Irradiation compared with chlorination for elimination of Escherichia coli O157:H7 internalized in lettuce leaves: influence of lettuce variety

Photocatalytically Enhanced Inactivation of Internalized Pathogenic Bacteria in Fresh Produce Using UV Irradiation with Nano-Titanium Dioxide

ABSTRACT

Once pathogens are internalized in fresh produce, they pose a challenging food safety issue because they are not effectively inactivated by conventional rinsing or sanitization. To protect food safety and public health, the objectives of this study were to examine internalized levels of foodborne pathogens in different types of fresh produce and to investigate the effectiveness of photocatalytically enhanced inactivation of internalized pathogens in fresh produce using UV irradiation with titanium dioxide (TiO2). For this, green fluorescent protein-labeled Salmonella Typhimurium and Escherichia coli O157:H7 were inoculated on the leaf surface of four types of fresh produce (∼108 CFU per leaf), and varying concentrations of TiO2 suspension (0.50, 0.75, 1.00, 1.25, and 1.50 μg/mL) were applied to the surface of contaminated leaves. Depending on the nature of each vegetable, the internalized bacterial level differed (2 to 5 log CFU/g of leaf). When UV irradiation (6,000 J/m2) was applied, the internalized Salmonella Typhimurium and E. coli levels were reduced by 0.8 to 2.4 log CFU per leaf; with TiO2, the reduction was 1.1 to 3.7 log CFU per leaf. The inactivation efficiency increased as the TiO2 concentration increased (up to 1.50 μg per leaf). These results indicate that TiO2 application enhanced the photocatalytic inactivation of internalized foodborne pathogens. The application of TiO2 would be most practical before UV irradiation and before distributing the produce. This study established a platform for future research on the inactivation of various internalized pathogens for protecting public health and scaling up fresh produce treatments by the food industry.

HIGHLIGHTS

Advanced Oxidation Process as a Postharvest Decontamination Technology To Improve Microbial Safety of Fresh Produce

Fresh produce is frequently associated with outbreaks of foodborne diseases; thus, there is a need to develop effective intervention technologies and antimicrobial treatments to improve the microbial safety of fresh produce. Washing with chemical sanitizers, commonly used by the industry, is limited in its effectiveness and is viewed as a possible cross-contamination opportunity. This review discuses the advanced oxidation process (AOP), which involves generating highly reactive hydroxyl radicals to inactivate human pathogens. Ionizing irradiation, ultraviolet (UV) light, and cold plasma can be regarded as AOP; however, AOPs employing combinations of UV, H₂O₂, cold plasma, and ozone may be more promising because higher amounts of hydroxyl radicals are produced in comparison to the individual treatments and the combinative AOPs may be more consumer friendly than ionizing irradiation. When applied as a gaseous/aerosolized treatment, AOPs may have advantages over immersion treatments, considering the reactivity of hydroxyl radicals and presence of organic materials in wash water. Gaseous/aerosolized AOPs achieve up to 5 log reductions of pathogenic bacteria on fresh produce compared to reductions of 1-2 logs with aqueous sanitizers. Further research needs to be conducted on specific AOPs before being considered for commercialization, such as reduced formation of undesirable chemical byproducts, impact on quality, and scaled up studies.

Evaluation by Flow Cytometry of Escherichia coli Viability in Lettuce after Disinfection

Foodborne outbreaks due to the consumption of ready-to-eat vegetables have increased worldwide, with Escherichia coli (E. coli) being one of the main sources responsible. Viable but nonculturable bacteria (VBNC) retain virulence even after some disinfection procedures and constitute a huge problem to public health due to their non-detectability through conventional microbiological techniques. Flow cytometry (FCM) is a promising tool in food microbiology as it enables the distinction of the different physiological states of bacteria after disinfection procedures within a short time. In this study, samples of lettuce inoculated with E. coli were subject to disinfection with sodium hypochlorite at free chlorine concentrations of 5, 10, 25, 50, and 100 mg·L⁻¹ or with 35% peracetic acid at concentrations of 5, 10, 25, and 50 mg·L⁻¹. The efficiency of these disinfectants on the viability of E. coli in lettuce was evaluated by flow cytometry with LIVE/DEAD stains. Results from this study suggest that FCM can effectively monitor cell viability. However, peracetic acid is more effective than sodium hypochlorite as, at half the concentration, it is enough to kill 100% of bacteria and always induces a lower percentage of VBNC. Finally, we can conclude that the recommended levels of chemical disinfectants for fresh fruit and vegetables are adequate when applied in lettuce. More importantly, it is possible to ensure that all cells of E. coli are dead and that there are no VBNC cells even with lower concentrations of those chemicals. These results can serve as guidance for lettuce disinfection, improving quality and the safety of consumption.

