Gamma irradiation as a means to eliminate Listeria monocytogenes from frozen chicken meat

Cells of Listeria monocytogenes ATCC 35152 were sensitive to gamma irradiation in phosphate buffer, pH 7.00 (D₁₀, dose required for 10% survival—0.15 kGy) at 0–5°C. The cells showed higher radiation survival when irradiated under frozen condition, with a D₁₀ of 0.3 kGy. The protection offered by shrimp/chicken/kheema homogenates (100 g litre^?1) was evidenced by even higher D₁₀ values (0.5 kGy) at both 0–5°C and cryogenic temperature. Boneless chicken meat samples were artificially inoculated with L monocytogenes ATCC 35152 cells at low (5 × 10³) colony-forming unit (cfu) g^?1 and high (5 × 10⁶ cfu g^?1) concentrations and irradiated at 1, 3, 4, 6 kGy doses under cryogenic conditions. The efficacy of the radiation process was evaluated by detecting L monocytogenes during storage at 2–4°C in the irradiated samples. These studies, when repeated with three other serotypes of L monocytogenes, clearly suggested the need for a dose of 3 kGy for elimination of 10³ cfu cells of L monocytogenes g^?1 from air-packed frozen chicken meat.     

Sensitivity of multidrug-resistant Salmonella typhimurium DT104 to organic acids and thermal inactivation in liquid egg products

A mixture of four Salmonella typhimurium DT104 strains and a mixture of four S. typhimurium non-DT104 strains were examined for their ability to grow in tryptic soy broth (TSB) acidified with acetic, lactic, citric, or malic acids at pH 5·4, 4·4, and 3·7. Significantly (P<0·05) higher numbers of S. typhimurium DT104 cells were detected at pH 4·4 and 4·0 in TSB acidified with acetic acid and at pH 4·4 and 3·7 in TSB acidified with lactic acid compared to non-DT104 cells. Acid-shocked and non-shocked (control) cells were plated on TSA (pH 7·3) acidified with lactic acid at pH 5·4, 4·4, and 4·0 and on TSA (pH 7·0±0·2) containing 0·5, 2·5, and 5% sodium chloride. Populations of acid-shockedS. typhimurium DT104 and non DT104 cells recovered on acidified or salt-supplemented TSA were significantly (P<0·05) lower than those of non-shocked cells. A significantly lower number of acid-shocked non-DT104 cells recovered on TSA at pH 5·4, compared to acid-shocked DT104 cells, suggests that DT104 cells may be more resistant to acid shock and subsequent exposure to acid pH. D values and z values of acid-shocked or non-shocked cells of DT104 and non-DT104 strains in liquid whole egg (WE), egg yolk (EY), egg white (EW), whole egg+10% salt (WES), and egg yolk+10% salt (EYS) were determined. Differences in thermal sensitivity of the two types of cells were few. Rates of thermal inactivation of S. typhimurium DT104 cells indicate that the USDA pasteurization process would eliminate >8 log₁₀cfu ml⁻¹of EW heated at 57°C and >11 log₁₀cfu ml⁻¹of WE, EY, WES, or EYS heated at 61°C. D values of acid-shocked DT104 and non-DT104 cells heated in liquid egg products were significantly (P<0·05) lower than those of respective non-shocked cells.     

Effect of pH and Ionic Strength on the Heat Stability of Rapeseed 12S Globulin (Cruciferin) by the ANS Fluorescence Method

The heat stability of rapeseed 12S globulin (cruciferin) was examined using 8-anilinonaphthalene-1-sulphonic acid (ANS) as a fluorescence probe. Heating cruciferin (0·06–0·3 mg ml⁻¹ in 10 mM glycyl–glycyl piperizine buffer, pH 7·0, with 0·1–1·0 M NaCl) for 20 min increased its hydrophobicity as monitored by ANS fluorescence measurements. The mid-point temperature for the heat effect (T_m) increased linearly with increasing solvent pH (T_m (°C)=4·16 pH+41 (μ=0.1)) or sodium chloride concentration (T_m (°C)=14·7 [NaCl]+71 (pH=7·0)). The range of T_m values for cruciferin was 45–96°C. At 20°C cruciferin was unstable at pH<3·0 but relatively stable under alkaline conditions (pH 8–10). Though possessing an oligomeric structure, cruciferin appears to heat denature in accordance with the two-stage deactivation model for simple globular proteins.     

