Growth of Escherichia coli O157:H7 in bruised apple (Malus domestica) tissue as influenced by cultivar, date of harvest, and source

Four of five apple cultivars (Golden Delicious, Red Delicious, McIntosh, Macoun, and Melrose) inoculated with Escherichia coli O157:H7 promoted growth of the bacterium in bruised tissue independent of the date of harvest (i.e., degree of apple ripening) or the source of the apple (i.e., tree-picked or dropped fruit). Apple harvest for this study began 4 September 1998 and ended 9 October, with weekly sampling. Throughout this study, freshly picked (<2 days after harvest) McIntosh apples usually prevented the growth of E. coli O157:H7 for 2 days. Growth of E. coli O157:H7 did occur following 6 days of incubation in bruised McIntosh apple tissue. However, the maximum total cell number was approximately 80-fold less than the maximum total cell number recovered from Red Delicious apples. When fruit was stored for 1 month at 4 degrees C prior to inoculation with E. coli O157:H7, all five cultivars supported growth of the bacterium. For each apple cultivar, the pH of bruised tissue was significantly higher and degrees Brix was significantly lower than the pH and degrees Brix of undamaged tissue regardless of the source. In freshly picked apples, changes in the pH did not occur over the harvest season. Bruised Golden Delicious, McIntosh, and Melrose apple tissue pHs were not significantly different (tree-picked or dropped), and the degrees Brix values of McIntosh, Macoun, and Melrose apple tissue were not significantly different. Single-cultivar preparations of cider did not support growth of E. coli, and the cell concentration of inoculated cider declined over an 11-day test period. The rate of decline in E. coli cell concentration in the McIntosh cider was greater than those in the other ciders tested. The findings of this study suggested that the presence of some factor besides, or in addition to, pH inhibited E. coli growth in McIntosh apples.     

Influence of Apple Cultivars on Inactivation of Different Strains of Escherichia coli O157:H7 in Apple Cider by UV Irradiation

This study examined the effect of different apple cultivars upon the UV inactivation of Escherichia coli O157:H7 strains within unfiltered apple cider. Apple cider was prepared from eight different apple cultivars, inoculated with approximately 10⁶ to 10⁷ CFU of three strains of E. coli O157:H7 per ml (933, ATCC 43889, and ATCC 43895), and exposed to 14 mJ of UV irradiation per cm². Bacterial populations for treated and untreated samples were then enumerated by using nonselective media. E. coli O157:H7 ATCC 43889 showed the most sensitivity to this disinfection process with an average 6.63-log reduction compared to an average log reduction of 5.93 for both strains 933 and ATCC 43895. The highest log reduction seen, 7.19, occurred for strain ATCC 43889 in Rome cider. The same cider produced the lowest log reductions: 5.33 and 5.25 for strains 933 and ATCC 43895, respectively. Among the apple cultivars, an average log reduction range of 5.78 (Red Delicious) to 6.74 (Empire) was observed, with two statistically significant (α ≤ 0.05) log reduction groups represented. Within the paired cultivar-strain analysis, five of eight ciders showed statistically significant (α ≤ 0.05) differences in at least two of the E. coli strains used. Comparison of log reductions among the E. coli strains to the cider parameters of °Brix, pH, and malic acid content failed to show any statistically significant relationship (R² ≥ 0.95). However, the results of this study indicate that regardless of the apple cultivar used, a minimum 5-log reduction is achieved for all of the strains of E. coli O157:H7 tested.     

Fate of Escherichia coli O157:H7 on Fresh-Cut Apple Tissue and Its Potential for Transmission by Fruit Flies

Pathogenic Escherichia coli O157:H7, as well as nonpathogenic strains ATCC 11775 and ATCC 23716, grew exponentially in wounds on Golden Delicious apple fruit. The exponential growth occurred over a longer time period on fruit inoculated with a lower concentration of the bacterium than on fruit inoculated with a higher concentration. The bacterium reached the maximum population supported in the wounds regardless of the initial inoculum concentrations. Populations of E. coli O157:H7 in various concentrations of sterilized apple juice and unsterilized cider declined over time and declined more quickly in diluted juice and cider. The decline was greater in the unsterilized cider than in juice, which may have resulted from the interaction of E. coli O157:H7 with natural populations of yeasts that increased with time. Experiments on the transmission of E. coli by fruit flies, collected from a compost pile of decaying apples and peaches, were conducted with strain F-11775, a fluorescent transformant of nonpathogenic E. coli ATCC 11775. Fruit flies were easily contaminated externally and internally with E. coli F-11775 after contact with the bacterium source. The flies transmitted this bacterium to uncontaminated apple wounds, resulting in a high incidence of contaminated wounds. Populations of the bacterium in apple wounds increased significantly during the first 48 h after transmission. Further studies under commercial conditions are necessary to confirm these findings.     

