Produce Wash Water Research Articles

The influence of pH on the efficacy of oxidation-reduction potential (ORP) to predict chlorine disinfection of surrogate bacteria, Escherichia coli O157:H7 and Listeria monocytogenes in oxidant demand free conditions and fresh produce wash water

Oxidation-reduction potential (ORP) is commonly used as a rapid measurement of the antimicrobial potential of free chlorine during industrial fresh produce washing. The current study tested the hypothesis that ORP can act as a “single variable” measurement of bacterial (vegetative and endospores) inactivation effectiveness with free chlorine irrespective of the water pH value. This situation has on occasion been assumed but never confirmed nor disproven. Chlorine-dosed pH 6.5 and 8.5 phosphate buffer solutions were inoculated with Escherichia coli (E. coli), Listeria innocua (L. innocua), or Bacillus subtilis (B. subtilis) endospores. ORP, free chlorine (FC), and log reduction were monitored after 5 s (for E. coli and L. innocua) and up to 30 min (for B. subtilis spores) of disinfection. Logistic and exponential models were developed to describe how bacteria reduction varied as a function of ORP at different pH levels. Validation tests were performed in phosphate buffered pH 6.5 and 8.5 cabbage wash water periodically dosed with FC, cabbage extract and a cocktail of Escherichia coli O157:H7 (E. coli O157:H7) and Listeria monocytogenes (L. monocytogenes). The built logistic and exponential models confirmed that at equal ORP values, the inactivation of the surrogate strains was not consistent across pH 6.5 and pH 8.5, with higher reductions at higher pH. This is the opposite of the well-known free chlorine-controlled bacterial inactivation, where the antibacterial effect is higher at lower pH. The validation test results indicated that in the cabbage wash water, the relationship between disinfection efficiency and ORP was consistent with the oxidant demand free systems. The study suggests that ORP cannot serve as a reliable single variable measurement to predict bacterial disinfection in buffered systems. When using ORP to monitor and control the antibacterial effectiveness of the chlorinated wash water, it is crucial to take into account (and control) the pH.

Multi-criteria decision-making framework on process water treatment of minimally processed leafy greens

The use and application possibilities of the multi-criteria decision analysis (MCDA) methodology, as a decision support tool in food safety risk management was demonstrated in a case study on the washing of minimally processed leafy greens, a multidimensional food safety decision problem involving public health, economic, environmental and socio-cultural aspects. The washing procedure remains a critical step in the production chain of ready-to-eat leafy vegetables, as the process water could act as a vector for cross-contamination of pathogens. The use of chemical sanitizers in produce wash water is scattered over EU member states and regulations remain unharmonized. The most appropriate washing methodology was determined, for the Belgian situation, based on an evaluation of weighted criteria, following the PROMETHEE II algorithm. Hereto multiple sources of information were consulted, including scientific and grey literature, expert evaluations and information provided by vegetable processing companies. Data generated over the past 10 years and published in scientific literature were selected and aggregated into probabilistic distributions using @Risk® software. An online stakeholder consultation, organized in Belgium, had the aim to measure stakeholders' preferences. The MCDA case study indicated that washing leafy greens in potable (ice) water, should be considered the most appropriate method for the Belgian case. The MCDA methodology can be applied for building evidence-based risk management decisions in the field of food safety, as it provides a transparent framework for the collection of evidence and the inclusion of stakeholders' interests.

Sequential Application of Peracetic Acid and UV Irradiation (PAA–UV/PAA) for Improved Bacterial Inactivation in Fresh-Cut Produce Wash Water

Sequential Application of Peracetic Acid and UV Irradiation (PAA–UV/PAA) for Improved Bacterial Inactivation in Fresh-Cut Produce Wash Water

Assessment of Preparation Methods to Produce a Postharvest Spinach Wash Water Model for Sanitizer Validation Studies and Comparison of Sanitizer Quantitation Methods

Currently no standard benchtop preparation method exists for simulated produce wash water, which makes it challenging to compare sanitizer efficacy reports and provide guidance for growers regarding water quality monitoring and free chlorine quantification. This work compares benchtop preparation methods for spinach-based model wash water (blender vs stomacher), metrics for organic load standardization (chemical oxygen demand (COD) vs nephelometric turbidity units (NTU)), and free chlorine quantitation methods (N,N-diethyl-p-phenylenediamine (DPD) vs iodometric titration (IOD)). It was found that COD is a more reliable metric for organic load standardization than NTU. Blender- and stomacher-generated wash water had similar physicochemical properties at organic loads up to 1000 mg/L COD, so both methods are acceptable, and DPD titration reflected expected patterns of free chlorine consumption in wash water more accurately than IOD. These results support the use of select wash water preparation and free chlorine detection methods, informing the development of a standardized protocol.

