Enhanced removal of Escherichia coli O157:H7 and Listeria innocua from fresh lettuce leaves using surfactants during simulated washing

Removal of foodborne bacterial pathogens from fresh produce during washing and sanitation process can significantly improve inactivation of the bacteria. Currently, produce wash systems mainly rely on mechanical forces to aid in removal of bacteria attached to the produce surface during washing and sanitation. This study evaluates the potential of surfactants to enhance removal of pathogens from the surface of fresh produce. Influence of three types of commercial food-grade surfactants, including Tween-20, sodium dodecyl sulfate (SDS), and lauric arginate (LAE), on the mechanical removal of pathogenic bacteria and viruses from fresh lettuce leaves in the presence of soil was evaluated. The addition of surfactants did not increase the removal of T7 phages from lettuce leaf surface (P > 0.05). The improvement of bacterial removal by addition of surfactants to wash water is corresponding to the decrease of the contact angle between wash water and leaf surface. The most effective Escherichia coli O157:H7-lux removal was obtained by washing with 0.1% LAE, followed by 0.1% Tween-20, then 0.1% SDS. The most enhanced detachment of Listeria innocua was achieved by 0.1% LAE, followed by 0.1% SDS, then 0.1% Tween-20. The presence of soil resulted in an increased resistance of bacterial cells to the washing process. There was no significant difference in the cell persistence on the lettuce surface within the extended incubation period (P > 0.05). The evaluation of lettuce quality indicated that the introduction of surfactants during the washing procedure may affect the firmness of leaves, but the color and electrolyte leakage rate were not affected by the exposure to wash water with surfactants (P > 0.05). Overall, these results suggest the potential of food grade surfactants to enhance the removal of bacteria particularly foodborne pathogens from the surface of fresh produce.     

Evaluation of chlorine,benzalkonium chloride and lactic acid as sanitizers for reducing Escherichia coli O157:H7 and Yersinia enterocolitica on fresh vegetables

The effectiveness in the assurance of fresh vegetable microbiological quality of wash solutions containing 200 ppm free chlorine, 0.1 mg/ml benzalkonium chloride, 0.2% and 1% lactic acid was assessed on Escherichia coli O157:H7 and Yersinia enterocolitica contaminated lettuce and tomatoes. Y. enterocolitica reduction on tomatoes (5.08, 4.77 and 4.21 log after 0.2% lactic acid, 200 ppm chlorine and 0.1 mg/ml benzalkonium chloride treatments, respectively) were significantly higher than those for Y. enterocolitica on lettuce and E. coli O157:H7 on both vegetables. Antimicrobial treatment effects on bacterial counts and product quality after subsequent 7 day storage (4 °C and 22 °C) were determined. No pathogens were found in natural microflora of fresh vegetables.     

Effects of organic load,sanitizer pH and initial chlorine concentration of chlorine-based sanitizers on chlorine demand of fresh produce wash waters

Chlorine-based sanitizers are widely used in fresh produce industry. However, maintaining sufficient free chlorine residual in wash solution in the presence of organic matter is a challenge. The purpose of this study was to evaluate the effects of organic load, sanitizer pH and initial chlorine concentration on chlorine demand of fresh produce wash water. A full factorial design was used to study the chlorine demand of romaine lettuce wash water with different organic load when reacting with NaOCl solution at different pH (2.5, 4.0, 6.0, 8.0 and 9.5) and initial chlorine concentration (50, 75 and 100 mg L⁻¹ free chlorine residual). The results showed that the chlorine demand of lettuce wash water significantly increased (P ≤ 0.05) with increasing organic load. The significant effect (P ≤ 0.05) of initial chlorine concentration on chlorine demand was only detected at high organic load. Increasing the pH of NaOCl from 2.5 to 9.5 led to decrease in the chlorine demand except slight increase from 6.0 to 8.0. Equations for predicting chlorine demand of various fresh produce wash waters at different organic load for different sanitizer pH and initial chlorine concentration were developed, and verified using four types of produce (romaine lettuce, iceberg lettuce, strawberry and grape) and two types of chlorine-based sanitizers (NaOCl and electrolyzed water). Our study demonstrated that organic load, sanitizer pH and initial chlorine concentration all affected the chlorine demand of produce wash water, and chlorine demand prediction equations developed can be used for different types of fresh produce and chlorine-based sanitizers.     