Control of Yersinia enterocolitica in raw pork and pork products by γ-irradiation

γ-Radiation response of Y. enterocolitica 5692 and 152 was studied at 0°C and at −40°C in phosphate buffer (pH 7.00) as well as in 10% raw meat/salami homogenate. The strains investigated did not differ in their response and were found to be sensitive to γ-radiation but exhibited a tailing phenomenon in the survival curve. The D₁₀ in homogenate was 0.25 kGy at 0°C. This response was not affected at −40°C. Storage studies of packs, inoculated artificially with heavy inoculum of Y. enterocolitica (10⁶ cfu/g) showed that while samples of salami and cooked ham could be decontaminated at doses of 4 and 3 kGy respectively; cells could not be eliminated from raw pork meat even at the higher dose of 6 kGy. The role of different treatments given prior to irradiation for revival of Y. enterocolitica after irradiation storage was also studied. The dose of 1 kGy at −40°C was efficient in eradicating low numbers (<10³) of naturally occuring Y. enterocolitica from raw pork meat without any revival during storage at refrigeration temperature.     

The effect of EDTA and trisodium phosphate,alone and in combination with nisin,on the growth of Arcobacter butzleri in culture

Arcobacter butzleri is a common contaminant of foods of animal origin and there is increasing evidence linking the organism to human illness. This study investigated the effectiveness of ethylene diamine tetra-acetic acid (EDTA) and trisodium phosphate (TSP) alone and in combination with nisin, on the growth of A. butzleri in culture. At 1 mM, 5 mM, 10 mM and 20 mM, EDTA inhibited growth, both alone and combined with 500 IU ml⁻¹nisin, but only at 20 mM did the addition of nisin produce a statistically significant difference compared with EDTA alone (4·3 log₁₀and 5·6 log₁₀reduction respectively). Short-term simultaneous exposure with EDTA and 500 IU ml⁻¹nisin was more effective than sequential treatment on both logarithmically growing and stationary phase cells.Growth of A. butzleri was inhibited by 0·5 mM TSP with simultaneous treatment being more effective than sequential treatment. After 10 min simultaneous treatment, only at 40 mM TSP (but not at lower concentrations) was there a statistically significant difference between TSP treatment alone and TSP plus 500 IU ml⁻¹nisin (5·2 log₁₀compared with 7·7 log₁₀reduction).Thirty-minute treatment with 20 mM EDTA or 10 min with 10% TSP at 4°C, followed by incubation in the presence of 500 IU ml⁻¹nisin, resulted in no viable cells being recovered after 24 h (>99·9% cell kill) indicating that a multiple hurdle approach is the most effective method of reducing growth and survival of A. butzleri in culture.     

Influence of gamma irradiation on growth and survival of Escherichia coli O157:H7 and quality of cig kofte, a traditional raw meat product

Cig kofte is a traditional Turkish food containing raw ground meat. Samples inoculated with Escherichia coli O157:H7 were irradiated at 0.5–6 kGy with a ⁶⁰Co source and stored at 4 and 25 °C. Total aerobic mesophilic count decreased with increasing irradiation doses, D₁₀ value was 0.83 kGy. Escherichia coli O157:H7 count decreased from 5.1 log₁₀ CFU g^?1 to an undetectable level (<1 log₁₀ CFU g^?1) after 1‐day storage at 4 °C following irradiation at 2 kGy, D₁₀‐value was 0.29 kGy. Irradiation doses up to 2 kGy did not affect sensory quality after 1 day. There was colour loss in samples irradiated at 2 kGy or above and stored for longer periods. Storage of the irradiated products at abused temperature must be avoided for safety assurance. Irradiation at 2 kGy has a great potential for extending the shelf‐life of cig kofte and assuring safety by decreasing the number of E. coli O157:H7 and other bacteria, but further studies with suitable package designs are needed to decrease quality degradation during extended storage.     

