A comparison of different chemical sanitizers for inactivating Escherichia coli O157:H7 and Listeria monocytogenes in solution and on apples, lettuce, strawberries, and cantaloupe

Assembly and Characterization of a Pathogen Strain Collection for Produce Safety Applications: Pre-growth Conditions Have a Larger Effect on Peroxyacetic Acid Tolerance Than Strain Diversity

Effective control of foodborne pathogens on produce requires science-based validation of interventions and control strategies, which typically involves challenge studies with a set of bacterial strains representing the target pathogens or appropriate surrogates. In order to facilitate these types of studies, a produce-relevant strain collection was assembled to represent strains from produce outbreaks or pre-harvest environments, including Listeria monocytogenes (n = 11), Salmonella enterica (n = 23), shiga-toxin producing Escherichia coli (STEC) (n = 13), and possible surrogate organisms (n = 8); all strains were characterized by whole genome sequencing (WGS). Strain diversity was assured by including the 10 most common S. enterica serotypes, L. monocytogenes lineages I-IV, and E. coli O157 as well as selected "non-O157" STEC serotypes. As it has previously been shown that strains and genetic lineages of a pathogen may differ in their ability to survive different stress conditions, a subset of representative strains for each "pathogen group" (e.g., Salmonella, STEC) was selected and assessed for survival of exposure to peroxyacetic acid (PAA) using strains pre-grown under different conditions including (i) low pH, (ii) high salt, (iii) reduced water activity, (iv) different growth phases, (v) minimal medium, and (vi) different temperatures (21°C, 37°C). The results showed that across the three pathogen groups pre-growth conditions had a larger effect on bacterial reduction after PAA exposure as compared to strain diversity. Interestingly, bacteria exposed to salt stress (4.5% NaCl) consistently showed the least reduction after exposure to PAA; however, for STEC, strains pre-grown at 21°C were as tolerant to PAA exposure as strains pre-grown under salt stress. Overall, our data suggests that challenge studies conducted with multi-strain cocktails (pre-grown under a single specific condition) may not necessarily reflect the relevant phenotypic range needed to appropriately assess different intervention strategies.

Oxidation of amino acids by peracetic acid: Reaction kinetics, pathways and theoretical calculations

Peracetic acid (PAA) is a sanitizer with increasing use in food, medical and water treatment industries. Amino acids are important components in targeted foods for PAA treatment and ubiquitous in natural waterbodies and wastewater effluents as the primary form of dissolved organic nitrogen. To better understand the possible reactions, this work investigated the reaction kinetics and transformation pathways of selected amino acids towards PAA. Experimental results demonstrated that most amino acids showed sluggish reactivity to PAA except cysteine (CYS), methionine (MET), and histidine (HIS). CYS showed the highest reactivity with a very rapid reaction rate. Reactions of MET and HIS with PAA followed second-order kinetics with rate constants of 4.6 ± 0.2, and 1.8 ± 0.1 M^-1⋅s^-1 at pH 7, respectively. The reactions were faster at pH 5 and 7 than at pH 9 due to PAA speciation. Low concentrations of H₂O₂ coexistent with PAA contributed little to the oxidation of amino acids. The primary oxidation products of amino acids with PAA were [O] addition compounds on the reactive sites at thiol, thioether and imidazole groups. Theoretical calculations were applied to predict the reactivity and regioselectivity of PAA electrophilic attacks on amino acids and improved mechanistic understanding. As an oxidative disinfectant, the reaction of PAA with organics to form byproducts is inevitable; however, this study shows that PAA exhibits lower and more selective reactivity towards biomolecules such as amino acids than other common disinfectants, causing less concern of toxic disinfection byproducts. This attribute may allow greater stability and more targeted actions of PAA in various applications.

Effect of post-harvest interventions on surficial carrot bacterial community dynamics, pathogen survival, and antibiotic resistance

Strategies to mitigate antibiotic-resistant bacteria (ARB), including human pathogens, on raw vegetables are needed to reduce incidence of resistant infections. The objective of this research was to determine the effectiveness of standard post-harvest interventions, sanitizer washing and cold storage, to reduce ARB, including antibiotic resistant strains of the human pathogen E. coli O15:H7 and a common spoilage bacterium Pseudomonas, on raw carrots. To provide a background inoculum representing potential pre-harvest carryover of ARB, carrots were dip-inoculated in dairy cow manure compost slurry and further inoculated with known ARB. Inoculated carrots were washed with one of three treatments: sodium hypochlorite (50 ppm free chlorine), peroxyacetic acid (40 ppm peroxyacetic acid; 11.2% hydrogen peroxide), tap water (no sanitizer), or no washing (control). Washed carrots were air dried, packaged and stored at 10 °C for 7d or 2 °C for up to 60 d. Enumeration was performed using total heterotrophic plate counts (HPC), HPCs on antibiotic-containing media ("ARBs"), E. coli O157:H7, and Pseudomonas sp. immediately after washing (0 d) and after 7 d of storage. In addition to the cultured bacteria, changes to the surficial carrot microbiota were profiled by sequencing bacterial 16S rRNA gene amplicons to determine the effect of sanitizer wash, storage temperature, and time of storage (0, 1, 7, 14 and 60 d). Storage temperature, addition of a sanitizer during wash, and duration of storage significantly affected the bacterial microbiota (Wilcoxon, p < 0.05). Inclusion of either sanitizer in the wash water significantly reduced the log CFU/g of E. coli O157:H7 and Pseudomonas sp., as well as HPCs enumerated on cefotaxime- (10 μg/ml), sulfamethoxazole- (100 μg/ml), or tetracycline (3 μg/ml) supplemented media compared to the unwashed control (ANOVA, p < 0.05). However, no significant reductions to bacteria resistant to vancomycin or clindamycin occurred after washing and storage. Members of the Proteobactetria, Firmicutes, Actinobacteria, Planctomycetes, and Acidobacteria comprised the bacterial carrot microbiota. The diversity of the carrot microbiota was significantly affected by the temperature of storage and by extended storage (60 d), when spoilage began to occur. There were no significant differences to the relative abundance of bacterial groups associated with the type of sanitizer used for washing. Results of this study indicate that inclusion of a sanitizer in wash water, followed by storage at 2 °C, might be an effective strategy to prevent re-growth of pathogenic E. coli O157:H7 and reduce levels of bacteria resistant to certain antibiotics on carrots.