Inactivation of Escherichia coli O157:H7 in vitro and on the surface of spinach leaves by biobased antimicrobial surfactants

This study was conducted to evaluate the effect of biosurfactants on the populations of Escherichia coli O157:H7 in suspension and on spinach leaves. Eight surfactants including four soybean oil-based biosurfactants, sodium dodecyl sulfate (SDS), polyoxyethylene sorbitan monooleate (Tween 80), sophorolipid (SO) and thiamine dilauryl sulfate (TDS) at concentrations of 0.1%, 0.5% and 1.0% were tested in bacterial suspension, and the most effective biosurfactants were applied on spinach leaves. Results showed that the soybean oil-based biosurfactants, SDS or Tween 80 did not significantly affect E. coli O157:H7 populations. SO and TDS at concentrations of 1.0% were effective in reducing E. coli O157:H7 populations in bacterial suspension. E. coli O157:H7 with an initial population of 7.1 log CFU/mL was not detectable (detection limit: 1 log CFU/mL) after 1 min in 1.0% TDS or after 2 h in 1.0% SO. On spinach leaves, SO at 1% did not significantly affect E. coli when compared to a water wash during 7 days post-treatment storage at 4 °C. However, TDS (1.0%) wash was as effective as 200 ppm chlorine in reducing population of spot inoculated E. coli O157:H7, achieving 3.1 and 2.7 log CFU/per leaf at day 0, and 1.4 and 1.9 log CFU/leaf at day 7 when compared with a water wash. No apparent change in spinach visual quality was observed. None of treatments caused changes in visual quality of spinach. Electron micrographs suggested ultrastructural damage of bacterial cells such as separation of the outer membrane from the cytoplasmic membrane. Overall, our results showed that SO and TDS may be potential sanitizers in inactivating human pathogens such as E. coli O157:H7 in wash water and on fresh produce.     

Efficacy of near neutral and alkaline pH electrolyzed oxidizing waters to control Escherichia coli O157:H7 and Salmonella Typhimurium DT 104 from beef hides

In this study efficacy of near neutral and alkaline pH electrolyzed oxidizing waters to reduce aerobic plate counts (AC) and Enterobacteriaceae (EC) from uninoculated fresh cattle hides and Escherichia coli O157:H7 and Salmonella Typhimurium DT 104 from inoculated hides were determined. Fresh hides were cut in to 15 by 20 cm pieces and subjected to a total of eight different treatment solutions; near neutral pH EO water (NEW-pH 6.5 at room temperature, 150 mg/L available chlorine), alkaline pH EO water (AEO-pH 11.6 at room temperature), hot alkaline pH EO water at 43 °C (HAEO-pH 11.60), alkaline pH EO water spray followed by 150 mg/L available chlorine containing near neutral pH EO water spray (A-NEW-both at room temperature), Blitz™ (PAA, pH 3.02 at room temperature), 5% lactic acid (LA, pH 2.04 at room temperature), deionized water (W) and no treatment (Control). For each treatment, 60 ml treatment solution was sprayed on hide using a hand held sprayer. Similar treatment protocol was employed to treat hide pieces inoculated with E. coli O157:H7 and S. Typhimurium DT 104. Five percent lactic acid spray treatment was found to be the most effective treatment and achieved 2.77, 2.74, 2.75 and 2.98 log CFU/cm² of AC, EC, E. coli O157:H7 and S. Typhimurium DT 104 reductions, respectively. All EO water treatments were equally effective in reducing all target microorganisms, except E. coli O157:H7. HAEO and A-NEW treatments yielded significant reduction of E. coli O157:H7 compared to other EO water treatments. These results indicate that various EO water treatments could become viable options to reduce pathogens on hide during slaughter.     

Simulation of decontamination and transmission of Escherichia coli O157:H7, Salmonella Enteritidis,and Listeria monocytogenes during handling of raw vegetables in domestic kitchens

Epidemiological data indicates that a large number of foodborne illnesses are attributed to cross-contamination during food preparation in the domestic kitchen. The objectives of this study were to evaluate the efficiency of household washing practices in removing Escherichia coli O157:H7, Listeria monocytogenes, and Salmonella Enteritidis on artificially contaminated lettuce and to determine the transfer rate of these three foodborne pathogens from contaminated lettuce to wash water, tomato, cabbage, and cutting boards during washing and cutting processes. Washing under the running tap water with scrubbing for 60 s was the most effective method in reducing pathogen populations by 1.86–2.60 log₁₀ CFU/g. Also, final rinsing and scrubbing practices were found to enhance the efficiency of washing treatment. In this study, the transfer rates of S. Enteritidis, E. coli O157:H7, and L. monocytogenes from cutting board to cabbage and tomato via cutting process (17.5–31.7%) were higher (P < 0.05) than from wash water to cabbage and tomato (0.8–23.0%) during washing treatment. Overall, our findings suggest that wash water and cutting board can be potential vehicles in the dissemination of foodborne pathogens. Therefore, there is a need to promote consumer awareness for proper handling practices in the kitchen to minimise the risk of foodborne infection.     

