Viability of Clostridium perfringens,Escherichia coli,and Listeria monocytogenes surviving mild heat or aqueous ozone treatment on beef followed by heat,alkali, or salt stress

The threat of pathogen survival following ozone treatment of meat necessitates careful evaluation of the microorganisms surviving under such circumstances. The objective of this study was to determine whether sublethal aqueous ozone treatment (3 ppm of O3 for 5 min) of microorganisms on beef surfaces would result in increased or decreased survival with respect to subsequent heat, alkali, or NaCl stress. A mild heat treatment (55 degrees C for 30 min) was used for comparison. Reductions in three-strain cocktails of Clostridium perfringens, Escherichia coli O157:H7, and Listeria monocytogenes on beef following the heat treatment were 0.14, 0.77, and 1.47 log10 CFU/g, respectively, whereas reductions following ozone treatment were 1.28, 0.85, and 1.09 log10 CFU/g, respectively. C. perfringens cells exhibited elevated heat resistance at 60 degrees C (D60 [time at 60 degrees C required to reduce the viable cell population by 1 log10 units or 90%] = 17.76 min) following heat treatment of beef (55 degrees C for 30 min) but exhibited reduced viability at 60 degrees C following ozone treatment (D60 = 7.64 min) compared with the viability of untreated control cells (D60 = 13.84 min). The D60-values for L. monocytogenes and E. coli O157:H7 following heat and ozone exposures were not significantly different (P > 0.05). C. perfringens cells that survived ozone treatment did not exhibit increased resistance to pH (pH 6 to 12) relative to non-ozone-treated cells when grown at 37 degrees C for 24 h. The heat treatment also resulted in decreased numbers of surviving cells above and below neutral pH values for both E. coli O157:H7 and L. monocytogenes relative to those of non-heat-treated cells grown at 37 degrees C for 24 h. There were significant differences (P < 0.05) in C. perfringens reductions with increasing NaCl concentrations. The effects of NaCl were less apparent for E. coli and L. monocytogenes survivors. It is concluded that pathogens surviving ozone treatment of beef are less likely to endanger food safety than are those surviving sublethal heat treatments.     

Fate of Escherichia coli O157:H7, Salmonella typhimurium DT 104, and Listeria monocytogenes in fresh meat decontamination fluids at 4 and 10 degrees C.

Bacterial pathogens may colonize meat plants and increase food safety risks following survival, stress hardening, or proliferation in meat decontamination fluids (washings). The objective of this study was to evaluate the ability of Escherichia coli O157:H7, Salmonella Typhimurium DT 104, and Listeria monocytogenes to survive or grow in spray-washing fluids from fresh beef top rounds sprayed with water (10 or 85 degrees C) or acid solutions (2% lactic or acetic acid, 55 degrees C) during storage of the washings at 4 or 10 degrees C in air to simulate plant conditions. Inoculated Salmonella Typhimurium DT 104 (5.4 +/- 0.1 log CFU/ml) died off in lactate (pH 2.4 +/- 0.1) and acetate (pH 3.1 +/- 0.2) washings by 2 days at either storage temperature. In contrast, inoculated E. coli O157:H7 (5.2 +/- 0.1 log CFU/ml) and L. monocytogenes (5.4 +/- 0.1 log CFU/ml) survived in lactate washings for at least 2 days and in acetate washings for at least 7 and 4 days, respectively; their survival was better in acidic washings stored at 4 degrees C than at 10 degrees C. All inoculated pathogens survived in nonacid (pH > 6.0) washings, but their fate was different. E. coli O157:H7 did not grow at either temperature in water washings, whereas Salmonella Typhimurium DT 104 failed to multiply at 4 degrees C but increased by approximately 2 logs at 10 degrees C. L. monocytogenes multiplied (0.6 to 1.3 logs) at both temperatures in water washings. These results indicated that bacterial pathogens may survive for several days in acidic, and proliferate in water, washings of meat, serving as potential cross-contamination sources, if pathogen niches are established in the plant. The responses of surviving pathogens in meat decontamination waste fluids to acid or other stresses need to be addressed to better evaluate potential food safety risks.     

