Modeling non-linear survival curves to calculate thermal inactivation of salmonella in poultry of different fat levels.

Survival curves of a cocktail of eight serotypes of Salmonella in ground poultry of different fat levels (1-12%), when heated rapidly to specified temperatures (58-65 degrees C), were examined. Because many of the survival curves were concave, values for two parameters: the asymptotic D-value and the "lag" times were estimated and used to develop secondary models for estimating the time needed to obtain a 7 log10 relative reduction as a function of fat level and temperature. To compute the necessary time, at a given temperature and fat level, the estimated lag time should be added to the product of 7 and the estimated asymptotic D-value. A model was also developed for estimating the standard error of the estimated times, so that upper confidence bounds for the necessary times can be computed. It was found that lag times increase with higher fat levels. The effect of fat on D-values depended on the species; it is estimated that, for a given increase of fat level, the increase of the D-value would be greater for ground chicken than that for ground turkey. In addition, there was a statistically significant species effect on D-values, with higher D-values for ground turkey than for ground chicken at the higher temperatures studied. The thermal death curves displayed a non-linear tendency, however, for estimation purposes, a linear curve was assumed. There was not a statistically significant interaction effect of fat levels and temperatures on D-values, thus, for modeling, it was assumed that z-values were not dependent on the fat levels. The z-values for ground chicken and turkey were estimated to be 5.5 degrees C and 6.1 degrees C, respectively, and are statistically significantly different. These findings should have substantial practical importance to food processors of cooked poultry, allowing them to vary their thermal treatment of ready-to-eat poultry products in a safe manner.

Cold shock induction of thermal sensitivity in Listeria monocytogenes.

Cold shock at 0 to 15 degrees C for 1 to 3 h increased the thermal sensitivity of Listeria monocytogenes. In a model broth system, thermal death time at 60 degrees C was reduced by up to 45% after L. monocytogenes Scott A was cold shocked for 3 h. The duration of the cold shock affected thermal tolerance more than did the magnitude of the temperature downshift. The Z values were 8.8 degrees C for controls and 7.7 degrees C for cold-shocked cells. The D values of cold-shocked cells did not return to control levels after incubation for 3 h at 28 degrees C followed by heating at 60 degrees C. Nine L. monocytogenes strains that were cold shocked for 3 h exhibited D(60) values that were reduced by 13 to 37%. The D-value reduction was greatest in cold-shocked stationary-phase cells compared to cells from cultures in either the lag or exponential phases of growth. In addition, cold-shocked cells were more likely to be inactivated by a given heat treatment than nonshocked cells, which were more likely to experience sublethal injury. The D values of chloramphenicol-treated control cells and chloramphenicol-treated cold-shocked cells were no different from those of untreated cold-shocked cells, suggesting that cold shock suppresses synthesis of proteins responsible for heat protection. In related experiments, the D values of L. monocytogenes Scott A were decreased 25% on frankfurter skins and 15% in ultra-high temperature milk if the inoculated products were first cold shocked. Induction of increased thermal sensitivity in L. monocytogenes by thermal flux shows potential to become a practical and efficacious preventative control method.

Heat inactivation of Salmonella typhimurium DT104 in beef as affected by fat content.

The heat resistance of an eight-strain cocktail of Salmonella typhimurium DT104 was determined at 58-65 degrees C in beef containing 7, 12, 18 or 24% fat. Inoculated beef was packaged in bags completely immersed in a circulating water bath and held at 58, 60, 62.5 and 65 degrees C for a predetermined length of time. The surviving cell population was enumerated by spiral plating heat-treated samples onto tryptic soy agar supplemented with 0.6% yeast extract and 1% sodium pyruvate. Preliminary studies on thermal inactivation of the Salmonellae isolates in chicken broth indicated no correlation between heat resistance and origin of the isolates. While linear survival curves were observed in chicken broth, inactivation kinetics in beef showed deviations from the first order kinetics, represented by an initial lag period or shoulder before any death occurred with time. Overall, increased fat levels in beef resulted in longer lag periods and lower D-values, suggesting that the lag periods must be taken into account and added to the D-values for calculating the time required at a specific temperature for achieving a specific lethality for Salm. typhimurium DT104 in beef. Thermal death times from this study will assist the retail food industry to design cooking regimes that ensure safety of beef contaminated with Salm. typhimurium DT104.