Integration of electron beam technology into fresh produce wash water line: Effect of inoculum suspension medium and water quality parameters on the radioresistance of Salmonella Typhimurium ATCC 13311

AbstractElectron beam technology may be integrated into the fresh produce wash water line, as an advanced oxidation process (AOP). However, prior to implementation, effectiveness of the technology must be assessed. Hydroxyl radical scavengers were used to quantify the role of yield of hydroxyl radicals in inactivation of the selected pathogen in aqueous solution. The combined lethal effect of e‐beam treatment with aqueous hydrogen peroxide (H2O2) solution was also investigated. The radioresistance of Salmonella Typhimurium ATCC 13311 in deionized (DI) water, that is radiation D10 values, increased (p < .05) from 0.15 ± 0.00 kGy to 0.18 ± 0.00 kGy and to 0.19 ± 0.06 kGy when peptone water (PW) and phosphate buffered saline (PBS) was dissolved into DI water, respectively. Membrane‐permeable ethanol provided better protection for this pathogen than nonpermeable PEG. Results suggest that to minimize the protective effects of inoculating suspending media used for the preparation of Salmonella suspension, 1.0 mM phosphate buffer (PB) should be used in microbial studies of irradiated foods. Nitrate and dissolved organic carbon (DOC) reacting with hydrated electrons (eaq−) and hydroxyl radicals (·OH), respectively, induced more sequestering of ·OH. E‐beam treatment with 0.60 kGy dose in aqueous solution with H2O2 led to a reduction of 6.44 ± 0.03 log CFU/mL f S. Typhimurium ATCC 13311.

Managing microbes

Managing microbes

Characterization and mitigation of chemical oxygen demand and chlorine demand from fresh produce wash water

Washing is essential in fresh-cut produce industry to remove debris and ensure desirable shelf life. The intensive water usage during washing and water scarcity in the production regions necessitate proper water reuse and reconditioning. However, the organic compounds accumulated in the water elevate chlorine demand (CLD) and chemical oxygen demand (COD), thus complicating food sanitization and subsequent water treatment. Hereby, we examined the major sources of COD and CLD during fresh-cut produce washing, and explored the mitigation of CLD by treating select water constituents for the first time. Results showed that sugars as the predominant composition contribute to over 80% of COD in the wash water, whereas minor constituents including proteins/peptides, acids, and phenolic compounds account for over 80% of total CLD. The actual CLDs by those compounds depend highly on produce type. Moreover, while removing large molecules from the wash water led to the greatest CLD reduction for romaine lettuce, absorption of anionic molecules proved the most effective for iceberg lettuce, onion, and carrot wash water. This study suggests the possibility of mitigating CLD by more targetted and cost-effective water treatment procedures without the need for removing sugars. It also recommends prudent and product-specific design for produce sanitization and water treatments.

Detection of Cyclospora cayetanensis in produce irrigation and wash water using large-volume sampling techniques

The recent increase of reported cyclosporiasis outbreaks associated with fresh produce has highlighted the need for understanding environmental transmission of Cyclospora cayetanensis in agricultural settings and facilities. Conducting such environmental investigations necessitates robust sample collection and analytical methods to detect C. cayetanensis in water samples. This study evaluated three sample collection methods for recovery of C. cayetanensis oocysts from water samples during seeded recovery experiments. Two filtration-based methods, dead-end ultrafiltration (DEUF) and USEPA Method 1623.1, were evaluated for oocyst recovery from irrigation water. A non-filter-based method, continuous flow centrifugation (CFC), was evaluated separately for recovery from creek water and spent produce wash water. Median C. cayetanensis recovery efficiencies were 17% for DEUF and 16–22% for Method 1623.1. The DEUF method proved to be more robust than Method 1623.1, as the recovery efficiencies were less variable and the DEUF ultrafilters were capable of filtering larger volumes of high-turbidity water without clogging. Median C. cayetanensis recovery efficiencies for CFC were 28% for wash water and 63% for creek water, making it a viable option for processing water with high turbidity or organic matter. The data from this study demonstrate the capability of DEUF and CFC as filter-based and non-filter-based options, respectively, for the recovery of C. cayetanensis oocysts from environmental and agricultural waters.

