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.     

Pulsed electric field inactivation of E. coli O157:H7 and non-pathogenic surrogate E. coli in strawberry juice as influenced by sodium benzoate,potassium sorbate,and citric acid

Current FDA regulations require that juice processors achieve a 5 log CFU/ml reduction of a target pathogen prior to distributing products. Whereas thermal pasteurization reduces the sensory characteristics of juice, pulsed electric field (PEF) treatments can be conducted at lower temperatures and may preserve sensory characteristics.Escherichia coli O157:H7 (ATCC 43895) and a non-pathogenic E. coli (ATCC 35218), respectively, were inoculated into single-strength strawberry juice with or without 750 ppm sodium benzoate (SB), 350 ppm potassium sorbate (PS), and 2.7% citric acid (CA). Juice was treated at outlet temperatures of 45, 50 and 55 °C at a field strength of 18.6 kV/cm for 150 μs with a laboratory-scale PEF unit. Inactivation of surrogate E. coli at 45, 50, and 55 °C were 2.86, 3.12, and 3.79 log CFU/ml, respectively, in plain juice (pH 3.4), and 2.75, 3.52, and 5.11 with the addition of benzoic and sorbic acids (pH 3.5). Inactivation of E. coli O157:H7 under the same conditions were 3.09, 4.08, and 4.71 log CFU/ml, respectively, and 2.27, 3.29, and 5.40 with antimicrobials. E. coli O157:H7 in juice with antimicrobials and 2.7% CA (pH 2.7) treated with PEF was reduced by 2.60, 4.32 and 6.95 log CFU/ml at 45, 50 and 55 °C while the surrogate E. coli decreased by 3.54, 5.69, and 7.13 log under the same conditions. When juice (pH 2.7) was held for 6 h without PEF treatment, higher numbers of E. coli 35218 (7.17 log CFU/ml) were inactivated than of acid-resistant E. coli O157:H7 (3.89 log). Slightly greater PEF inactivation of E. coli O157:H7 than of the surrogate bacterium indicates that E. coli ATCC 35218 provides a margin of safety when used as a surrogate for O157:H7 in plain strawberry juice or in juice + SB + PS at 45–50 °C, or with SB + PS and CA at 55 °C.     

Balsamic vinegar from Modena: An easy and effective approach to reduce Listeria monocytogenes from lettuce

The microbiological safety of fresh produce is a significant concern of consumers and industry. After applying at an inoculated level (6–7 log CFU/mL) of Listeria monocytogenes on Iceberg lettuce, the antilisterial properties of balsamic vinegar from Modena, white wine vinegar and acetic acid solutions were investigated.Different proportions of the vinegars, acetic acid (58.7 g/L), and deionized water were evaluated to determine the role of those solutions at the stage of washing Iceberg lettuce to remove L. monocytogenes. The maximum observed log reduction of L. monocytogenes was 2.15 ± 0.04 for balsamic vinegar (50% (v/v)), 1.18 ± 0.06 for white wine vinegar ((50% (v/v)) and 1.13 ± 0.06 for acetic acid ((50% (v/v)). Washing with water only reduces 0.05 ± 0.04 log CFU/mL of L. monocytogenes numbers.Listeria reductions observed for balsamic vinegar are similar or higher than those of chlorine-based sanitizers evaluated in other studies with lettuce. In the case of balsamic vinegar solutions, Listeria inhibition followed a linear reduction according to the model: Log (N/N₀) = −4.09 × balsamic vinegar proportion % (v/v) − 0.13; R² = 0.95. Balsamic vinegar washings may be a promising method for reducing other foodborne pathogens present in produce or other foods, at home and retail environments.     