Acid tolerance of acid-adapted and nonadapted Escherichia coli O157:H7 following habituation (10 degrees C) in fresh beef decontamination runoff fluids of different pH values

This study evaluated survival of Escherichia coli O157:H7 strain ATCC 43895 during exposure to pH 3.5 following its habituation for 2 or 7 days at 10 degrees in fresh beef decontamination waste runoff fluid mixtures (washings) containing 0, 0.02, or 0.2% of lactic or acetic acids. Meat washings and sterile water (control) were initially inoculated with approximately 5 log CFU/ml of acid- and nonadapted E. coli O157:H7 cells cultured (30 degrees C, 24 h) in broth with and without 1% glucose, respectively. After 2 days, E. coli O157:H7 survivors from acetate washings (pH 3.7 to 4.7) survived at pH 3.5 better than E. coli O157:H7 survivors from lactate washings (pH 3.1 to 4.6), especially when the original inoculum was acid adapted. Also, although E. coli O157:H7 habituated in sterile water for 2 days survived well at pH 3.5, the corresponding survivors from nonacid water meat washings (pH 6.8) were rapidly killed at pH 3.5, irrespective of acid adaptation. After 7 days, E. coli O157:H7 survivors from acetate washings (pH 3.6 to 4.7) continued to resist pH 3.5, whereas those from lactate washings died off. This loss of acid tolerance by E. coli O157:H7 was due to either its low survival in 0.2% lactate washings (pH 3.1) or its acid sensitization in 0.02% lactate washings, in which a Pseudomonas-like natural flora showed extensive growth (> 8 log CFU/ml) and the pH increased to 6.5 to 6.6. Acid-adapted E. coli O157:H7 populations habituated in water washings (pH 7.1 to 7.3) for 7 days continued to be acid sensitive, whereas nonadapted populations increased their acid tolerance, a response merely correlated with their slight (< 1 log) growth at 10 degrees C. These results indicate that the expression of high acid tolerance by acid-adapted E. coli O157:H7 can be maintained or enhanced in acid-diluted meat decontamination waste runoff fluids of pH levels that could permit long-term survival at 10 degrees C. Previous acid adaptation, however, could reduce the growth potential of E. coli O157:H7 at 10 degrees C in nonacid waste fluids of high pH and enriched in natural flora. These conditions might further induce an acid sensitization to stationary E. coli O157:H7 cells.     

Bactericidal effects of Lactobacillus reuteri and allyl isothiocyanate on Escherichia coli O157:H7 in refrigerated ground beef

Two naturally occurring antimicrobial agents were tested in packages of refrigerated ground beef for their ability to reduce the viability of Escherichia coli O157:H7 during storage. Allyl isothiocyanate (AITC) and Lactobacillus reuteri were tested separately and together for their action against a cocktail of five strains of E. coli O157:H7 in ground beef held at 4 degrees C for 25 days. Ground beef prepared from whole, raw inside round beef roasts was inoculated with low (3 log CFU/g) or high (6 log CFU/g) levels of the E. coli O157:H7 mixture. The beef was treated with AITC (about 1,300 ppm), L. reuteri, or both, along with 250 mM of glycerol per kg of meat at two levels (3 and 6 log CFU/g) and according to a design that yielded 8 controls plus 10 different treatments. Samples were analyzed for E. coli O157:H7 survivors, numbers of total bacteria, and lactic acid bacteria on days 0 to 25 at 5-day intervals. L. reuteri at both input levels with glycerol killed E. coli O157:H7 at both inoculated levels before day 20. AITC completely eliminated E. coli O157:H7 at the low-inoculum level (3 log CFU/g) and reduced viability >4.5 log CFU/g at the high-inoculum level (6 log CFU/g) by the end of the storage period. The combination of L. reuteri and AITC did not yield an additive effect against E. coli O157:H7 viability. L. reuteri in the presence of glycerol was highly effective against E. coli O157:H7 in ground beef during refrigerated storage (4 degrees C) in modified atmosphere packages. Sensory testing is planned to evaluate effects of treatments.     

Validation of time and temperature values as critical limits for the control of Escherichia coli O157:H7 during the production of fresh ground beef

In order to provide beef processors with valuable data to validate critical limits set for temperature during grinding, a study was conducted to determine Escherichia coli o157:H7 growth at various temperatures in raw ground beef. Fresh ground beef samples were inoculated with a cocktail mixture of streptomycin-resistant E. coli O157:H7 to facilitate recovery in the presence of background flora. Samples were held at 4.4, 7.2, and 10 degrees C, and at room temperature (22.2 to 23.3 degrees C) to mimic typical processing and holding temperatures observed in meat processing environments. E. coli O157:H7 counts were determined by direct plating onto tryptic soy agar with streptomycin (1,000 microg/ml), at 2-h intervals over 12 h for samples held at room temperature. Samples held under refrigeration temperatures were sampled at 4, 8, 12, 24, 48, and 72 h. Less than one log of E. coli O157:H7 growth was observed at 48 h for samples held at 10 degrees C. Samples held at 4.4 and 7.2 degrees C showed less than one log of E. coli O157:H7 growth at 72 h. Samples held at room temperature showed no significant increase in E. coli O157:H7 counts for the first 6 h, but increased significantly afterwards. These results illustrate that meat processors can utilize a variety of time and temperature combinations as critical limits in their hazard analysis critical control point plans to minimize E. coli O157:H7 growth during the production and storage of ground beef.     

