Growth patterns of Escherichia coli O157:H7 in ground beef treated with nisin,chelators, organic acids and their combinations immobilized in calcium alginate gels

The effects of antimicrobial substances including nisin, acetic acid, lactic acid, potassium sorbate and chelators (disodium ethylenediamine tetraacetic acid [EDTA] and sodium hexametaphosphate [HMP]), alone or in combination and, with or without immobilization in calcium alginate gels, on the growth of Escherichia coli O157:H7 in ground beef were investigated. Results showed that acetic acid and potassium sorbate could inhibit the growth of E. coli O157:H7 effectively at 10°C and at 30°C. Both EDTA and HMP did not halt the growth of E. coli O157:H7. In an antimicrobial system immobilized with calcium alginate, most of the antimicrobials could not inhibit the growth of E. coli O157:H7 in ground beef at 10°C and at 30°C, with the exception of acetic acid and lactic acid. Immobilization did not enhance the effectiveness of acetic acid against E. coli O157:H7 in ground beef at 10°C and at 30°C (P>0.05) but it did enhance the effectiveness of lactic acid at 10°C. In a system combining different antimicrobials, treatment with nisin /EDTA or nisin/potassium sorbate at 10°C revealed a significantly lower population change of E. coli O157:H7 compared to samples treated with nisin, EDTA or potassium sorbate alone. The use of calcium alginate immobilization further enhanced the effectiveness of the combination system of nisin/EDTA, nisin/acetic acid and nisin/potassium sorbate on the growth of E. coli O157:H7 in ground beef at 10°C but it was not effective at 30°C.     

Development of an Effective Treatment for A 5‐Log Reduction of Escherichia coli in Refrigerated Pickle Products

Refrigerated cucumber pickle products cannot be heat processed due to the loss of characteristic sensory attributes. Typically brined refrigerated pickles contain less than 100 mM acetic acid with pH values of 3.7 to 4.0. Refrigeration (4 to 10 °C) helps to inhibit the growth of spoilage bacteria and maintain flavor, texture, and appearance of the pickles. Previous research has shown that pathogenic Escherichia coli strains are unusually acid resistant and survive better in refrigerated acid solutions than at higher temperatures. We found that E. coli O157:H7 can survive for 1 mo or longer at 4 °C in brines typical of commercial refrigerated pickles. Our objective was to develop methods to assure a 5‐log reduction of pathogenic E. coli in these types of products, while maintaining the sensory characteristics. A novel brine formulation was developed, based on current commercial refrigerated pickle brines, which contained 25 mM fumaric acid, 5 mM benzoic acid, 70 mM acetic acid, and 342 mM (2%) sodium chloride, with a pH of 3.8. Sensory data indicate that this formulation did not affect flavor or other sensory attributes of the product, compared to traditional formulations. We achieved a 5‐log reduction of E. coli O157:H7 at 30 °C for 1.52 ± 0.15 d, at 20 °C for 3.12 ± 0.34 d, or at 10 °C for 8.83 ± 0.56 d. Growth of lactic acid bacteria was also inhibited. These results can be used by manufacturers to assure a 5‐log reduction in cell numbers of E. coli O157:H7 and Salmonella without a heat process during the manufacture of refrigerated pickle products. Practical Application : While refrigerated acidified vegetable products are exempt from the acidified foods regulations, we have shown that the vegetative microbial pathogens E. coli O157:H7 can survive for up to 1 mo in these products, given current commercial production practices. To improve the safety of refrigerated pickle products, a brine formulation with reduced acetic acid, but containing fumaric acid, was developed to assure a 5‐log reduction in cell numbers of E. coli O157:H7 without a heat process. The formulation can be used to assure the safety of refrigerated pickled vegetables without altering sensory characteristics.     