Susceptibility of Escherichia coli O157:H7 to Disinfectants In Vitro and in Simulated Footbaths Amended with Manure

Escherichia coli O157:H7 is a human pathogen associated with gastrointestinal disease and hemolytic uremic syndrome. Direct contact with ruminants and their feces at agritourism or livestock interaction events is a known source of transmission. Footbath use is a pathogen reduction strategy that may decrease the transmission of E. coli O157:H7 at these interactions. The efficacy of chemical disinfectants in footbaths is not well reported. Our objective was to determine the susceptibility of E. coli O157:H7 toward commonly used disinfectants in vitro and within contaminated footbaths. The minimum inhibitory concentration and the minimum bactericidal concentration (MIC and MBC) and the time-to-kill were determined in vitro for seven E. coli O157:H7 strains using five disinfectant compounds (didecyldimethylammonium chloride [DDAC], glutaraldehyde, hydrogen peroxide, phenol, and sodium hypochlorite). Time-kill assays were performed within simulated footbaths at 22°C, 37°C, and 42°C with and without organic contamination using three commercial disinfectants with similar active ingredients (0.26% Clorox^® Bleach, 0.034% Virex^® II 256, and 1% Virkon^™ S). The MBCs of disinfectants toward E. coli O157:H7 were 3.2, 625, 40, 5000, and 320 ppm of DDAC, glutaraldehyde, hydrogen peroxide, phenol, and sodium hypochlorite, respectively. At 2 × MIC, E. coli O157:H7 reached a 3 log₁₀(colony-forming unit [CFU]/mL) reduction on contact with glutaraldehyde, by 20 min with DDAC and sodium hypochlorite, and by 4 h with phenol and hydrogen peroxide. In simulated footbaths, the commercial disinfectants reduced concentrations by 3 log₁₀(CFU/mL) on contact in the absence of organic contamination, but viable E. coli O157:H7 was recovered from organically contaminated Clorox Bleach and Virex II 256 footbaths. No E. coli O157:H7 was recovered from the Virkon S footbaths after 10 min. This study highlights the ability for organic contamination to compromise the efficacy of disinfectants in footbaths and the importance of choosing an appropriate footbath disinfectant to retain the efficacy.

Development of a Chlorine Dosing Strategy for Fresh Produce Washing Process to Maintain Microbial Food Safety and Minimize Residual Chlorine

The residual free chlorine level in fresh produce wash solution is closely correlated to the chemical and microbial safety of produce. Excess amount of free chlorine can quickly react with organic matters to form hazardous disinfection by-products (DBPs) above EPA-permitted levels, whereas deficiency of residual chlorine in produce wash solution may result in incompletely removing pathogens on produce. The purpose of this study was to develop a chlorine dosing strategy to optimize the chlorine dosage during produce washing process without impacting the microbial safety of fresh produce. Prediction equations were developed to estimate free chlorine needed to reach targeted residual chlorine at various sanitizer pH and organic loads, and then validated using fresh-cut iceberg lettuce and whole strawberries in an automated produce washer. Validation results showed that equations successfully predicted the initial chlorine concentration needed to achieve residual chlorine at 10, 30, 60, and 90 mg/L for both lettuce and strawberry washing processes, with the root mean squared error at 4.45 mg/L. The Escherichia coli O157:H7 reductions only slightly increased on iceberg lettuce and strawberries with residual chlorine increasing from 10 to 90 mg/L, indicating that lowering residual chlorine to 10 mg/L would not compromise the antimicrobial efficacy of chlorine-based sanitizer. Based on the prediction equations and E. coli O157:H7 reduction results, a chlorine dosing strategy was developed to help the produce industry to maintain microbial inactivation efficacy without adding excess amount of free chlorine.

PRACTICAL APPLICATION

The chlorine dosing strategy can be used for fresh produce washing process to enhance the microbial food safety and minimize the DBPs formation potential.

Survival of Listeria innocua on Fuji apples under commercial cold storage with or without low dose continuous ozone gaseous

This study evaluated the fate of Listeria innocua, a non-pathogenic species closely related to Listeria monocytogenes, on Fuji apple fruit surfaces during commercial cold storage with and without continuous low doses of gaseous ozone. Unwaxed Fuji apples of commercially acceptable maturity were inoculated with 6.0-7.0 Log₁₀ CFU L. innocua/apple, and subjected to refrigerated air (RA, 33 °F), controlled atmosphere (CA, 33 °F, 2% O₂, 1% CO₂), or CA with low doses of ozone gas (50.0 -87.0 ppb ) storage in a commercial facility for 30 weeks. A set of uninoculated apples was simultaneously subjected to the above storage conditions for total plate count and yeasts and molds enumeration. L. innocua survival under RA and CA storage was similar, which led to 2.5-3.0 Log₁₀ CFU/apple reduction during storage. Continuous gaseous ozone application decreased L. innocua population on Fuji apples to ∼1.0 Log₁₀ CFU/apple after 30-week storage, and suppressed apple native flora. CA storage delayed apple fruit ripening through reduction of apple firmness and titratable acidity loss, and low dose gaseous ozone application had no negative influence on apple visual quality, including both external and internal disorders. In summary, L. innocua decreased on Fuji apple surfaces during commercial long-term RA and CA storage. Ozone gas has the potential to be used as a supplemental intervention method to control Listeria spp. and to ensure fresh apple safety.

