Influence of simulated apple cider composition on the heat resistance of Escherichia coli O157:H7

In light of recent outbreaks involving Escherichia coli O157:H7 in unpasteurized apple cider, this study was undertaken to determine the effect of apple cider composition on the heat resistance of this pathogen, and to develop regression models to predict its inactivation under varying conditions of pH (2.82-4.06), titratable acidity (TA) (31.14-111.80), and Brix (8.86-15.32). Heating was conducted in capillary tubes submerged in a stirred oil bath, and viability was determined by plating on tryptic soy agar. A central rotatable composite design was used which consisted of 18 treatment combinations at each of three heating temperatures (52, 54 and 56 degreesC). The time (min) for a 2-log reduction (T(2D)) and a 4-log reduction (T(4D)) were calculated for each of the 54 treatment combinations by fitting a smoothing spline to the survival curve. Two regression models were developed to predict the effect of the factors on T(2D) and T(4D), and regression coefficients of 0.95 and 0.90 were obtained, respectively. The effects of pH and TA on T(2D) and T(4D) were significant (Pless than or equal to0.05) at all heating temperatures. Increasing Brix to 15degrees had a protective effect on E. coli O157:H7. The interaction between pH and TA was significant, as was the combined effect of Brix and pH. The combination of Brix and TA was not significant. The results support the use of a nonparametric model such as a smoothing spline for the analysis of survival curves when parametric models are not adequate.