Combined sustainable sanitising treatments to reduce Escherichia coli and Salmonella Enteritidis growth on fresh-cut kailan-hybrid broccoli

The effects of peroxyacetic acid (PA), neutral electrolysed water (NEW), ultraviolet C light (UV-C) and superatmospheric O₂ packaging (HO), single or combined, on the growth rate of Escherichia coli and S. Enteritidis inoculated onto fresh-cut kailan-hybrid broccoli were studied throughout 14 days of shelf life at 5 and 10 °C. As controls, unwashed, water-washed and NaClO-washed were used. PA and NEW showed a better sanitising effect than NaClO against both E. coli/S. Enteritidis with reductions of 2.2/2.7 and 2.6/2.6 log CFU g⁻¹, respectively. UV-C attained E. coli/S. Enteritidis decreases of 1.3–1.4/2.1–2.2 log CFU g⁻¹ compared to unwashed samples. The bacteriostatic effects of PA or NEW were improved when they were combined with UV-C, decreasing E. coli and S. Enteritidis counts by about 3 log CFU g⁻¹. After 14 days, PA and NEW-treated samples attained E. coli/S. Enteritidis reductions at 5 °C of 2.3/1.4 and about 1 log CFU g⁻¹, respectively. Storage of those samples at 10 °C masked that sanitising effect. At both storage temperatures, UV-C-treated samples registered lower E. coli and S. Enteritidis reductions (or higher increases for S. Enteritidis at 10 °C) regarding conventional NaClO-sanitized samples or other single treatments. However, PA, NEW or UV-C-treated samples under HO at both storage temperatures showed better bacterial control than individual treatments with the greatest effectiveness for UV-C + HO. Triple combination did not imply an enhanced benefit over double combinations. Conclusively, PA, NEW and UV-C seems to be effective emergent alternatives to NaClO to reduce initial E. coli and S. Enteritidis populations in fresh-cut kailan-hybrid broccoli. Storage at 5 or 10 °C under HO greatly controlled microbial growth.     

Antimicrobial activity of ascorbic acid alone or in combination with lactic acid on Escherichia coli O157:H7 in laboratory medium and carrot juice

The antimicrobial effects of ascorbic acid alone and in combination with lactic acid, against Escherichia coli O157:H7 in Brain Heart Infusion (BHI) broth and in carrot juice as a food model was investigated. In control samples, E. coli O157:H7 continued to growth from 3.98 ± 0.2 log CFU and reached to 8.88 ± 0.1 log₁₀ CFU after 8 h at 37° C; however, bacterial population was undetectable level in BHI in the presence of 0.4% ascorbic acid and 0.2% lactic acid. In carrot juice, E. coli O157:H7 continue to grow from initial population count of 4.41 ± 0.9 log₁₀ CFU to 8.75 ± 0.07 log₁₀ CFU at 37 °C for 24 h. Similarly, the bacteria population was undetectable when 0.2 or 0.4% ascorbic acid and 0.2% lactic acid applied. Our findings suggest the application of ascorbic acid, in combination with lactic acid, may have potential as preservative to inhibit the growth of E. coli O157:H7 in food.     

Wash water disinfection of a full-scale leafy vegetables washing process with hydrogen peroxide and the use of a commercial metal ion mixture to improve disinfection efficiency

Hydrogen peroxide (H₂O₂) was used to maintain the microbial wash water quality of a full-scale leafy vegetables (radicchio, sugar loaf, curled endive, lollo, lollo rosso) wash water process. Despite addition of 300 L/h of 1.8% H₂O₂ to a 450 L washing bath (333 ± 50 kg/h fresh-cut produce introduction speed), the H₂O₂ quickly decreased and a lower wash water contamination of aerobic psychrotrophic plate count (APC) and enterococci than without addition of H₂O₂ could not be maintained. There was no significant difference between the APC on fresh-cut leafy vegetables washed with H₂O₂ and those washed with water.In a second part, lab-scale experiments were performed to assess the impact of a commercial metal ion formulation (Bacsan^®, containing a. o. Cu²⁺, Zn²⁺, Ag⁺) on the stability of H₂O₂ in artificial wash water, made from iceberg lettuce and tap water. Bacsan improved the stability of H₂O₂ in artificial lettuce wash water and fresh-cut leafy vegetables wash water from a processing company and synergistically increased the disinfection efficiency of APC and Escherichia coli (E. coli) compared to H₂O₂ or Bacsan. Increasing chemical oxygen demand (COD) had detrimental effect on the H₂O₂ stability and disinfection efficiency. Addition of Ag⁺ to Bacsan further synergistically enhanced the H₂O₂ stability.H₂O₂ is not suited as an in situ wash water disinfectant to avoid cross-contamination in fresh-cut leafy vegetables washing processes due to the slow water disinfection kinetics and the rapid H₂O₂ consumption. However, H₂O₂/Bacsan shows potential for use in off-line processes.     

