Heat resistance ofCitrobacter freundii in media with various water activities

Summary

The heat resistance ofCitrobacter freundii NCTC 9750 between 45–65°C in media with various water activities has been determined.

At a water activity of nearly 1.00, the Arrhenius plot of the death rate shows a sharp breakpoint at 56.5°C, suggesting the existence of at least two different thermal inactivation processes causing lethality of the bacterial cell. The activation energy below 56.5°C is 0.4186 MJ/mol (100 000 cal/mol), above 56.5°C it is 0.1863 MJ/mol (44 500 cal/mol). After addition of sucrose (1.8 mol/l) or NaCl (0.77 mol/l) to the heating medium, such a breakpoint is not observed. The activation energy for these processes are, for sucrose; 0.2097 MJ/mol, for NaCl; 0.3641 MJ/mol. However, at an NaCl concentration of 1.54 mol/l there is a breakpoint at 53.3°C.

The influence of the sucrose concentration on the heat resistance can be described by the formula: ln k_S=ln k_O−a [sucrose]. Such a simple correlation does not exist for the influence of NaCl or glycerol.

The heat inactivation of whole cells ofC. freundii was also measured with a differential scanning calorimeter. The first irreversible conformation change took place at 323 K, the main conformation change at 343 K.