The enzymic release of fatty acids from phosphatidylcholine in green peas (Pisum sativum)

‘Phospholipid acyl-hydrolase’ (PLAH), an enzymic activity releasing fatty acid from phosphatidylcholine (PC), has been identified and characterised in green peas. The K_m value for PC dipalmitoyl ester was 0·167 mm. The enzymic activity possessed a pH optimum of 5·6 and was stable for 20 min only at that pH value. The optimum temperature was 45°C and thermal sensitivity was indicated by a 94% decrease in activity upon exposure of the enzyme to 55°C for 3 min, and by an exponential decrease in activity upon storage at 4°C for 1 week. The enzyme was optimally activated by 2·0 mm calcium chloride at pH 5·6, and the optimal concentration of sodium dodecyl sulphate was 0·75 mg ml^?1. Pea PLAH was non-competitively inhibited by sodium cyanide, EDTA and p-chloromercuribenzoate, with no activity in the presence of mercuric chloride. The results from this study are related to those of other workers on lipid-degrading enzymes in peas, and a pathway is proposed for the enzymic degradation of endogenous lipids in fresh or unblanched frozen peas during post-harvest storage.     

Survival of Campylobacter coli in porcine liver

Enteropathogenic Campylobacter jejuni, Campylobacter coli and Campylobacter lari are currently the most common causes of acute infectious diarrhoeal illness in the UK and in most developed countries. Many domestic animals, including pigs, act as natural reservoirs of these organisms and infection may occur through the ingestion of contaminated foodstuffs. Therefore, the safety of porcine liver produced in Northern Ireland was assessed in relation toCampylobacter spp. Storage trials showed that Campylobacter spp. were not able to proliferate in liver at 37°C, but could persist at 4°C and 15°C. Survival was better, however, during storage at 4°C than at 15°C.Campylobacter were rapidly killed in raw liver homogenates and distilled water at 37°C, but not at 4°C. An initial inoculum of 8 log₁₀cfu g⁻¹C. coli was undetectable in liver homogenates after 24h storage at 37°C. Campylobacter coli were sensitive to freezing on liver slices at −18°C and were reduced by 5 log₁₀cycles after 7 days storage. Cells survived better on chilled liver slices and in autoclaved liver homogenates than in raw liver homogenates at all temperatures, which indicates the presence of a heat-labile antagonistic agent in raw liver homogenates. Growth and survival of C. coli was not affected by Lactobacillus plantarum, as C. coli was able to reach 8·5 log₁₀cfu ml⁻¹in 7 days and maintain its viability in the presence of 8·0 log₁₀cfu ml⁻¹L. plantarum. Thus, storage of C. coli on porcine liver at 4°C selected for the survival of this pathogen compared to similar storage at 37°C.Such information may be useful in identifying conditions and treatments that could be integrated in HACCP strategies, or be used to design processes that prevent proliferation and/or destroy Campylobacter spp. that may be present in liver.     

Synergistic effect of Sodium Chloride,temperature and pH on growth of a cocktail of spoilage yeasts: a research note

The effect of sodium chloride (0·5–10% w/v), pH (2·6–6·3) and temperature (1–22°C) were studied on the growth of a cocktail of food spoilage yeasts. The length of the lag phase and the time taken to reach the level of 10⁶cfu ml⁻¹were calculated for each set of conditions. It was found that the lag phase constituted as little as 21% of the total time to reach 10⁶cfu ml⁻¹when the yeasts were grown in favourable conditions and as much as 62% of the total time when more extreme conditions were used. It was concluded that the lag phase was the most important factor affecting the spoilage potential of chilled foods with low pH and high salt values. The single most effective factor in reducing the growth rate of yeasts was temperature. The lag phase was 15, 38, 270, 630 and 875 h when the temperature was 15, 8, 4, 2 and 1°C respectively. At any single temperature, there appeared to be a synergistic effect of NaCl and pH and under the most extreme conditions tested (1°C; pH 5·8; 6% NaCl), the lag phase was over 1000 h. These data have implications for the spoilage potential of high salt, reduced pH foods stored at chill temperatures.     