Acid tolerance of acid-adapted and nonacid-adapted Escherichia coli O157:H7 strains in beef decontamination runoff fluids or on beef tissue

This study assessed the acid tolerance response (ATR) of stationary phase, acid-adapted (tryptic soy broth [TSB]+1% glucose) or nonacid-adapted (glucose-free TSB) Escherichia coli O157:H7 strains (ATCC43889, ATCC43895, ATCC51658 and EO139), grown individually or in a mixed culture, prior to inoculation of beef or meat decontamination runoff (washings) fluids (acidic [pH 4.95] or nonacidic [pH 7.01]). The inoculated beef was left untreated or treated by dipping for 30s in hot water (75 degrees C) followed by 2% lactic acid (55 degrees C). Inoculated beef samples and washings were stored aerobically at 4 or 15 degrees C for 6d, and at set intervals (0, 2, and 6d) were exposed (for 0, 60, 120, and 180min) to pH 3.5 (adjusted with lactic acid) TSB plus 0.6% yeast extract. Overall, there were no significant (P0.05) differences in responses of cultures prepared as individual or mixed strains. Decontamination of meat did not affect the subsequent ATR of E. coli O157:H7 other than resulting in lower initial pathogen levels exposed to acidic conditions. In this study, E. coli O157:H7 appeared to become more tolerant to acid following incubation in acidic washings of sublethal pH (4.89-5.22) compared to nonacidic washings (pH 6.97-7.41) at 4 degrees C or in both types of washings incubated at 15 degrees C. The ATR of the pathogen inoculated into washings was enhanced when cells were previously acid-adapted and incubated at 4 degrees C. Similarly, the ATR on meat was increased by previous acid-adaptation of the inoculum in broth and enhanced by storage at 4 degrees C. Populations on treated meat were consistently lower than those on untreated meat during storage and following exposure to acid. Although on day-0 there were no significant (P0.05) differences in ATR between acid-adapted and nonacid-adapted populations on meat, acid-adapted cells displayed consistently higher resistance through day-6. This suggests that acid-adapted E. coli O157:H7 introduced on meat may become resistant to subsequent lactic acid exposure following storage at 4 degrees C.     

Antibacterial effect of lactoferricin B on Escherichia coli O157:H7 in ground beef.

The antibacterial activity of lactoferricin B on enterohemorrhagic Escherichia coli O157:H7 in 1% peptone medium and ground beef was studied at 4 and 10 degrees C. In 1% peptone medium, 50 and 100 microg of lactoferricin B per ml reduced E. coli O157:H7 populations by approximately 0.7 and 2.0 log CFU/ml, respectively. Studies comparing the antibacterial effect of lactoferricin B on E. coli O157:H7 in 1% peptone at pH 5.5 and 7.2 did not reveal any significant difference (P > 0.5) at the two pH values. Lactoferricin B (100 microg/g) reduced E. coli O157:H7 population in ground beef by about 0.8 log CFU/g (P < 0.05). No significant difference (P > 0.5) was observed in the total plate count between treatment and control ground beef samples stored at 4 and 10 degrees C. The antibacterial effect of lactoferricin B on E. coli O157:H7 observed in this study is not of sufficient magnitude to merit its use in ground beef for controlling the pathogen.     

