Benne S. Marmer

Thermal destruction of Escherichia coli O157:H7 in beef and chicken: determination of D- and Z-values

Thermal inactivation of a four-strain mixture of E. coli O157:H7 was determined in 90% lean ground beef, and lean ground chicken. Inoculated meat was packaged in bags which were completely immersed in a circulating water bath and held at 55, 57.5, 60, 62.5, and 65 degrees C for predetermined lengths of time. D-values, determined by linear regression, in beef were 21.13, 4.95, 3.17, 0.93 and 0.39 min, respectively (z = 6.0 degrees C). Using a survival model for non-linear survival curves, D-values in beef ranged from 20.45 min (D1; and there was no D2) at 55 degrees C to 0.16 min (D1) and 1.45 min (D2) at 65 degrees C. When E. coli O157:H7 four-strain cocktail was heated in chicken, D-values calculated by both approaches were consistently less at all temperatures. The heat resistance of E. coli O157:H7 was not altered after refrigerated or frozen storage of inoculated beef for 48 h. The results of this study will be beneficial to the food industry in designing HACCP plans to effectively eliminate E. coli O157:H7 in the meat products used in this study.

Thermal destruction ofListeria monocytogenes in liver sausage slurry

Abstract Thermal destruction ofListeria monocytogenes was determined in a liver sausage slurry (1:1, liver sausage batter and water) using a submerged ampule technique.D-values forL. monocytogenes Scott A grown at 37°C were 8·9 min at 57·2°C, 2·4 min at 60·0°C, and 1·1 min at 62·8°C (Z=6·2°C) based on analysis of the linear portion of the survivor curves.D-values of 6·6, 1·6, and 0·4 min (Z=4·65°C) were obtained when the data were analyzed using non-linear techniques.L. monocytogenes strain V7 (D60=1.0min) was more thermosensitive than Scott A (D60=1·6min) or HO-VJ-S (D60=1·6min). When Scott A was grown at 19°C, there was a decrease in thermal resistance (D60=0·8min). These data indicate thatL. monocytogenes has a thermal resistance in liver sausage comparable to that observed in other food systems.

Heat Resistance and Fatty Acid Composition of Listeria monocytogenes: Effect of pH, Acidulant, and Growth Temperature

The objective of this study was to determine the influence of pH, acidulant, and growth temperature history on the heat resistance and fatty acid composition of Listeria monocytogenes Scott A. Cells were grown to late exponential phase (OD600 = 0.6) at 10, 19, or 37 degrees C in brain heart infusion broth acidified to pH 5.4 or 7 with either acetic or lactic acid. Thermal death times at 60 degrees C subsequently were determined by using a submerged-coil heating apparatus. The surviving cell population was enumerated by spiral-plating heated samples onto tryptic soy agar supplemented with 0.6% yeast extract and 1% sodium pyruvate. The thermal resistance of cells cultured at a particular temperature was significantly lower (P < 0.05) when lactic acid was used to acidify the medium of pH 5.4. Regardless of acid identity, D values significantly decreased (P < 0.05) with increased growth temperature when the pH of the growth medium was 5.4, whereas D values significantly increased (P < 0.05) with increased temperature at pH 7. At pH 5.4 adjusted with lactic acid, D values were 1.30, 1.22, and 1.14 min for cells grown at 10, 19, and 37 degrees C, respectively. At pH 5.4 adjusted with acetic acid, L. monocytogenes failed to grow at 10 degrees C; the D values were 1.32 and 1.22 min when the cells were grown at 19 and 37 degrees C, respectively. At pH 7, the D values were 0.95, 1.12, and 1.28 min with lactic acid and 0.83, 0.93, and 1.11 min with acetic acid at 10, 19, and 37 degrees C, respectively. The most abundant fatty acids (44 to 82%) were branched-chain saturated fatty acids (anteiso-and iso-C15:0 and iso-C17:0) regardless of pH, acidulant, or growth temperature. However, there was an increase in C15:0 isomers at the expense of iso-C17:0 when the growth temperature was lowered from 37 to 10 degrees C. While variable changes in longer-chain fatty acids were found, the percentage of longer-chain (C16 and C18) fatty acids was greatest when L. monocytogenes was grown at 37 degrees C regardless of pH or acidulant. This study demonstrates that the heat resistance of L. monocytogenes depends upon its growth conditions.