Attachment of Escherichia coli O157:H7 to the Surfaces and Internal Structures of Apples as Detected by Confocal Scanning Laser Microscopy

Confocal scanning laser microscopy (CSLM) was used to demonstrate the attachment of Escherichia coli O157:H7 transformed with a plasmid encoding for green fluorescent protein (GFP) to the surface and within the internal structures of nonwaxed Red Delicious cv. apples. Apples at 2 or 25°C were inoculated with an E. coli O157:H7 cell suspension at 2 or 25°C. The effect of a negative temperature differential (cold inoculum, warm apple), a positive differential (warm inoculum, cold apple), and no differential (warm inoculum, warm apple), in combination with a pressure differential (atmospheric versus 10,130 Pa), on the attachment and infiltration of cells was determined. CSLM stereo images of external surfaces of apples subjected to all combinations of test parameters showed preferential cellular attachment to discontinuities in the waxy cuticle on the surface and to damaged tissue surrounding puncture wounds, where the pathogen was observed at depths up to 70 μm below the skin surface. Attachment to lenticels was sporadic but was occasionally observed at depths of up to 40 μm. Infiltration through the floral tube and attachment to seeds, cartilaginous pericarp, and internal trichomes were observed in all apples examined, regardless of temperature differential during inoculation. The pressure differential had no effect on infiltration or attachment of E. coli O157:H7. Image analysis to count cells at various depths within tissues was used to quantitatively compare the extent of infiltration into various apple structures as well as the effects of the temperature differential. Puncture wounds harbored greater numbers of the pathogen at greater depths than did other sites examined. Attachment or infiltration of cells was greater on the intact skin and in lenticels, russet areas, and the floral tube of apples inoculated under a negative temperature differential compared to those inoculated under no temperature differential. The results suggest that E. coli O157:H7 attached to internal core structures or within tissues of apples may evade decontamination treatments. Interventions designed to deliver disinfectants to these locations or to remove viable cells of E. coli O157:H7 and other pathogens from apples by other means need to be developed and validated.     

Combinations of Intervention Treatments Resulting in 5-Log10-Unit Reductions in Numbers of Escherichia coli O157:H7 and Salmonella typhimurium DT104 Organisms in Apple Cider

The U.S. Food and Drug Administration (FDA) recently mandated a warning statement on packaged fruit juices not treated to reduce target pathogen populations by 5 log₁₀ units. This study describes combinations of intervention treatments that reduced concentrations of mixtures of Escherichia coli O157:H7 (strains ATCC 43895, C7927, and USDA-FSIS-380-94) or Salmonella typhimurium DT104 (DT104b, U302, and DT104) by 5 log₁₀ units in apple cider with a pH of 3.3, 3.7, and 4.1. Treatments used were short-term storage at 4, 25, or 35°C and/or freeze-thawing (48 h at −20°C; 4 h at 4°C) of cider with or without added organic acids (0.1% lactic acid, sorbic acid [SA], or propionic acid). Treatments more severe than those for S. typhimurium DT104 were always required to destroy E. coli O157:H7. In pH 3.3 apple cider, a 5-log₁₀-unit reduction in E. coli O157:H7 cell numbers was achieved by freeze-thawing or 6-h 35°C treatments. In pH 3.7 cider the 5-log₁₀-unit reduction followed freeze-thawing combined with either 6 h at 4°C, 2 h at 25°C, or 1 h at 35°C or 6 h at 35°C alone. A 5-log₁₀-unit reduction occurred in pH 4.1 cider after the following treatments: 6 h at 35°C plus freeze-thawing, SA plus 12 h at 25°C plus freeze-thawing, SA plus 6 h at 35°C, and SA plus 4 h at 35°C plus freeze-thawing. Yeast and mold counts did not increase significantly (P < 0.05) during the 6-h storage at 35°C. Cider with no added organic acids treated with either 6 h at 35°C, freeze-thawing or their combination was always preferred by consumers over pasteurized cider (P < 0.05). The simple, inexpensive intervention treatments described in the present work could produce safe apple cider without pasteurization and would not require the FDA-mandated warning statement.     

