Fourier transform infrared spectroscopy, detection and identification of Escherichia coli O157:H7 and Alicyclobacillus strains in apple juice

Fourier Transform Infrared (FT-IR) spectroscopy (4000-400 cm(-1)) combined with multivariate statistical methods were used to identify and detect Escherichia coli O157:H7 from Alicyclobacillus spp. recovered from apple juice. Four treatments and a control in triplicate experiments (N=3) were studied; the first three treatments of pasteurized apple juice were inoculated with E. coli O157:H7 ATCC 35150, Alicyclobacillus acidoterrestris 1016 and Alicyclobacillus spp. C-Fugi-6 respectively. The fourth treatment was a 1:1 (v:v) mixed culture of both A. acidoterrestris 1016 and Alicyclobacillus spp. C-Fugi-6. The control was uninoculated pasteurized apple juice. The second derivative transformation and loadings plot over the range of 1800-900 cm(-1) highlighted the most distinctive variations among bacterial spectra. Loadings 1 and 2 were distinctively representative of the bacterial spectral data and accounted for 73% of the total variability. Treatments were noticeably segregated with distinct clustering by principal component analysis (PCA). Using soft independent modeling of class analogy (SIMCA) analysis, 88.3% of (E. coli O157:H7 ATCC 35150) spectra, 75.0% of (A. acidoterrestris 1016) spectra, 88.3% of (Alicyclobacillus spp. C-Fuji-6) spectra, and 80.0% of the mixed culture of both Alicyclobacillus strains spectra were correctly classified. Using the spectral features of bacterial cellular constituents such as nucleic acids, proteins, phospholipids, peptidoglycan, and lipopolysaccharides from examined bacterial cells, pure and mixed cultures of Alicyclobacillus spp. cells, and the pathogenic E. coli cells could be detected in apple juice.     

Heat resistance of Escherichia coli O157:H7 in apple juice.

The objective was to determine the effect of cider composition on the heat resistance of Escherichia coli O157:H7. The average D52 value in a model Empire apple juice was 18 min with a z value of 4.8 degrees C. Increasing the Brix from 11.8 to 16.5 degrees had no effect on thermal resistance, while increasing L-malic acid from 0.2 to 0.8%, or reducing the pH from 4.4 to 3.6 sensitized the cells to heat. The greatest effect on heat resistance was afforded by the preservatives benzoic and sorbic acids: D50 values in ciders containing 1,000 mg/l were 5.2 min in the presence of sorbic acid and only 0.64 min in the presence of benzoic acid. Commercial apple juice concentrates yielded lower numbers of survivors than single-strength juices even though their higher sugar concentrations of about 46 degrees Brix increased heat resistance.     

Phosphate buffer increases recovery of Escherichia coli O157:H7 from frozen apple juice

It is common practice to dilute food products in 0.1% peptone before microbiological analysis. However, this diluent may not be appropriate for detection of injured organisms present in acidic foods. Shelf-stable unclarified apple juice (pH 3.6) was inoculated with approximately 1 x 10(7) CFU/ml of Escherichia coli O157:H7 and held at 23 +/- 2 degrees C (control) or frozen to -20 +/- 2 degrees C for 24 h to induce injury before sampling. Unfrozen or frozen and thawed juice was diluted 1:1 or 1:10 in 0.1% (wt/vol) peptone (pH 6.1) or 0.1 M phosphate buffer (pH 7.2). Juice samples were plated onto tryptic soy agar with 0.1% (wt/vol) sodium pyruvate (TSAP) to measure survival or onto sorbitol MacConkey agar (SMA) to indicate injury. Counts on TSAP or SMA were the same for control samples held in peptone or phosphate buffer for up to 45 min. However, populations of E. coli in frozen and thawed samples declined rapidly upon dilution in 0.1% peptone. Within 20 min, E. coli underwent a >1-log10 CFU/ml reduction in viability as measured on TSAP and a >2-log10 CFU/ml reduction to below the limit of detection (1.6 or 2.3 log10 CFU/ml) on SMA. In contrast, populations of E. coli in frozen and thawed samples diluted in phosphate buffer did not decrease significantly on TSAP and decreased by <0.6 log CFU/ml on SMA during a 45-min holding period. The acidity of apple juice appears to interfere with the recovery of freeze-thaw-injured E. coli O157:H7 during sampling. Using 0.1 M phosphate buffer (pH 7.2) as a diluent results in superior recovery of these organisms on both selective and nonselective plating media.     