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

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

Gamma Irradiation Influences the Survival and Regrowth of Antibiotic-Resistant Bacteria and Antibiotic-Resistance Genes on Romaine Lettuce

Contamination of romaine lettuce with human pathogens, antibiotic-resistant bacteria (ARB), and antibiotic resistance genes (ARGs) occurs during production. Post-harvest interventions are emplaced to mitigate pathogens, but could also mitigate ARB and ARGs on vegetables. The objective of this research was to determine changes to lettuce phyllosphere microbiota, inoculated ARB, and the resistome (profile of ARGs) following washing with a sanitizer, gamma irradiation, and cold storage. To simulate potential sources of pre-harvest contamination, romaine lettuce leaves were inoculated with compost slurry containing antibiotic-resistant strains of pathogenic (Escherichia coli O157:H7) and representative of spoilage bacteria (Pseudomonas aeruginosa). Various combinations of washing with sodium hypochlorite (50 ppm free chlorine), packaging under modified atmosphere (98% nitrogen), irradiating (1.0 kGy) and storing at 4°C for 1 day versus 14 days were compared. Effects of post-harvest treatments on the resistome were profiled by shotgun metagenomic sequencing. Bacterial 16S rRNA gene amplicon sequencing was performed to determine changes to the phyllosphere microbiota. Survival and regrowth of inoculated ARB were evaluated by enumeration on selective media. Washing lettuce in water containing sanitizer was associated with reduced abundance of ARG classes that confer resistance to glycopeptides, β-lactams, phenicols, and sulfonamides (Wilcoxon, p < 0.05). Washing followed by irradiation resulted in a different resistome chiefly due to reductions in multidrug, triclosan, polymyxin, β-lactam, and quinolone ARG classes (Wilcoxon, p < 0.05). Irradiation followed by storage at 4°C for 14 days led to distinct changes to the β-diversity of the host bacteria of ARGs compared to 1 day after treatment (ANOSIM, R = 0.331; p = 0.003). Storage of washed and irradiated lettuce at 4°C for 14 days increased the relative abundance of Pseudomonadaceae and Carnobacteriaceae (Wilcoxon, p < 0.05), two groups whose presence correlated with detection of 10 ARG classes on the lettuce phyllosphere (p < 0.05). Irradiation resulted in a significant reduction (∼3.5 log CFU/g) of inoculated strains of E. coli O157:H7 and P. aeruginosa (ANOVA, p < 0.05). Results indicate that washing, irradiation and storage of modified atmosphere packaged lettuce at 4°C are effective strategies to reduce antibiotic-resistant E. coli O157:H7 and P. aeruginosa and relative abundance of various ARG classes.

Antimicrobial Particle-Based Novel Sanitizer for Enhanced Decontamination of Fresh Produce