The microbiological quality of ready-to-eat food in Siu Mei and Lo Mei shops in Hong Kong

The safety of ready-to-eat food is an important issue. Improper handling of ready-to-eat food items may result in foodborne outbreaks. In this study, Chinese barbecued pork (Char Siu in Chinese) was selected as the target ready-to-eat food item for a microbial survey. The aim of this study was to evaluate the microbiological quality of Chinese barbecued pork sold in licensed Siu Mei and Lo Mei shops in Hong Kong. A total of 115 samples were collected from supermarkets or wet markets in the 18 districts. They were tested for aerobic plate counts (APC), Escherichia coli and Staphylococcus aureus counts, and the presence of Salmonella spp. for assessing their safety level. Results showed APC ranging from 1.97 to 6.84 log CFU/g, with a mean of 5.05 log CFU/g; E. coli counts ranging from none detected to 3.10 log CFU/g, with a mean of 1.78 log CFU/g; and S. aureus counts ranging from none detected to 1.42 log CFU/g, with a mean of 0.15 log CFU/g. The mean APC and E. coli counts of samples from supermarkets were found to be significantly lower than those from wet markets (p < 0.05) indicating that supermarkets had better microbiological quality than wet markets. Salmonella spp. were isolated from 39% of the samples analyzed, indicating that cross-contamination was quite a serious problem in Siu Mei and Lo Mei shops in Hong Kong. Based on these results, recommendations such as routine inspections and training of vendors were suggested to improve the microbiological quality of products sold in licensed Siu Mei and Lo Mei shops in Hong Kong so as to minimize risks of foodborne outbreaks.     

Comparative simulation of Escherichia coli O157:H7 behaviour in packaged fresh-cut lettuce distributed in a typical Canadian supply chain in the summer and winter

The ability to predict the risk associated with Escherichia coli O157:H7 in packaged fresh-cut lettuce hinges on the availability of realistic times and temperatures encountered in commercial supply chains. We report here on temperature profiles measured in winter and summer months in a retail supply chain and their predicted impact on the fate of the pathogen in fresh-cut lettuce. Time and temperature data were recorded in individual packages within cases of lettuce in the bottom, middle and top layers of pallets distributed from a processing facility to retail storage. Measurements indicated there were occasional increases in temperature during either season and that temperatures were ≥5 °C for longer periods of time during the summer. A stochastic simulation model based on time and temperature distributions was constructed in @RISK™ to predict the fate of E. coli O157:H7. The output from the model demonstrated a range of possible outcomes for the time-temperature profiles measured in the commercial supply chain, ranging from slight growth to die-off. The outputs of the stochastic simulations were similar to analytical simulations based on actual times and temperatures experienced by product in the supply chain. The distributions presented in the current model could be incorporated in quantitative risk assessments to improve predictions about the prevalence and fate of E. coli O157:H7 in fresh-cut lettuce supply chains.     