Antibacterial activity of acidified sodium benzoate against Escherichia coli O157:H7, Salmonella enterica, and Listeria monocytogenes in tryptic soy broth and on cherry tomatoes

Concerns about undesirable by-products from chlorine sanitation of fresh produce and the limited efficacy with the presence of organic matter, have led to studies on alternative washing solutions. The aim of this study was to evaluate the antibacterial activities of acidified sodium benzoate (NaB) solutions against Escherichia coli O157:H7, Salmonella enterica and Listeria monocytogenes in growth medium and on cherry tomatoes. Experimentally, the minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs, >3 Log reduction) of NaB against E. coli O157:H7 ATCC 43895, S. Enteritidis, and L. monocytogenes Scott A were determined at pH 7.0-4.0 using micro-broth dilution method and agar plating method, respectively. The reduction of the three bacteria in tryptic soy broth (TSB) by 500 and 1000 ppm NaB at pH 2.0, 2.5 and 3.0 for 30 min at 21 °C was compared. Residual bacterial cocktails inoculated on cherry tomatoes were determined after soaking in 3000 ppm NaB solution adjusted to pH 2.0 for 3 min at 21 °C. Results showed that the MBC of NaB reduced from >10,000 ppm at pH 7.0 to 1000 ppm at pH 4.0 and was identical for the three bacteria. The log reduction of bacteria in TSB indicated that 1000 ppm NaB at pH 2.0 was the most effective in killing the three pathogens. The respective reduction of E. coli O157:H7 and S. enterica cocktails inoculated on cherry tomatoes immersed in 3000 ppm NaB (pH 2.0) at 21 °C for 3 min was 4.99 ± 0.57 and 4.08 ± 0.65 log CFU/g, which was significantly higher (p < 0.05) than the treatments of 200 ppm free chlorine at pH 6.5. Conversely, the reduction of L. monocytogenes on tomatoes by 3000 ppm NaB (4.88 ± 0.73 log CFU/g) was similar (p > 0.05) to 200 ppm chlorine. Furthermore, the reduction of bacterial cocktails on tomatoes by 3000 ppm NaB at pH 2.0 was not affected after adding 1% tomato puree, and bacteria were not detected in NaB washing solutions with and without 1% tomato puree and on following un-inoculated tomatoes. This study showed that acidified NaB solution may be used as an alternative post-harvest wash of produce.

Effectiveness of different antimicrobial washes combined with freezing against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes inoculated on blueberries

To ensure the safety of produce, including blueberries, elimination of potential pathogens is critical. This study evaluated the efficacy of antimicrobial washes when coupled with frozen storage against Escherichia coli O157:H7, Salmonella Typhimurium and Listeria monocytogenes on wild blueberries. Inoculated blueberries were sprayed with antimicrobial solutions at different concentrations for various contact times (chlorine dioxide -2.5, 5, 10, and 15 ppm for 10 s, 1, 5, and 10 min; chlorine -100, 150, and 200 ppm for 10s, 1, 5, and 10 min; lactic acid 1 and 2% for 5, 10 and 20 min) and following treatment, stored at -12 °C for 1 week. Compared to antimicrobial washing alone, the additional freezing significantly reduced pathogens (P < 0.05). Concentrations of all three antimicrobials combined with freezing reduced L. monocytogenes to undetectable levels (detection limit < 1 log CFU/g). The greatest reduction of E. coli O157:H7 (4.4 log CFU/g) and Salmonella (5.4 log CFU/g) was achieved by 2% lactic acid or 200 ppm Cl₂ followed with frozen storage. These antimicrobials maintained the visual quality of blueberries and did not leave detectable residues. In conclusion, antimicrobial washes, when combined with frozen storage, effectively reduce the risk of pathogen contamination on blueberries.

Efficacy of Peracetic Acid in Inactivating Foodborne Pathogens on Fresh Produce Surface

Washing treatment with effective sanitizer is one of the critical steps in ensuring fresh produce safety. This study was to evaluate the efficacy of peracetic acid (PAA; VigorOx® 15 F&V), chlorine-based sanitizers (acidic electrolyzed water [AEO], near neutral electrolyzed water and bleach), lactic acid, and deionized (DI) water to reduce Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella Typhimurium DT104 from fresh produce surfaces. A 5-strain cocktail of E. coli O157:H7, L. monocytogenes, and S. Typhimurium DT104 was separately prepared and used for surface inoculation on produce samples (E. coli O157:H7 on romaine lettuce, lemons, tomatoes, and blueberries; L. monocytogenes on romaine lettuce and cantaloupe; S. Typhimurium DT104 on lemons, tomatoes, cantaloupe, and blueberries). PAA at 45, 85, and 100 mg/L; AEO, NNEO, and bleach at 100 mg/L of free chlorine; lactic acid at 2%; and DI water were used for washing inoculated produce in an automated produce washer for 5 min. In general, PAA at 100 mg/L achieved the highest microbial inactivation of E. coli O157:H7 (lettuce, lemon, tomato, and blueberry at 2.2, 5.7, 5.5, and 6.7 log CFU/g, respectively), S. Typhimurium DT104 (lemon, tomato, cantaloupe, blueberry at 5.4, 6.8, 4.5, and 5.9 log CFU/g, respectively), and L. monocytogenes (lettuce and cantaloupe at 2.4 and 4.4 log CFU/g, respectively). Efficacy of sanitizers on produce with coarse surface (for example, lettuce and cantaloupe) was lower than produce with smooth texture (lemon, tomato, and blueberry). Cross-contamination of E. coli O157:H7 among romaine lettuce heads during simulated retail crisping process was greatly reduced by the application of PAA and NNEO.

PRACTICAL APPLICATION

NNEO and PAA showed high efficacy in foodborne pathogen removal from fresh produce. Produce surface texture plays an important role in pathogen removal. NNEO and PAA effectively prevented cross-contamination during the crisping process.