Influence of agitation, inoculum density, pH, and strain on the growth parameters of Escherichia coli O157:H7—relevance to risk assessment

Foods may differ in at least two key variables from broth culture systems typically used to measure growth kinetics of enteropathogens: initial population density of the pathogen and agitation of the culture. The present study used nine Escherichia coli O157:H7 strains isolated from beef and associated with human illness. Initial kinetic experiments with one E. coli O157:H7 strain in brain-heart infusion (BHI) broth at pH 5.5 were performed in a 2 x 2 x 3 factorial design, testing the effects of a low (ca. 1-10 colony-forming units [CFU]/ml) or high (ca. 1000 CFU/ml) initial population density, culture agitation or no culture agitation, and incubation temperatures of 10, 19, and 37 degrees C. Kinetic data were modeled using simple linear regression and the Baranyi model. Both model forms provided good statistical fit to the data (adjusted r(2)>0.95). Significant effects of agitation and initial population density were identified at 10 degrees C but not at 19 or 37 degrees C. Similar growth patterns were observed for two additional strains tested under the same experimental design. The lag, slope, and maximum population density (MPD) parameters were significantly different by treatment. Further tests were conducted in a 96-well microtiter plate system to determine the effect of initial population density and low pH (4.6-5.5) on the growth of E. coli O157:H7 strains in BHI at 10, 19, and 37 degrees C. Strain variability was more apparent at the boundary conditions of growth of low pH and low temperature. This study demonstrates the need for growth models that are specific to food products and environments for plausible extrapolation to risk assessment models.

Thermal Destruction of Escherichia coli O157:H7 in Hamburger†

The inactivation of E. coli O157:H7 in ground beef patties cooked in a skillet was investigated. Ground beef patties inoculated with a mixture of five strains of E. coli O157:H7 were cooked in a Farberware skillet set at a temperature of 275°F (137°C). Eight type K thermocouples connected to a data logger were used to record the temperatures at eight points within the patty. The cooking times studied ranged from 2.25 min to 4 min. Tryptic soy agar plates overlaid with sorbitol MacConkey agar were used for recovery of E. coli O157:H7. Heating of ground beef patties to an internal temperature endpoint of 155°F (68.3°C) resulted in 4-log cycle reductions of the organism. The results of this investigation conducted under conditions simulating those that occur in the retail food industry provide a basis for ensuring safety against E. coli O157:H7 in ground beef patties.

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

Abstract 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.

Thermal resistance of nonproteolytic type B and type E Clostridium botulinum spores in phosphate buffer and turkey slurry.

The heat resistance of nonproteolytic type B and type E Clostridium botulinum spores in phosphate buffer and turkey slurry was determined from 70 to 90°C. Thermal-death times were determined in vials heated using a water bath. Recovery of heat-injured spores was on reinforced clostridial medium (RCM) and tryptic soy agar (TSA) with and without added lysozyme (10 μg/ml). Decimal-reduction times (D-values) were determined by fitting a survival model to the data using a curve-fitting program. The apparent or measured heat resistance was maximum with RCM supplemented with lysozyme. The D-values at 80°C for type E spores in buffer ranged from 1.03 min for strain Whitefish to 4.51 min for strain Saratoga. The D-value for the most heat-resistant nonproteolytic type B strain KAP B5 in buffer was 4.31 min at 80°C. The z-values in buffer for all strains were very similar, ranging from 8.35 to 10.08°C.Turkey slurry offered protection to the spores with a concomitant increase in heat resistance. The D-values in turkey slurry ranged from 51.89 min at 70°C to 1.18min at 85°C for type E strain Alaska (z = 9.90°C) and from 32.53 min at 75°C to 0.80 min at 90°C for nonproteolytic type B strain KAP B5 (z = 9.43°C). Thermal-death-time values from this study will assist food processors to design thermal processes that ensure safety against nonproteolytic C. botulinum in cook/chill foods.