Viability of acid-adapted Escherichia coli O157:H7 in ground beef treated with acidic calcium sulfate

The effects of lactic acid, acetic acid, and acidic calcium sulfate (ACS) on viability and subsequent acid tolerance of three strains of Escherichia coli O157:H7 were determined. Differences in tolerance to acidic environments were observed among strains, but the level of tolerance was not affected by the acidulant to which cells had been exposed. Cells of E. coli O157:H7 adapted to grow on tryptic soy agar acidified to pH 4.5 with ACS were compared to cells grown at pH 7.2 in the absence of ACS for their ability to survive after inoculation into ground beef treated with ACS, as well as untreated beef. The number of ACS-adapted cells recovered from ACS-treated beef was significantly (alpha = 0.05) higher than the number of control cells recovered from ACS-treated beef during the first 3 days of a 10-day storage period at 4 degrees C, suggesting that ACS-adapted cells might be initially more tolerant than unadapted cells to reduced pH in ACS-treated beef. Regardless of treatment of ground beef with ACS or adaptation of E. coli O157:H7 to ACS before inoculating ground beef, the pathogen survived in high numbers.     

VALIDATION OF HEATING CONDITIONS IN PRODUCTION OF DIRECT ACIDIFIED BEEF SUMMER SAUSAGE FOR ELIMINATION OF ESCHERICHIA COLI O157:H7

The effectiveness of a typical production process for eliminating Escherichia coli O157:H7 in directly acidified all‐beef summer sausage was evaluated for formulations of different fat contents (approximately 8 and 17%) and types of direct acidulant (encapsulated citric or lactic acid). Raw batter inoculated with E. coli O157:H7 to an initial level of ca. 7.4 log cfu/g was stuffed into 64‐mm casings and processed according to a thermal processing schedule used by a small commercial processor for directly acidified summer sausage products (maximum internal product temperature of 70C, followed by cold showering). For all‐beef summer sausage, log reductions ranged from 5.3 to 5.5 cfu/g when product reached 64.4C (148F) internal temperature (IT) and 70C (158F) IT, and from 6.3 to 6.5 log cfu/g reductions when product reached 37.8C (100F) IT after thermal processing and cold showering. No differences in E. coli O157:H7 counts were observed for products with different fat or acid contents.     

Inactivation ofEscherichia coliO157:H7 by combinations of GRAS chemicals and temperature

Reported outbreaks of foodborne illness involvingEscherichia coliO157:H7 have increased in the United States during the last decade, with a variety of food products being implicated as vehicles of infection. Studies were carried out to determine the efficacy of combinations of various GRAS chemicals and moderate temperatures to killE. coliO157:H7. A five-strain mixture ofE. coliO157:H7 of approximately 10⁸cfu ml⁻¹was inoculated into 0·1% peptone solutions containing 1·0 or 1·5% lactic acid plus 0·1% hydrogen peroxide, 0·1% sodium benzoate or 0·005% glycerol monolaurate. The solutions were incubated at 8°C for 0, 15 and 30 min; at 22°C for 0, 10 and 20 min; or at 40°C for 0, 10 and 15 min; populations ofE. coliO157:H7 were determined at each sampling time. At 40°C, the pathogen was inactivated to undetectable levels within 10 min of incubation in the presence of 1·0 or 1·5% lactic acid plus hydrogen peroxide, and within 15 min of incubation in the presence of 1·5% lactic acid plus sodium benzoate or glycerol monolaurate. At 22°C, complete inactivation ofE. coliO157:H7 was observed after 20 min of exposure to 1·5% lactic acid plus 0·1% hydrogen peroxide, whereas a reduction of 5 log₁₀cfu ml⁻¹was observed with a treatment of 1·5% lactic acid plus glycerol monolaurate. None of the treatments resulted in total inactivation of the pathogen at 8°C. The aforementioned treatments could potentially be used to inactivate or reduceE. coliO157:H7 populations on raw produce.     

Evaluation of Brining Ingredients and Antimicrobials for Effects on Thermal Destruction of Escherichia coli O157:H7 in a Meat Model System