Microbial food safety of raw or minimally processed fresh produce is a significant challenge. The current sanitation processes are effective for inactivation of bacteria in wash water but have limited efficacy (<2 log/g reduction) for inactivation of microbes on the surfaces of fresh produce. This study demonstrates a novel concept to enhance effectiveness of chlorine using a particle-based sanitizer to improve decontamination of fresh produce. In this concept, enhanced effectiveness is achieved by localized high concentration of chlorine bound to the surfaces of silica particles and improved surface contact of microparticles with the produce surface using mechanical shear during a washing process. The results of this study demonstrate that 500 ppm active chlorine can be bound to the surfaces of modified silica particles. These modified particles maintain over 90% of their initial chlorine content during extended storage in aqueous solution and provide improved inactivation of both Escherichia coli O157:H7, Listeria innocua, and Pseudomonas fluorescens in the presence of organic content in contrast to conventional chlorine sanitizer. The modified particles exhibit effective sanitation of fresh produce (>5-log reduction) in the presence of relatively high organic content (chemical oxygen demand of 500 mg/liter), demonstrating a potential to address a significant unmet need to improve fresh produce sanitation. The particle-based sanitizer had no significant effect on the quality of fresh lettuce.IMPORTANCE The limitation of current sanitation processes for inactivation of microbes on the surfaces of fresh produce is due to nonspecific consumption of sanitizers by reactions with the food matrix and complexity of surface chemistries and structural features of produce surfaces. This study demonstrates a novel approach to enhance sanitation effectiveness of fresh produce using a particle-based sanitizer. The particle-based sanitizer concept provides localized high concentration delivery of chlorine to the surfaces of fresh produce and enables more than 5 logs of inactivation of inoculated bacteria on fresh produce surfaces without significant changes in produce quality. The results of this study illustrate the potential of this approach to address the unmet need for improving sanitation of fresh produce. Further validation of this approach using a scaled-up produce washing system will enable commercialization of this novel concept.

Viable-but-Nonculturable Listeria monocytogenes and Salmonella enterica Serovar Thompson Induced by Chlorine Stress Remain Infectious

The microbiological safety of fresh produce is monitored almost exclusively by culture-based detection methods. However, bacterial food-borne pathogens are known to enter a viable-but-nonculturable (VBNC) state in response to environmental stresses such as chlorine, which is commonly used for fresh produce decontamination. Here, complete VBNC induction of green fluorescent protein-tagged Listeria monocytogenes and Salmonella enterica serovar Thompson was achieved by exposure to 12 and 3 ppm chlorine, respectively. The pathogens were subjected to chlorine washing following incubation on spinach leaves. Culture data revealed that total viable L. monocytogenes and Salmonella Thompson populations became VBNC by 50 and 100 ppm chlorine, respectively, while enumeration by direct viable counting found that chlorine caused a <1-log reduction in viability. The pathogenicity of chlorine-induced VBNC L. monocytogenes and Salmonella Thompson was assessed by using Caenorhabditis elegans Ingestion of VBNC pathogens by C. elegans resulted in a significant life span reduction (P = 0.0064 and P < 0.0001), and no significant difference between the life span reductions caused by the VBNC and culturable L. monocytogenes treatments was observed. L. monocytogenes was visualized beyond the nematode intestinal lumen, indicating resuscitation and cell invasion. These data emphasize the risk that VBNC food-borne pathogens could pose to public health should they continue to go undetected.IMPORTANCE Many bacteria are known to enter a viable-but-nonculturable (VBNC) state in response to environmental stresses. VBNC cells cannot be detected by standard laboratory culture techniques, presenting a problem for the food industry, which uses these techniques to detect pathogen contaminants. This study found that chlorine, a sanitizer commonly used for fresh produce, induces a VBNC state in the food-borne pathogens Listeria monocytogenes and Salmonella enterica It was also found that chlorine is ineffective at killing total populations of the pathogens. A life span reduction was observed in Caenorhabditis elegans that ingested these VBNC pathogens, with VBNC L. monocytogenes as infectious as its culturable counterpart. These data show that VBNC food-borne pathogens can both be generated and avoid detection by industrial practices while potentially retaining the ability to cause disease.

Quantitative Microbial Risk Assessment for Escherichia coli O157:H7 in Fresh-Cut Lettuce

Leafy green vegetables, including lettuce, are recognized as potential vehicles for foodborne pathogens such as Escherichia coli O157:H7. Fresh-cut lettuce is potentially at high risk of causing foodborne illnesses, as it is generally consumed without cooking. Quantitative microbial risk assessments (QMRAs) are gaining more attention as an effective tool to assess and control potential risks associated with foodborne pathogens. This study developed a QMRA model for E. coli O157:H7 in fresh-cut lettuce and evaluated the effects of different potential intervention strategies on the reduction of public health risks. The fresh-cut lettuce production and supply chain was modeled from field production, with both irrigation water and soil as initial contamination sources, to consumption at home. The baseline model (with no interventions) predicted a mean probability of 1 illness per 10 million servings and a mean of 2,160 illness cases per year in the United States. All intervention strategies evaluated (chlorine, ultrasound and organic acid, irradiation, bacteriophage, and consumer washing) significantly reduced the estimated mean number of illness cases when compared with the baseline model prediction (from 11.4- to 17.9-fold reduction). Sensitivity analyses indicated that retail and home storage temperature were the most important factors affecting the predicted number of illness cases. The developed QMRA model provided a framework for estimating risk associated with consumption of E. coli O157:H7-contaminated fresh-cut lettuce and can guide the evaluation and development of intervention strategies aimed at reducing such risk.