Growth and sporulation potential of Clostridium perfringens in aerobic and vacuum-packaged cooked beef

Growth of Clostridium perfringens in aerobic-and anaerobic-(vacuum) packaged cooked ground beef was investigated. Autoclaved ground beef was inoculated with ~3.0-log10 CFU/g of C. perfringens , packaged and stored at various temperatures. Vegetative cells and heat-resistant spores were enumerated by plating unheated and heated (75°C for 20 min) meat samples on tryptose-sulfite-cycloserine agar. Clostridium perfringens grew to >7 logs within 12 h at 28, 37 and 42°C under anaerobic atmosphere and at 37 and 42°C under aerobic conditions. At 28°C under aerobic conditions, growth was relatively slow and total viable count increased to >6 logs within 36 h. Similarly, growth at 15°C in air was both slower and less than under vacuum. Regardless of packaging, the organism either declined or did not grow at 4, 8 and 12°C. Spores were not found at <12°C. Spores were detected as early as 8 h at 42°C under anaerobic conditions, but in general, the type of atmosphere had little influence on sporulation at ≥28°C. Temperature abuse (28°C storage) of refrigerated products for 6 h will not permit C. perfringens growth. However, cyclic and static temperature abuse of such products for relatively long periods may lead to high and dangerous numbers of organisms. Reheating such products to an internal temperature of 65°C before consumption would prevent food poisoning since the vegetative cells were killed.

Growth of Clostridium perfringens from spore inocula in sous-vide turkey products

Clostridium perfringens growth from a spore inoculum was investigated in vacuum-packaged, cook-in-bag ground turkey (pH 6) that included 0.3% (w/w) sodium pyrophosphate, and sodium chloride at 0, 1, 2, or 3% (w/w). The packages were processed to an internal temperature of 71.1 degrees C, ice chilled and stored at various temperatures. The total C. perfringens population was determined by plating diluted samples on tryptose-sulfite-cycloserine agar followed by anaerobic incubation at 37 degrees C for 48 h. At 28 degrees C, the addition of 3% salt in turkey was effective in delaying growth for 12 h. At 15 degrees C, growth occurred at a relatively slow rate in the presence of 1-2% salt. Vegetative cells were not observed even after 28 days of storage in the presence of 3% salt. C. perfringens growth was not observed at 4 degrees C regardless of salt levels. The D-values ranged from 23.2 min (no salt) to 17.7 min (3% salt). Cyclic and static temperature abuse of refrigerated products for 8 h did not lead to growth by C. perfringens from a spore inoculum.

Transfer Coefficient Models for Escherichia coli O157:H7 on Contacts between Beef Tissue and High-Density Polyethylene Surfaces†

Risk studies have identified cross-contamination during beef fabrication as a knowledge gap, particularly as to how and at what levels Escherichia coli O157:H7 transfers among meat and cutting board (or equipment) surfaces. The objectives of this study were to determine and model transfer coefficients (TCs) between E. coli O157:H7 on beef tissue and high-density polyethylene (HDPE) cutting board surfaces. Four different transfer scenarios were evaluated: (i) HDPE board to agar, (ii) beef tissue to agar, (iii) HDPE board to beef tissue to agar, and (iv) beef tissue to HDPE board to agar. Also, the following factors were studied for each transfer scenario: two HDPE surface roughness levels (rough and smooth), two beef tissues (fat and fascia), and two conditions of the initial beef tissue inoculation with E. coli O157:H7 (wet and dry surfaces), for a total of 24 treatments. The TCs were calculated as a function of the plated inoculum and of the cells recovered from the first contact. When the treatments were compared, all of the variables evaluated interacted significantly in determining the TC. An overall TC-per-treatment model did not adequately represent the reduction of the cells on the original surface after each contact and the interaction of the factors studied. However, an exponential model was developed that explained the experimental data for all treatments and represented the recontamination of the surfaces with E. coli O157:H7. The parameters for the exponential model for cross-contamination with E. coli O157:H7 between beef tissue and HDPE surfaces were determined, allowing for the use of the resulting model in quantitative microbial risk assessment.