ABSTRACT: Escherichia coli O157:H7 may become internalized during brine injection of meat. This study evaluated the effect of brining ingredients on E. coli O157:H7 in a meat model system after simulated brining, storage, and cooking. Fresh knuckles (5.3 ± 2.4% fat) or beef shoulder (15.3 ± 2.2% fat) were ground individually, mixed with an 8-strain composite of rifampicin-resistant E. coli O157:H7 (7 log CFU/g) and brining solutions. Treatments included no brining, distilled water, sodium chloride (NaCl, 0.5%), sodium tripolyphosphate (STP, 0.25%), sodium pyrophosphate (SPP, 0.25%), NaCl + STP, NaCl + SPP, NaCl + STP + potassium lactate (PL, 2%), NaCl + STP + sodium diacetate (SD, 0.15%), NaCl + STP + PL + SD, NaCl + STP + lactic acid (0.3%), NaCl + STP + acetic acid (0.3%), NaCl + STP + citric acid (0.3%), NaCl + STP + EDTA (20 mM) + nisin (0.0015%) or pediocin (1000 AU/g), NaCl + STP + sodium metasilicate (0.2%), NaCl + STP + cetylpyridinium chloride (CPC; 0.5%), and NaCl + STP + hops beta acids (0.00055%). Samples (30 g) were analyzed for pH, and total microbial and rifampicin-resistant E. coli O157:H7 (inoculum) populations immediately after mixing, storage (24 h at 4 °C), and cooking to 65 °C. Fat and moisture contents and water activity were measured after storage and cooking only; cooking losses also were determined. The effect of beef type on microbial counts, pH, and water activity was negligible. No reductions in microbial counts were obtained by the brining solutions immediately or after storage, except for samples treated with CPC, which reduced (P < 0.05) pathogen counts after storage by approximately 1 log cycle. Cooking reduced pathogen counts by 1.5 to 2.5 logs, while CPC-treated samples had the lowest (P < 0.05) counts compared to any other treatment. These data may be useful in developing/improving brining recipes for control of E. coli O157:H7 in moisture-enhanced beef products.     

Lethality of heat to Escherichia coli O157:H7: D- and z-value determinations in turkey,lamb and pork

Thermal inactivation of a four-strain mixture of E. coli O157:H7 was determined in lean ground turkey, lamb and pork. Inoculated meat was packaged in bags completely immersed in a circulating water bath and held at 55, 57.5, 60, 62.5, and 65°C for predetermined lengths of time. The surviving cell population was enumerated by spiral plating meat samples on tryptic soy agar overlaid with Sorbitol MacConkey agar. D-values, determined by linear regression, in turkey were 11.51, 3.59, 1.89, 0.81 and 0.29 min at 55, 57.5, 60, 62.5 and 65°C, respectively (z=6.5°C). When a survival model was fitted to the non-linear survival curves, D-values in turkey ranged from 11.26 min at 55°C to 0.23 min at 65°C (z=6°C). When the E. coli O157:H7 four-strain cocktail was heated in ground pork or lamb, D-values calculated by both approaches were similar at all temperatures. Thermal-death-times from this study will assist the retail food industry to design cooking regimes that ensure safety of ground muscle foods contaminated with E. coli O157:H7.     

Antimicrobial Effects of Quillaja saponaria Extract Against Escherichia coli O157:H7 and the Emerging Non‐O157 Shiga Toxin‐Producing E. coli

Natural alternate methods to control the spread of Shiga toxin‐producing Escherichia coli (STEC) are important to prevent foodborne outbreaks. Quillaja saponaria aqueous bark extracts (QE), cleared by the U.S. Food and Drug Administration as a natural flavorant, contain bioactive polyphenols, tannins, and tri‐terpenoid saponins with anti‐inflammatory and antimicrobial activity. The objective of this study was to determine the effects of commercial QE against E. coli O157:H7 and non‐O157 strains over 16 h at 37 °C and RT. Overnight cultures of 4 E. coli O157:H7 strains and 6 non‐O157 STECs in Tryptic Soy Broth (TSB) were washed and resuspended in phosphate‐buffered saline (PBS, pH 7.2), and treated with QE and controls including citric acid (pH 3.75), sodium benzoate (0.1% w/w), acidified sodium benzoate (pH 3.75) or PBS for 6 h or 16 h. Recovered bacteria were enumerated after plating on Tryptic Soy Agar, from duplicate treatments, replicated thrice and the data were statistically analyzed. The 4 QE‐treated E. coli O157:H7 strains from initial ～7.5 log CFU had remaining counts between 6.79 and 3.5 log CFU after 16 h at RT. QE‐treated non‐O157 STECs showed lower reductions with remaining counts ranging from 6.81 to 4.55 log CFU after 16 h at RT.  Incubation at 37 °C caused reduction to nondetectable levels within 1 h, without any significant reduction in controls. Scanning electron microscopy studies revealed damaged cell membranes of treated bacteria after 1 h at 37 °C. QE shows potential to control the spread of STECs, and further research in model food systems is needed.     