A probabilistic assessment of the contribution of wastewater-irrigated lettuce to Escherichia coli O157:H7 infection risk and disease burden in Kumasi, Ghana

Wastewater use for vegetable production is widespread across the cities of many developing countries. Studies on the microbial health risks associated with the practice have largely depended on faecal indicator organisms with potential underestimation or overestimation of the microbial health risks and disease burdens. This study assessed the Escherichia coli O157:H7 infection risk and diarrhoeal disease burden measured in disability-adjusted life years (DALYs) associated with the consumption of wastewater-irrigated lettuce in Kumasi, Ghana using data on E. coli O157:H7 in ready-to-harvest, wastewater-irrigated lettuce. Two exposure scenarios - best case and worst case - associated with a single consumption of wastewater-irrigated lettuce were assessed. The assessment revealed wastewater-irrigated lettuce is contributing to the transmission of E. coli O157:H7 in Kumasi, Ghana. The mean E. coli O157:H7 infection risk and DALYs in the wet and dry seasons, irrespective of the exposure scenario, were above the World Health Organization tolerable daily infection risk of 2.7 × 10⁻⁷ per person per day and 10⁻⁶ DALYs per person per year. It is recommended that legislation with clear monitoring indicators and penalties is implemented to ensure that farmers and food sellers fully implement risk mitigating measures.

Effect of gamma radiation on the reduction of Salmonella strains, Listeria monocytogenes, and Shiga toxin-producing Escherichia coli and sensory evaluation of minimally processed spinach (Tetragonia expansa)

This study evaluated the effects of irradiation on the reduction of Shiga toxin-producing Escherichia coli (STEC), Salmonella strains, and Listeria monocytogenes, as well as on the sensory characteristics of minimally processed spinach. Spinach samples were inoculated with a cocktail of three strains each of STEC, Salmonella strains, and L. monocytogenes, separately, and were exposed to gamma radiation doses of 0, 0.2, 0.4, 0.6, 0.8, and 1.0 kGy. Samples that were exposed to 0.0, 1.0, and 1.5 kGy and kept under refrigeration (4°C) for 12 days were submitted to sensory analysis. D10 -values ranged from 0.19 to 0.20 kGy for Salmonella and from 0.20 to 0.21 for L. monocytogenes; for STEC, the value was 0.17 kGy. Spinach showed good acceptability, even after exposure to 1.5 kGy. Because gamma radiation reduced the selected pathogens without causing significant changes in the quality of spinach leaves, it may be a useful method to improve safety in the fresh produce industry.

Salmonella internalization in mung bean sprouts and pre- and postharvest intervention methods in a hydroponic system

Mung bean sprouts, typically consumed raw or minimally cooked, are often contaminated with pathogens. Internalized pathogens pose a high risk because conventional sanitization methods are ineffective for their inactivation. The studies were performed (i) to understand the potential of internalization of Salmonella in mung bean sprouts under conditions where the irrigation water was contaminated and (ii) to determine if pre- and postharvest intervention methods are effective in inactivating the internalized pathogen. Mung bean sprouts were grown hydroponically and exposed to green fluorescence protein-tagged Salmonella Typhimurium through maturity. One experimental set received contaminated water daily, while other sets received contaminated water on a single day at different times. For preharvest intervention, irrigation water was exposed to UV, and for postharvest intervention-contaminated sprouts were subjected to a chlorine wash and UV light. Harvested samples were disinfected with ethanol and AgNO3 to differentiate surface-associate pathogens from the internalized ones. The internalized Salmonella Typhimurium in each set was quantified using the plate count method. Internalized Salmonella Typhimurium was detected at levels of 2.0 to 5.1 log CFU/g under all conditions. Continuous exposure to contaminated water during the entire period generated significantly higher levels of Salmonella Typhimurium internalization than sets receiving contaminated water for only a single day (P < 0.05). Preintervention methods lowered the level of internalized Salmonella by 1.84 log CFU/g (P < 0.05), whereas postintervention methods were ineffective in eliminating internalized pathogens. Preintervention did not completely inactivate bacteria in sprouts and demonstrated that the remaining Salmonella Typhimurium in water became more resistant to UV. Because postharvest intervention methods are ineffective, proper procedures for maintaining clean irrigation water must be followed throughout production in a hydroponic system.