Evaluation of Peroxyacetic Acid for Reducing Low Levels of Salmonella on Laboratory-Inoculated and Naturally Contaminated In-Shell Hazelnuts

In-shell hazelnuts may be exposed to many sources of Salmonella during harvest and postharvest processing. Chemical sanitizers, such as peroxyacetic acid (PAA), are used by some postharvest processors in an attempt to reduce microbial loads; however, data are limited on its efficacy to reduce Salmonella on in-shell hazelnuts under conditions relevant to the industry. This study analyzed the efficacy of PAA to reduce small numbers of Salmonella on laboratory-inoculated (3.9 most probable number [MPN] per hazelnut) and naturally contaminated in-shell hazelnuts (8.2 MPN/33 g). Batches of hazelnuts (333 to 1,500 g) were agitated in water or PAA (80 or 200 ppm) for 1 min. Inoculated hazelnuts were analyzed for the presence of Salmonella on an individual nut basis. Naturally contaminated hazelnuts were analyzed for the presence of Salmonella in larger sample sizes (33 g). Water and PAA (80 ppm) were effective at reducing Salmonella contamination on laboratory-inoculated hazelnuts (untreated, 85.5% positive; water, 38.7%; and PAA, 29.5%). Neither water nor PAA treatments (80 and 200 ppm) were effective in reducing Salmonella on naturally contaminated hazelnuts. The use of PAA in wash lines is unlikely to reduce Salmonella contamination on in-shell hazelnuts; however, PAA may reduce cross-contamination during postharvest washing activities.

Fate of Listeria monocytogenes on Fresh Apples under Different Storage Temperatures

Fresh apples are typically stored for up to 1 year commercially; different apple varieties require different storage temperatures to maintain their quality characteristics. There is sparse information available about Listeria monocytogenes survival on fresh apples under various storage temperatures. The objective of this study was to comprehensively evaluate the effect of storage temperature on apple fruit decay and L. monocytogenes survival. Unwaxed apple fruits of selected varieties (Fuji and Granny Smith) were dip inoculated in a three-strain L. monocytogenes cocktail to establish ∼3.5 and 6.0 Log10 CFU/apple. Twenty-four hours post-inoculation, apples were subjected to 1, 4, 10, or 22°C storage for up to 3 months. Apples under the different storage treatments were sampled at 1-, 4-, 7- and 14-day for short-term storage under all four tested temperatures, and 2-, 4-, 8-, and 12-week for long-term storage at 1, 4, and 10°C. A set of uninoculated and unwaxed apples were simultaneously subjected to the previously mentioned storage temperatures and sampled biweekly for their total bacterial count (TPC) and yeasts/molds (Y/M) count. During the 2-week short-term storage, L. monocytogenes population on organic Granny Smith apples stored at 1, 4, or 10°C was reduced by 0.2-0.3 Log. When apples were stored at 22°C, there was a 0.5-1.2 Log10 CFU/apple reduction 14-day post storage dependent on the initial inoculation level. During the 12-week cold storage under 1, 4, and 10°C, L. monocytogenes count on organic Granny Smith apples decreased by 0.5-1.5 Log10 CFU/apple for both inoculation levels. L. monocytogenes had similar survival pattern on conventional Granny Smith and Fuji apples with 0.8-2.0 Log10 CFU/apple reduction over a 3-month cold storage period. Interestingly, both TPC and Y/M count were stable regardless of apple variety or cultivation practice during the 12-week storage at all tested temperatures. In summary, while L. monocytogenes did not proliferate on apple surfaces during 12 weeks of refrigerated storage, only a limited reduction of L. monocytogenes was observed in this study. Therefore, the apple industry cannot rely on cold storage alone to control this pathogen. Additional interventions are needed to eradicate Listeria on fresh apples during long-term cold storage.

Slightly acidic electrolyzed water combined with chemical and physical treatments to decontaminate bacteria on fresh fruits

Effect of sequential combination of slightly acidic electrolyzed water (SAEW) with chemical and physical treatments on bacterial decontamination on fruits was investigated in this study. Effect of treatments on microbial and sensory quality was also analyzed after subsequent storage at 4 °C and room temperature (RT, 23 ± 0.15 °C). Whole apple and tomato fruits were inoculated with cocktail strains of Escherichia coli O157:H7 and Listeria monocytogenes. Uninoculated and inoculated fruits were washed first with distilled water (DW), calcium oxide (CaO), fumaric acid (FA), and SAEW at RT for 3 min. Combinations were performed by adding treatment one at a time to SAEW as following FA + SAEW, CaO + FA + SAEW, and CaO + FA + SAEW + ultrasonication (US) or microbubbles (MB). All the sanitizer treatments resulted in significant (p < 0.05) bacterial reduction compared to DW used as control. Increasing the treatments in combination from FA + SAEW to CaO + FA + SAEW + US resulted in an increased bacterial decontamination. The cavitation induced by ultrasonication in FA + SAEW solution resulted in a higher additive effect in decontamination of Escherichia coli O157:H7 and Listeria monocytogenes compare to the agitation generated by microbubble generator in FA + SAEW solution. CaO + FA + SAEW and CaO + FA + SAEW + US were effective in improving the microbial safety and quality of apple fruits. However, additional treatment of US impacted on the quality of tomato fruits during storage at RT. Therefore, a combination of SAEW with sanitizers (CaO and FA) and mechanical force (Ultrasonication) has the potential to be used in postharvest sanitation processing in the fresh fruit industry.