Determination of antimicrobial effect of mint and basil essential oils on survival of E. coli O157:H7 and S. typhimurium in fresh-cut lettuce and purslane

In this research, mint and basil essential oils at concentrations of 0.01 ml/L, 0.032 ml/L or 0.08 ml/L were used for disinfection treatments of fresh-cut lettuce and purslane samples inoculated with Salmonella typhimurium or Escherichia coli 0157:H7. Disinfection treatment time was applied as 10 min (short) or 15 min (long). Disinfected samples were packaged aerobically and stored at refrigerator +4 °C for 7 days. It was observed that mint and basil essential oils showed antimicrobial effect on the survival of E. coli O157:H7 and S. typhimurium inoculated into lettuce and purslane samples during refrigerated storage. Mint essential oil showed higher antimicrobial effect on pathogens when compared to basil essential oil. Mint and basil essential oils at concentration of 0.08 ml/L were the most effective antimicrobial treatment against pathogens in two different vegetable. S. typhimurium was more resistant against basil oil in lettuce samples when compared to its resistance against basil oil in purslane samples, whereas E. coli O157:H7 was more resistant to mint oil in purslane samples when compared to its resistance against mint oil in lettuce samples.     

Water reconditioning by high power ultrasound combined with residual chemical sanitizers to inactivate foodborne pathogens associated with fresh-cut products

The suitability of high power ultrasound (HPU, 20 kHz, 0.28 kW/l) combined with residual chemical sanitizers for water reconditioning was studied. A synergetic disinfection effect was observed when HPU was combined with peroxyacetic acid (PAA) or a commercial mix of organic acids and phenolic compounds (OA/PC). In recycled water (RW) with a chemical oxygen demand (COD) of 500 mg O₂/l, PAA inactivated 2 log units of Escherichia coli O157:H7 at concentrations of 3.2, 6.4, 16 mg/l after 7 min, 2 min, 29 s, respectively. The OA/PC or HPU treatments alone needed 26 min treatments to achieve the same reduction. The addition of TiO₂ (5 g/l) to HPU (sonocatalysis) did not improve E. coli O157:H7 inactivation. However, when HPU was combined with a residual concentration of PAA (3.2 mg/l), the total inactivation of E. coli O157:H7 and Salmonella (6 log unit reductions) occurred after 11 min, but for Listeria monocytogenes only 1.7 log reductions were detected after 20 min. When HPU was combined with OA/PC, a synergistic effect for the inactivation of E. coli O157:H7 was also observed, but this sanitizer significantly modified the physical-chemical quality characteristics of the RW. These results show that the residual PAA concentration that can be found in the wash water combined with HPU could result in an environmentally friendlier and toxicologically safer strategy for water reconditioning of the fresh-cut industry. The use of the sanitizer alone requires higher concentrations and/or longer contacts times. Even though the residual PAA in combination with HPU was adequate for water reconditioning, it is not appropriate for the process wash water because this wash water must be instantaneously disinfected.     

Effect of hydrogen peroxide vapor treatment for inactivating Salmonella Typhimurium,Escherichia coli O157:H7 and Listeria monocytogenes on organic fresh lettuce

In this study, the efficacy of hydrogen peroxide vapor (HPV) for reducing Salmonella Typhimurium, Escherichia coli O157:H7 and Listeria monocytogenes on lettuce was investigated as well as its effect on lettuce quality. Lettuce was inoculated with a cocktail containing three strains of each pathogen then treated with vaporized hydrogen peroxide for 0, 2, 4, 6, 8 and 10 min. The concentrations of hydrogen peroxide used were 0, 1, 3, 5 and 10%. With increasing treatment time and hydrogen peroxide concentration, HPV treatment showed significant (P < 0.05) reduction compared to the control (0%, treated with vaporized distilled water). In particular, vaporized 10% hydrogen peroxide treatment for 10 min was the most effective combination for reducing the three pathogens on lettuce. The reduction levels of S. Typhimurium, E. coli O157:H7 and L. monocytogenes on lettuce were 3.12, 3.15 and 2.95 log₁₀ CFU/g, respectively. Furthermore, there were no significant (P > 0.05) quality changes (color and texture) of lettuce among all tested samples, and hydrogen peroxide residues were not detected after 36 h storage time in any of the treated samples. These results suggest that HPV treatment could be an alternative method for reducing S. Typhimurium, E. coli O157:H7 and L. monocytogenes on fresh produce.     