Practical applications of sensor-based methodologies for monitoring peracetic acid (PAA) as a disinfectant of fresh produce wash water

An accurate sensor for peracetic acid (PAA) is needed to monitor and control the disinfection of fresh produce wash water. Inaccurate measurements can result in underdosing and unsafe produce or overdosing with huge cost implications. In this study, five methods were compared for measuring PAA in process water from washing apples, tomatoes, red sweet peppers, and lemons. A reversed phase HPLC-DAD method for the determination of PAA in water was adapted for process wash water and used as a reference method. The results showed that HPLC offered high selectivity for PAA and precise and reproducible measurement for its quantification. However, as HPLC is not suitable for in-line applications in processing plants and requires highly skilled technicians, installation and maintenance costs, other analytical methods were examined including a redox titration, a reflectometric method and two electrochemical methods (an amperometric in-line probe and a chronoamperometric method with disposable single-use electrode sensor). The drop titration kit overestimated PAA because of the interferences from other oxidizing agents such as hydrogen peroxide (HP) apart from it required substantial time to perform the measurements plus the extensive training of the technical personnel to ensure confidence in the results. The reflectometric method with disposable test strips, although fast and objective, overestimated PAA due to the wash water with a high content of suspended organic matter that needed the addition of high concentrations of PAA, coexisting with HP. From the two electrochemical methods studied, the in-line amperometric sensor underestimated the PAA concentration in lemon wash water as the PAA could not freely diffuse through the membrane through the active electrode because of the organic matter present in the wash water released from the lemon peel. In contrast, the chronoamperometric sensor showed good selectivity for PAA without interferences, with reliable results across all the wash water types and PAA concentrations.

Rapid detection of Escherichia coli using bacteriophage-induced lysis and image analysis.

Rapid detection of bacterial pathogens is a critical unmet need for both food and environmental samples such as irrigation water. As a part of the Food safety Modernization Act (FSMA), The Produce Safety rule has established several requirements for testing for the presence of generic Escherichia coli in water, but the current method available for testing (EPA M1603) demands specified multiple colony verification and highly trained personnel to perform these tests. The purpose of the study was to assess a phage induced bacterial lysis using quantitative image analysis to achieve rapid detection of E. coli at low concentrations within 8 hours. This study aimed to develop a simple yet highly sensitive and specific approach to detect target bacteria in complex matrices. In the study, E. coli cells were first enriched in tryptic soy broth (TSB), followed by T7 phage induced lysis, concentration, staining and fluorescent imaging. Image analysis was conducted including image pre-processing, image segmentation and quantitatively analysis of cellular morphological features (area, eccentricity and full width at half maximum). Challenge experiments using realistic matrices, including simulated fresh produce wash water, coconut water and spinach wash water, demonstrated the method can be applied for use in situations that occur in food processing facilities. The results indicated E. coli cells that are lysed by T7 phages demonstrated significantly (P < 0.05) higher extracellular DNA release, altered cellular shape (from rod to circular) and diffused fluorescent signal intensity. Using this biosensing strategy, a sensitivity to detect Escherichia coli at 10 CFU/ml within 8 hours was achieved, both in laboratory medium and in complex matrices. The proposed phage based biosensing strategy enables rapid detection of bacteria and is applicable to analysis of food systems. Furthermore, the steps involved in this assay can be automated to enable detection of target bacteria in food facilities without extensive resources.

