Equal numbers of bacteria placed in physiologic saline or nutrient broth at the same pH are not destroyed with the same ease by heat. Some 12 factors or parameters of microorganisms and their environment have been studied for their effects on heat destruction, and they are presented below.22 Water The heat resistance of microbial cells increases with decreasing humidity, moisture, or water activity (aw), and this is illustrated in Table 17–1 for spores of Bacillus cereus.
For example, at aw of 1.00 and pH 6.5, D95 was 2.386 minutes while at aw of 0.86, D95 was 13.842 minutes.18 Dried microbial cells placed in test tubes and then heated in a water bath are considerably more heat resistant than moist cells of the same type.
Because it is well established that protein denaturation occurs at a faster rate when heated in water than in air, it is suggested that protein denaturation is either the mechanism of death by heat or is closely associated with it (see a later section in this chapter). The precise manner in which water facilitates heat denaturation of proteins is not entirely clear, but it has been pointed out that the heating of wet proteins causes the formation of free–SH groups with a consequent increase in the water-binding capacity of proteins. The presence of water allows for thermal breaking of peptide bonds, a process that requires more energy in the absence of water and, consequently, confers a greater refractivity to heat.
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The effect of salt on the heat resistance of microorganisms is variable and dependent on the kind of salt, concentration employed, and other factors. Some salts have a protective effect on microorganisms, and others tend to make cells more heat sensitive. It has been suggested that some salts may decrease water activity and thereby increase heat resistance by a mechanism similar to that of drying, whereas others may increase water activity (e.g., Ca2+ and Mg2+) and, consequently, increase sensitivity to heat. It has been shown that supplementation of the growth medium of Bacillus megaterium spores with CaCl2 yields spores with increased heat resistance, whereas the addition of l-glutamate, l-proline, or increased phosphate content decreases heat resistance