The thermal inactivation characteristics of Bacillus subtilis A spores suspended in skim milk with the use of large-scale ultrahigh temperature (UHT) processing equipment were investigated in terms of survival as measured with two plating media. Data on survival immediately after UHT treatments were recorded in temperature-survivor curves, time-survivor curves, and decimal reduction time (DRT) curves. The temperature-survivor curves emphasized that inactivation is accelerated more by increases in the treatment temperature than by increases in the exposure time. Time-survivor curves and DRT curves were not linear. Generally, exceedingly concave time-survivor curves were observed with the standard plating medium; however, only slightly concave curves were observed when CaCl2 and sodium dipicolinate were added to the medium. For a given UHT sample, larger D values were obtained by use of the medium with the added CaCl2 and sodium dipicolinate. The DRT curves of all data were concave and appeared to have two discrete slopes (ZD values). The ZD values observed in the upper UHT range (above 260 F; 127 C) were twice those observed at lower test temperatures.

Injury of test cultures was quantitated by differences in colony counts obtained with a complete medium and those obtained on conventional selective media. Staphylococcus aureus, Streptococcus faecalis, and several strains of Escherichia coli were injured when exposed to the quaternary ammonium compound methylalkyltrimethyl ammonium chloride. Representative hypochlorite sanitizers also caused injury of E. coli ML30. Sanitizer concentration appeared to be the main factor in the cause of death and injury, a higher concentration being needed to cause death. Increases in temperature did not result in substantial increases in injury; however, the lethal effect was greater at higher temperatures. Varying the cell concentration from 107 to 109 cells per ml did not change the fraction of cell population killed or injured. The inability or failure of common selective media to detect injured bacteria in food could have serious public health consequences.