The effect of a competitive microflora,pH and temperature on the growth kinetics of Escherichia coli O157:H7

Growth curves were generated for Escherichia coli O157:H7 in brain–heart infusion broth incubated at 37 or 15°C in the presence of individual and combinations of competing microflora. Broths were inoculated withE. coli O157:H7 (log₁₀3·00 cfu ml⁻¹) and competitors (log₁₀4·00 cfu ml⁻¹) and the initial pH of the broth was either neutral (7·0) or adjusted to 5·8 and then sequentially reduced to 4·8 over 10 h to simulate fermentation conditions. Growth curves were also generated for the competitors in these cultures, including Pseudomonas fragi, Hafnia alvei, Pediococcus acidilactici (pepperoni starter culture) and Brochothrix thermosphacta . Gompertz equations were fitted to the data and growth kinetics including lag phase duration, exponential growth rates and maximum population densities (MPD) calculated. In pure culture, the growth parameters for E. coli O157:H7 in neutral pH broths were significantly different from those recorded in simulated fermentation broths (P<0·05). The presence of competitors in the broth also had a significant effect on the growth kinetics of the pathogen. H. alvei significantly inhibited the growth (lag phase, growth rate and MPD) of E. coli O157:H7 at 37°C, neutral pH and outgrew the pathogen under these conditions. In neutral pH cultures, two other competitors, B. thermosphacta and P. acidilactici also inhibited the lag phase of the pathogen but had no effect on the other growth parameters. In simulated fermentation broths, the growth rate of E. coli O157:H7 was consistently slower and the MPD lower in the presence of a competitive microflora than when grown individually. At 15°C, only one competitor, P. fragi significantly inhibited the lag phase of the pathogen. The implications of these findings for food safety are discussed.     

Hardaliye: fermented grape juice as a traditional Turkish beverage

Twenty-six aged hardaliye samples, collected from different spots of the Kirklareli province of Turkey and hardaliye produced in laboratory conditions using the traditional method were investigated in this study. The pH ranged from 3·21 to 3·97. Red colour (Hunter Lab a_Lvalue) of samples ranged from 1·33 to 9·66. The total bacterial count ranged from 3·5×10²to 8×10⁵cfu ml⁻¹. The lactic acid bacteria counts of the samples were found to be between 1·0×10²and 4·0×10⁴cfu ml⁻¹. Yeasts and moulds, which were found in 21 samples out of 26, ranged from 1·0×10²to 8·1×10⁴cfu ml⁻¹. Coliforms and Escherichia coli were found in none of the samples. The changes of some microbiological and chemical properties of hardaliye during fermentation were investigated. The pH of hardaliye dropped from 3·86 to 3·39. The ethanol content of the end product was determined as 595·50 mg dl⁻¹. During the fermentation process, the total bacteria count, lactic acid bacteria count and yeast count changed from 2·1×10⁵, 6·0×10⁴and 1·2×10⁵cfu ml⁻¹to 1·3×10², 1·2×10³and 1·1×10³cfu ml⁻¹, respectively.Lactobacilli isolated from hardaliye samples were characterized by API 50 CH and other phenotypic criteria. A succession of Lactobacillus species, dominated by L. paracasei subsp. paracasei and L. casei subsp. pseudoplantarum were found during the fermentation process.     

In-milk inactivation of Escherichia coli O157:H7 by the environmental lytic bacteriophage ECPS-6

Shiga toxin-producing Escherichia coli is a common foodborne pathogen which transmission includes dairy products. In the search for novel biocontrol methods, bacteriophages have become important candidates for the eradication of foodborne pathogens. The aim of this study was to evaluate the bacteriophage-mediated reduction of E. coli O157:H7 in raw and filtered milk. Laboratory-scale tests showed that the bacteriophage ECPS-6 efficiently adsorbed to E. coli O157: H7 cells. Furthermore, ECPS-6 remained stable when heated at 70°C for 20 min and in a wide pH range from 3.0 to 11.0. The trials on contaminated milk were performed using filtered and unfiltered raw milk contaminated with 1 × 10⁵ CFU × ml⁻¹ of E. coli O157: H7. Bacteriophage was added at multiplicity of infection (MOI) 5 and 50. The ECPS-6 reached the highest lytic activity at MOI = 5 (25°C) which resulted in 4.74 Log₁₀ CFU × ml⁻¹ and 7.3 Log₁₀ CFU × ml⁻¹ reduction after 10 days for both tested strains, respectively. Under refrigerated conditions (4°C) the quantity of E. coli decreased to 1.5 Log₁₀ CFU × ml⁻¹ and 3.04 Log₁₀ CFU × ml⁻¹ for these strains, respectively. Usage of MOI = 50 for the treatment unfiltered milk led to the reduction of E. coli O157:H7 A-2 below the detection limit after 6 hr.     