Heat inactivation of Escherichia coli O157:H7 in apple juice exposed to chlorine

Exposure of Escherichia coli O157:H7 to chlorine before heat treatment results in increased production of heat shock proteins. Current heating regimens for pasteurizing apple cider do not account for chlorine exposure in the wash water. This research determined the effect of sublethal chlorine treatment on thermal inactivation of E. coli O157:H7. D58-values were calculated for stationary-phase cells exposed to 0.6 mg/liter of total available chlorine and unchlorinated cells in commercial shelf-stable apple juice (pH 3.6). D58-values for unchlorinated and chlorine-exposed cells in buffer were 5.45 and 1.65 min, respectively (P < 0.01). Death curves of chlorine-exposed and unchlorinated cells in apple juice were not completely linear. Unchlorinated cells heated in apple juice exhibit a 3-min delay before onset of linear inactivation. Chlorine treatment eliminated this shoulder, indicating an overall loss of thermotolerance. The linear portion of each curve represented a small fraction of the total population. D58-values calculated from these populations are 0.77 min for unexposed cells and 1.19 min for chlorine-exposed cells (P = 0.05). This indicates that a subpopulation of chorine-treated cells is possibly more resistant to heat because of chlorine treatment. The effect of chlorine treatment, however, is insignificant when compared with the effect of losing the shoulder. This is illustrated by the time required to kill the initial 90% of the cell population. This is observed to be 3.14 min for unchlorinated versus 0.3 min for chlorine-exposed cells (P < 0.001). These observations indicate that current heat treatments need not be adjusted for the effect of chlorine treatment.     

Antibacterial effect of monocaprylin on Escherichia coli O157:H7 in apple juice

The antibacterial effect of low concentrations of monocaprylin on Escherichia coli O157:H7 in apple juice was investigated. Apple juice alone (control) or containing 2.5 mM (0.055%) or 5 mM monocaprylin was inoculated with a five-strain mixture of E. coli O157:H7 at approximately 6.0 log CFU/ml. The juice samples were stored at 23 or 4 degrees C for 14 or 21 days, respectively, and the population of E. coli O157:H7 was determined on tryptic soy agar plates supplemented with 0.6% yeast extract. At both storage temperatures, the population of E. coli O157:H7 in monocaprylin-supplemented juice samples was significantly lower (P < 0.05) than that in the control samples. The concentration of monocaprylin and the storage temperature had a significant effect on the inactivation of E. coli O157:H7 in apple juice. Monocaprylin at 5 mM was significantly more effective than 2.5 mM monocaprylin for killing E. coli O157:H7 in apple juice. Inactivation of E. coli O157:H7 by monocaprylin was more pronounced in juice stored at 23 degrees C than in the refrigerated samples. Results of this study indicated that monocaprylin is effective for killing E. coli O157:H7 in apple juice, but detailed sensory studies are needed to determine the organoleptic properties of apple juice containing monocaprylin.     