Survival of Escherichia coli O157:H7 and Salmonella in apple cider and orange juice as affected by ozone and treatment temperature

Inactivation of Escherichia coli O157:H7 and Salmonella in apple cider and orange juice treated with ozone was evaluated. A five-strain mixture of E. coli O157:H7 or a five-serovar mixture of Salmonella was inoculated (7 log CFU/ml) into apple cider and orange juice. Ozone (0.9 g/h) was pumped into juices maintained at 4 degrees C, ambient temperature (approximately 20 degrees C), and 50 degrees C for up to 240 min, depending on organism, juice, and treatment temperature. Samples were withdrawn, diluted in 0.1% peptone water, and surface plated onto recovery media. Recovery of E. coli O157:H7 was compared on tryptic soy agar (TSA), sorbitol MacConkey agar, hemorrhagic coli agar, and modified eosin methylene blue agar; recovery of Salmonella was compared on TSA, bismuth sulfite agar, and xylose lysine tergitol 4 (XLT4) agar. After treatment at 50 degrees C, E. coli O157:H7 populations were undetectable (limit of 1.0 log CFU/ml; a minimum 6.0-log CFU/ml reduction) after 45 min in apple cider and 75 min in orange juice. At 50 degrees C, Salmonella was reduced by 4.8 log CFU/ml (apple cider) and was undetectable in orange juice after 15 min. E. coli O157:H7 at 4 degrees C was reduced by 4.8 log CFU/ml in apple cider and by 5.4 log CFU/ml in orange juice. Salmonella was reduced by 4.5 log CFU/ml (apple cider) and 4.2 log CFU/ml (orange juice) at 4 degrees C. Treatment at ambient temperature resulted in population reductions of less than 5.0 log CFU/ml. Recovery of E. coli O157:H7 and Salmonella on selective media was substantially lower than recovery on TSA, indicating development of sublethal injury. Ozone treatment of apple cider and orange juice at 4 degrees C or in combination with mild heating (50 degrees C) may provide an alternative to thermal pasteurization for reduction of E. coli O157:H7 and Salmonella in apple cider and orange juice.     

Survival of Escherichia coli O157:H7 in ground-beef patties during storage at 2, -2, 15 and then -2 degrees C, and -20 degrees C

The survival of Escherichia coli O157:H7 and of a nonpathogenic control strain of E. coli was monitored in raw ground beef that was stored at 2 degrees C for 4 weeks, -2 degrees C for 4 weeks, 15 degrees C for 4 h and then -2 degrees C for 4 weeks, and -20 degrees C. Irradiated ground beef was inoculated with one E. coli control strain or with a four-strain cocktail of E. coli O157:H7 (ca. 10(5) CFU/g), formed into patties (30 to 45 g), and stored at the appropriate temperature. The numbers of the E. coli control strain decreased by 1.4 log 10 CFU/g, and pathogen numbers declined 1.9 log 10 CFU/g when patties were stored for 4 weeks at 20 degrees C. When patties were stored at -2 degrees C for 4 weeks, the numbers of the E. coli control strain and the serotype O157:H7 strains decreased 2.8 and 1.5 log 10 CFU/g, respectively. Patties stored at 15 degrees C for 4 h prior to storage at -2 degrees C for 4 weeks resulted in 1.6 and 2.7 log 10-CFU/g reduction in the numbers of E. coli and E. coli O157:H7, respectively. Storage of retail ground beef at 15 degrees C for 4 h (tempering) did not result in increased numbers of colony forming units per gram, as determined with violet red bile, MRS lactobacilli, and plate-count agars. Frozen storage (-20 degrees C) of ground-beef patties that had been inoculated with a single strain of E. coli resulted in approximately a 1 to 2 log 10-CFU/g reduction in the numbers of the control strain and individual serotype O157:H7 strains after 1 year. There was no significant difference between the survival of the control strain and the O157:H7 strains, nor was there a difference between O157:H7 strains. These data demonstrate that tempering of ground-beef patties prior to low-temperature storage accelerated the decline in the numbers of E. coli O157:H7.     

Fate of Escherichia coli O157:H7 in coleslaw during storage

An outbreak of Escherichia coli O157:H7 infection associated with the consumption of coleslaw in several units of a restaurant chain prompted a study to determine the fate of the pathogen in two commercial coleslaw preparations (pH 4.3 and 4.5) held at 4, 11, and 21 degrees C for 3 days. At an initial population of 5.3 log10 CFU/g of coleslaw, E. coli O157:H7 did not grow in either coleslaw stored at the three temperatures. Rather, the population of E. coli O157:H7 decreased by 0.1 to 0.5 log10 CFU/g within 3 days. The greatest reduction (0.4 and 0.5 log10 CFU/g) in population occurred at 21 degrees C, whereas only slight decreases (0.1 to 0.2 log10 CFU/g) occurred at 4 and 11 degrees C. A pH of 4.3 to 4.5 of coleslaw had little effect on reducing E. coli O157:H7 populations. Results suggest that the tolerance of E. coli O157:H7 to acid pH, not temperature abuse, is a major factor influencing the pathogen's fate in restaurant-prepared coleslaw.     