Reduction of Escherichia coli O157:H7 and Salmonella Typhimurium DT 104 on fresh produce using an automated washer with near neutral electrolyzed (NEO) water and ultrasound

Automated produce washers can be a useful processing aid when treating fresh produce contaminated with pathogens. The use of near neutral pH electrolyzed (NEO) water as a wash or sanitizing solution has been shown to lead to significant reductions of Escherichia coli O157:H7 and Salmonella on fresh produce. To further enhance reported pathogen reductions, the effects of a combined NEO water (155 mg/L free chlorine, pH 6.5) and ultrasound wash protocol on lettuce and tomatoes inoculated with E. coli O157:H7 and S. Typhimurium DT 104 were studied. The effects of the pH of NEO water and washer agitation on pathogen reductions were also assessed. Inoculated tomatoes and lettuce leaves were treated with either chilled deionized water or NEO water, with or without 20 kHz ultrasound (130 W and 210 W). Tomatoes were treated for 1, 3 and 5 min while lettuce was treated for 5, 10 and 15 min. Ultrasound significantly increased the oxidation-reduction potential (ORP) of NEO water (p < 0.05) but did not affect the pH and free chlorine concentration (p > 0.05). Increased washing time and higher ultrasonic power led to significantly greater reductions of both pathogens on produce items (p < 0.05). NEO water combined with 210 W ultrasonication for 15 min led to 4.4 and 4.3 log reductions of E. coli O157:H7 and S. Typhimurium on lettuce, respectively, while 210 W ultrasound for 5 min completely inactivated both pathogens on tomatoes. Both pathogens were completely inactivated in NEO water solutions, suggesting that its use presents little chance of cross-contamination.     

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.     

The influence of acid stress on the growth of Listeria monocytogenes and Escherichia coli O157:H7 on cooked ham

Acid solutions are increasingly being used for decontaminating meat surfaces. On the surfaces of acid-treated meat, the population of microorganism is reduced due to the low pH of acids, and the subsequent growth of the microorganism is reduced due to the residual acids on meat surfaces. Microbial cells on meat surfaces subjected to acid treatments may cross-contaminate untreated meat surfaces, e.g., microorganisms on the surfaces of acid-treated cooked ham cross contaminate the untreated surfaces during slicing. The objective of this study was to examine this scenario in determining the subsequent growth of acid-treated Listeria monocytogenes and Escherichia coli O157:H7 on the surfaces of untreated meat. Cells of multiple-strain L. monocytogenes or E. coli O157:H7 were exposed to HCl solutions of pH 3, 4, or 5 and deionized water at room temperature for 24 h. The acid or deionized water-treated cells were inoculated separately onto cooked ham. Samples inoculated with L. monocytogenes were stored at 4 and 8 °C and samples inoculated with E. coli O157:H7 were stored at 10 and 12 °C. Populations of the pathogens on ham were enumerated during storage, and the lag phase durations (LPD, h) and growth rates (GR, log CFU/h) of the pathogens were determined. The populations of L. monocytogenes and E. coli O157:H7 in pH 5, 4, and 3 solutions were 1.2–3.1 and 0.6–2.4 log CFU/ml, respectively, lower than those in deionized water, indicating an increased acid stress on both microorganisms at lower pHs. L. monocytogenes subjected to pH 3 and pH 4 stresses and E. coli O157:H7 subjected to pH 3 stress exhibited significantly (p < 0.05) extended LPDs and reduced GRs on cooked ham. The growth of L. monocytogenes on ham was more readily reduced by acid stress than that of E. coli O157:H7. This study showed that acid treatments reduced the viability of L. monocytogenes and E. coli O157:H7and the acid stress reduced their subsequent growth ability on untreated ham. Therefore, cross-contamination of L. monocytogenes or E. coli O157:H7 cells from acid-treated meat surfaces onto untreated meat surfaces may not impose increased risk to the product.     