Properties of polyphenol oxidase in mango (Mangifera indica) kernel

Polyphenol oxidase (PPO) activity of filtered extract of ground mango kernel suspension (400 g litre⁻¹) was studied spectrophotometrically at 420 nm using catechol as substrate. The enzyme was most active at pH 6·0 and 25°C. Activity was reduced by 50% at pH values of 5·0 and 7·1, and also at temperatures of 14°C and 30°C. The calculated activation energy and the Michaelis constant (K_m) were 21·4 kcal mol⁻¹ °C⁻¹ and 24·6 mM , respectively. The V_max value was 2·14 units g⁻¹ mango kernel. The time to heat inactivate PPO decreased rapidly to < 10 min with increasing temperature of ⩾ 70°C at 50% activity. © 1998 SCI.     

Destruction of Salmonella enteritidis inoculated in liquid whole egg by high hydrostatic pressure: comparative study in selective and non-selective media

The destruction of Salmonella enteritidis inoculated in liquid whole egg at approximately 10⁷−10⁸cfu ml^−lwas studied under combinations of pressure (350 and 450 MPa), temperature (50, 20, 2 and −15°C) and time (5, 10, 15 min and cycles of 5+5 and 5+5+5 min). One non-selective medium (tryptone soy agar) and two selective media (brilliant green agar and salmonella-shigella) were used to evaluate viability of S. enteritidis after pressurization. The inactivation rate increased with pressure and exposure time, being minimal at 350 MPa and −15°C for 5 min (over 1 log₁₀of reduction) and reaching total inactivation (8 log₁₀of reduction) in several treatments at 50°C. Treatments in cycles showed greater effectiveness than continuous treatments of the same total time. The effect of pressure was enhanced by elevated temperatures. The higher counts were obtained in the non-selective medium, indicating the presence of injured cells after pressure treatment. D -values obtained for two temperatures (2 and 20°C) and different times (0–60 min) under controlled pressure (400 MPa) showed that microbial inactivation followed a first-order kinetics with a decimal reduction time evaluated in tryptone soy agar medium of 9·5 min at 2°C and 8·8 min at 20°C.     

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.     

A new process for preparing transparent alkalised duck egg and its quality

When fresh duck (Anas plotyrhyncus) eggs (pH 8·0–8·5) are heated, their albumen develops a turbid gel. Through appropriate alkalisation (pH 11·5–12·8), the gel's transparency can be increased. The transparency of the heated duck egg-white is affected by pH value, heating temperature, heating rate and salt concentration. This research deals with the process for preparing the transparent alkalised duck egg and the change in its quality when stored. If fresh duck eggs are pickled in a solution of 42 g NaOH+50 g NaCl litre^?1 (25·3°C) for 8 days, removed, put in a water bath and heated at 70°C for 10 min they become transparent, their hardness and penetration increasing with storage. Total bacterial count and volatile basic nitrogen also increase with storage. The total bacterial count and the volatile basic nitrogen were 4·6 × 10⁶ cfu g^?1, 0·32 mg g^?1 when stored at a temperature of under 25°C for 4 weeks, respectively.     

Isolation of Yersinia enterocolitica from ready-to-eat foods and pork by a simple two step procedure

Out of 119 ready-to-eat food samples and pork processed for isolation of Yersinia enterocolitica, 15 samples (12·6%) were positive for the presence of Y. enterocolitica by a two step procedure in a modified trypticase soy broth containing 0·25% yeast extract, 0·2% bile salts and 4 μg ml^-1 Irgasan at pH 7·6 and upon incubating at 10°C and 22°C for 6 days. Only five samples (4·2%) were positive by cold enrichment in trypticase soy broth containing 0·2% yeast extract, incubated at 4°C for 14 days. Four strains of Y. enterocolitica and one strain of Y. intermedia were isolated from pork samples processed by cold enrichment. Out of 15 strains of Y. enterocolitica isolated from pork (four strains) and ready-to-eat food samples (10 strains) by two step procedure only three strains belonged to pathogenic serotype O:3 and which were isolated only from pork samples. Overall recovery of Y. enterocolitica was much better by the two step procedure as compared to cold enrichment (P < 0·05).     