Persistence of Escherichia coli O157:H7 during pilot-scale processing of iceberg lettuce using flume water containing peroxyacetic acid-based sanitizers and various organic loads

In order to minimize cross-contamination during leafy green processing, chemical sanitizers are routinely added to the wash water. This study assessed the efficacy of peroxyacetic acid and mixed peracid against E. coli O157:H7 on iceberg lettuce, in wash water, and on equipment during simulated commercial production in a pilot-scale processing line using flume water containing various organic loads. Iceberg lettuce (5.4kg) inoculated to contain 106CFU/g of a 4-strain cocktail of non-toxigenic, GFP-labeled, ampicillin-resistant E. coli O157:H7, was shredded using a commercial shredder, step-conveyed to a flume tank, washed for 90s using water alone or two different sanitizing treatments (50ppm peroxyacetic acid or mixed peracid) in water containing organic loads of 0, 2.5, 5 or 10% (w/v) blended iceberg lettuce, and then dried using a shaker table and centrifugal dryer. Thereafter, three 5.4-kg batches of uninoculated iceberg lettuce were identically processed. Various product (25g) and water (50ml) samples collected during processing along with equipment surface samples (100cm2) from the flume tank, shaker table and centrifugal dryer were then assessed for numbers of E. coli O157:H7. Organic load rarely impacted (P>0.05) the efficacy of either peroxyacetic acid or mixed peracid, with typical reductions of >5logCFU/ml in wash water throughout processing for all organic loads. Increases in organic load in the wash water corresponded to changes in total solids, chemical oxygen demand, turbidity, maximum filterable volume, and oxidation/reduction potential. After 90s of exposure to flume water, E. coli O157:H7 reductions on inoculated lettuce ranged from 0.97 to 1.74logCFU/g using peroxyacetic acid, with an average reduction of 1.35logCFU/g for mixed peracid. E. coli O157:H7 persisted on all previously uninoculated lettuce following the inoculated batch, emphasizing the need for improved intervention strategies that can better ensure end-product safety.

Correlation of Conformational Changes and Protein Degradation with Loss of Lysozyme Activity Due to Chlorine Dioxide Treatment

Chlorine dioxide (ClO₂) is a potent oxidizing agent used for the treatment of drinking water and decontamination of facilities and equipment. The purpose of this research is to elucidate the manner in which ClO₂ destroys proteins by studying the effects of ClO₂ on lysozyme. The degree of enzyme activity lost can be correlated to the treatment time and levels of the ClO₂ used. Lysozyme activity was drastically reduced to 45.3% of original enzyme activity when exposed to 4.3 mM ClO₂ in the sample after 3 h. Almost all activities were lost in 3 h after exposure to higher ClO₂ concentrations of up to 16.8 and 21.9 mM. Changes in protein conformation and amount as a result of ClO₂ treatment were determined using the Raman spectroscopy and gel electrophoresis. Raman shifts and the alteration of spectral features observed in the ClO₂-treated lysozyme samples are associated with loss of the α-helix secondary structure, tertiary structure, and disulfide bond. Progressive degradation of the denatured lysozyme by increasing levels of chlorine dioxide was also observed in gel electrophoresis. Hence, ClO₂ can effectively cause protein denaturation and degradation resulting in loss of enzyme activity.

A Dual Filtration-Based Multiplex PCR Method for Simultaneous Detection of Viable Escherichia coli O157:H7, Listeria monocytogenes, and Staphylococcus aureus on Fresh-Cut Cantaloupe

Fresh-cut cantaloupe is particularly susceptible to contamination with pathogenic bacteria, such as Escherichia coli O157:H7, Listeria monocytogenes, and Staphylococcus aureus. Therefore, development of rapid, yet accurate detection techniques is necessary to ensure food safety. In this study, a multiplex PCR system and propidium monoazide (PMA) concentration were optimized to detect all viable pathogens in a single tube. A dual filtration system utilized a filtration membrane with different pore sizes to enrich pathogens found on fresh-cut cantaloupe. The results revealed that an optimized multiplex PCR system has the ability to effectively detect three pathogens in the same tube. The viable pathogens were simultaneously detected for PMA concentrations above 10 μg/ml. The combination of a nylon membrane (15 μm) and a micro pore filtration membrane (0.22 μm) formed the dual filtration system used to enrich pathogens. The achieved sensitivity of PMA-mPCR based on this dual filtration system was 2.6 × 103 cfu/g for L. monocytogenes, 4.3 × 10 cfu/g for E. coli O157:H7, and 3.1 × 102 cfu/g for S. aureus. Fresh-cut cantaloupe was inoculated with the three target pathogens using concentrations of 103, 102, 10, and 1 cfu/g. After 6-h of enrichment culture, assay sensitivity increased to 1 cfu/g for each of these pathogens. Thus, this technique represents an efficient and rapid detection tool for implementation on fresh-cut cantaloupe.

Effectiveness of Washing Procedures in Reducing Salmonella enterica and Listeria monocytogenes on a Raw Leafy Green Vegetable (Eruca vesicaria)

Vegetables are an important source of nutrients, but they can host a large microbial population, particularly bacteria. Foodborne pathogens can contaminate raw vegetables at any stage of their production process with a potential for human infection. Appropriate washing can mitigate the risk of foodborne illness consequent to vegetable consumption by reducing pathogen levels, but few data are available to assess the efficacy of different practices. In the present work, six different washing methods, in the presence or absence of sanitisers (peracetic acid and percitric acid, sodium bicarbonate, sodium hypochlorite) and vinegar, were tested for their effectiveness in reducing Salmonella and Listeria counts after artificial contamination of raw rocket (Eruca vesicaria). Results showed that washing with sodium hypochlorite (200 mg/L) was the only method able to produce a significant 2 Log reduction of Salmonella counts, but only in the case of high initial contamination (7 Log CFU/g), suggesting potential harmful effects for consumers could occur. In the case of Listeria monocytogenes, all the examined washing methods were effective, with 200 mg/L sodium hypochlorite solution and a solution of peracetic and percitric acids displaying the best performances (2 and 1.5 Log reductions, respectively). This highlights the importance of targeting consumers on fit for purpose and safe washing practices to circumvent vegetable contamination by foodborne pathogens.