Destruction of Escherichia coli O157:H7 by vanillic acid in unpasteurized juice from six apple cultivars

The behavior of Escherichia coli O157:H7 in Granny Smith, Gala, Empire, McIntosh, Red Delicious, and Golden Delicious apple juice with or without supplementation with 5 or 10 mM vanillic acid was examined over a storage period of 7 days at 4 and 15 degrees C. The consequences of supplementation on sensory difference and preference were also determined by triangle testing. Juices made from the six apple cultivars had pH values ranging between pH 3.13 and 3.92. Vanillic acid exerted a concentration, pH, and time-dependent lethal effect toward E. coli O157:H7 in unpasteurized apple juice. Supplementation with 10 mM vanillic acid led to a 5-logarithm reduction in populations after 7 days at both temperatures, but sensory analysis revealed significant differences from and preference for unsupplemented juices. Supplementation with 5 mM vanillic acid accelerated death of E. coli O157:H7, but population reductions ranged from 5 log CFU/ml in low pH juices to none in high pH juices, particularly at 4 degrees C. No sensory difference or preference was detected in two of the six juices at this level of supplementation.     

Evaluation of Fourier transform infrared (FT-IR) spectroscopy and chemometrics as a rapid approach for sub-typing Escherichia coli O157:H7 isolates

The importance of tracking outbreaks of foodborne illness and the emergence of new virulent subtypes of foodborne pathogens have created the need for rapid and reliable sub-typing methods for Escherichia coli O157:H7. Fourier transform infrared (FT-IR) spectroscopy coupled with multivariate statistical analyses was used for sub-typing 30 strains of E. coli O157:H7 that had previously been typed by multilocus variable number tandem repeat analysis (MLVA) and pulsed field gel electrophoresis (PFGE). Hierarchical cluster analysis (HCA) and canonical variate analysis (CVA) of the FT-IR spectra resulted in the clustering of the same or similar MLVA types and separation of different MLVA types of E. coli O157:H7. The developed FT-IR method showed better discriminatory power than PFGE in sub-typing E. coli O157:H7. Results also indicated the spectral relatedness between different outbreak strains. However, the grouping of some strains was not in complete agreement with the clustering based on PFGE and MLVA. Additionally, HCA of the spectra differentiated the strains into 30 sub-clusters, indicating the high specificity and suitability of the method for strain level identification. Strains were also classified (97% correct) based on the type of Shiga toxin present using CVA of the spectra. This study demonstrated that FT-IR spectroscopy is suitable for rapid (≤16 h) and economical sub-typing of E. coli O157:H7 with comparable accuracy to MLVA typing. This is the first report of using an FT-IR-based method for sub-typing E. coli O157:H7.     

Inactivation of Escherichia coli O157:H7 and Escherichia coli 8739 in apple juice by pulsed electric fields.

The effect of high voltage pulsed electric field (PEF) treatment on Escherichia coli O157:H7 and generic E. coli 8739 in apple juice was investigated. Fresh apple juice samples inoculated with E. coli O157:H7 and E. coli 8739 were treated by PEF with selected parameters including electric field strength, treatment time, and treatment temperature. Samples were exposed to bipolar pulses with electric field strengths of 30, 26, 22, and 18 kV/cm and total treatment times of 172, 144, 115, and 86 micros. A 5-log reduction in both cultures was determined by a standard nonselective medium spread plate laboratory procedure. Treatment temperature was kept below 35 degrees C. Results showed no difference in the sensitivities of E. coli O157:H7 and E. coli 8739 against PEF treatment. PEF is a promising technology for the inactivation of E. coli O157:H7 and E. coli 8739 in apple juice.     

The effects of freezing and thawing on the survival of Escherichia coli O157:H7 in apple juice

Unpasteurized apple juice, adjusted to pH 3.6 to 7.0 was inoculated (10(7) CFU/ml) with single strains of E. coli O157:H7 to evaluate the effect of frozen storage on the viability of this organism. Samples were stored under frozen conditions (-20+/-2 degrees C) for up to 16 days. Cell populations were determined at regular intervals by plating onto tryptic soy agar with added pyruvate (TSAP) or onto sorbitol MacConkey agar (SMA). Populations in the neutralized juice remained unchanged during frozen storage. Populations in non-neutralized juice decreased by 1-3 log10 CFU/ml depending on the strain tested and the pH of the juice. The greatest population decrease was observed with the first freeze/thaw cycle of frozen storage (24 h) and a slow decline in survival occurred thereafter. Injury was observed after 2 weeks of storage when juice pH was at or below pH 4.2. When samples were subjected to multiple freeze/thaw cycles, loss of viability and injury increased with each freeze/thaw cycle.     