Inactivation of internalized Salmonella Typhimurium in lettuce and green onion using ultraviolet C irradiation and chemical sanitizers

AIMS

The internalized human pathogens in fresh produce are not effectively removed during conventional washing, and therefore, it may cause foodborne illness when the produce is consumed raw. Thus, effective nonthermal processes are needed to prevent this risk.

METHODS AND RESULTS

Green fluorescence protein-tagged Salmonella Typhimurium was either sprayed on the surface of iceberg lettuce or injected into the bottom part (bulb) of green onions to induce bacterial internalization. The contaminated vegetables were collected after 2 days and subjected to surface disinfection. Different fluencies of UV-C radiation (75-900 mJ cm(-2)) and two fluencies of UV-C (450, 900 mJ cm(-2)) combined with chlorine and peracetic acid (PAA) were applied to the produce to examine the inactivation efficiency of internalized bacteria. A range of 1·96-2·52 log reduction in the internalized Salmonella was achieved when the lettuce was treated with higher UV-C fluency (150, 450, 900 mJ cm(-2)) or UV-C combined with chemical disinfectants. Significant reduction (1·00-1·49 log CFU g(-1)) in internalized Salmonella was observed in green onion treated with UV-C with the fluency of 150 or 900 mJ cm(-2) or UV-C-chlorine/PAA. No significant reduction was observed in either lettuce or green onion treatments when chlorine or PAA was used alone. The food quality measured with firmness was not changed during any treatments. However, a slight colour change was observed in lettuce only when UV-C was used at 900 mJ cm(-2).

CONCLUSION

High fluency UV-C can significantly inactivate the internalized Salmonella in lettuce and green onion while maintaining the food quality.

SIGNIFICANCE AND IMPACT OF THE STUDY

This research provides applicable research outcomes for developing nonthermal methods to inactivate internalized pathogens in fresh produce.

Factors affecting radiation D-values (D₁₀) of an Escherichia coli cocktail and Salmonella Typhimurium LT2 inoculated in fresh produce

This study evaluated the effect of produce type, resuspension medium, dose uniformity ratio (DUR), and sample preparation conditions (tissue exposure, MAP, anoxia) on the D₁₀ -value of an Escherichia coli cocktail (BAA-1427, BAA-1428, and BAA-1430) and Salmonella Typhimurium LT2 inoculated on the surfaces of tomato, cantaloupe, romaine lettuce, and baby spinach. Produce at room temperature were irradiated using a 1.35 MeV Van de Graaf electron beam accelerator at 0.2 to 0.9 kGy. The D₁₀-values for E. coli and Salmonella were 0.20 ± 0.01 kGy and 0.14 ± 0.01 kGy, respectively. Bacterial inactivation was not affected by produce type as long as the samples were irradiated in unsealed bags, the bacteria were suspended in broth, and the sample tissue was exposed. Sample location in front of the e-beam source during exposure is crucial. A 20% increase in DUR yielded a 53% change in the D₁₀- values. Variations in sample preparation, microbiological methods and irradiation set-up, result in variable D₁₀-values for different microorganisms on fresh produce.

PRACTICAL APPLICATIONS

Most irradiation studies disregard the effect of sample handling and processing parameters on the determination of the D₁₀-value of different microorganisms in fresh and fresh-cut produce. This study shows the importance of exposure of sample, resuspension medium, available oxygen, and dose uniformity ratio. D₁₀-values can differ by 35% to 53% based on these factors, leading to considerable under- or over-estimation of the irradiation treatment. Results from this study will help to lay firm groundwork for future studies on D₁₀-values determination for different pathogens on fruits and vegetables.