Simultaneous quantification of peracetic acid and hydrogen peroxide in different water matrices using HPLC-UV

With low potential to generate harmful by-products, peracetic acid (PAA) has drawn increasing attention as an alternative oxidant for disinfection and advanced oxidation processes in wastewater treatment. Commercial formulations contain significant concentrations of both PAA and H2O2 in aqueous solutions and a robust method to distinctively quantify the two respective oxidants simultaneously is needed. This study aimed to improve the accuracy of employing methyl p-tolyl sulfide (MTS) and triphenylphosphine (TPP) as indicators for PAA and H2O2, respectively, under various environmental conditions, by the detection of the oxidation products of sulfoxide MTSO and phosphine oxide TPPO using HPLC-UV. To improve the analytical method, the reaction rates of MTS and TPP with PAA and H2O2, impact of pH on the detection, and matrix effects of real wastewater effluents and produce wash water were evaluated to minimize the interference of H2O2 with PAA determination. The determined rate constants of PAA reaction with MTS (kPAA/MTS=34.6±0.4M−1s−1 in 2/1 H2O/acetonitrile (ACN) (v/v)) and H2O2 reaction with TPP (kH2O2/TPP=2.7±0.2M−1s−1 in 1/1 H2O/ACN (v/v)) provided the fundamental guidance to optimize the method. Overall, a highly accurate and sensitive method for simultaneous quantification of PAA and H2O2 (method quantification limit = 0.8 and 6.0 μM, respectively) is established and will be useful for various environmental samples with PAA and H2O2 applications.

Use of Chlorine Dioxide to Treat Recirculated Process Water in a Commercial Tomato Packinghouse: Microbiological and Chemical Risks

Antimicrobial treatment of fresh produce wash water can enable water reuse by avoiding microbial cross-contamination. In this study, the efficacy of chlorine dioxide (ClO2; 2-3 mg/L) to maintain the microbiological quality of process wash water in a tomato packinghouse was assessed. Different parameters were measured, including the physicochemical characteristics of the wash water (pH, oxidation-reduction potential (ORP), chemical oxygen demand (COD), absorbance at 254 nm (UVA254), temperature, electrical conductivity (EC)), the levels of different microorganisms present in water and in the product (aerobic mesophilic bacteria (AMB), total coliforms, yeasts and molds, E. coli), and the presence of chlorate as disinfection by-product. The treatment with ClO2 had a significant effect on the microbial populations present in the recirculated wash water and the tomatoes compared with the use of untreated recirculated water. The microbial load in tomatoes washed with tap water, and ClO2-treated recirculated water was similar. The transfer of chlorate from wash water to the fruits was very low under the conditions tested. The results obtained indicate that ClO2 could be a suitable option for the reuse of wash water in commercial packinghouses, as it reduces the microbial populations in wash water while not leading to the presence of disinfection by-products, such as chlorate, in the final product. This study shows that the monitorization and control of the microbiological quality of process wash water should be managed together with good practices able to guarantee hygienic conditions of the equipment by implementing efficient cleaning and disinfection systems.

Prevalence of Salmonella and Listeria monocytogenes in non-traditional irrigation waters in the Mid-Atlantic United States is affected by water type, season, and recovery method.

Irrigation water contaminated with Salmonella enterica and Listeria monocytogenes may provide a route of contamination of raw or minimally processed fruits and vegetables. While previous work has surveyed specific and singular types of agricultural irrigation water for bacterial pathogens, few studies have simultaneously surveyed different water sources repeatedly over an extended period of time. This study quantified S. enterica and L. monocytogenes levels (MPN/L) at 6 sites, including river waters: tidal freshwater river (MA04, n = 34), non-tidal freshwater river, (MA05, n = 32), one reclaimed water holding pond (MA06, n = 25), two pond water sites (MA10, n = 35; MA11, n = 34), and one produce wash water site (MA12, n = 10) from September 2016—October 2018. Overall, 50% (84/168) and 31% (53/170) of sampling events recovered S. enterica and L. monocytogenes, respectively. Results showed that river waters supported significantly (p < 0.05) greater levels of S. enterica than pond or reclaimed waters. The non-tidal river water sites (MA05) with the lowest water temperature supported significantly greater level of L. monocytogenes compared to all other sites; L. monocytogenes levels were also lower in winter and spring compared to summer seasons. Filtering 10 L of water through a modified Moore swab (MMS) was 43.5 (Odds ratio, p < 0.001) and 25.5 (p < 0.001) times more likely to recover S. enterica than filtering 1 L and 0.1 L, respectively; filtering 10 L was 4.8 (p < 0.05) and 3.9 (p < 0.05) times more likely to recover L. monocytogenes than 1L and 0.1 L, respectively. Work presented here shows that S. enterica and L. monocytogenes levels are higher in river waters compared to pond or reclaimed waters in the Mid-Atlantic region of the U.S., and quantitatively shows that analyzing 10 L water is more likely recover pathogens than smaller samples of environmental waters.