High pressures in combination with antimicrobials to reduce Escherichia coli O157:H7 and Salmonella Agona in apple juice and orange juice

The effect of high pressure processing in conjunction with the chemical antimicrobials, dimethyl dicarbonate (DMDC), hydrogen peroxide, cinnamic acid, potassium sorbate, and sodium benzoate (NaB) on E. coli O157:H7 strain E009 and Salmonella enterica serovar Agona was investigated in apple juice and orange juice, respectively. Juices were inoculated with approximately 10(6) CFU/ml and subjected to pressures of 550 MPa (E. coli O157:H7 samples) and 400 MPa (Salmonella Agona samples) for 2 min at 6 degrees C (initial temperature). Populations of each pathogen were determined before pressurization, immediately after pressurization, and after samples had been held after treatment for 24 h at 4 degrees C. The most effective treatment for E. coli O157:H7, as determined by plating immediately after pressurization, was 125 ppm of DMDC, which caused a >4.98-log reduction. Other treatments that were significantly different from the sample with no added antimicrobial were 62.5 ppm of DMDC, 300 ppm of hydrogen peroxide, and 500 ppm of NaB, which produced 4.97-, 5.79-, and 3.91-log total reductions, respectively. After 24 h at 4 degrees C, E. coli O157:H7 was undetectable in all treatment groups (and controls). In samples inoculated with Salmonella, the most effective treatment was 62.5 ppm of DMDC, which produced a 5.96-log decrease immediately after pressure treatment. The results for 1,000 ppm of NaB, which produced a 3.26-log decrease, also were significantly different from those for the sample containing no antimicrobials. After 24 h at 4 degrees C, all samples with added antimicrobials had near or more than a 5-log total reduction of Salmonella Agona.     

Rapid discrimination of Alicyclobacillus strains in apple juice by Fourier transform infrared spectroscopy

Alicyclobacillus spp. are thermoacidophilic, spore-forming bacteria. Some of which cause spoilage in pasteurized and heat-treated apple juice products through the production of guaiacol. Fourier transform infrared (FT-IR) spectroscopy was used to discriminate between eight Alicyclobacillus strains (WAC, 81-2, Oly#21, 51-1, KF, 1016, 1101, and A-Gala A4) in apple juice. FT-IR vibrational combination bands reflected compositional differences in the cell membranes of Alicyclobacillus strains in the "fingerprint region" at wavenumbers between 1500 and 800 cm(-1). Distinctive segregation among spectral sample clusters of different Alicyclobacillus strains was observed using principal component analysis (PCA). Two closely related strains (1016 and 1101) of Alicyclobacillus acidoterrestris could be distinguished, suggesting that this method can be highly selective. Results of soft independent modeling of class analogy (SIMCA) demonstrated that guaiacol-producing and non-guaiacol producing Alicyclobacillus strains could be differentiated up to 89% of the time. This technique may provide a tool for fruit juice producers to detect Alicyclobacillus rapidly and to monitor and control guaiacol formation.     

