Effect of thermal treatments in oils on bacterial spore survival

The heat resistance of Bacillus cereus F4165/75, Clostridium sporogenes PA 3679 and Cl. botulinum 62A spores suspended in buffer (pH 7˙2), olive oil and a commercial oil (a mixture of rapeseed oil and soy oil) was investigated. Linear survivor curves were obtained with B. cereus spores in the three menstrua and with 62A and PA 3679 spores suspended in buffer. However, the inactivation kinetics of the clostridial spores suspended in oils were concave upward with a characteristic tailing-off for 62A spores suspended in olive oil. These deviations from the semi-log model could not be ascribed to a heterogeneity in heat resistance of the spore population or to the variation of a_w during heating. Spore resistance to heat increased in the order: buffer ⋖ commercial oil < olive oil. The greater heat resistance of oil-suspended spores was ascribed to the low a_w (0˙479 and 0˙492 for commercial oil and olive oil, respectively) and to the composition of the oils. The difference in z values ( ca 28°C in oils and 10°-12°C in buffer) suggested that the mechanism of inactivation differs for spores suspended in lipids and in aqueous systems. The thermodynamic data were consistent with this hypothesis.