Effect of organic load on the efficacy of activated persulfate in inactivating Escherichia coli O157:H7 and the production of halogenated by-products

Activated persulfate is highly effective in inactivating foodborne pathogens. There is great potential to use this chemical in fresh produce sanitation. The present study evaluated the potential effect of organic load in fresh produce wash water on the bactericidal efficacy of activated persulfate. Romaine lettuce, iceberg lettuce, strawberries, and grapes were used to prepare the produce extracts which were later diluted with water to simulate produce wash water containing different levels of organic matters, represented by the Chemical Oxygen Demand (COD, 100–2000 mg/L). Ferrous activated persulfate (PS/Fe2+) and alkaline activated persulfate (PS/OH−) were prepared in simulated wash water. A five-strain cocktail of Escherichia coli O157:H7 was added to the activated persulfate solution and treated for 2 min. Water properties, including COD, phenolics, protein content, UV254 absorbance, turbidity, pH, and oxidation reduction potential (ORP), were measured and correlated to the microbial reductions. Trihalomethanes, one of the major types of chlorinated disinfection byproducts, were also monitored. Romaine lettuce inoculated with E. coli O157:H7 was used to validate the organic load effect. The results showed that the organic loads in wash water significantly reduced the effectiveness of bacterial inactivation. The microbial reduction decreased from over 8.0 log CFU/mL to near 2.0 log CFU/mL. The organic load had a significantly larger effect on PS/Fe2+ than on PS/OH−. Based on the correlation coefficients, COD, phenolics, and UV254 were found to be the major indicators to predict the organic load effect. However, since UV254 measurement was quicker, UV254 was used as the primary indicator for real produce (romaine lettuce) application. When UV254 was adjusted to 0.75 in the simulated wash water, lower reductions of E. coli O157:H7 on romaine lettuce and higher cross-contamination potentials were observed during washing with PS/Fe2+, PS/OH−, and NaOCl as compared with the lettuce in wash water without additional organic load. In addition, no significant amount of THMs was generated in simulated wash water spiked with produce extracts during activated persulfate treatment. In conclusion, to maintain a high efficacy of activated persulfate for fresh produce washing, the organic load of wash water must be carefully monitored. Activated persulfate is advantageous to traditional chlorine sanitizers as no toxic chlorinated disinfection by-products will be generated.

Use of 254 nm ultraviolet light for decontamination of fresh produce and wash water

Fresh produce has been identified as the transmission vehicle for many foodborne illness outbreaks every year. They could be contaminated by pathogens at any point within the farm-to-fork food chain. Washing is often used to remove dirt and pathogens on fresh produce. Moreover, an effective decontamination method is required for wash water to prevent cross-contamination. Our group developed a water-assisted ultraviolet (UV) decontamination system (WUV) which combined washing and UV treatments. We evaluated the influence of turbidity (0–100 NTU) and chemical oxygen demand (COD; 0–2500 ppm) on UV inactivation of Salmonella in simulated wash water with different produce extracts. In the worst condition tested (turbidity of 100 NTU and COD of 2500 ppm), Salmonella in simulated wash water could be eliminated by UV (23–28 mW/cm2; wavelength peak at 254 nm) in 90 s (>6 log CFU/mL reduction), indicating the potential of UV for fresh produce wash water disinfection. Within the ranges tested, the effectiveness of UV inactivation of Salmonella was significantly decreased with increasing COD, but not with turbidity. In addition, ultraviolet lamps were installed in different locations (above and/or in the washing tank).Three WUV configurations showed similar Salmonella reduction on blueberries (1.8–2.0 log) and tomatoes (2.4–2.9 log). However, the Overhead and Dual WUV configurations reduced significantly (P < 0.05) more Salmonella on lettuce than the Submersible WUV configuration.

Dynamic changes in the physicochemical properties of fresh-cut produce wash water as impacted by commodity type and processing conditions.