CONDITIONS AFFECTING AUTOINDUCER‐2‐BASED QUORUM‐SENSING OF ESCHERICHIA COLI O157:H7 ON FRESH BEEF

ABSTRACT

This study evaluated whether inoculated (none, 1, 5 log colony‐forming units [cfu]/cm²) Escherichia coli O157:H7 would result in detection of autoinducer (AI)‐2‐like activity on beef. Inoculated fresh beef, containing low (LNB) or high (HNB) initial levels of natural flora, was analyzed for bacterial populations and AI‐2‐like activity during aerobic or vacuum‐packaged storage (4, 10, 25C). As expected, no growth of E. coli O157:H7 was detected at 4C, while at 10C, growth was detected only on LNB samples stored aerobically; AI‐2‐like activity was minimal (P ≥ 0.05) at both temperatures. E. coli O157:H7 showed more growth in LNB than HNB, and in aerobically than vacuum‐packaged samples inoculated with 1 log cfu/cm² of the pathogen during storage at 25C. AI‐2‐like activity was generally higher in LNB than HNB samples stored aerobically at 25C, while no significant AI‐2‐like activity was detected in samples stored in vacuum packages. The results indicated that E. coli O157:H7 may exhibit AI‐2‐like activity on aerobically stored beef in the presence of lower initial levels of natural flora, and at temperatures allowing prolific growth of the pathogen. Thus, AI‐2‐based quorum‐sensing of E. coli O157:H7 may not be of importance in beef stored at low temperatures.

PRACTICAL APPLICATIONS

This study presents evidence that Escherichia coli O157:H7 showed autoinducer (AI)‐2 activity and involved in quorum‐sensing on fresh beefcontaining low initial levels of natural flora during aerobic storage at abusive storage temperatures. Thus, AI‐2‐based quorum‐sensing of E. coli O157:H7 may not be important in beef stored at recommended low temperatures.     

Reduction of Pathogens Using Hot Water and Lactic Acid on Beef Trimmings

Beef trimmings from young or mature beef cattle were obtained commercially. Trimmings within age type then were inoculated with about 6.0 log₁₀ CFU/mL of rifampicin-resistant. Escherichia coli O157:H7 and Salmonella typhimurium (ATCC 13311) were randomly assigned to 1 of 3 treatments (control; 95 °C hot water alone, or with 2% L-lactic acid). After treatment, trimmings were ground, held for 0, 14, 28, or 42 d in chub packages at 4 °C, and total aerobic plat counts, E. coli O157:H7, and S. typhimurium counts were determined. Non-inoculated trimmings were also treated and samples were evaluated for pH, fat, moisture, TBA, meat color by colorimeter, and meat color, and odor by trained sensory panels. Trimmings treated with water or hot water plus lactic acid reduced levels of E. coli O157:H7 and S. typhimurium and tended to be darker after treatment. Meat odor in the final product was not affected by treatment.     

Sensitivity of acid-adapted and acid-shocked Shigella flexneri to reduced pH achieved with acetic, lactic, and propionic acids.

Survival and growth characteristics of unadapted, acid-adapted, and acid-shocked Shigella flexneri 2a cells in acidified (pH 3.5 to 5.5) tryptic soy broth with 0.25% glucose (TSB) and tryptic soy agar (TSA) were determined. S. flexneri was grown at 37 degrees C for 18 h in tryptic soy broth without glucose (TSBNG) (unadapted) and TSBNG supplemented with 1% glucose (TSBG) (acid-adapted). Cells grown in TSBNG were acid shocked by adjusting 16-h cultures to pH 5.05 +/- 0.05 with lactic acid. Cells were then inoculated into TSB acidified with acetic, lactic, or propionic acids to pH 5.5, 4.5, or 3.5 and incubated at 37 degrees C for 6 h. The order of lethality at a given pH was lactic acid < acetic acid < propionic acid. Significantly (P < or = 0.05) higher numbers of acid-adapted cells, compared to acid-shocked and unadapted cells, were recovered from TSB acidified (pH 3.5) with lactic or acetic acids. None of the cells survived a 30-min exposure in TSB acidified with propionic acid to pH 3.5. When the three cell types were plated on TSA acidified with lactic, acetic, or propionic acids at pH < or = 4.5, < or = 5.5, and < or = 5.5, respectively, visible colonies were not detected. Viable unadapted, acid-adapted, and acid-shocked cells were, however, recovered from TSA acidified with all three acids at pH > or = 4.5. Acid-adapted and, to a lesser extent, acid-shocked cells survived at lower pH than did unadapted cells, indicating that prior exposure to mild acidic environment results in increased acid resistance. Survival of S. flexneri at a given pH was influenced by the type of acidulant used, a response characteristic exhibited by other gram-negative enteric pathogens.     