Acid Resistance Systems Required for Survival of Escherichia coli O157:H7 in the Bovine Gastrointestinal Tract and in Apple Cider Are Different

Escherichia coli O157:H7 is a highly acid-resistant food-borne pathogen that survives in the bovine and human gastrointestinal tracts and in acidic foods such as apple cider. This property is thought to contribute to the low infectious dose of the organism. Three acid resistance (AR) systems are expressed in stationary-phase cells. AR system 1 is σ^S dependent, while AR systems 2 and 3 are glutamate and arginine dependent, respectively. In this study, we sought to determine which AR systems are important for survival in acidic foods and which are required for survival in the bovine intestinal tract. Wild-type and mutant E. coli O157:H7 strains deficient in AR system 1, 2, or 3 were challenged with apple cider and inoculated into calves. Wild-type cells, adapted at pH 5.5 in the absence of glucose (AR system 1 induced), survived well in apple cider. Conversely, the mutant deficient in AR system 1, shown previously to survive poorly in calves, was susceptible to apple cider (pH 3.5), and this sensitivity was shown to be caused by low pH. Interestingly, the AR system 2-deficient mutant survived in apple cider at high levels, but its shedding from calves was significantly decreased compared to that of wild-type cells. AR system 3-deficient cells survived well in both apple cider and calves. Taken together, these results indicate that E. coli O157:H7 utilizes different acid resistance systems based on the type of acidic environment encountered.     

Nanocapsular Dispersion of Thymol for Enhanced Dispersibility and Increased Antimicrobial Effectiveness against Escherichia coli O157:H7 and Listeria monocytogenes in Model Food Systems

Essential oils are marginally soluble in water, making it challenging to evenly disperse them in foods and resulting in an increased tendency to bind with food lipids and proteins, resulting in lowered antimicrobial efficacy. In the current study, free and nano-dispersed (ND) thymol were compared in terms of their antimicrobial efficacies against Escherichia coli O157:H7 ATCC 43889 and 43894 and Listeria monocytogenes strains Scott A and 101 in apple cider and 2% reduced-fat milk. Apple cider was adjusted to pHs 5.5 and 3.5, and antimicrobial tests were performed at 0.3-, 0.5-, 0.75-, and 1.0-g/liter thymol concentrations at 35, 32, 25, and 4°C. Overall, 0.5 and 1.0 g/liter thymol in nano-dispersion and along with free thymol were inhibitory and bactericidal, respectively, against bacterial strains under all treatment conditions. At pH 5.5, 0.5 g/liter ND thymol was bacteriostatic against L. monocytogenes and E. coli for up to 48 h. At pH 3.5, L. monocytogenes controls did not survive beyond 12 h but E. coli survived and was inhibited by 0.5 g/liter ND thymol after 12 and 48 h in apple cider. E. coli strains were significantly sensitive to 4°C and pH 3.5 (P < 0.05). When bacteria were tested in 2% reduced-fat milk at 35 or 32°C, ND and free thymol demonstrated inhibition at 4.5 g/liter. Thus, the current technology seems to be promising and novel, enabling thymol-containing nano-dispersions that are not only transparent but also effective against pathogens in food applications, especially in clear beverages.     

Influence of apple cultivars on inactivation of different strains of Escherichia coli O157:H7 in apple cider by UV irradiation

This study examined the effect of different apple cultivars upon the UV inactivation of Escherichia coli O157:H7 strains within unfiltered apple cider. Apple cider was prepared from eight different apple cultivars, inoculated with approximately 10(6) to 10(7) CFU of three strains of E. coli O157:H7 per ml (933, ATCC 43889, and ATCC 43895), and exposed to 14 mJ of UV irradiation per cm(2). Bacterial populations for treated and untreated samples were then enumerated by using nonselective media. E. coli O157:H7 ATCC 43889 showed the most sensitivity to this disinfection process with an average 6.63-log reduction compared to an average log reduction of 5.93 for both strains 933 and ATCC 43895. The highest log reduction seen, 7.19, occurred for strain ATCC 43889 in Rome cider. The same cider produced the lowest log reductions: 5.33 and 5.25 for strains 933 and ATCC 43895, respectively. Among the apple cultivars, an average log reduction range of 5.78 (Red Delicious) to 6.74 (Empire) was observed, with two statistically significant (alpha < or = 0.05) log reduction groups represented. Within the paired cultivar-strain analysis, five of eight ciders showed statistically significant (alpha < or = 0.05) differences in at least two of the E. coli strains used. Comparison of log reductions among the E. coli strains to the cider parameters of (o)Brix, pH, and malic acid content failed to show any statistically significant relationship (R(2) > or = 0.95). However, the results of this study indicate that regardless of the apple cultivar used, a minimum 5-log reduction is achieved for all of the strains of E. coli O157:H7 tested.     

