Comparison of Capillary and Test Tube Procedures for Analysis of Thermal Inactivation Kinetics of Mold Spores

Characteristics of capillary and test tube procedures for thermal inactivation kinetic analysis of microbial cells were studied for mold spores. During heating, capillaries were submerged in a water bath and test tubes were held with their caps positioned above the level of the heating medium. Thermal inactivation curves of Aspergillus niger spores in capillaries at around 60°C consisted of a shoulder and a fast linear decline, whereas curves in test tubes consisted of a shoulder, a fast linear decline, and a horizontal tail. There were no significant differences in values of the rate and the delay of fast declines in curves between the procedures. Some experiments were done to clarify the cause for tailing with test tubes. There were no tails with test tubes whose inner walls were not contaminated by A. niger spores, suggesting that tails arise from A. niger spores contaminating the inner walls of test tubes. Temperature of the inner wall at the level of a heating medium was lower than that of the medium. Further, there were no tails for test tubes submerged in the heating medium. These results showed that the reason for survival of contaminants on the upper wall of test tubes was that cells were not subjected to sufficient inactivation temperature. Finally, thermal inactivation curves of A. niger spores in capillaries at various constant temperatures were studied. Curves consisted of a shoulder and a fast linear decline at 57°C and above, whereas curves at below 57°C consisted of a shoulder, a fast linear decline, and a sloping tail.