Thermal Inactivation of Escherichia coli O157:H7 in Strawberry Puree and Its Effect on Anthocyanins and Color

Raw whole strawberries, if contaminated with pathogens, such as Escherichia coli O157:H7, must be pasteurized prior to consumption. Therefore, the objective of this research was to investigate the thermal inactivation kinetics of E. coli O157:H7 in strawberry puree (SP), and evaluate the changes in anthocyanins and color, and the survival of yeasts and molds (YM) after thermal processing. Inoculated with a 5‐strain cocktail, fresh SP, with or without added sugar (20 and 40 °Brix), was heated at 50, 52, 54, 57.5, 60, and 62.5 °C to determine the thermal resistance of E. coli O157:H7. In raw SP, the average D‐values of E. coli O157:H7 were 909.1, 454.6, 212.8, 46.1, and 20.2 s at 50, 52, 54, 57.5, and 60 °C, respectively, with a z‐value of 5.9 °C. While linearly decreasing with temperature, the log D‐values of E. coli O157:H7 increased slightly with sugar concentration. The log degradation rates of anthocyanins increased linearly with temperature, but decreased slightly with sugar concentrations. These results suggest that sugar may provide some protection to both E. coli O157: H7 and anthocyanins in SP. The browning index was not affected by heating at 50 and 52 ºC at low sugar concentrations, but increased by an average of 1.28%, 2.21%, and 10.1% per min when SP was exposed to heating at 54, 57.5, and 60 °C, respectively. YM was also inactivated by heating. This study demonstrated that properly designed thermal processes can effectively inactivate E. coli O157:H7 and YM in contaminated SP, while minimizing the changes in anthocyanins and color.     

Acid Resistance and Molecular Characterization of Escherichia coli O157:H7 and Different Non‐O157 Shiga Toxin‐Producing E. coli Serogroups

The objective of this study was to compare the acid resistance (AR) of non‐O157 Shiga toxin‐producing Escherichia coli (STEC) strains belonging to serogroups O26, O45, O103, O104, O111, O121, and O145 with O157:H7 STEC isolated from various sources in 400 mM acetic acid solutions (AAS) at pH 3.2 and 30 °C for 25 min with or without glutamic acid. Furthermore, the molecular subgrouping of the STEC strains was analyzed with the repetitive sequence‐based PCR (rep‐PCR) method using a DiversiLab^TM system. Results for a total of 52 strains ranged from 0.31 to 5.45 log reduction CFU/mL in the absence of glutamic acid and 0.02 to 0.33 CFU/mL in the presence of glutamic acid except for B447 (O26:H11), B452 (O45:H2), and B466 (O104:H4) strains. Strains belonging to serogroups O111, O121, and O103 showed higher AR than serotype O157:H7 strains in the absence of glutamic acid. All STEC O157:H7 strains exhibited a comparable DNA pattern with more than 95% similarity in the rep‐PCR results, as did the strains belonging to serogroups O111 and O121. Surprisingly, the DNA pattern of B458 (O103:H2) was similar to that of O157:H7 strains with 82% similarity, and strain B458 strain showed the highest AR to AAS among the O103 strains with 0.44 log reduction CFU/mL without glutamic acid. In conclusion, STEC serotypes isolated from different sources exhibited diverse AR and genetic subtyping patterns. Results indicated that some non‐O157 STEC strains may have higher AR than STEC O157:H7 strains under specific acidic conditions, and the addition of glutamic acid provided enhanced protection against exposure to AAS.     