Effect of organic acids,hydrogen peroxide and mild heat on inactivation of Escherichia coli O157:H7 on baby spinach

Minimally processed baby spinach contaminated with Escherichia coli O157:H7 has been associated with multiple outbreaks of foodborne illnesses recently. Chlorinated water is widely used to wash vegetables commercially, but this washing procedure has limited efficacy and can lead to the formation of carcinogenic substances. This study was conducted to determine the effects of organic acids and hydrogen peroxide alone and in binary combinations with or without mild heat (40 and 50 °C) on the inactivation of Escherichia coli O157:H7 on baby spinach. Baby spinach leaves were dip-inoculated with E. coli O157:H7 to a level of 6 log CFU/g and stored at 4 °C for 24 h before treatment. Individual washing solutions (1% and 2% lactic acid [LA], citric acid [CA], malic acid [MA], tartaric acid [TA], acetic acid [AA], hydrogen peroxide [H₂O₂] as well as binary combinations of LA, CA, MA and H₂O₂ at final concentrations of 1% were used to decontaminate spinach leaves at 22, 40 or 50 °C for 2–5 min to test their efficacy in reducing E. coli O157:H7. Chlorinated water (200 ppm free chlorine) decreased the population of E. coli O157:H7 on baby spinach by only 1.2–1.6 log CFU/g, which was not significantly different from DI water washing. Washing with 1% LA at 40 °C for 5 min was the most effective treatment achieving a 2.7 log reduction of E. coli O157:H7 which is significantly higher than chlorine washing. Washing with LA + CA or LA + HP at 40 °C for 5 min was equally effective against E. coli O157:H7, resulting in a 2.7 log reduction of E. coli O157:H7. The application of mild heat significantly enhanced the efficacy of washing solutions on the inactivation of E. coli O157:H7. There was, however, no significant difference between treatments at 40 °C for 5 min and 50 °C for 2 min. The results suggested that the use of organic acids in combination with mild heat can be a potential intervention to control E. coli O157:H7 on spinach.     

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.     

Effects of Quillaja saponaria extract and Nα-lauroyl-l-arginine ethyl ester on reducing selected foodborne pathogens in vitro and maintaining quality of fresh-cut endive (Cichorium endivia L.) at pilot plant scale

The aim of the present study was to evaluate the applicability of Quillaja saponaria extract (QSE) and N^α-lauroyl-l-arginine ethyl ester (LAE) as antimicrobial wash water additives in fresh-cut lettuce processing. Antibacterial activities of LAE and QSE against selected strains of the foodborne pathogens Salmonella enterica, Bacillus cereus, Staphylococcus aureus, Pseudomonas aeruginosa, and Listeria monocytogenes were examined in vitro. Minimum inhibitory concentrations determined by broth microdilution assay demonstrated that LAE exhibited a strong antimicrobial activity with MICs between 4 and 32 μg/mL against all tested strains, whereas QSE showed a weaker antimicrobial activity with MICs >512 μg/mL. On a pilot-plant scale, the effects of warm water washing at 45 °C for 120 s with and without 40 mg/L QSE or 100 mg/L LAE as well as cold water washing at 4 °C for 120 s with QSE or LAE, respectively, of shredded endive (Cichorium endivia L.) were investigated regarding microbiological and sensory quality as well as physiological properties. Samples were analyzed for headspace O₂ and CO₂ levels, phenylalanine-ammonia-lyase activity and contents of nitrite and nitrate during nine days of cold storage at 4 °C. By analogy to its antimicrobial effect against the foodborne pathogens in vitro, LAE allowed up to 4 log₁₀ cfu/mL reduction of the microbial load in the washing water of the pilot plant, and might therefore reduce cross-contamination while saving water. The addition of LAE to warm washing water impaired sensory properties of fresh-cut endive during storage, which was predicted by chlorophyll fluorescence imaging analyses. QSE treatment combined with warm water washing best retained sensory appearance throughout our study, being possibly suitable for the production of premium products.     