Experimental use of a gas sensor-based instrument for differentiation of Escherichia coli O157:H7 from non-O157:H7 Escherichia coli field isolates

The rapid and economical detection of human pathogens in animal and food production systems would enhance food safety efforts. An instrument based on gas sensors coupled with an artificial neural network (ANN) was developed for the detection of and differentiation between laboratory isolates of Escherichia coli O157:H7 and non-O157:H7 E. coli. The purpose of this study was to use field isolates of E. coli to further evaluate the sensor system. This gas sensor-based, computer-controlled detection system was used to monitor gas emissions from 12 isolates of E. coli O157:H7 and 8 non-O157:H7 E. coli isolates. A standard concentration of each isolate was grown in 10 ml of nutrient broth at 37 degrees C for 16 h, and gas sampling was carried out every 5 min. Readings were continuously plotted to generate gas signatures. A back-propagation ANN algorithm was used to interpret the gas patterns. By analysis of the response of the ANN, the sensitivity and specificity of the instrument were calculated. Detectable differences between the gas signatures of the E. coli O157:H7 isolates and the non-O157:H7 isolates were observed. The instruments degree of sensitivity was high for E. coli O157:H7 isolates, but a lower degree of accuracy was observed for non-O157:H7 isolates because of increased strain variation. The sensitivity of the detection system was improved by the normalization of the data generated from the gas sensors. Because of its ability to detect differences in gas patterns, this instrument has a broad range of potential food safety applications.     

Occurrence of Escherichia coli O157:H7 and other enterohemorrhagic Escherichia coli in the environment

Enterohemorrhagic Escherichia coli (EHEC) (O157 and other serotypes) are zoonotic pathogens linked with severe human illnesses. The main virulence factors of EHEC are the Shiga toxins, among others. Most of the genes coding for these toxins are bacteriophage-encoded. Although ruminants are recognized as their main natural reservoir, water has also been documented as a way of transmission of EHEC. E. coli O157:H7 and other EHEC may contaminate waters (recreational, drinking or irrigation waters) through feces from humans and other animals. Indeed, the occurrence of EHEC carrying the stx2 gene in raw municipal sewage and animal wastewater from several origins has been widely documented. However, the evaluation of the persistence of naturally occurring EHEC in the environment is still difficult due to methodological problems. Methods proposed for the detection and isolation of stx-encoding bacteria, ranging from the classic culture-based methods to molecular approaches, and their application in the environment, are discussed here. Most virulence factors associated with these strains are linked to either plasmids or phages, and consequently they are likely to be subject to horizontal gene transfer between species or serotypes. Moreover, the presence of infectious stx-phages isolated as free particles in the environment and their high persistence in water systems suggest that they may contribute to the spread of stx genes, as they are directly involved in the emergence of new pathogenic strains, which might have important health consequences.     

Effect of vanillin on the fate of Listeria monocytogenes and Escherichia coli O157:H7 in a model apple juice medium and in apple juice

The effects of vanillin on the fates of Listeria monocytogenes and Escherichia coli O157:H7 at pH values between 3.5 and 4.5 were verified in a model apple juice (MAJ) medium and in apple juice incubated at 4 or 15 degrees C. Viable E. coli O157:H7 cells were recovered from MAJ for up to 10 days, but L. monocytogenes did not survive at pH 3.5. Supplementation with 40 mm vanillin exerted a lethal effect that was species, concentration, pH and temperature dependant. E. coli O157:H7 was more sensitive to vanillin than L. monocytogenes, and viable cells could not be recovered after 2 days incubation at either temperature. L. monocytogenes and E. coli O157:H7 were inoculated (10(5) cfu/ml) in pH adjusted (pH 4.00) or unadjusted (pH 3.42) juice from Granny Smith apples that was supplemented with 40 mm vanillin. Neither species were recovered after 3 days incubation at 4 or 15 degrees C. These findings indicate that vanillin could be useful as a preservative for minimally processed apple products.     

