Influence of heat transfer with tube methods on measured thermal inactivation parameters for Escherichia coli

The purpose of this study was to investigate the influence of heat transfer on measured thermal inactivation kinetic parameters of bacteria in solid foods when using tube methods. The bacterial strain selected for this study, Escherichia coli K-12, had demonstrated typical first-order inactivation characteristics under isothermal test conditions. Three tubes of different sizes (3, 13, and 20 mm outer diameter) were used in the heat treatments at 57, 60, and 63 degrees C with mashed potato as the test food. A computer model was developed to evaluate the effect of transit heat transfer behavior on microbial inactivation in the test tubes. The results confirmed that the survival curves of E. coli K-12 obtained in 3-mm capillary tubes were log linear at the three tested temperatures. The survival curves observed under nonisothermal conditions in larger tubes were no longer log linear. Slow heat transfer alone could only partially account for the large departures from log-linear behavior. Tests with the same bacterial strain after 5 min of preconditioning at a sublethal temperature of 45 degrees C revealed significantly enhanced heat resistance. Confirmative tests revealed that the increased heat resistance of the test bacterium in the center of the large tubes during the warming-up periods resulted in significantly larger D-values than those obtained with capillary tube methods.