Effect of Using Milk as a Heating Menstruum on the Apparent Heat Resistance of Bacillus stearothermophilus Spores

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Abstract
Heat resistance at 121.1 C (250 F) of Bacillus stearothermophilus spores was studied using two heating menstrua. D values of 3.8 and 3.5 min were obtained when spores were heated in 0.01 M PO4 buffer, pH 6.5, and in skimmilk, pH 6.5, respectively. With buffer as a heating menstruum. increasing the pH from 6.5 to 7.2 resulted in an increase in the D value from 3.8 to 4.1 min. When the pH of skimmilk was increased from 6.5 to 7.2, D values increased from 3.5 to 5.2 min. Skimmilk as a component of the enumeration medium inhibited germination and/or outgrowth of B. stearothermophilus spores; however, this inhibition was not influenced over the pH range of 6.0 to 7.2. Addition of 10% skimmilk, pH 6.5, to the medium for enumeration of spores heated in buffer at pH 6.5 or 7.2, in each instance reduced the number of spores that could be recovered but did not change the slopes of survival curves.