Minimization of water consumption in fresh-cut salad washing by UV-C light

The decontamination effect of UV-C light at increasing fluence up to 1.2 kJ/m² was studied with reference to natural microflora and inoculated pathogens in lamb's lettuce wash water. UV-C light dose of 0.4 kJ/m² allowed to inactivate most of the native microflora and to achieve more than 5- Log-reductions in inoculated microorganisms (Salmonella enterica, Listeria monocytogenes and Escherichia coli). In multiple washing cycles up to 5, UV-C light treatment allowed to obtain more than 3-log reductions in native microflora in wash water. Spectral properties indicated that, independently on the number of recycling cycles, a considerable amount of UV-C light can penetrate wash water if its thickness is lower than circa 1 cm.     

Microbiological contamination linked to implementation of good agricultural practices in the production of organic lettuce in Southern Brazil

Interviews were conducted with the owners of three organic lettuce farms in the state of Rio Grande do Sul in southern Brazil using a standardized self-assessment questionnaire to ascertain the status of implementation of good agricultural practices and management systems in place. In addition, on each farm 132 samples (manure, field soil, water, workers' hands and equipment, lettuce seedlings, and crops) were collected during four visits throughout the lettuce crop production cycle and subjected to analysis for hygiene indicators (Escherichia coli, coliforms) and presence of Salmonella and E. coli O157. E. coli O157 was detected twice (in irrigation water and in rinse water) out of 27 analyzed water samples. Salmonella spp. was detected in one out of nine manure samples applied as organic fertilizer. In addition, generic E. coli was frequently present in numbers exceeding 10 cfu/g in manure, manured soil, and lettuce samples or more than 1 cfu/100 ml in water. No E. coli O157 was detected in any of the lettuce samples (n = 36), but Salmonella spp. was detected once in lettuce taken during the crop cycle 2 weeks before harvest. It was demonstrated that the combination of the self-assessment questionnaire and microbiological sampling and analysis could identify weak points in current organic farming practices in this region of southern Brazil. It was noted that manure composting was not adequately controlled and appropriate waiting times before application as an organic fertilizer to crop were not respected. Also the selection of the water source and the sanitary quality of the water used for irrigation were not under control. The washing step (with sanitizer) of lettuce crops at harvest, generally considered a potential reduction step for microbial contamination, was often not verified for its efficiency. This may detract from the sanitary quality of the produce and are risk factors for the introduction of pathogens in the lettuce sent to market. The study, combining interviews, sampling, and analysis, contributed to increasing the farmers' awareness of enteric pathogens as a food safety issue in leafy greens. Further communication and training on good agricultural practices are recommended to remediate the weak points identified in the current management system.     

Should chlorate residues be of concern in fresh-cut salads?

Chlorine remains the most popular method used by the fresh produce industry for decontamination. However, the occurrence of disinfection by-products (DBP) derived from chlorine-based disinfectants has been highlighted as a problem. After recent reports, chlorate residues in fresh produce are of concern in Europe. This study evaluated the chlorate accumulation in process wash water and the residues in fresh-cut lettuce when sodium hypochlorite was used as a wash aid. At a commercial processing facility, total chlorine was continually added to achieve a free chlorine level of 1–80 mg L⁻¹ for water disinfection as the organic load measured as chemical oxygen demand (COD) increased over time (1000–1500 mg O₂ L⁻¹). This resulted in chlorate accumulation (19–45 mg L⁻¹) in the process water. When fresh-cut lettuce was washed in that water, chlorate residues were detected in the lettuce and the concentrations increased linearly with the repeated use of the same process water, reaching concentrations of 4.5–5.0 mg kg⁻¹. To understand the chlorate accumulation in the process wash water, several experiments were performed at a pilot plant scale with different levels of COD and free chlorine. There was a significant (p < 0.001) correlation (R = 0.91) between the total added chlorine and the chlorate accumulation in the process water. We demonstrated that the added chlorine needed to maintain a free chlorine level in the process water was the contributing factor to chlorate accumulation. Chlorate residues in the washed fresh-cut lettuce after rinsing for 1 min in tap water and in commercial bags were below the limit of quantification. This study contributes to the knowledge of chlorate accumulation in the process water when sodium hypochlorite was used as a sanitizer.     