A mathematical risk model for Escherichia coli O157:H7 cross-contamination of lettuce during processing

A stochastic simulation modelling approach was taken to determine the extent of Escherichia coli O157:H7 contamination in fresh-cut bagged lettuce leaving the processing plant. A probabilistic model was constructed in Excel to account for E. coli O157:H7 cross contamination when contaminated lettuce enters the processing line. Simulation of the model was performed using @Risk Palisade© Software, providing an estimate of concentration and prevalence in the final bags of product. Three different scenarios, named S1, S2, and S3, were considered to represent the initial concentration on the contaminated batch entering the processing line which corresponded to 0.01, 1 and 100 cfu/g, respectively. The model was satisfactorily validated based on Standard Error of Prediction (SEP), which ranged from 0.00-35%. ANOVA analysis performed on simulated data revealed that the initial concentration in the contaminated batch (i.e., S1, S2, and S3) did not influence significantly (p=0.4) the E. coli O157:H7 levels in bags derived from cross contamination. In addition, significantly different (p<0.001) prevalence was observed at the different levels simulated (S1; S2 and S3). At the lowest contamination level (0.01 cfu/g), bags were cross-contaminated sporadically, resulting in very low E. coli O157:H7 populations (mean: ≤2 cfu/bag) and prevalence levels (<1%). In contrast, higher average prevalence levels were obtained for S2 and S3 corresponding to 3.05 and 13.39%, respectively. Furthermore, the impact of different interventions on E. coli O157:H7 cross-contamination (e.g., pathogen testing, chlorination, irradiation, and cleaning and disinfection procedures) was evaluated. Model showed that the pathogen was able to survive and be present in the final bags in all simulated interventions scenarios although irradiation (0.5 KGy) was a more effective decontamination step in reducing prevalence than chlorination or pathogen testing under the same simulated conditions.

Low-dose irradiation improves microbial quality and shelf life of fresh mint (Mentha piperita L.) without compromising visual quality

The effects of low-dose irradiation (0.25 to 2 kGy) and postirradiation storage (at 4 degrees C) on microbial and visual quality, color values (L*, a*, b*, chroma, and hue [ degrees ]), and chlorophyll content (Chl a, Chl b, and total Chl) of fresh mint were evaluated. Samples inoculated with E. coli O157:H7, Salmonella, and MS2 bacteriophage were irradiated and evaluated. E. coli O157:H7 and Salmonella populations were reduced by 2 to 2.4, 3.5, and 5.8 log CFU/g, respectively, 1 d after treatment with 0.25, 0.60, and 1 kGy, respectively, and were completely eliminated at 2 kGy. None of the irradiation doses (P < 0.0001) reduced MS2 bacteriophage populations by more than 0.60 log PFU/g. Irradiation doses did not affect visual quality and samples remained of excellent to good quality (score 7.75 to 9) for up to 9 d of storage. Irradiation at 0.60, 1, and 2 kGy increased (P < 0.0001) Chl a, Chl b, and total Chl. Both total Chl and Chl a decreased significantly after 3 d of storage. Significant decreases in Chl b were not observed until day 12 of storage. Color values (L*, b*, and chroma) were not significantly different until day 6 of storage and hue ( degrees ) remained unchanged (179 degrees ) for the entire storage period of 12 d. Overall, irradiation did not change L*, a*, b*, or chroma. These results demonstrate that irradiation of fresh mint at 2 kGy has the potential to improve its microbial quality and extend its shelf life without compromising its visual quality and color.

PRACTICAL APPLICATION

Mints and other raw fresh herbs are widely used for flavoring as well as garnish in a variety of dishes without further cooking. However, mint is one considered as one of the high-risk herbs when it comes to microbial contamination. We have evaluated the use of gamma irradiation treatment at very low doses ranging from 0 to 2 kGy to eliminate seeded Salmonella spp, E. coli O157:H7, and MS2 bacteriophage, a surrogate of hepatitis A virus. We found that low-dose irradiation (1.0 to 2.0 kGy) appears to be a promising method for improving the microbiological quality of fresh mint without compromising its visual and color attributes. This method may be applied to many popular fresh culinary herbs that are commonly used as garnishes in Asian cuisine.