Use of mustard flour to inactivate Escherichia coli O157:H7 in ground beef under nitrogen flushed packaging

This study was undertaken to determine whether the glucosinolates naturally present in non-deheated mustard flour could serve as a source of allyl and other isothiocyanates in sufficient quantity to kill Escherichia coli O157:H7 inoculated in ground beef at three different levels, during refrigerated storage of the meat under nitrogen. Mustard flour was mixed at 5%, 10% or 20% (w/w) with freshly ground beef, then the beef was inoculated with a cocktail of five strains of E. coli O157:H7 at either 3, 6 or < or =1.6 log10 cfu/g. The ground beef was formed into 100 g patties and each was placed in a bag of Nylon/EVOH/PE, which was back-flushed with 100% N2, heat-sealed and stored at 4 degrees C for < or =21 days. During storage, the allyl isothiocyanate (AIT) levels in package headspaces were determined by gas liquid chromatography. By 21 days, the levels present in treatments were not significantly different. After 21 days storage, there were 0.5, 3 and 5.4 log10 decreases in numbers of E. coli O157:H7 from the initial levels of 6 log10 cfu/g in meat containing 5%, 10% and 20% mustard flour, respectively. When inoculated at 3 log10 cfu/g, E. coli O157:H7 was reduced to undetectable levels after 18, 12 and 3 days with 5%, 10% and 20% mustard flour, respectively. When immunomagnetic separation (IMS) was used for E. coli recovery following its inoculation at < or =1.6 log10 cfu/g, 5% mustard did not completely eliminate the pathogen from ground beef stored for 6 days. The natural microflora of the ground beef which developed in vacuum packages was unaffected by the addition of 5% mustard flour but some inhibition was found at higher concentrations. Sensory evaluation of the cooked ground beef showed that there were no significant differences in the acceptability of meat treated with 5 or 10% mustard flour. However, panelists could distinguish untreated controls from mustard treatments, but considered the mustard-treated meat to be acceptable. These results showed that it is possible to use mustard flour at levels of >5-10% to eliminate E. coli O157:H7 from fresh ground beef.     

A model study of Escherichia coli O157:H7 survival in fermented dry sausages--influence of inoculum preparation, inoculation procedure, and selected process parameters

The influence of inoculum preparation, inoculation level, and inoculation procedure on Escherichia coli O157:H7 inactivation during the manufacture of fermented sausage was evaluated in a model study. Prior growth in glucose-enriched tryptone soya broth, which provided exposure to mildly acidic conditions (pH 4.8), had no effect on the later survival of E. coli O157: H7 strains 5-1 and ATCC 43894 under extremely acidic conditions (pH 2), but the same strains became sensitive to acidity after 7 days of incubation on the surface of refrigerated beef (as per the normal contamination route from slaughter to further processing). In subsequent sausage production trials, the extent of destruction observed for E. coli O157:H7 strains F-90, 5-1, and ATCC 43894 inoculated directly into the meat batter was unchanged when the inoculation level was decreased from 7.3 to 4.7 log CFU/g, but the level of inactivation was ca. 1 log higher when the surfaces of beef cuts, rather than the batter, were inoculated 7 days prior to processing. Regardless of processing conditions (fermentation to a pH of < or = 5.0 at 24 or 37 degrees C, drying at 14 degrees C to a water activity [a(w)] value of 0.91 or 0.79), strains F-90, 5-1, and ATCC 43894 showed similar survival capacities during the manufacture of sausage. A approximately 2-log reduction in pathogen numbers was generally obtained after samples were dried to an a(w) of 0.91, irrespective of fermentation temperature. The addition of a 5-day predrying holding stage at the fermentation temperature significantly (P < 0.05) increased pathogen inactivation when fermentation was carried out at 37 degrees C (but not when it was carried out at 24 degrees C). However, significant pathogen reductions (4 to 5 log CFU/g) were achieved only for extensively dried products (a(w) = 0.79).     

