Thermal Inactivation of Bacteriophages Active Against Lactic Streptococci

Thermal destruction studies on two bacteriophages active against Streptococcus lactis were carried out in milk at pH 6.2. Inactivation curves were similar at all temperatures studied; a relatively rapid initial decline in titer was followed by a slower reduction and leveling off of numbers, until complete inactivation was approached. When inactivation was nearly complete, the populations sometimes again declined rapidly. A longer time was required for complete destruction of the bacteriophage than would be anticipated from a straightline extrapolation of the initial portion of the thermal inactivation curve. Destruction times for one bacteriophage at 70, 72.8, 75, and 77.8C were 200, 70, 20, and 6min, respectively and, for the other, 170, 60, 25, and 9min, respectively. Plotting the thermal death times of the two bacteriophages yielded z values of 8.75 and 10.5.