Organic materials in fresh-cut produce wash water deplete free chlorine that is required to prevent pathogen survival and cross-contamination. This research evaluated water quality parameters frequently used to describe organic load for their fitness to predict chlorine demand (CLD) and chemical oxygen demand (COD), which are major needs identified by the industry-led produce food safety taskforce. Batches of romaine lettuce, iceberg lettuce, or carrot of different cut sizes and shapes were washed in 40 liters of water. Physicochemical properties of wash water including CLD, COD, total organic carbon (TOC), total suspended solids (TSS), total dissolved solids (TDS), turbidity, total sugar content, and pH, were monitored. Results indicate that pH is primarily commodity dependent, while organic load is additionally impacted by cutting and washing conditions. Significant linear increases in COD, TOC, CLD, TDS, and turbidity resulted from increasing product-to-water ratio, and decreasing cut size. Physicochemical parameters, excluding pH, showed significant positive correlation across different cut sizes within a commodity. High correlations were obtained between CLD and COD and between COD and TOC for pooled products. The convenient measurement of TDS, along with its strong correlation with COD and CLD, suggests the potential of TDS for predicting organic load and chlorine reactivity. Finally, the potential application and limitation of the proposed models in practical produce processing procedures are discussed extensively.

The use of redox potential to estimate free chlorine in fresh produce washing operations: Possibilities and limitations

Maintaining free chlorine (FC) residual at appropriate pH values is a control approach used to prevent pathogen cross-contamination during tomato dump tank handling and fresh-cut produce washing operations. Oxidation reduction potential (ORP) is a rapid measurement of oxidant-based sanitizer strength, and has been used to estimate FC residual. However, factors, in addition to FC and pH, which influence ORP are not fully understood. This study examined the relationship between ORP and FC under chlorine demand (CLD) free conditions and during fresh produce washing. An equation predictive of FC was developed in the form logFC = f(ORP, ORP2, ORP.pH). A good correlation between ORP and logFC was maintained when other variables changed, but the resulting ORP-logFC curve changed (slope, intercept). A decrease in pH or temperature led to an increase in ORP. Using tap water to wash the produce instead of distilled water significantly changed the ORP. For different types of tested produce, i.e., fresh-cut carrot, onion, romaine and iceberg lettuce, and for whole tomatoes, increasing the product-to-water ratio (i.e., increasing the organics transferred into the water) led to a decrease in ORP for a specific FC residual. The choice of acidulant during washing also influenced ORP. Overall, the correlation of ORP with logFC is more reliable at the lower end (5 mg/L FC) than at the higher end (100 mg/L FC) of the FC range used in fresh produce washing. However, since the ORP in fresh produce wash water is affected significantly in multiple ways by the wash water and process conditions, the predicted FC values with ORP under certain fresh-cut produce washing conditions cannot be generalized for other conditions.

Operational limits of sodium hypochlorite for different fresh produce wash water based on microbial inactivation and disinfection by-products (DBPs)

The goal of the present study was to establish free chlorine operational limits for the disinfection of different types of fresh-cut produce wash water using sodium hypochlorite (NaClO). The proposed operational limits in the wash water are based on both the effectiveness of microbial inactivation and the occurrence of disinfection by-products (DBPs). For this purpose, lab-scale dynamic experiments were performed simulating the conditions of commercial fresh-cut processing lines. Different free chlorine (FC) concentrations (≈0, ≈10, ≈20, and ≈30 mg/L) were maintained in process wash water (PWW) of shredded iceberg lettuce, baby leaves (rocket, red oak leaf, and red swiss chard), shredded red cabbage and diced onions by means of the constant addition of the different doses of NaClO. Physicochemical characteristics of PWWs were monitored during the experiments including FC, total chlorine (TC), pH, temperature, oxidation-reduction potential (ORP), electrical conductivity (EC), chemical oxygen demand (COD), absorbance at 254 nm (UV254), and the presence of DBPs such as trihalomethanes (THMs), haloacetic acids (HAAs) and chlorate. The antimicrobial capacity of FC doses was evaluated by the inactivation of indigenous total aerobic bacteria (TAB). In all cases, FC had a significant antimicrobial effect (p < 0.05), but the operation limit differed depending on the type of PWW. A lower FC operational limit based on TAB counts of 10 mg/L was effective for onion PWW, while levels up to 20–25 mg/L were established for lettuce and cabbage PWWs. However, the levels of TAB in PWW of baby leaves were not controlled even at the highest FC concentration of 30 mg/L. The establishment of a maximum operational limit for FC based on the concentration of DBPs in PWW was not possible as even the lowest FC concentration (10 mg/L) would potentially lead to unacceptable levels of chlorate in the different types of product washed including lettuce, baby leaves, red cabbage and onion.