Inactivation of Escherichia coli O157:H7 in Apple Juice by Irradiation

Three strains (932, Ent-C9490, and SEA13B88) of Escherichia coli O157:H7 were used to determine the effectiveness of low-dose gamma irradiation for eliminating E. coli O157:H7 from apple juice or cider and to characterize the effect of inducing pH-dependent, stationary-phase acid resistance on radiation resistance. The strains were grown in tryptic soy broth with or without 1% dextrose for 18 h to produce cells that were or were not induced to pH-dependent stationary-phase acid resistance. The bacteria were then transferred to clarified apple juice and irradiated at 2°C with a cesium-137 irradiator. Non-acid-adapted cells had radiation D values (radiation doses needed to decrease a microbial population by 90%) ranging from 0.12 to 0.21 kGy. D values increased to 0.22 to 0.31 kGy for acid-adapted cells. When acid-adapted SEA13B88 cells were tested in five apple juice brands having different levels of suspended solids (absorbances ranging from 0.04 to 2.01 at 550 nm), radiation resistance increased with increasing levels of suspended solids, with D values ranging from 0.26 to 0.35 kGy. Based on these results, a dose of 1.8 kGy should be sufficient to achieve the 5D inactivation of E. coli recommended by the National Advisory Committee for Microbiological Criteria for Foods.     

Modeling of Combined Processing Steps for Reducing Escherichia coli O157:H7 Populations in Apple Cider

Probabilistic models were used as a systematic approach to describe the response of Escherichia coli O157:H7 populations to combinations of commonly used preservation methods in unpasteurized apple cider. Using a complete factorial experimental design, the effect of pH (3.1 to 4.3), storage temperature and time (5 to 35°C for 0 to 6 h or 12 h), preservatives (0, 0.05, or 0.1% potassium sorbate or sodium benzoate), and freeze-thaw (F-T; −20°C, 48 h and 4°C, 4 h) treatment combinations (a total of 1,600 treatments) on the probability of achieving a 5-log₁₀-unit reduction in a three-strain E. coli O157:H7 mixture in cider was determined. Using logistic regression techniques, pH, temperature, time, and concentration were modeled in separate segments of the data set, resulting in prediction equations for: (i) no preservatives, before F-T; (ii) no preservatives, after F-T; (iii) sorbate, before F-T; (iv) sorbate, after F-T; (v) benzoate, before F-T; and (vi) benzoate, after F-T. Statistical analysis revealed a highly significant (P < 0.0001) effect of all four variables, with cider pH being the most important, followed by temperature and time, and finally by preservative concentration. All models predicted 92 to 99% of the responses correctly. To ensure safety, use of the models is most appropriate at a 0.9 probability level, where the percentage of false positives, i.e., falsely predicting a 5-log₁₀-unit reduction, is the lowest (0 to 4.4%). The present study demonstrates the applicability of logistic regression approaches to describing the effectiveness of multiple treatment combinations in pathogen control in cider making. The resulting models can serve as valuable tools in designing safe apple cider processes.     

Plant Lesions Promote the Rapid Multiplication of Escherichia coli O157:H7 on Postharvest Lettuce