Preliminary Study on the Effect of Fermented Cheese Whey on Listeria monocytogenes, Escherichia coli O157:H7, and Salmonella Goldcoast Populations Inoculated onto Fresh Organic Lettuce

Cheese whey fermented by an industrial starter consortium of lactic acid bacteria was evaluated for its antibacterial capacity to control a selection of pathogenic bacteria. For their relevance on outbreak reports related to vegetable consumption, this selection included Listeria monocytogenes, serotype 4b, Escherichia coli O157:H7, and Salmonella Goldcoast. Organically grown lettuce was inoculated with an inoculum level of ∼10(7) colony-forming unit (CFU)/mL and was left for about 1 h in a safety cabinet before washing with a perceptual solution of 75:25 (v/v) fermented whey in water, for 1 and 10 min. Cells of pathogens recovered were then counted and their number compared with that obtained for a similar treatment, but using a chlorine solution at 110 ppm. Results show that both treatments, either with chlorine or fermented whey, were able to significantly reduce (p < 0.05) the number of bacteria, in a range of 1.15-2.00 and 1.59-2.34 CFU/g, respectively, regarding the bacteria tested. Results suggest that the use of fermented whey may be as effective as the solution of chlorine used in industrial processes in reducing the pathogens under study (best efficacy shown for Salmonella), with the advantage of avoiding health risks arising from the formation of carcinogenic toxic chlorine derivates.

Fate of Listeria monocytogenes in Fresh Apples and Caramel Apples

An outbreak of listeriosis in late 2014 and early 2015 associated with caramel apples led to questions about how this product became a vector for Listeria monocytogenes. This investigation aimed to determine information about the survival and growth of L. monocytogenes in both fresh apples and caramel apples, specifically examining the effects of site and level of inoculation, inoculum drying conditions, and storage temperature. At a high inoculation level (7 log CFU per apple), L. monocytogenes inoculated at the stem end proliferated on Gala caramel apples at both 5 and 25°C and on Granny Smith caramel apples at 25°C by as much as 3 to 5 log CFU per apple. Fresh apples and caramel apples inoculated at the equatorial surface supported survival but not growth of the pathogen. Growth rates (μmax) for apples inoculated at the stem end, as determined using the Baranyi and Roberts growth model, were 1.64 ± 0.27 and 1.38 ± 0.20 log CFU per apple per day for Gala and Granny Smith caramel apples, respectively, stored at 25°C. At a low inoculation level (3 log CFU per apple), L. monocytogenes inoculated at the stem end and the equatorial surface survived but did not grow on fresh Gala and Granny Smith apples stored at 25°C for 49 days; however, on caramel apples inoculated at the stem end, L. monocytogenes had significant growth under the same conditions. Although certain conditions did not support growth, the pathogen was always detectable by enrichment culture. The inoculation procedure had a significant effect on results; when the inoculum was allowed to dry for 24 h at 5°C, growth was significantly slowed compared with inoculum allowed to dry for 2 h at 25°C. Variation in stick materials did affect L. monocytogenes survival, but these differences were diminished once sticks were placed into caramel apples.

Transfer of Pathogens from Cantaloupe Rind to Preparation Surfaces and Edible Tissue as a Function of Cutting Method

Whole and cut cantaloupes have been implicated as vehicles in foodborne illness outbreaks of norovirus, salmonellosis, and listeriosis. Preparation methods that minimize pathogen transfer from external surfaces to the edible tissue are needed. Two preparation methods were compared for the transfer of Listeria monocytogenes, Salmonella enterica serovar Typhimurium LT2, murine norovirus, and Tulane virus from inoculated cantaloupe rinds to edible tissue and preparation surfaces. For the first method, cantaloupes were cut into eighths, and edible tissue was separated from the rind and cubed with the same knife used to open the cantaloupes. For the second method, cantaloupes were scored with a knife around the circumference sufficient to allow manual separation of the cantaloupes into halves. Edible tissue was scooped with a spoon and did not contact the preparation surface touched by the rind. Bacteria and virus were recovered from the rinds, preparation surfaces, and edible tissue and enumerated by culture methods and reverse transcription, quantitative PCR, respectively. Standard plate counts were determined throughout refrigerated storage of cantaloupe tissue. Cut method 2 yielded approximately 1 log lower recovery of L. monocytogenes and Salmonella Typhimurium from edible tissue, depending on the medium in which the bacteria were inoculated. A slight reduction was observed in murine norovirus recovered from edible tissue by cut method 2. The Tulane virus was detected in approximately half of the sampled cantaloupe tissue and only at very low levels. Aerobic mesophilic colony counts were lower through day 6 of storage for buffered peptone water-inoculated cantaloupes prepared by cut method 2. No differences were observed in environmental contamination as a function of cutting method. Although small reductions in contamination of edible tissue were observed for cut method 2, the extent of microbial transfer underscores the importance of preventing contamination of whole cantaloupes.

Inactivation of Listeria monocytogenes, Salmonella spp. and Escherichia coli O157:H7 on cantaloupes by octenidine dihydrochloride

The efficacy of a new generation disinfectant, octenidine dihydrochloride (OH), as wash and coating treatments for reducing Listeria monocytogenes (LM), Salmonella spp. (SAL), and Escherichia coli O157:H7 (EC) on cantaloupe was investigated. Cantaloupe rind plugs inoculated separately with the three bacterial species (∼8 log CFU/cm(2)) were washed for 1, 3, 5 min at 25 °C in water, or chlorine (200 ppm), ethanol (1%), OH (0.01, 0.05, 0.1%) and surviving populations were measured after treatment. Additionally, inoculated cantaloupe rind plugs were coated with 2% chitosan or chitosan containing OH (0.01, 0.05, 0.1%) and sampled for surviving pathogens. Subsequently, the antimicrobial efficacy of OH wash and coating (0.1, 0.2%) on whole cantaloupes was determined. All OH wash reduced LM, SAL, and EC on cantaloupe rinds by > 5 log CFU/cm(2) by 2 min, and reduced populations to undetectable levels (below 2 log CFU/cm(2)) by 5 min (P < 0.05). Similarly, OH coating on cantaloupe rinds reduced the pathogens by 3-5 log /cm(2) (P < 0.05). Washing and coating whole cantaloupes with OH reduced the three pathogens by at least 5 log and 2 log CFU/cm(2), respectively (P < 0.05). Results suggest that OH could be used as antimicrobial wash and coating to reduce LM, SAL, and EC on cantaloupes.