Effects of cold storage and heat–acid shocks on growth and verotoxin 2 production ofEscherichia coliO157:H7

Escherichia coliO157:H7 was cold-stored (4°C) either in nutritious menstruum [buffered Brain Heart Infusion (BHI) broth] or with starvation (buffered saline) at pH 7.0 or 5.5. Cultures grown in BHI broth at 37°C for 24h served as non-cold-stored controls. After 4-weeks cold storage, bacterial cells were shocked by heat (45°C for 5min) and acid (pH 2.5 for 30min at 37°C) and subsequently moved to optimal conditions (BHI broth of pH 7.4 incubated at 37°C). The results showed: (a) both lag-phase duration and growth rate of this pathogen at 37°C significantly increased after cold-storage with starvation, but not after cold storage in the nutritious menstruum; (b) combined heat–acid shocks increased growth rates at 37°C of both previously cold-stored and non-cold-stored bacterial cells; (c) final concentrations of verotoxin produced by bacterial cells at 37°C were not affected by previous cold storage in the nutritious menstruum; (d) verotoxin production by bacterial cells at 37°C increased after cold storage with starvation, and heat–acid shocks further enhanced that production. Further research is needed to evaluate the food safety implications of these results, i.e. whether cells ofE. coliO157:H7 originating from nutrient-poor/lower-pH environments may be more harmful to humans than those from nutrient-rich/higher-pH foods.     

Effect of stress induced by suboptimal growth factors on survival of Escherichia coli O157:H7.

This study investigated the growth and survival of E. coli O157:H7 exposed to a combination of suboptimal factors (22 degrees C, 7 degrees C, -18 degrees C/0.5% NaCl, 5.0% NaCl/pH 7.0, pH 5.4, pH 4.5/addition of lactic acid) in a simulation medium for red meat (beef gravy). Prolonged survival was noted as the imposed stress was more severe, and as multiple growth factors became suboptimal. At a defined temperature (7 degrees C or -18 degrees C), survival was prolonged at the more acid, more suboptimal pH (pH 4.5 > pH 5.4 > pH 7.0) while at a defined pH (pH 4.5), better survival was observed at 7 degrees C than at 22 degrees C. This suggests that application of the hurdle concept for preservation of food may inhibit outgrowth but induce prolonged survival of E. coli O157:H7 in minimal processed foods. At both 22 degrees C and 7 degrees C, the addition of lactic acid instead of HCl to reduce pH (to pH 4.5) resulted in a more rapid decrease of E. coli O157:H7. High survival was observed in beef gravy, pH 5.4 at -18 degrees C (simulation of frozen meat)-reduction of log 3.0 to log 1.9 after 43 days--and in beef gravy, pH 4.5 and 5% NaCl at 7 degrees C (simulation of a fermented dried meat product kept in refrigeration)--less than 1 log reduction in 43 days. In these circumstances, however, a high degree of sublethal damage of the bacterial cells was noted. The degree of sublethal damage can be estimated from the difference in recovery of the pathogen on the non-selective TSA medium and the selective SMAC medium.     

Effect of gelation factors on the formation and characteristics of protease‐induced whey protein gel

Whey protein gel formed at 10% (w/v) whey protein concentration, 0.5% E/S, pH 7.0, 55°C and 2.5 mM CaCl₂ concentration had an average particle size of 23.46 μm, hardness of 0.46, cohesiveness of 0.13 and adhesiveness of 1.40, and the gel showed semisolid, smooth and creamy texture. There were no distinct changes in gel textural properties after heating at 80 and 90°C for 5 min, respectively, or being kept at 4°C for 1 month. The textural properties of the gel showed no significant difference after its pH was adjusted to 4.5, 5.5 and 7.5 compared with that of pH 6.5 (control gel). However, the average particle size significantly increased after being adjusted to pH 4.5 and pH 5.5. Transmission electron micrographs showed that protease‐induced gel possessed much looser aggregate structure compared with heat‐induced compact gel, which may give support to its potential application in low‐fat foods that no need of extensive heating.     

