour, and cooked in the oven set at 350°F (176.7°C) for 40 min followed by 15 min of ambient air cooling. For calculating D-values, brownie batter was transferred into thermal-death-time disks, sealed, and placed in hot-water baths. The samples were held for pre-determined time intervals in hot-water baths and immediately transferred to cold-water baths. Microbial populations were enumerated using injury-recovery media. At the end of baking, Salmonella and L. monocytogenes populations decreased by 6.3 and 5.9 log CFU/g, respectively. D-values of Salmonella and L. monocytogenes cocktails were 53.4 and 37.5 min at 64°C; 27.2 and 16.9 min at 68°C; 10.7 and 9.1 min at 72°C; and 4.6 and 7.3 min at 76°C; respectively. The z-values of Salmonella and L. monocytogenes cocktails were 11.1 and 16.4°C, respectively. This study can be used as a supporting document for the validation of similar brownie baking processes to control Salmonella and L. monocytogenes. The data from this study can also be employed for developing basic prediction models for the survival and thermal resistance of these pathogens during brownie baking step.

Effect of Physical Structures of Food Matrices on Heat Resistance of Enterococcus faecium NRRL-2356 in Wheat Kernels, Flour and Dough

Nonpathogenic surrogate microorganisms, with a similar or slightly higher thermal resistance of the target pathogens, are usually recommended for validating practical pasteurization processes. The aim of this study was to explore a surrogate microorganism in wheat products by comparing the thermal resistance of three common bacteria in wheat kernels and flour. The most heat-resistant Enterococcus faecium NRRL-2356 rather than Salmonella cocktail and Escherichia coli ATCC 25922 was determined when heating at different temperature-time combinations at a fixed heating rate of 5 °C/min in a heating block system. The most heat-resistant pathogen was selected to investigate the influences of physical structures of food matrices. The results indicated that the heat resistance of E. faecium was influenced by physical structures of food matrices and reduced at wheat kernel structural conditions. The inactivation of E. faecium was better fitted in the Weibull distribution model for wheat dough structural conditions while in first-order kinetics for wheat kernel and flour structural conditions due to the changes of physical structures during heating. A better pasteurization effect could be achieved in wheat kernel structure in this study, which may provide technical support for thermal inactivation of pathogens in wheat-based food processing.

Salmonella enterica and Escherichia coli in Wheat Flour: Detection and Serotyping by a Quasimetagenomic Approach Assisted by Magnetic Capture, Multiple-Displacement Amplification, and Real-Time Sequencing

Food safety is a new area for novel applications of metagenomics analysis, which not only can detect and subtype foodborne pathogens in a single workflow but may also produce additional information with in-depth analysis capabilities. In this study, we applied a quasimetagenomic approach by combining short-term enrichment, immunomagnetic separation (IMS), multiple-displacement amplification (MDA), and nanopore sequencing real-time analysis for simultaneous detection of Salmonella and Escherichia coli in wheat flour. Tryptic soy broth was selected for the 12-h enrichment of samples at 42°C. Enrichments were subjected to IMS using beads capable of capturing both Salmonella and E. coli MDA was performed on harvested beads, and amplified DNA fragments were subjected to DNA library preparation for sequencing. Sequencing was performed on a portable device with real-time basecalling adaptability, and resulting sequences were subjected to two parallel pipelines for further analysis. After 1 h of sequencing, the quasimetagenomic approach could detect all targets inoculated at approximately 1 CFU/g flour to the species level. Discriminatory power was determined by simultaneous detection of dual inoculums of Salmonella and E. coli, absence of detection in control samples, and consistency in microbial flora composition of the same flour samples over several rounds of experiments. The total turnaround time for detection was approximately 20 h. Longer sequencing for up to 15 h enabled serotyping for many of the samples with more than 99% genome coverage, which could be subjected to other appropriate genetic analysis pipelines in less than a total of 36 h.IMPORTANCE Enterohemorrhagic Escherichia coli (EHEC) and Salmonella are of serious concern in low-moisture foods, including wheat flour and its related products, causing illnesses, outbreaks, and recalls. The development of advanced detection methods based on molecular principles of analysis is essential to incorporate into interventions intended to reduce the risk from these pathogens. In this work, a quasimetagenomic method based on real-time sequencing analysis and assisted by magnetic capture and DNA amplification was developed. This protocol is capable of detecting multiple Salmonella and/or E. coli organisms in the sample within less than a day, and it can also generate sufficient whole-genome sequences of the target organisms suitable for subsequent bioinformatics analysis. Multiplex detection and identification were accomplished in less than 20 h and additional whole-genome analyses of different nature were attained within 36 h, in contrast to the several days required in previous sequencing pipelines.

Reduction in pathogenic load of wheat by tempering with saline organic acid solutions at different seasonal temperatures

As a raw agricultural commodity, wheat is exposed to microbial contamination; therefore, enteric pathogens may be among its microbiota creating a food safety risk in milled products. This research evaluates (1) the effectiveness of organic acids dissolved in saline solutions to reduce the counts of pathogenic microorganisms in soft and hard wheat, and also investigates the effect of seasonal temperature on (2) survivability of pathogens in wheat kernels and on (3) pathogen inactivation during tempering with saline organic acid solutions. Wheat samples were inoculated with cocktails of either 5 serovars of Salmonella enterica, 5 E. coli O157:H7 or 6 non-O157 Shiga toxin-producing E. coli (STEC) strains to achieve a concentration of ~7 log CFU/g. Inoculated samples were allowed to stand for 7-days at temperatures (2.0, 10.8, 24.2, 32 °C) corresponding to those experienced during winter, spring/fall, and summer (average and maximum) in the main wheat growing regions in the state of Nebraska, USA. Besides water, solutions containing acid (acetic or lactic 2.5% or 5.0% v/v) and NaCl (~26% w/v) were used for tempering the wheat to 15.0% (soft) and 15.5% (hard) moisture at the different seasonal temperatures. The survival of pathogenic microorganisms throughout the resting period, and before and after tempering was analyzed by plating samples on injury-recovery media. The survival rate of pathogenic microorganisms on wheat kernels was higher at temperatures experienced during the winter (2.0 °C) and spring/fall (10.8 °C) months. Regardless of tempering temperature, the initial pathogen load was reduced significantly by all solutions when compared to the control tempered with water (P ≤ .05). The combination of lactic acid (5.0%) and NaCl was the most effective treatment against Salmonella enterica, E. coli O157:H7 and non-O157 STEC, with average reduction values of 1.8, 1.8 and 1.6 log CFU/g for soft wheat and 2.6, 2.4 and 2.4 log CFU/g for hard wheat, respectively. Implementation of organic acids and NaCl in tempering water may have the potential to reduce the risk of pathogen contamination in milled products.