Ozonized water with plant antimicrobials: An effective method to inactivate Salmonella enterica on iceberg lettuce in the produce wash water

Post-harvest washing of produce is performed to remove physical debris and to lower microbial load. The use of ozone in combination with plant-based antimicrobials was evaluated as an alternative to conventional sanitizers such as chlorine. Plant based antimicrobials that were evaluated in combination with ozone included oregano oil, carvacrol, Quillaja saponin and olive extract. Ozone was dispersed in phosphate buffered saline (PBS), following which individual antimicrobials or their combinations were added. Iceberg lettuce leaves (10 g portions) inoculated with Salmonella enterica serotype Newport (6.5 ± 1 log CFU/g) were added to the wash suspension. The leaves were tested for reduction in S. Newport population after 60, 90 and 120 min of treatment. Exposure to ozonized water for 120 min resulted in a 2.1 log CFU/g (p < 0.05) reduction in S. Newport population. The addition of 0.1% oregano oil to ozonized water resulted in 3 log CFU/g reduction after 120 min but a 4.1 log CFU/g reduction after 60 min, indicating that the antioxidant property of oregano oil might have diminished ozone activity and resuscitated injured S. Newport cells. The addition of 5% olive extract to ozonized water resulted in 4.2 log CFU/g reduction of S. Newport after 120 min (p < 0.05) of treatment. While 5% olive extract did not confer protection to S. Newport cells from ozone, 1% olive extract resulted in higher S. Newport survival after 120 min treatment than the 60 min treatment. The use of carvacrol (0.1%, 0.3% and 0.5%) in ozonized water reduced the pathogen population to below the limit of detection (10 CFU/g) (p < 0.05) which was in excess of 6 log CFU/g. These results indicate that the efficacy of ozone is compounded by the addition of certain plant-based antimicrobials when used at optimum concentrations. Ozone combined with plant antimicrobials could serve as an effective alternative to sanitizers currently used for washing and processing of produce.

Factors affecting the performance and monitoring of a chlorine wash in preventing Escherichia coli O157:H7 cross-contamination during postharvest washing of cut lettuce

This study examined factors that may affect the performance of a produce wash in preventing E. coli O157:H7 cross-contamination. Fresh-cut romaine lettuce (8 or 20 g) inoculated with ca. 7 log CFU/g of E. coli O157:H7 expressing green fluorescence protein was washed with uninoculated cut lettuce (800 or 2000 g) in 40 L of water for 2 min. Washing trials were performed in both sterile tap water and spent wash water collected from a commercial leafy greens processing facility under different prewash free chlorine levels (0, 5, 20 and 30 ppm), water temperatures (3 and 20 °C), leaf-to-water ratios (1:20 and 1:50), and flow rates (fast, medium, and slow). Performance of the chlorine wash was greatly affected by free chlorine level and wash water quality. Temperature, lettuce-to-water ratio, or water flow rate did not appear to have a significant impact. A prewash free chlorine concentration of 10 or 20 ppm was effective in preventing E. coli O157:H7 cross-contamination when lettuce was washed in tap water in the two washing systems assembled; 30 ppm of chlorine was needed to prevent cross-contamination during washing in industry water. Small-scale (100 ml) inactivation studies were performed to examine the impact of organic and solid contents (with the addition of lettuce juice and play sand) on the antimicrobial efficacy of chlorinated water. Increases in organic matter resulted in greater depletion of chlorine and the presence of solids enabled the pathogen to better survive chlorination; both led to a lower antimicrobial efficacy observed. ORP did not correlate linearly with free chlorine. E. coli O157:H7 cross-contamination was observed when ORP exceeded 650 mV, indicating that a threshold level of 650 mV may not ensure the safety of produce wash water. Since effective chlorine level is highly influenced by wash water quality, monitoring of produce washing systems may best be performed by measuring wash water quality in conjunction with rapid measurement of free chlorine.