Effectiveness of Broad-Spectrum Chemical Produce Sanitizers against Foodborne Pathogens as In Vitro Planktonic Cells and on the Surface of Whole Cantaloupes and Watermelons

Over the past few years, foodborne disease outbreaks linked to enteric pathogens present on cantaloupe and watermelon surfaces have raised concerns in the melon industry. This research evaluated the effectiveness of commercially available produce sanitizers against selected foodborne pathogens, both in cell suspensions and on the outer rind surface of melons. The sanitizers (65 and 200 ppm of chlorine, 5 and 35% hydrogen peroxide, 5 and 50 ppm of liquid chlorine dioxide, various hydrogen peroxide-acid combinations, 0.78 and 2.5% organic acids, and 300 ppm of quaternary ammonium) were tested against Escherichia coli O157:H7, Listeria monocytogenes, Salmonella, and non-O157 Shiga toxin-producing E. coli (O26, O45, O103, O111, O121, and O145). The cell suspension study revealed the ability of all tested sanitizers to reduce all selected pathogens by 0.6 to 9.6 log CFU/ml in vitro. In the melon study, significant differences in pathogen reduction were observed between sanitizers but not between melon types. The most effective sanitizers were quaternary ammonium and hydrogen peroxide-acid combinations, with 1.0- to 2.2-log CFU/g and 1.3- to 2.8-log CFU/g reductions, respectively, for all pathogens. The other sanitizers were less effective in killing the pathogens, with reductions ranging from 0.0 to 2.8 log CFU/g depending on pathogen and sanitizer. This study provides guidance to the melon industry on the best produce sanitizers for use in implementing a broad-spectrum pathogen intervention strategy.

Efficacy of oxidizing disinfectants at inactivating murine norovirus on ready-to-eat foods

Noroviruses are the leading cause of foodborne illness, and ready-to-eat foods are frequent vehicles of their transmission. Studies of the disinfection of fruits and vegetables are becoming numerous. It has been shown that strong oxidizing agents are more effective than other chemical disinfectants for inactivating enteric viruses. The aim of this study was to evaluate the efficacy of oxidizing disinfectants (sodium hypochlorite, chloride dioxide and peracetic acid) at inactivating noroviruses on fruits and vegetables, using a norovirus surrogate, namely murine norovirus 3, which replicates in cell culture. Based on plaque assay, solutions of peracetic acid (85 ppm) and chlorine dioxide (20 ppm) reduced the infectivity of the virus in suspension by at least 3 log10 units after 1 min, while sodium hypochlorite at 50 ppm produced a 2-log reduction. On the surface of blueberries, strawberries and lettuce, chlorine dioxide was less effective than peracetic acid and sodium hypochlorite, which reduced viral titers by approximately 4 logs. A surprising increase in the efficacy of sodium hypochlorite on surfaces fouled with artificial feces was noted.

Risk Factors for Salmonella, Shiga Toxin-Producing Escherichia coli and Campylobacter Occurrence in Primary Production of Leafy Greens and Strawberries

The microbiological sanitary quality and safety of leafy greens and strawberries were assessed in the primary production in Belgium, Brazil, Egypt, Norway and Spain by enumeration of Escherichia coli and detection of Salmonella, Shiga toxin-producing E. coli (STEC) and Campylobacter. Water samples were more prone to containing pathogens (54 positives out of 950 analyses) than soil (16/1186) and produce on the field (18/977 for leafy greens and 5/402 for strawberries). The prevalence of pathogens also varied markedly according to the sampling region. Flooding of fields increased the risk considerably, with odds ratio (OR) 10.9 for Salmonella and 7.0 for STEC. A significant association between elevated numbers of generic E. coli and detection of pathogens (OR of 2.3 for STEC and 2.7 for Salmonella) was established. Generic E. coli was found to be a suitable index organism for Salmonella and STEC, but to a lesser extent for Campylobacter. Guidelines on frequency of sampling and threshold values for E. coli in irrigation water may differ from region to region.

Effectiveness of levulinic acid and sodium dodecyl sulfate employed as a sanitizer during harvest or packing of cantaloupes contaminated with Salmonella Poona

Freshly harvested Eastern variety cantaloupes (Cucumis melo L. var. reticulatus cv. Athena) were subjected to three different harvest and wash treatments to examine conditions under which the efficacy of the sanitizer, levulinic acid (LV) plus sodium dodecyl sulfate (SDS), could be enhanced to reduce Salmonella contamination. In treatment set one, cantaloupes were spot inoculated with Salmonella enterica serovar Poona (prepared from solid or liquid media cultures) before or after a 1-min dip treatment in LV (2.5, 5.0, 7.5, or 10%) and 2.5% SDS. S. Poona initial populations on rind tissue (4.26-5.04 log CFU/sample) were reduced to detection by enrichment culture when cantaloupes were subsequently exposed to any of the LV/SDS solutions. When S. Poona was introduced after cantaloupes had been dip-treated, greater decreases in pathogen populations at the stem scar were observed when cantaloupes were treated with increasing concentrations of LV. In treatment set two, the response of S. Poona dip-treated with 5% LV/2.5% SDS was compared to a simulated commercial dump tank treatment incorporating 200 ppm chlorine as well as a two-stage treatment employing both the chlorine tank and LV/SDS dip treatments. S. Poona levels (log CFU/sample or # positive by enrichment culture/# analyzed) after treatments were 5.25, 3.07, 7/10, 5/10 (stem scar) and 3.90, 25/40, 28/40, 20/40 (rind) for non-treated, chlorine tank, LV/SDS dip, and tank plus dip treatments, respectively. In treatment set three, freshly harvested cantaloupes were first treated in the field using a needle-free stem scar injection (200 μl, 7.5% LV/1.0% SDS, 60 psi) and a cantaloupe spray (30 ml, 7.5% LV/0.5% SDS). Cantaloupe stem scar and rind tissue were then spot-inoculated with S. Poona using either a liquid or soil-based medium followed by a simulated dump tank treatment incorporating either 200 ppm chlorine or 5% LV/2% SDS. S. Poona inoculated on field-treated cantaloupe rind decreased by 4.7 and 5.31 (liquid) and 3.27 and 3.36 (soil) log CFU/sample after simulated chlorine and LV/SDS tank treatments, respectively. In the case of stem scar tissue, S. Poona populations exhibited a 1.0 log greater reduction when cantaloupes were treated with LV/SDS compared to chlorine in the dump tank (P<0.05). Based on this study, application of multiple hurdles is warranted, as additional decreases in S. Poona populations were obtained when cantaloupes were subjected to a chlorine dump tank followed by a LV/SDS dip treatment.