Dynamics of low (1–4 cells) vs high populations of Listeria monocytogenes and Salmonella Typhimurium in fresh-cut salads and their sterile liquid or solidified extracts

The growth variability of low or high populations of Listeria monocytogenes and Salmonella Typhimurium in fresh-cut lettuce and cabbage was studied. The salads were inoculated with a few (1–4) or 1000 cells/sample of the above organisms and stored at 8 °C. Their liquid or solidified sterile extracts were also inoculated with the same cell numbers to evaluate the behavior of pathogens in the presence or absence of the epiphytic flora and illustrate the effect of the micro-environment structure, where growth may occur. The sterile extracts were stored at 8–10 °C simulating marginal temperature fluctuations. Inoculum of 1000 cells/sample increased with limited variation (SD < 0.5 log CFU/g) on vegetable salads, as opposed to the great variability (<0.7–3.4 log CFU/g) in the growth from 1 to 4 cells/sample. Total logarithmic increase of 1000 cells/sample of the pathogens on the salads ranged from 1.8 to 2.1 log CFU/g, contrary to 1–4 cells/sample, which exhibited higher increase (2.7–3.4 log CFU/g). The latter suggests that “fail-dangerous” implications may derive from challenge tests with unrealistic high inocula. Different batches of vegetables used for preparation of sterile extracts, introduced high variability in the growth of 1–4 cells/sample, suggesting nutrient-dependent effect on growth of pathogens. Low inoculum of L. monocytogenes did not increase in sterile cabbage extracts, whereas they increased from 1 to 3.5 logs in cabbage salad, probably due to the stimulatory effect of indigenous flora. In contrast, 1–4 cells/sample of S. Typhimurium grew only on the solidified extract of cabbage but not on the salad, indicating competitive activity of the indigenous microflora against the pathogen. Salmonella showed no growth at 8 °C but increased 4 logs at 10 °C, illustrating the impact of boundary storage conditions. Monte Carlo simulation of bacterial growth based on broth data overestimated growth of L. monocytogenes on lettuce, while remarkably underestimated the actual increase in cabbage. Significant deviation between model and data is likely when extrapolating broth-based simulations of growth from low populations in foods, due to the various factors affecting the microbial growth on a real food, which are (inevitably) ignored by broth-based models.     

Effect of γ irradiation on quality-maintaining of fresh-cut lettuce

The effects of irradiation on microorganisms and physiological quality of fresh-cut lettuce were evaluated during storage at 4 °C. The total bacterial counts on fresh-cut lettuce irradiated with 1.0 kGy were reduced by the order of 2.35 Log CFU/g, and the total coliform group were lowered to less than 30 MPN (most probably number)/100 g. The polyphenoloxidase activity of fresh-cut lettuce was significantly inhibited by irradiation. In addition, the loss of vitamin C of fresh-cut lettuce irradiated with 1.0 kGy was significantly (α = 0.05) lower than that of non-irradiated. The best treatment of maintaining quality of fresh-cut lettuce appeared to be 1.0 kGy irradiation.     

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.     

Effects of processing and storage variables on penetration and survival of Escherichia coli O157:H7 in fresh-cut packaged carrots

The effects of severity of slicing and peeling, and of storage atmosphere and temperature on the survival of Escherichia coli O157:H7 at and below the cut surfaces of fresh-cut carrot discs were determined. Slicing with a blunt machine blade enhanced penetration of E. coli O157:H7 and its subsequent survival during storage at 8 °C. Significantly (P < 0.05) higher numbers of cells (0.5 log CFU g⁻¹) penetrated deeper (475–500, 725–750 and 975–1000 μm) into carrot tissue sliced with a blunt machine blade compared to those sliced with a razor blade. Counts on Day 3 and Day 5 of storage remained higher in carrot tissue sliced with a blunt machine blade at all depths sampled. There were no significant effects of peeling method on penetration and survival. Storage of carrot slices in sub-optimal atmospheres (20%CO₂/1%O₂ resulted in increased survival of cells at the surface and within the tissue compared to storage in more optimal atmospheres (5% CO₂/3% O₂). Increasing storage temperature from 4 to 10 °C resulted in growth of E. coli O157:H7 at all sample depths. For all experiments, E. coli O157:H7 cells colonising the surface generally survived better than cells that penetrated into the tissue. The data are relevant to improving microbial safety in the fresh-cut sector by demonstrating that cutting with sharp blades (e.g. during harvesting and processing) reduces the depth of potential contamination by E. coli O157:H7. This may also have implications for more efficient anti-microbial dipping and reduced pathogen survival during storage.     