Recovery of heat-stressed E. coli O157:H7 from ground beef and survival of E. coli O157:H7 in refrigerated and frozen ground beef and in fermented sausage kept at 7°C and 22°C

A study was undertaken to obtain information on survival of Escherichia coli O157:H7 in ground beef subjected to heat treatment, refrigeration and freezing and on survival of E. coli O157:H7 in fermented sausage kept at 7°C and 22°C. For the challenge test, a mixture of E. coli O157:H7 strains (EH 321, EH 385, EH 302) was used and enumeration was performed on an isolation medium suitable for recovery of stressed organisms: modified Levine's eosin methylene blue agar (mEMB). Heat resistance of E. coli O157:H7 decreased after pre-incubation at a reduced temperature.Escherichia coli O157:H7 was more susceptible to heat inactivation after storage at 7°C and die-off was even more enhanced if cultures were frozen prior to heat inactivation. The enhanced reduction of the pathogen at 56°C after prior storage under refrigeration was confirmed in a test with inoculated ground beef.Escherichia coli O157:H7 was able to survive in ground beef at 7°C for 11 days and at −18°C for 35 days showing maximal one log reduction during the storage period. Thus, ground beef contaminated with E. coli O157:H7 will remain a hazard even if the ground beef is held at low or freezing temperatures. At both 7°C and 22°C, a gradual reduction of E. coli O157:H7 was noticed in fermented sausage over the 35 days storage period resulting in a 2 log decrease of the high inoculum (10⁶cfu 25 g⁻¹). For the low inoculum (10³cfu 25 g⁻¹) a 2·5 log reduction was obtained in 7 and 28 days storage at respectively 22 and 7°C. Application of good hygienic practices and implementation of HACCP in the beef industry are important tools in the control of E. coli O157:H7.     

Gas‐producing and spoilage potential of Enterobacteriaceae and lactic acid bacteria isolated from chilled vacuum‐packaged beef

This study aimed to enumerate and identify lactic acid bacteria and Enterobacteriaceae from spoiled and nonspoiled chilled vacuum‐packaged beef and determine their potential to cause blown pack spoilage. These microbial groups were also enumerated in nonspoiled samples and detected in abattoir samples. The potential of isolates to cause ‘blown pack’ spoilage of vacuum‐packaged beef stored at chilled temperature (4 °C) and abuse temperature (15 °C) was investigated. Populations of lactic acid bacteria in exudate of spoiled and nonspoiled samples were not significantly different (P > 0.05), whereas the number of lactic acid bacteria on the surface was significantly higher (P < 0.05) in spoiled samples as compared to nonspoiled samples. The population of Enterobacteriaceae species in exudate and on the surface of samples were significantly higher (P < 0.05) in spoiled packs in comparison with nonspoiled packs. Results of the deterioration potential showed that ‘blown pack’ spoilage was noticeable after 7 days at 15 °C and after 6 weeks at 4 °C for samples inoculated with Hafnia alvei.     

Survival of Listeria monocytogenes, Yersinia enterocolitica and Escherichia coli O157:H7 and Quality Changes After Irradiation of Beef Steaks and Ground Beef

Beef steaks and ground beef were inoculated with Listeria monocytogenes, Yersinia enterocolitica, or Escherichia coli O157:H7. Samples were packaged in air or under vacuum and irradiated at low (0.60 to 0.80 kGy) or medium (1.5 to 2.0 kGy) doses, with each dose delivered at either a low (2.8 M/min conveyor speed) or high (6.9 M/min) dose rate. Medium-dose irradiation accompanied by 7°C storage resulted in no Y. enterocolitica or E. coli O157:H7 survivors being detected. There was no effect on survival of the pathogens by dose rate or storage atmosphere. No difference (P>0.05) was observed in meat pH or color, but TBA values increased after 7 days storage.     

Critical Factors Affecting the Destruction of Escherichia coli O157:H7 in Apple Cider Treated with Fumaric Acid and Sodium Benzoate

Destruction of Escherichia coli O157:H7 in apple cider treated with fumaric acid and sodium benzoate (0.15% and 0.05% w/v, respectively) was determined under pH and storage temperatures that commonly occur in apple cider. At 5°C storage, while destruction of E. coli O157:H7 in the presence of preservatives increased with time, there was little decline in E. coli O157:H7 populations in the absence of the preservatives. Increasing storage temperatures to 15°C and 25°C significantly increased the rate of destruction of E. coli O157:H7 in cider with the preservatives (P < 0.05). Increasing the pH of cider (from 3.2 to 4.7) decreased the rate of destruction of E. coli O157:H7.