Effect of Disinfectants on Preventing the Cross-Contamination of Pathogens in Fresh Produce Washing Water

The potential cross-contamination of pathogens between clean and contaminated produce in the washing tank is highly dependent on the water quality. Process wash water disinfectants are applied to maintain the water quality during processing. The review examines the efficacy of process wash water disinfectants during produce processing with the aim to prevent cross-contamination of pathogens. Process wash water disinfection requires short contact times so microorganisms are rapidly inactivated. Free chlorine, chlorine dioxide, ozone, and peracetic acid were considered suitable disinfectants. A disinfectant’s reactivity with the organic matter will determine the disinfectant residual, which is of paramount importance for microbial inactivation and should be monitored in situ. Furthermore, the chemical and worker safety, and the legislative framework will determine the suitability of a disinfection technique. Current research often focuses on produce decontamination and to a lesser extent on preventing cross-contamination. Further research on a sanitizer’s efficacy in the washing water is recommended at the laboratory scale, in particular with experimental designs reflecting industrial conditions. Validation on the industrial scale is warranted to better understand the overall effects of a sanitizer.

Belgian and Spanish consumption data and consumer handling practices for fresh fruits and vegetables useful for further microbiological and chemical exposure assessment

A consumer survey was organized in Spain and Belgium to obtain consumption data and to gain insight into consumer handling practices for fresh vegetables consumed raw or minimally processed (i.e., heads of leafy greens, bell peppers, tomatoes, fresh herbs, and precut and packed leafy greens) and fruits to be consumed without peeling (i.e., apples, grapes, strawberries, raspberries, other berries, fresh juices, and precut mixed fruit). This information can be used for microbiological and/or chemical food safety research. After extensive cleanup of rough databases for missing and extreme values and age correction, information from 583 respondents from Spain and 1,605 respondents from Belgium (18 to 65 years of age) was retained. Daily intake (grams per day) was calculated taking into account frequency and seasonality of consumption, and distributions were obtained that can be used in quantitative risk assessment for chemical hazards with chronic effects on human health. Data also were recalculated to obtain discrete distributions of consumption per portion and the corresponding frequency of consumption, which can be used in acute microbiological risk assessment or outbreak investigations. The ranked median daily consumption of fruits and vegetables was similar in Spain and Belgium: apple > strawberry > grapes > strawberries and raspberries; and tomatoes > leafy greens > bell peppers > fresh herbs. However, vegetable consumption was higher (in terms of both portion and frequency of consumption) in Spain than in Belgium, whereas the opposite was found for fruit consumption. Regarding consumer handling practices related to storage time and method, Belgian consumers less frequently stored their fresh produce in a refrigerator and did so for shorter times compared with Spanish consumers. Washing practices for lettuce heads and packed leafy greens also were different. The survey revealed differences between these two countries in consumption and consumer handling practices, which can have an impact on outcomes of future microbiological or chemical risk assessment studies.

Survival of Salmonella and Escherichia coli O157:H7 on strawberries, basil, and other leafy greens during storage

The survival of Salmonella and Escherichia coli O157:H7 on strawberries, basil leaves, and other leafy greens (spinach leaves, lamb and butterhead lettuce leaves, baby leaves, and fresh-cut iceberg lettuce) was assessed at cold (<7 °C) and ambient temperatures. All commodities were spot inoculated with E. coli O157:H7 or Salmonella to obtain an initial inoculum of 5 to 6 log and 4 to 5 log CFU/g for strawberries and leafy greens, respectively. Samples were air packed. Strawberries were stored at 4, 10, 15, and 22 °C and basil leaves and other leafy greens at 7, 15, and 22 °C for up to 7 days (or less if spoiled before). Both Salmonella and E. coli O157:H7 showed a gradual decrease in numbers if inoculated on strawberries, with a similar reduction observed at 4, 10, and 15 °C (2 to 3 log after 5 days). However, at 15 °C (and 10 °C for E. coli O157:H7), the survival experiment stopped before day 7, as die-off of both pathogens below the lower limit of detection was achieved or spoilage occurred. At 22 °C, strawberries were moldy after 2 or 4 days. At that time, a 1- to 2-log reduction of both pathogens had occurred. A restricted die-off (on average 1.0 log) and increase (on average , 0.5 log) of both pathogens on basil leaves occurred after 7 days of storage at 7 and 22 °C, respectively. On leafy greens, a comparable decrease as on basil was observed after 3 days at 7 °C. At 22 °C, both pathogens increased to higher numbers on fresh-cut iceberg and butterhead lettuce leaves (on average 1.0 log), probably due to the presence of exudates. However, by using spot inoculation, the increase was rather limited, probably due to minimized contact between the inoculum and cell exudates. Avoiding contamination, in particular, at cultivation (and harvest or postharvest) is important, as both pathogens survive during storage, and strawberries, basil, and other leafy green leaves are consumed without inactivation treatment.