Heat Resistance of Bacillus cereus Spores Located between Seals and Seal Surfaces

The heat resistance of Bacillus cereus ATCC 14579 spores enclosed between a silicone-rubber seal and a stainless-steel surface was determined in a temperature range from 90°C to 112°C. Under these conditions spores exhibited a markedly higher resistance than in distilled water. Attachment of spores to one of the surfaces had no effect on the heat resistance if the spores were still in direct contact with the heating medium (water or saturated steam). At 90°C suspended spores were killed 1.7 times, at 98°C 2.2 times, at 106°C 14 times, and at 112°C 18 times faster than the enclosed spores. The minor temperature dependence of the heat destruction under the seals resulted in an Arrhenius activation energy of 135 kJ/mol, in comparison with 285 kJ/mol for the suspended spores. It is suggested that the free water initially present is completely replaced by the elastic seal as it is pressed onto the seal surface. The environment of the enclosed spores therefore might be compared to an environment of reduced water activity which is known to bring about a drastic increase in heat resistance.