Predictive thermal inactivation model for Listeria monocytogenes with temperature, pH, NaCl, and sodium pyrophosphate as controlling factors.

The effects and interactions of heating temperature (55 to 65 degrees C), pH (4 to 8), salt (NaCl; 0 to 6%, wt/vol), and sodium pyrophosphate (SPP; 0 to 0.3%, wt/vol) on the heat inactivation of a four-strain mixture of Listeria monocytogenes in beef gravy were examined. A factorial experimental design comparing 48 combinations of heating temperature, salt concentration, pH value, and SPP content was used. Heating was carried out using a submerged-coil heating apparatus. The recovery medium was plate count agar supplemented with 0.6% yeast extract and 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 SPP levels. Whereas increasing the NaCl concentration protected L. monocytogenes against the lethal effect of heat, high SPP concentrations increased heat sensitivity. Also, low pH values increased heat sensitivity of L. monocytogenes. The four variables interacted to affect the inactivation of the pathogen. Thermal resistance of L. monocytogenes can be lowered by combining these intrinsic factors. A predictive model that described the combined effect of temperature, pH, NaCl, and SPP levels on thermal resistance of L. monocytogenes was developed. The model can predict D-values for any combination of temperature, pH, NaCl, and SPP that are within the range of those tested. Using this predictive model, food processors should be able to design adequate thermal regimes to eliminate L. monocytogenes in thermally processed foods.

Application and evaluation of male-specific bacteriophage as a process integrity or faecal contamination indicator in a pork slaughterhouse environment.

A male-specific bacteriophage plaque assay was evaluated as a faecal contamination or process integrity indicator for aspects of the pork slaughter process. Over 400 samples were tested including: sponge swabs from animal hauling trailer floors and dressed carcass surfaces; faecal material; water from slaughter sites; and water from each stage of wastewater treatment. Bacteriophage were observed in wastewater, trailers, slaughter process water and swine faeces. No bacteriophage were observed on dressed carcasses. Numbers of phage plaque-forming units per gram or millilitre showed greater variation and were usually lower than standard indicators, including total coliform or Escherichia coli counts. Among the applications studied, male-specific bacteriophage appear to be best suited for process control verification for wastewater treatment.

Survival of Escherichia coli O157:H7 during the manufacture of pepperoni.

This study investigated the growth and survival of Escherichia coli O157:H7 during the manufacture of pepperoni to determine whether a 5-log10-unit decline in numbers, as recommended by the U.S. Food Safety and Inspection Service (FSIS), could be achieved. A range of pepperoni formulations with variations in salt (2.5 to 4.8%) and sodium nitrite (100 to 400 ppm) levels, and with pH (4.4 to 5.6) adjusted by manipulation of dextrose concentrations were prepared. The batters produced were inoculated with E. coli O157:H7 380-94 at a level of approximately 6.70 log10 CFU/g; changes in pathogen numbers, pH, titratable acidity, and sodium nitrite concentrations were monitored during fermentation and drying. With the standard commercial formulation (i.e., 2.5% salt, 100 ppm sodium nitrite, pH 4.8) E. coli O157:H7 numbers declined by approximately 0.41 log10 CFU/g during fermentation and a further 0.43 log10 CFU/g during subsequent drying (7 days). A regression equation was fitted to the data which showed significantly (P < 0.001) greater reductions in pathogen numbers in samples with increased salt and sodium nitrite contents and lowered pH. However declines were in all cases less than the target reduction of 5 log10 CFU/g.