Growth and survival of Escherichia coli O157:H7 and Listeria monocytogenes in egg products held at different temperatures

Growth and survival of Escherichia coli O157:H7 and Listeria monocytogenes in steamed eggs and scrambled eggs held at different temperatures (5, 18, 22, 37, 55, and 60 degrees C) were investigated in the present study. Among the holding temperatures tested, both pathogens multiplied best at 37 degrees C followed by 22, 18, and 5 degrees C. In general, E. coli O157:H7 grew better in the egg products than L. monocytogenes did at all the storage temperatures tested except at 5 degrees C. E. coli O157:H7 did not grow in steamed eggs and scrambled eggs held at 5 degrees C. L. monocytogenes showed a slight population increase of approximately 0.6 to 0.9 log CFU/g in these egg products at the end of the 36-h storage period at 5 degrees C. The population of both pathogens detected in the egg products was affected by the initial population, holding temperature, and length of the holding period. It was also noted that L. monocytogenes was more susceptible than E. coli O157:H7 in steamed eggs held at 60 degrees C. After holding at 60 degrees C for 1 h, no detectable viable cells of L. monocytogenes with a population reduction of 5.4 log CFU/g was observed in steamed eggs, whereas a lower population reduction of only approximately 0.5 log CFU/ml was noted for E. coli O157:H7.     

Identification of Escherichia coli O157:H7 meat processing indicators for fresh meat through comparison of the effects of selected antimicrobial interventions

Fresh meat products can become contaminated with the pathogen Escherichia coli O157:H7 during the slaughter process; therefore, an E. coli O157:H7 indicator to verify the effectiveness of process controls in slaughter establishments would be extremely useful. The hides of 20 beef cattle were sampled, and 113 bacterial isolates were obtained. Thirteen of these isolates representing four genera, Escherichia, Enterobacter, Providencia, and Serratia, were selected based on growth and biochemical characteristics similar to those of five clinical strains of E. coli O157:H7. The temperature sensitivity was determined for the individual isolates and the five E. coli O157:H7 strains at 55 and 65 degrees C. D65-values for all 13 isolates were not significantly different from D65-values of the E. coli O157:H7 strains. E. coli isolates were the only isolates whose D55-values were not significantly different from those of the E. coli O157:H7 strains. E. coli isolates P3 and P68 were more resistant to the effects of 55 degrees C than were the other E. coli isolates but were not significantly different from E. coli O157:H7 WS 3331 (P > 0.05). The remaining E. coli isolates (P1, P8, and P14) were not significantly different from E. coli O157:H7 strains ATCC 35150, ATCC 43894, ATCC 43895, and WS 3062 (P > 0.05). Prerigor lean and adipose beef carcass tissue was artificially contaminated with stationary-phase cultures of the five E. coli beef cattle isolates or a cocktail of five E. coli O157:H7 strains in a fecal inoculum. Each tissue sample was processed with the following microbial interventions: 90 degrees C water; 90 degrees C water followed by 55 degrees C 2% lactic acid; 90 degrees C water followed by 20 degrees C 2% lactic acid; 20 degrees C water followed by 20 degrees C 2% lactic acid; 20 degrees C water followed by 20 degrees C 20 ppm chlorine; and 20 degrees C water followed by 20 degrees C 10% trisodium phosphate. The appropriateness of the E. coli isolates as potential E. coli O157:H7 indicators was dependent upon the microbial intervention utilized. For all microbial intervention methods applied irrespective of tissue type, the mean log reductions of at least two E. coli isolates were not significantly different from the mean log reduction of the E. coli O157:H7 cocktail (P > 0.05). Because of the frequent employment of multiple microbial interventions in the cattle industry, no single isolate can realistically represent the effectiveness of all microbial interventions for reduction of E. coil O157:H7. Thus, the use of a combination of E. coli isolates may be required to accurately predict the effectiveness of microbial intervention methods on the reduction of E. coli O157:H7 in beef carcass tissue.     

Thermal inactivation of acid, cold, heat, starvation, and desiccation stress-adapted Escherichia coli O157:H7 in moisture-enhanced nonintact beef