Optimization of a fluorescence sandwich enzyme-linked immunosorbent assay for detection of Escherichia coli O157:H7 in apple juice

Sandwich enzyme-linked immunosorbent assay, especially when coupled with biosensor technology, is a simple methodology that can rapidly screen juices for Escherichia coli O157:H7 contamination. However, sampling directly from apple juice and ciders has been postulated to reduce immunoassay sensitivity. In fluorescence sandwich enzyme-linked immunosorbent assays using commercially available polyclonal or monoclonal antibodies, sampling pasteurized apple juice spiked with E. coli O157:H7 compared to spiked phosphate-buffered saline shifted the range of detection. The spiked apple juice range of detection was 10(4) to 10(6) CFU/ml, whereas that for spiked phosphate-buffered saline was 10(6) to 10(8) CFU/ml, representing a hundredfold difference in sensitivity. Apple juice also increased background fluorescence intensity (P < 0.001) while reducing the net fluorescence intensity per CFU (P < 0.001). The addition of the polymer polyvinylpyrrolidone to apple juice significantly improved assay performance by increasing sensitivity and net fluorescence intensity per CFU and by reducing background fluorescence. Adjusting pH of apple juice from 3.9 to 7.4 improved assay performance but not to the degree seen with phosphate-buffered saline or polyvinylpyrrolidone-treated apple juice samples. The apple juice polyphenol, epicatechin, reduced net fluorescence intensity in a concentration-dependent manner, a change that was reversed by polyvinylpyrrolidone. Taken all together, these results suggest that polyvinylpyrrolidone can improve detection of O157:H7 in juices by reducing the effect of polyphenols on fluorescence sandwich enzyme-linked immunosorbent assay performance.     

Detection of Escherichia coli O157:H7 and Salmonella typhimurium using filtration followed by Fourier-transform infrared spectroscopy

Fourier-transform infrared spectroscopy has been successfully used as a nondestructive method for identifying, distinguishing, and classifying pathogens. In this study, a less time-consuming Fourier-transform infrared procedure was developed to identify Escherichia coli O157:H7 and Salmonella Typhimurium. Samples containing 10(9) CFU/ml were prepared in tryptic soy broth and then serially diluted (up to eight times) to obtain bacterial solutions of 10(9) to 10 CFU/ml. These dilutions were incubated at 37 degrees C for 6 h, samples were filtered through a Metricel filter hourly (for 0 to 6 h), and spectra were obtained using a ZnSe contact attenuated total reflectance accessory on a Continu mum infrared microscope. Midinfrared spectra (4,000 to 700 cm(-1)) of Salmonella Typhimurium and E. coli O157:H7 were generated, and peak areas in the region of 1,589 to 1,493 cm(-1) were used to detect the pathogens. Initially, detection limits were between 10(6) and 10(7) CFU/ml without preenrichment, and samples starting with 500 CFU/ml were detectable following incubation for 6 h, when counts reached at least 10(6) CFU/ ml. Compared with results of previously published studies in which Fourier-transform infrared spectroscopy was used to identify select pathogens, this method is more rapid and less expensive for practical large-scale sample analysis.     

Inactivation of Escherichia coli O157:H7, Salmonella typhimurium and Listeria monocytogenes in apple juice with gaseous ozone

This research was initiated to assess the efficacy of gaseous ozone for inactivation Escherichia coli O157:H7, Salmonella typhimurium and Listeria monocytogenes in apple juice. Juice samples with solids content of 18, 36, and 72 °Brix inoculated with a culture cocktail of three foodborne pathogens were treated with gaseous ozone at a flow rate of 3.0 L/min and an ozone generation rate of 0.10, 0.90, 3.51, and 5.57 g/h for 0.5, 1, 5, and 10 min, respectively. The inactivation kinetics of gaseous ozone on foodborne pathogens conformed to the Weibull model. The time required to achieve a 5 log reduction (t_5d) was estimated using the parameters of the Weibull model. The t_5d increased with increasing solids content of apple juice. The ozone generation rate did not impart a significant effect (p > 0.05) on t_5d. Gaseous ozone is effective at inactivating foodborne pathogens in apple juice but the efficacy is dependent on the solids content of the juice sample.     

