Increased heat resistance of Bacillus subtilis spores in oil

Abstract

The heat resistance of Bacillus subtills 168 spores suspended in several kinds of oil was investigated. In soybean oil, the D value of the thermal death of spores at 98.4°C was 53 min, being about 7 times greater than that in potassium phosphate buffer. The Z value was 71°C in soybean oil but 11°C in the buffer. A similar increase in the D value was observed in other edible oils and n-hexadecane. The differences in the heat resistance of spores among these oils seemed to depend upon the water content of oils even at considerably low levels. Furthermore, the dehydration of oils by dry heat increased the heat resistance of spores. In addition, freeze-dried spores were more resistant to heat than frozen spores in soybean oil. In an open heating system, the spore resistance was higher than that in a closed system. These results suggest that the water content of oil substantially determines the heat resistance of spores suspended in it.