This study was conducted to compare thermal inactivation of stress-adapted and nonadapted Escherichia coli O157:H7 in nonintact beef moisture enhanced with different brine formulations and cooked to 65°C. Coarsely ground beef was mixed with acid, cold, heat, starvation, or desiccation stress-adapted or nonadapted rifampin-resistant E. coli O157:H7 (eight-strain mixture, 5 to 6 log CFU/g) and a brine solution for a total moisture enhancement level of 10%. The brine treatments included distilled water (control), sodium chloride (0.5% NaCl) plus sodium tripolyphosphate (0.25% STP), or NaCl + STP combined with cetylpyridinium chloride (0.2% CPC), lactic acid (0.3% LA), or sodium metasilicate (0.2% SM). The treated meat was extruded into bags (15 cm diameter), semifrozen (-20°C for 4.5 h), and cut into 2.54-cm (1-in.)-thick portions. Samples were individually vacuum packaged, frozen (-20°C for 42 h), and tempered at 4°C for 2.5 h before cooking. Partially thawed (-1.8 ± 0.4°C) samples were pan broiled to an internal temperature of 65°C. Pathogen counts of partially thawed (before cooking) samples moisture enhanced with brines containing CPC, LA, or SM were 0.7 to 1.1, 0.0 to 0.4, and 0.2 to 0.4 log CFU/g, respectively, lower than those of the control. Compared with microbial count reductions obtained after pan broiling of beef inoculated with nonadapted E. coli O157:H7 cells, count reductions during cooking of meat inoculated with cold and desiccation stress-adapted, acid stress-adapted, and heat and starvation stress-adapted cells indicated sensitization, cross protection, and no effect, respectively, of these stresses on the pathogen during subsequent exposure to heat. Among all stressed cultures, CPC-treated samples (0.8 to 3.6 log CFU/g) and LA-treated samples (0.8 to 3.5 log CFU/g) had the lowest numbers of E. coli O157:H7 survivors after cooking.     

Influence of sodium chloride on growth of lactic acid bacteria and subsequent destruction of Escherichia coli O157:H7 during processing of Lebanon bologna

Due to undesirable quality changes, Lebanon bologna is often processed at temperatures that do not exceed 48.8 degrees C (120 degrees F). Therefore, it is important to study parameters that influence the destruction of Escherichia coli O157:H7 in Lebanon bologna. The objective of the present study was to determine the influence of curing salts (NaCl and NaNO2) on the destruction of E. coli O157:H7 during Lebanon bologna processing. Fermentation to pH 4.7 at 37.7 degrees C reduced populations of E. coli O157:H7 by approximately 0.3 log10, either in the presence or absence of curing salts. Subsequent destruction of E. coli O157:H7 during heating of fermented product to 46.1 degrees C was significantly reduced by the presence of 3.5% NaCl and 156 ppm NaNO2, compared to product without curing salts (P < 0.01). The presence of a higher level of NaCl (5%) in Lebanon bologna inhibited the growth of lactic acid bacteria (LAB), which yielded product with higher pH (approximately 5.0) and significantly reduced the destruction of E. coli O157:H7 even further (P < 0.05). Lower concentrations of NaCl (0, 2.5%) yielded Lebanon bologna with higher LAB counts and lower pHs, compared to product with 5% NaCl. When lactic acid was used to adjust pH in product containing different levels of NaCl, it was determined that low pH was directly influencing destruction of E. coli O157:H7, not NaCl concentration.     

Survival of Escherichia coli O157:H7 in dry fermented sausages containing micro-encapsulated probiotic lactic acid bacteria

Escherichia coli O157:H7 is capable of surviving the rigorous processing steps during the manufacture of dry fermented sausages. The effect of adding two probiotic organisms, Lactobacillus reuteri and Bifidobacterium longum as co-cultures with the meat starter cultures Pediococcus pentosaceus and Staphylococcus carnosus on the viability of E. coli O157:H7 in dry fermented sausages was studied. A 5 strain cocktail of E. coli O157:H7 was added at 7.4 log cfu/g to the sausage batter and challenged with either or both Lb. reuteri or B. longum before or after they were micro-encapsulated. Sausages were fermented at < or = 26 degrees C and 88% relative humidity (RH) followed by drying at 75% RH and 13 degrees C for 25 d. The pH, water activity (aw), protein, moisture, and numbers of all inoculated organisms were monitored during processing. The pH and aw decreased from 5.7 and 0.98 to 4.9 and 0.88 at the end of fermentation and drying, respectively. These processes reduced E. coli O157:H7 by 1.0 and 0.7 log cfu/g at the end of fermentation and drying, respectively. Unencapsulated Lb. reuteri with or without B. longum reduced E. coli O157:H7 by 3.0 log cfu/g and B. longum caused a 1.9 log cfu/g reduction. While micro-encapsulation increased survival of Lb. reuteri and B. longum, it reduced their inhibitory action against E. coli O157:H7.     

Effect of acid adaptation on survival of Escherichia coli O157:H7 in meat decontamination washing fluids and potential effects of organic acid interventions on the microbial ecology of the meat plant environment.