Several outbreaks of Escherichia coli O157:H7 infections have been associated with minimally processed leafy vegetables in the United States. Harvesting and processing cause plant tissue damage. In order to assess the role of plant tissue damage in the contamination of leafy greens with E. coli O157:H7, the effect of mechanical, physiological, and plant disease-induced lesions on the growth of this pathogen on postharvest romaine lettuce was investigated. Within only 4 h after inoculation, the population sizes of E. coli O157:H7 increased 4.0-, 4.5-, and 11.0-fold on lettuce leaves that were mechanically bruised, cut into large pieces, and shredded into multiple pieces, respectively. During the same time, E. coli O157:H7 population sizes increased only twofold on leaves that were left intact after harvest. Also, the population size of E. coli O157:H7 was 27 times greater on young leaves affected by soft rot due to infection by Erwinia chrysanthemi than on healthy middle-aged leaves. Confocal microscopy revealed that leaf tip burn lesions, which are caused by a common physiological disorder of lettuce, harbored dense populations of E. coli O157:H7 cells both internally and externally. Investigation of the colonization of cut lettuce stems by E. coli O157:H7 showed that the pathogen grew 11-fold over 4 h of incubation after its inoculation onto the stems, from which large amounts of latex were released. The results of this study indicate that plant tissue damage of various types can promote significant multiplication of E. coli O157:H7 over a short time and suggest that harvesting and processing are critical control points in the prevention or reduction of E. coli O157:H7 contamination of lettuce.     

Fate ofescherichia coli andE. coli O157∶H7 in apple juice treated with propolis extract

Fruit juices are targets of spoilage moulds, yeasts and acid tolerant bacteria. They might be contaminated with bacteria from raw materials, environment, packaging and during the handling of the product. These contaminations have frequently resulted in the spoilage of fruit juice and consequently commercial losses. The objective of this study was to determine the influence of propolis in apple juice againstEscherichia coli andE. coli O157:H7 strains of the spoilage and pathogenic bacteria. For this purpose, apple juice was obtained from fresh apples and then was pasteurised. The pH value, titrable acidity (as % malic acid) and Brix degree of this apple juice were 3.72±0.10, 0.67±0.05% and 12.1±0.01, respectively. Propolis extract at 1,2 and 5% concentrations were tested to determine ofE. coli andE. coli O157:H7 inhibition using paper disc diffusion method. The control treatment had no propolis extract. The apple juices were contaminated with these bacteria, and the activity of propolis was observed at first, 18^th, 24^th, 48^th and 72^nd hours at 4 and 25°C. The number of cells in the tubes was counted using serial dilution method. Results indicated that propolis extract at 2 and 5% concentrations had significant antimicrobial activity againstE. coli andE. coli O157:H7, therefore we can conclude that propolis extract is worthy of further study as a natural preservative for the foods prone to microbial spoilage.     

Genetics of phytopathogenic fungi XIX. Responses of four apple varieties to inoculation with auxotrophic mutants of Penicillium expansum

Summary Variations in content and/or availability of the amino acids methionine, cysteine, arginine, leucine, and histidine in the apple varieties, Jonathan, McIntosh, Red Delicious, and Golden Delicious influence the fruit's susceptibility to amino acid-dependent mutants ofPenicillium expansum. Differences among these varieties in the levels of several vitamins, or of the purines, adenine, or hypoxanthine, do not alter susceptibility. The growth in vitro of 7 arginine-requiring mutants at 1×10^–4 M arginine and the decay they cause in apples were directly correlated.     

Survival or Growth of Escherichia coli O157:H7 in a Model System of Fresh Meat Decontamination Runoff Waste Fluids and Its Resistance to Subsequent Lactic Acid Stress

A potential may exist for survival of and resistance development by Escherichia coli O157:H7 in environmental niches of meat plants applying carcass decontamination interventions. This study evaluated (i) survival or growth of acid-adapted and nonadapted E. coli O157:H7 strain ATCC 43895 in acetic acid (pH 3.6 ± 0.1) or in water (pH 7.2 ± 0.2) fresh beef decontamination runoff fluids (washings) stored at 4, 10, 15, or 25°C and (ii) resistance of cells recovered from the washings after 2 or 7 days of storage to a subsequent lactic acid (pH 3.5) stress. Corresponding cultures in sterile saline or in heat-sterilized water washings were used as controls. In acetic acid washings, acid-adapted cultures survived better than nonadapted cultures, with survival being greatest at 4°C and lowest at 25°C. The pathogen survived without growth in water washings at 4 and 10°C, while it grew by 0.8 to 2.7 log cycles at 15 and 25°C, and more in the absence of natural flora. E. coli O157:H7 cells habituated without growth in water washings at 4 or 10°C were the most sensitive to pH 3.5, while cells grown in water washings at 15 or 25°C were relatively the most resistant, irrespective of previous acid adaptation. Resistance to pH 3.5 of E. coli O157:H7 cells habituated in acetic acid washings for 7 days increased in the order 15°C > 10°C > 4°C, while at 25°C cells died off. These results indicate that growth inhibition by storage at low temperatures may be more important than competition by natural flora in inducing acid sensitization of E. coli O157:H7 in fresh meat environments. At ambient temperatures in meat plants, E. coli O157:H7 may grow to restore acid resistance, unless acid interventions are applied to inhibit growth and minimize survival of the pathogen. Acid-habituated E. coli O157:H7 at 10 to 15°C may maintain a higher acid resistance than when acid habituated at 4°C. These responses should be evaluated with fresh meat and may be useful for the optimization of decontamination programs and postdecontamination conditions of meat handling.     