Inactivation of Cronobacter sakazakii in head lettuce by using a combination of ultrasound and sodium hypochlorite

The present study investigated the synergistic effects of combined ultrasound (37 kHz, 380 W for 5–100 min) and sodium hypochlorite (NaOCl) (50–200 ppm) on reducing Cronobacter sakazakii KCTC 2949 in head lettuce. Ultrasound was not enough to inactivate C. sakazakii (0.01–0.58 log-reduction), whereas NaOCl significantly (P < 0.05) reduced C. sakazakii (0.58–2.77 log-reduction). Despite the significant reduction in C. sakazakii with NaOCl treatment (200 ppm), the combination of 100 min ultrasound and 200 ppm NaOCl resulted in an additional 1.67 log-reduction of C. sakazakii (4.44 log-reduction = 2.77 + 1.67 log). Synergistic reductions on C. sakazakii were observed in most combined treatments, although the most synergistic reduction values were <1.0 log₁₀ CFU/g. The highest synergistic value of reduction in head lettuce was 1.08 log₁₀ CFU/g when treated with a combination of 100 min ultrasound and 200 ppm NaOCl. Moreover, the pH and °Brix after combined treatment with 5–100 min ultrasound and 50–200 ppm NaOCl did not significantly differ from any single treated lettuce. These results indicate that combined treatment with 100 min ultrasound and 200 ppm NaOCl could be a potential approach to reduce C. sakazakii on post-harvest leaf vegetable during processing and enhance the shelf-life during distribution and storage.     

Fate of Listeria monocytogenes and Escherichia coli O157:H7 in the presence of natural background microbiota on conventional and organic lettuce

Foodborne illnesses due to the consumption of contaminated raw vegetables is a continuing food safety concern. The limited efficacy of chlorine products to disinfect in fresh-cut industries, has led to study other methods or strategies to improve the safety of processed fresh-cut products. It has been reported that the presence of competing microorganisms on the surfaces of fresh produce can contribute to the reduction of pathogens. The aim of this study was to evaluate the interactions between the natural background microbiota of shredded conventional and organic lettuce and Listeria monocytogenes and Escherichia coli O157:H7. The effect of different initial load of background microbiota (‘low’, ‘medium’ and ‘high’) was tested for its ability to reduce L. monocytogenes and E. coli O157:H7 populations on shredded lettuce during storage at 10 ± 1 °C for 8 days. After the different pre-conditioning steps in order to obtain different initial loads of microbiota, in general, we observed that varying its size had no effect on L. monocytogenes and E. coli O157:H7 survival/growth during the storage period. Only differences on the survival/growth of L. monocytogenes and E. coli O157:H7 inoculated onto organic and conventional lettuce, respectively at the end of storage period at 10 °C were found. These results highlight the necessity for corrective measures to avoid contamination of fresh-cut vegetables with foodborne pathogens.     

A novel electrochemical device as a disinfection system to maintain water quality during washing of ready to eat fresh produce

Electrolyzed water (EW) is known by its bactericidal efficacy and capability to oxidize organic matter. The present research evaluated the efficacy of recently developed electrolytic cells able to generate higher concentration of reactive oxygen species using lower power and salt concentration than conventional cells. This study tested the inactivation of Escherichia coli O157:H7, the organic matter depletion and trihalomethane (THM) generation by EW in process wash water under dynamic conditions. To achieve this, clean tap water was continuously added up to 60 min with artificial process water with high chemical oxygen demand (COD) inoculated with E. coli O157:H7, in experiments performed in a pilot plant that recirculated water through one electrolytic cell. Plate counts of E. coli O157:H7, COD, THMs, free, combined and total chlorine, pH, temperature and oxidation-reduction potential were determined. Results indicate that the novel electrolysis system combined with minimal addition of NaCl (0.05%) was able to suppress E. coli O157:H7 population build-up and decreased the COD accumulation in the process wash water. THM levels in the water were relatively high but its concentration in the washed product was marginal. Highly effective electrolysis has been proven to reduce the occurrence of foodborne diseases associated to cross-contamination in produce washers without having an accumulation of THMs in the washed product.     