The acid tolerance of Escherichia coli O157:H7 may be pH inducible. Correspondingly, organic acid meat decontamination washing fluids may enhance the establishment of acid-adapted E. coli O157:H7 strains in packing plants, especially in mixtures with water washings from meat that may be of sublethal pH. Acid-adapted and nonadapted cultures of a rifampin-resistant derivative of the acid-resistant E. coli O157:H7 strain ATCC 43895 were tested to evaluate their survival in meat-washing fluids over a wide pH range. The cultures were exposed (10(5) CFU/ml) to acidic (2% lactic acid. 2% acetic acid, or a mixture of the two with water washings at ratios of 1/1, 1/9, or 1/99 [vol/vol]) or nonacid (water) meat washings for up to 14 days at 4 or 10 degrees C storage. E. coli O157:H7 survived in water washings, but the low storage temperatures and predominant natural microbiota synergistically inhibited its growth. Compared with acid-adapted populations, nonadapted populations displayed greater potential for survival and a tendency to initiate growth in water meat washings at 10 degrees C. The pathogen survived in most of the acid washings throughout storage (14 days), sometimes with minimal population reductions. Overall. nonadapted populations declined faster than acid-adapted populations, while the declines increased as the acid concentration and temperature of storage increased and were more dramatic in lactate, compared to acetate, washings. Acid-containing washings were selective for growth of lactic acid bacteria and yeasts. indicating that organic acid treatments may alter the microbial ecology of meat plant environments and potentially that of the meat. These results should be considered when selecting decontamination technologies for meat.     

PREDICTIVE MODEL FOR THE COMBINED EFFECT OF TEMPERATURE,pH, SODIUM CHLORIDE,AND SODIUM PYROPHOSPHATE ON THE HEAT RESISTANCE OF ESCHERICHIA COLI O157:H71

The effects and interactions of heating temperature (5–62.5C), pH (4 – 8), NaCl (0 – 6%, w/v), and sodium pyrophosphate (0 – 0.3%, w/v) on the heat resistance of a four strain mixture of Escherichia coli O157:H7 in beef gravy were examined. Thermal death times were determined using a submerged coil heating apparatus. The recovery medium was plate count agar supplemented with 1% sodium pyruvate. Decimal reduction times (D-values) were calculated by fitting a survival model to the data with a curve fitting program. The D-values were analyzed by second order response surface regression for temperature, pH, NaCl and sodium pyrophosphate levels. The four variables interacted to affect the inactivation of the pathogen. Thermal resistance of E. coli O157:H7 can be lowered by combining these intrinsic factors. A mathematical model describing the combined effect of temperature, pH, NaCl and sodium pyrophosphate levels on the thermal inactivation of E. coli O157:H7 was developed. The model can predict D-values for any combinations of temperature, pH, NaCl and sodium pyrophosphate that are within the range of those tested.     

Effect of acid resistance of Escherichia coli O157:H7 on efficacy of buffered lactic acid to decontaminate chilled beef tissue and effect of modified atmosphere packaging on survival of Escherichia coli O157:H7 on red meat.

The present study examined the effect of pH-independent acid resistance of Escherichia coli O157:H7 on efficacy of buffered lactic acid to decontaminate chilled beef tissue. A varied level of acid resistance was observed among the 14 strains tested. Eight strains were categorized as acid resistant, four strains as acid sensitive, and two strains demonstrated acid-inducible acid resistance. The survival of an acid-resistant (II/45/4) and acid-sensitive (IX/8/16) E. coli O157:H7 strain on chilled beef tissue treated with 1 and 2% buffered lactic acid, sterile water, or no treatment (control) was followed. A gradual reduction of E. coli O157:H7 was noticed during the 10 days of storage at 4 degrees C for each of the treatments. Decontamination with 1 and 2% buffered lactic acid did not appreciably affect the pathogen. Differences in the pH-independent acid resistance of the strains had no effect on the efficacy of decontamination. The effect of modified atmosphere packaging (MAP) on survival of E. coli O157:H7 in red meat was also studied. MAP (40% CO2/60% N2) or vacuum did not significantly influence survival of E. coli O157:H7 on inoculated sliced beef (retail cuts) meat compared to packing in air. The relative small outgrowth of lactic acid bacteria during storage under vacuum for 28 days did not affect survival of E. coli O157:H7. Neither lactic acid decontamination nor vacuum or MAP packaging could enhance reduction of E. coli O157:H7 on beef, thus underlining the need for preventive measures to control the public health risk of E. coli O157:H7.