Demography and mass-rearing of Carposina sasakii Matsumura (Lepidoptera: Carposinidae) reared on Golden Delicious and Red Fuji apples in the laboratory

Carposina sasakii Matsumura (Lepidoptera: Carposinidae), is one of the most serious fruit-boring pests in over ten species of fruit trees, and is especially damaging apples in the northern of China. The application of new planting systems, i.e., high-density and dwarfing rootstock orchard systems with mixed apple varieties, makes it important to study the fitness of C. sasakii on these apple varieties to gain fundamental knowledge for use in pest management involving this insect. In this study, life table data of C. sasakii were collected using Golden Delicious and Red Fuji apples as hosts. The egg-larva duration of male C. sasakii reared on Golden Delicious apples (22.81 d) was significantly shorter than that reared on Red Fuji apples (24.27 d). The egg-larva mortality in Golden Delicious apples (59.00%) was lower than that in Red Fuji apples (72.49%). The mortality of the pupal stage, however, was higher in Golden Delicious (10.51%) than in Red Fuji (0%). The total oviposition period (TPOP) on Golden Delicious apples (32.94 d) was significantly shorter than in individuals reared on Red Fuji apples (34.19 d). The intrinsic rate of increase (r = 0.0581 d⁻¹), net reproductive rate (R₀ = 7.57 offspring), and finite rate of increase (λ = 1.0598 d⁻¹) were all higher on Golden Delicious than those on Red Fuji. When the net reproductive rate (R₀) was used, the harvest rate of pupae was higher (0.8678) when reared on Golden Delicious apples than when reared on Red Fuji apples (0.8398). When a large cohort size (n = 200) was used for effective bootstrap sample, the P_E values for C. sasakii reared on Golden Delicious apples and Red Fuji apples were both almost equal to 1. For C. sasakii culturing purposes, Golden Delicious apples would be more productive than Red Fuji.     

Selective Enrichment with a Resuscitation Step for Isolation of Freeze-Injured Escherichia coli O157:H7 from Foods

We studied injury of Escherichia coli O157:H7 cells in 11 food items during freeze storage and methods of isolating freeze-injured E. coli O157:H7 cells from foods. Food samples inoculated with E. coli O157:H7 were stored for 16 weeks at −20°C in a freezer. Noninjured and injured cells were counted by using tryptic soy agar and sorbitol MacConkey agar supplemented with cefixime and potassium tellurite. Large populations of E. coli O157:H7 cells were injured in salted cabbage, grated radish, seaweed, and tomato samples. In an experiment to detect E. coli O157:H7 in food samples artificially contaminated with freeze-injured E. coli O157:H7 cells, the organism was recovered most efficiently after the samples were incubated in modified E. coli broth without bile salts at 25°C for 2 h and then selectively enriched at 42°C for 18 h by adding bile salts and novobiocin. Our enrichment method was further evaluated by isolating E. coli O157:H7 from frozen foods inoculated with the organism prior to freezing. Two hours of resuscitation at 25°C in nonselective broth improved recovery of E. coli O157:H7 from frozen grated radishes and strawberries, demonstrating that the resuscitation step is very effective for isolating E. coli O157:H7 from frozen foods contaminated with injured E. coli O157:H7 cells.     

