The destruction of micro-organisms in foods using heat is a well-known phenomenon in the preservation techniques of foods. However the temperature response of vegetative cells and spores is far from uniform. Spores tend to be more heat resistant than vegetative cells which in turn range widely in their heat resistance. Even individual bacteria within a population of a given species show a normal distribution of heat resistance. Thus it is possible to allow heat resistant (or thermoduric) organisms to survive by using a heating regime which is sufficient to destroy bacteria of low to intermediate heat resistance but which fails to kill thermoduric bacteria. These may then thrive within a processing unit, for example, a blancher, and increase the microbial load on a subsequent sterilization operation.