Influence of pH and temperature upon calcium accumulation and release by bovine sarcoplasmic reticulum

Sarcoplasmic reticulum (SR) was prepared from fresh beef sternomandibularis muscle and shown to be relatively free from contamination by lysosomes, sarcolemma and mitochondrial membranes. Ca^{2 +} accumulation by SR from fresh and cold-shortened muscle was 51 and 39 nmoles/mg protein, respectively. The Ca^{2 +} accumulating ability of fresh SR vesicles decreased with lowering of pH (7·3, 6·8, 6·2, 5·5 and 5·0) at all temperatures (0, 15 and 38°C). Lowering the temperature from 38 to 0°C at pH 6·6 resulted in the release of 48% of the total accumulated Ca^{2 +}, whereas the corresponding value on lowering the temperature from 38 to 15°C at the same pH was only 12%. Thus, low temperatures accelerate the release of Ca^{2 +} by SR. Although simultaneously lowering pH and temperature also increased Ca^{2 +} release by SR, the amount of Ca^{2 +} released was less than if pH and temperature were altered independently. The findings are discussed in the light of explaining cold shortening.     

Effect of gamma and electron beam irradiation on the survival of pathogens inoculated into salted, seasoned, and fermented oyster

The objective of this study was to identify the efficacy of gamma and electron beam irradiation of the food-borne pathogens including 3-strain cocktail of Listeria monocytogenes (ATCC 19114, 19115, and 19111), Staphylococcus aureus (ATCC 6538, 25923, and 29213), and Vibrio parahaemolyticus (ATCC 17802, 33844, and 27969) in salted, seasoned, and fermented oyster (oyster Jeotkal, 8% salt), commercially available in the market. Irradiation (0, 0.5, 1, 2, and 5 kGy) significantly reduced the initial microbial level not only immediately after irradiation but also during storage at 10 °C for 4 weeks (P ≤ 0.05). No viable cell was detected at 5 kGy of irradiation at a detection limit of 10¹ CFU/g. Gamma irradiation was more effective than electron beam irradiation, and yielded D₁₀ values of 0.60, 0.71, and 0.29 kGy for L. monocytogenes, S. aureus, and V. parahaemolyticus, and those of electron beam irradiation were 0.69, 0.94, and 0.29 kGy, respectively. V. parahaemolyticus was most sensitive to irradiation and storage among all pathogens tested. Sensory quality was not affected by irradiation treatment. Results suggest that a low dose irradiation can improve the microbial quality and reduce the risk by the food-borne pathogens of oyster Jeotkal, which has limited alternative sterilization methods due to the temperature sensitivity of food products.     

Changes in heat tolerance of Escherichia coli O157:H7 after exposure to acidic environments

Acid-adapted bacterial cells are known to have enhanced tolerance to various secondary stresses. However, a comparison of heat tolerance of acid-adapted and acid-shocked cells of Escherichia coli O157:H7 has not been reported. D - and z -values of acid-adapted, acid-shocked, and control cells of an unusually heat-resistant strain (E0139) of E. coli O157:H7, as well as two other strains of E. coli O157:H7, were determined based upon the number of cells surviving heat treatment at 52, 54 or 56°C in tryptic soy broth (pH 7·2) for 0, 10, 20 or 30 min. The unusual heat tolerance of E. coli O157:H7 strain E0139 was confirmed. D -values for cells from 24-h cultures were 100·2, 28·3, and 6·1 min at 52, 54 and 56°C, respectively, with a z -value of 3·3°C. The highest D -values of other E. coli O157:H7 strains were 13·6 and 9·2 min at 52 and 54°C, respectively, whereas highest D -values of non-O157:H7 strains were 78·3 and 29·7 min at 52 and 54°C. D -values of acid-adapted cells were significantly higher than those of unadapted and acid-shocked cells at all temperatures tested. In a previous study, we observed that both acid-adapted cells and acid-shocked cells of strain E0139 had enhanced acid tolerance. This suggests that different mechanisms protect acid-adapted and acid-shocked cells against subsequent exposure to heat or an acidic environment. The two types of cells should be considered separately when evaluating survival and growth characteristics upon subsequent exposure to different secondary stress conditions.