Inactivation of Escherichia coli O157:H7 on lettuce, using low-energy X-ray irradiation

Low-energy X-ray irradiation was assessed as a means of eliminating Escherichia coli O157:H7 on lettuce. Round-cut iceberg lettuce samples (2.54-cm diameter) were dip or spot inoculated with a three-strain cocktail of E. coli O157:H7, stored for 24 h at 4 degrees C, and then irradiated at four dose levels up to 0.25 kGy using a prototype low-energy (70 kV) X-ray irradiator. E. coli O157:H7 survivors were quantified by plating on sorbitol MacConkey agar containing cefixime and tellurite. Dip inoculation yielded a D(10)-value of 0.040 +/- 0.001 kGy, which is 3.4 times lower than a previously reported value of 0.136 kGy using gamma radiation. The D(10)-value for E. coli O157:H7 on spot-inoculated samples was 0.078 +/- 0.008 kGy, which is about twice that of dip-inoculated samples. When 10 stacked leaves were irradiated from both sides, a dose of 0.2 kGy was achieved at the center of the stack with a surface dose of 1 kGy, corresponding to a approximately 5-log reduction of E. coli O157:H7 at the center of the stack. Based on these findings, low-energy X-ray irradiation appears to be a promising microbial inactivation strategy for leafy greens and potentially for other types of fresh produce.

Effects of X-ray radiation on Escherichia coli O157:H7, Listeria monocytogenes, Salmonella enterica and Shigella flexneri inoculated on shredded iceberg lettuce

The main goal of this investigation was to study the efficacy of X-ray doses (0.1, 0.2, 0.3, 0.5, 0.75, 1.0, 1.5 and 2.0 kGy) on inoculated Escherichia coli O157: H7, Listeria monocytogenes, Salmonella enterica and Shigella flexneri on shredded iceberg lettuce. The second goal was to study the effect of X-ray on the inherent microflora counts and visual color of shredded iceberg lettuce during storage at 4 degrees C for 30 days. Treatment with 1.0 kGy X-ray significantly reduced the population of E. coli O157: H7, L. monocytogenes, Salmonella enterica and S. flexneri on shredded iceberg lettuce by 4.4, 4.1, 4.8 and 4.4-log CFU 5 cm(-2), respectively. Furthermore, more than a 5 log CFU reduction of E. coli O157: H7, L. monocytogenes, S. enterica and S. flexneri was achieved with 2.0 kGy X-ray. Treatment with X-ray reduced the initial microflora on iceberg lettuce and kept them significantly (p < 0.05) lower than the control during storage at 4 degrees C and 90% RH for 30 days. Treatment with X-ray did not significantly (p > 0.05) change the green color of iceberg lettuce leaves. Treatment with X-ray significantly reduced selected pathogens and inherent microorganisms on shredded iceberg lettuce leaves, which could be a good alternative to other technologies for produce (lettuce) industry.

Viral Inactivation in Foods: A Review of Traditional and Novel Food-Processing Technologies

  Over one-half of foodborne illnesses are believed to be viral in origin. The ability of viruses to persist in the environment and foods, coupled with low infectious doses, allows even a small amount of contamination to cause serious problems. An increased incidence of foodborne illnesses and consumer demand for fresh, convenient, and safe foods have prompted research into alternative food-processing technologies. This review focuses on viral inactivation by both traditional processing technologies such as use of antimicrobial agents and the application of heat, and also novel processing technologies including high-pressure processing, ultraviolet- and gamma-irradiation, and pulsed electric fields. These industrially applicable control measures will be discussed in relation to the 2 most common causes of foodborne viral illnesses, hepatitis A virus and human noroviruses. Other enteric viruses, including adenoviruses, rotaviruses, aichi virus, and laboratory and industrial viral surrogates such as feline caliciviruses, murine noroviruses, bacteriophage MS2 and ΦX174, and virus-like particles are also discussed. The basis of each technology, inactivation efficacy, proposed mechanisms of viral inactivation, factors affecting viral inactivation, and applicability to the food industry with a focus on ready-to-eat foods, produce, and shellfish, are all featured in this review.