Synergetic antibacterial efficacy of cold nitrogen plasma and clove oil against Escherichia coli O157:H7 biofilms on lettuce

This study aims to evaluate the antimicrobial effects of cold nitrogen plasma (CNP) and clove oil against Escherichia coli O157:H7 (E. coli O157:H7) biofilm on lettuce. Both clove oil (1 mg/mL, 2 mg/mL and 4 mg/mL) and CNP (400–600 W) displayed significant eradication effect on E. coli O157:H7 biofilms in vitro (p < 0.001). Subsequently, the antibiofilm effect of combined treatment was studied as well. Compared with the respective treatment, combined treatment exhibited remarkable synergistic effect on eradicating E. coli O157:H7 biofilms. The confocal laser scanning microscopy (CLSM) and scanning electron microscopy (SEM) had also visually testified that the antibacterial effects of clove oil on E. coli O157:H7 biofilms (in vitro and on lettuce) were enhanced by CNP at 400 W for short treatment duration. The results of sensory evaluation indicated that combined treatment has mild negative effect on lettuce quality. Moreover, the synergetic antibacterial mechanism of clove oil and CNP against E. coli O157:H7 was concluded as that they could damage the bacterial cell wall and the outer membrane, leading to leakage of cellular components, such as nucleic acid and ATP.     

Assessment and speciation of chlorine demand in fresh-cut produce wash water

For the fresh-cut produce industry, a critical area of concern is potential pathogen cross-contamination during wash operations when wash water is reused and re-circulated in wash systems continuously imputed with fresh-cut produce. However, little research has focused on the chemical properties of wash water. Organic input from residual soil and vegetable material deteriorates water quality and creates increasing chlorine demand within this wash water.This study evaluated the origins of chlorine demand input and chlorine decay kinetics of fresh-cut produce wash water. Using a model system, vegetable juice released per kg of processed produce for shredded romaine lettuce, shredded iceberg lettuce, shredded carrot and baby spinach was 82.1 mL/kg, 94.5 mL/kg, 158 mL/kg, and 2.26 mL/kg, respectively. Batch water analysis revealed a rapid reaction between constituents in the wash water and chlorine where over a 90 min observation period, 50% of chlorine demand occurred within first 5 min, underscoring the challenge for any water treatment process to reduce chlorine demand once vegetables are deposited into washing systems. Moreover, the results also showed sustained chlorine demand over 90-min periods, indicating an accumulative effect on chlorine consumption with continuous organic input. Additionally, HPLC-SEC analysis showed that the constituents contributing to chlorine demand are predominantly dissolved small molecules (<3400 Da), which will challenge water reuse treatment approaches. This study provides quantitative information of chlorine demand origins and chlorine decay kinetics in wash water and provides baseline data critical for integrating water reuse in the fresh produce processing industry.     

Effects of UV-C treatment on inactivation of Salmonella enterica and Escherichia coli O157:H7 on grape tomato surface and stem scars,microbial loads,and quality

The purpose of this study was to investigate the effectiveness of ultraviolet-C (UV-C) light inactivation as affected by the location of pathogens on the surface and at stem scars of whole grape tomatoes. A mixed bacterial cocktail containing a three strain mixture of Escherichia coli O157:H7 (C9490, E02128 and F00475) and a three serotype mixture of Salmonella enterica (S. Montevideo G4639, S. Newport H1275, and S. Stanley H0558) were used. Tomatoes were spot inoculated using approximately 100 μL of inocula to achieve a population of about 10^7±1 CFU/tomato. Additionally, the effects of treatment on color, texture, lycopene content, and background microbial loads during post UV-C storage at 4 °C for 21 days were determined. Results showed that UV-C doses of 0.60–6.0 kJ/m² resulted in 2.3–3.5 log CFU per fruit reduction of E. coli O157:H7 compared to 2.15–3.1 log CFU per fruit reduction for Salmonella on the surfaces. Under the same conditions, log reductions achieved at stem scar were 1.7–3.2 logs CFU for E. coli O157:H7 and 1.9–2.8 logs CFU for Salmonella. The treatment was effective in controlling native microbial loads during storage at 4 °C as the total aerobic mesophilic organisms (PCA) and anaerobic lactic acid bacteria (LAB) counts of treated tomatoes were significantly (p < 0.05) lower during storage compared to the control group and the yeast and mold populations were reduced significantly below the detection limit. Furthermore, the firmness of tomato and its color was not affected by the UV-C doses during storage. UV-C radiation could potentially be used for sanitizing fresh tomatoes and extending shelf-life. The results of this study indicate that the specific location of pathogens on the produce influences the effectiveness of UV-C treatment, which should be taken into consideration for the design of UV-C systems for produce sanitization.     