Effect of Electropermeabilization by Ohmic Heating for Inactivation of Escherichia coli O157:H7, Salmonella enterica Serovar Typhimurium,and Listeria monocytogenes in Buffered Peptone Water and Apple Juice

The effect of electric field-induced ohmic heating for inactivation of Escherichia coli O157:H7, Salmonella enterica serovar Typhimurium, and Listeria monocytogenes in buffered peptone water (BPW) (pH 7.2) and apple juice (pH 3.5; 11.8 °Brix) was investigated in this study. BPW and apple juice were treated at different temperatures (55°C, 58°C, and 60°C) and for different times (0, 10, 20, 25, and 30 s) by ohmic heating compared with conventional heating. The electric field strength was fixed at 30 V/cm and 60 V/cm for BPW and apple juice, respectively. Bacterial reduction resulting from ohmic heating was significantly different (P < 0.05) from that resulting from conventional heating at 58°C and 60°C in BPW and at 55°C, 58°C, and 60°C in apple juice for intervals of 0, 10, 20, 25, and 30 s. These results show that electric field-induced ohmic heating led to additional bacterial inactivation at sublethal temperatures. Transmission electron microscopy (TEM) observations and the propidium iodide (PI) uptake test were conducted after treatment at 60°C for 0, 10, 20, 25 and 30 s in BPW to observe the effects on cell permeability due to electroporation-caused cell damage. PI values when ohmic and conventional heating were compared were significantly different (P < 0.05), and these differences increased with increasing levels of inactivation of three food-borne pathogens. These results demonstrate that ohmic heating can more effectively reduce bacterial populations at reduced temperatures and shorter time intervals, especially in acidic fruit juices such as apple juice. Therefore, loss of quality can be minimized in a pasteurization process incorporating ohmic heating.     

Survival of Escherichia coli O157:H7 ATCC 43895 in a Model Apple Juice Medium with Different Concentrations of Proline and Caffeic Acid

The effects of proline and caffeic acid on the survival of Shiga toxin-producing Escherichia coli (STEC) O157:H7 strain ATCC 43895 in a model apple juice medium were studied. It is hypothesized that the inhibitory effect of caffeic acid may explain why almost all outbreaks of STEC O157:H7 infections linked to apple juice or cider have occurred in October or November.     

A study of ethylene in apple, red raspberry, and cherry

High ethylene levels were associated with flower abscission in apple (Malus sylvestris) and cherry (Prunus avium and Prunus cerasus), “June drop” of immature cherries, and harvest drop of apple and red raspberry (Rubus idaeus). However, an increase in ethylene content was not associated with June drop of apples and harvest drop of cherries. During the period of fruit ripening on the plant, the largest increases in ethylene occurred in apple flesh and red raspberry receptacular tissue. Ethylene remained low throughout the period of sweet and tart cherry ripening. The data obtained indicated marked ethylene gradients between adjacent tissues. Increases of ethylene in some tissues may have resulted from ethylene diffusion from adjacent tissues containing high levels of ethylene.     

Bacteriophages Reduce Experimental Contamination of Hard Surfaces, Tomato, Spinach, Broccoli, and Ground Beef by Escherichia coli O157:H7

A bacteriophage cocktail (designated ECP-100) containing three Myoviridae phages lytic for Escherichia coli O157:H7 was examined for its ability to reduce experimental contamination of hard surfaces (glass coverslips and gypsum boards), tomato, spinach, broccoli, and ground beef by three virulent strains of the bacterium. The hard surfaces and foods contaminated by a mixture of three E. coli O157:H7 strains were treated with ECP-100 (test samples) or sterile phosphate-buffered saline buffer (control samples), and the efficacy of phage treatment was evaluated by comparing the number of viable E. coli organisms recovered from the test and control samples. Treatments (5 min) with the ECP-100 preparation containing three different concentrations of phages (10¹⁰, 10⁹, and 10⁸ PFU/ml) resulted in statistically significant reductions (P = <0.05) of 99.99%, 98%, and 94%, respectively, in the number of E. coli O157:H7 organisms recovered from the glass coverslips. Similar treatments resulted in reductions of 100%, 95%, and 85%, respectively, in the number of E. coli O157:H7 organisms recovered from the gypsum board surfaces; the reductions caused by the two most concentrated phage preparations were statistically significant. Treatment with the least concentrated preparation that elicited significantly less contamination of the hard surfaces (i.e., 10⁹ PFU/ml) also significantly reduced the number of viable E. coli O157:H7 organisms on the four food samples. The observed reductions ranged from 94% (at 120 ± 4 h posttreatment of tomato samples) to 100% (at 24 ± 4 h posttreatment of spinach samples). The data suggest that naturally occurring bacteriophages may be useful for reducing contamination of various hard surfaces, fruits, vegetables, and ground beef by E. coli O157:H7.