Effect of citral on the thermal inactivation of Escherichia coli O157:H7 in citrate phosphate buffer and apple juice

Inactivation and sublethal injury of Escherichia coli O157:H7 cells induced by heat in citrate phosphate buffer and apple juice (both at pH 3.8) were studied, and the effect of a combined preservation treatment using citral and heat treatments was determined. Heat resistance of E. coli O157:H7 was similar in both treatment media; after 27 min at 54°C, 3 log units of the initial cell population was inactivated in both treatment media. However, under less harsh conditions a protective effect of apple juice was found. Whereas inactivation followed linear kinetics in the citrate phosphate buffer, when cells were treated in apple juice the survival curves were concave downward. Heat treatment caused a great degree of sublethal injury; 4 min at 54°C inactivated less than 0.5 log CFU/ml but sublethally injured more than 3 log CFU/ml. The addition of 18 and 200 ppm of citral to the treatment medium acted synergistically with heat at 54°C to inactivate 3 × 10(4) and 3 × 10(7) CFU/ml, respectively. Addition of citral thus reduced the time needed to inactivate 1 log unit of the initial E. coli O157:H7 population from 8.9 to 1.7 min. These results indicate that a combined process of heat and citral can inactivate E. coli O157:H7 cells and reduce their potential negative effects.     

Inactivation of Salmonella typhimurium and Escherichia coli O157:H7 in apple juice by a combination of nisin and cinnamon

Pasteurized apple juice with nisin (0, 25, 50, 100, and 200 ppm, wt/vol) and cinnamon (0 and 0.3%, wt/vol) was inoculated with Salmonella Typhimurium and Escherichia coli O157:H7 at 10(4) CFU/ml and stored at 5 and 20 degrees C. Counts on tryptic soy agar (TSA), selective medium (xylose Lysine desoxycholate agar for Salmonella Typhimurium, and MacConkey sorbitol agar for E. coli O157:H7), and thin agar layer (TAL) were determined at 1 h and 1, 3, 7, and 14 days. The TAL method (selective medium overlaid with TSA) was used for recovery of sublethally injured cells. The pathogens were gradually inactivated by the acidic pH of apple juice. Nisin and cinnamon greatly contributed to the inactivation. The killing effect was more marked at 20 degrees C, with counts in all treated samples being undetectable by direct plating in 3 days for Salmonella Typhimurium and 7 days for E. coli O157:H7. Thus, several factors influenced the decrease in counts: low pH, addition of nisin and cinnamon, and storage temperature. The TAL method was as effective as TSA in recovering injured cells of the pathogens. The combination of nisin and cinnamon accelerates death of Salmonella Typhimurium and E. coli O157:H7 in apple juice and so enhances the safety of the product.     

Survival of Escherichia coli O157:H7 during storage in pressure-treated orange juice.

The effect of a high-pressure treatment on the survival of a pressure-resistant strain of Escherichia coli O157:H7 (NCTC 12079) in orange juice during storage at 3 degrees C was investigated over the pH range of 3.4 to 5.0. The pH of shelf-stable orange juice was adjusted to 3.4, 3.6, 3.9, 4.5, and 5.0 and inoculated with 10(8) CFU ml(-1) of E. coli O157:H7. The orange juice was then pressure treated at 400 MPa for 1 min at 10 degrees C or was held at ambient pressure (as a control). Surviving E. coli O157:H7 cells were enumerated at 1-day intervals during a storage period of 25 days at 3 degrees C. Survival of E. coli O157:H7 during storage was dependent on the pH of the orange juice. The application of high pressure prior to storage significantly increased the susceptibility of E. coli O157:H7 to high acidity. For example, after pressure treatment, the time required for a 5-log decrease in cell numbers was reduced from 13 to 3 days at pH 3.4, from 16 to 6 days at pH 3.6, and from >25 to 8 days at pH 3.9. It is evident that the use of high-pressure processing of orange juice in order to increase the juice's shelf-life and to inactivate pathogens has the added advantage that it sensitizes E. coli O157:H7 to the high acid conditions found in orange juice, which results in the survival of significantly fewer E. coli O157:H7 during subsequent refrigerated storage.