Quantitative study of cross-contamination with Escherichia coli,E. coli O157, MS2 phage and murine norovirus in a simulated fresh-cut lettuce wash process

The water management in fresh-cut produce processing is an important factor affecting the microbial quality and safety of fresh-cut produce. For this study, a commercial lettuce washing process was simulated using two subsequent washing baths (WB). The worst case scenario, when no sanitizers are used and still common in some European countries, was investigated to fully understand the potential for cross-contamination and to obtain baseline data for further risk assessment. The two cross-contamination processes (from lettuce to water and from water to lettuce) were included in the simulation study and the transfer of Escherichia coli, E. coli O157, MS2 phage and murine norovirus was quantified. The mean reduction of initially contaminated lettuce through the washing in two successive WB was limited: 0.3 ± 0.1 log CFU E. coli/g after washing in WB1 and an additional reduction of 0.2 ± 0.1 log after WB2. The microbial load of the water in the washing baths, initially starting off with potable water, increased rapidly during the washing process of the contaminated lettuce. Furthermore to quantify the transfer of the four implicated micro-organisms from contaminated water to the lettuce, the first washing bath was inoculated with either approximately 3.0, 4.0 and 5.0 log CFU E. coli/100 ml or 4.8, 5.6 or 6.7 log CFU E. coli O157/100 ml, 4.0, 5.1 or 6.5 log PFU MS2 phages/100 ml or 6.5 log PFU/100 ml norovirus surrogate MNV-1. The contamination of the subsequently washed lettuce portions resulted in levels of ca. 1.0 up to 1.9 log CFU E. coli/g after passing the two washing steps. In addition, after a rapid initial increase, due to spill over of water from WB 1 to WB2, the contamination of WB2 further augmented during the washing process to approximately 1.0 to 0.5 log below the inoculation level of WB1. Transfer of E. coli O157, MS2 phages or MNV-1 from the water to the lettuce was respectively 0.9% ± 0.3%, 0.5% ± 0.2% and 0.5 ± 0.1% after WB1 and resulted in a contamination level for the highest inoculum level of WB1 of respectively ca. 2.9 ± 0.1 log CFU/g, 3.7 ± 0.1 log PFU/g and 4.4 ± 0.1 log PFU/g lettuce. The quantitative data of lettuce contamination and transfers established in this study further highlight the vulnerability of fresh produce to cross-contamination during the washing stage and stresses that notwithstanding the use of initial potable water and partial refreshment of water but without the use of sanitizers, microbial pathogens (or indicator organisms) may easily be introduced and reside for prolonged periods in washing baths enabling cross-contamination to the final fresh-cut product.     

Disinfection of selected vegetables under nonthermal treatments: Chlorine,acid citric,ultraviolet light and ozone

Lettuce, tomatoes and carrots were evaluated under four disinfection methods. Chlorine (50, 100 and 200 ppm) was compared for effectiveness with citric acid (0.5, 1 and 1.5%), ultraviolet light (UV-C) (0.65 and 1.6 mW/cm²) and ozone (5 ppm) to inactivate Escherichia coli ATCC 11775. Processing times were from 3 min up to 60 min. Hunter color parameters, color functions (ΔE, hue, chroma), tomato color index (TCI) and whiteness index (WI) were evaluated after disinfection. Results showed that citric acid was not effective for inactivation of E. coli at the tested conditions. UV-C was effective in the inactivation of the microorganism when fluence was higher, being more effective in the smooth surface of tomato (2.7 log). Meanwhile, ozone was also able to inactivate bacteria in tomatoes (2.2 log) after only 3 min. Carrots and lettuce showed lower inactivation for all treatments because of their porous and roughened surfaces. UV-C was the treatment that most affected the color of the produce; it generated browning of lettuce, and increase of TCI and WI of carrots. Ozone also affected the greenness of lettuce. Concentration, dose and processing times of novel disinfection methods need to be evaluated not only for microbial counts, but also sensory properties.