Sanitizer efficacy in preventing cross-contamination during retail preparation of whole and fresh-cut cantaloupe

The objective of this study was to evaluate the efficacy of tap water (TW), commercial electrolyzed water (EW), and a commercial acid-based sanitizer (AS) in preventing cross-contamination of cantaloupe during processing in retail settings. A whole cantaloupe was dip-inoculated with a cocktail of Salmonella or L. monocytogenes to achieve approximately 5 log CFU/cm². One inoculated and two non-inoculated whole cantaloupes were treated in 76 L of TW, EW (free chlorine: 50–60 ppm), or AS (pH 2.8, combination of lactic acid and phosphoric acid) for 5 min. Subsequently, fresh-cut cantaloupe flesh from the inoculated and non-inoculated cantaloupes were soaked together in 76 L of TW, EW, or AS for 90 s. EW treatment resulted in an approximately 1.5 log reduction in both Salmonella and L. monocytogenes on the rind of whole cantaloupe, which was significantly greater than with the TW treatment (0.5 log reduction) (P < 0.05). Cross-contamination of non-inoculated whole cantaloupes occurred when washed with inoculated whole cantaloupe in TW (four of four cantaloupes positive for Salmonella and L. monocytogenes) or AS (four of four cantaloupes positive for Salmonella and two of four positive for L. monocytogenes). Cross-contamination did not occur when whole cantaloupes were washed in EW. Additional washing of mixed fresh-cut cantaloupe flesh from the inoculated and non-inoculated cantaloupes prepared after washing of whole cantaloupes demonstrated that the EW treatment reduced the likelihood of cross-contamination compared with TW and AS. No viable Salmonella or L. monocytogenes were detected from 100 mL sample of EW processing water, but were detected in TW and AS (L. monocytogenes only). The addition of a sanitizing agent to water used for the processing of whole and fresh-cut cantaloupe in a retail setting is recommended to prevent cross-contamination and reduce microbial load.     

Modeling the inactivation of Escherichia coli O157:H7 and Uropathogenic E. coli in ground beef by high pressure processing and citral

Escherichia coli O157:H7 are a well-known intestinal foodborne pathogen which were responsible for numerous foodborne illness outbreaks. Uropathogenic E. coli (UPEC) are common contaminants in meat and poultry, and may cause urinary tract infections after colonizing the gastrointestinal tract followed by accidental transfer of contaminated feces from the anus to the urethra. High pressure processing (HPP) has been demonstrated an effective means in reducing pathogenic E. coli levels in meat and poultry. Citral, an antimicrobial, also demonstrated some inactivation effect on pathogenic E. coli in ground beef (ca. 0.5–1.0 log CFU/g reduction at 1% w/w without HPP). With HPP alone, to achieve 5 log CFU/g reduction required a 500 MPa level and 15 min (for both O157:H7 and UPEC). However, 1% of citral addition may lower the pressure requirement to 380 MPa and 15 min which could reduce the food quality damage. To effectively inactivate E. coli O157:H7 and UPEC in meat, high pressure processing (HPP) in combination with the antimicrobial citral was studied. Ground beef inoculated with E. coli O157:H7 or UPEC were treated at different HPP conditions (250–350 MPa; 10–20 min), and citral (0.75–1.25%, w/w) following a central composite experimental design. Quadratic linear regression equations were developed to describe and predict the reductions of E. coli O157:H7 (R² = 0.93, p < 0.001) and UPEC (R² = 0.92, p < 0.001). Dimensionless nonlinear models consisting of three impact factors were also developed and compared with the linear models. These models were experimentally validated. Citral enhanced the inactivation of pathogenic E. coli, increasing the effectiveness of the HPP process. The models may assist the food industry and regulatory agencies in risk assessment of E. coli O157:H7 and UPEC on ground meats.