Heat inactivation of bacteriophage strains active against lactic streptococci.

The purpose of this investigation was to obtain further information on the time-temperature relationships in the heat inactivation of bacteriophage strains active against Streptococcus lactis and Streptococcus cremoris and to determine whether the heat resistance characteristics of various bacteriophage strains were related to other criteria used in bacteriophage classification. Harriman, in his unpublished thesis (1934), reported that heating at 60 C for 5 minutes destroyed the activity of bacteria-free filtrates which caused inhibition of acid production in butter cultures. These studies were carried out on acid filtrates of undesignated pH. Whitehead and Cox (1936) reported that one strain of lactic streptococcus was not resistant to 70 C for 30 minutes, and another was destroyed by 50 C for 30 minutes. Whitehead and Hunter (1937) reported that all the bacteriophages they investigated had a thermal death point of 70-75 C at pH 6.0, with the exception of one which was destroyed at 55-60 C. Yakovlev (1939) reported that a bacteriophage active against a strain of S. cremoris resisted 80 C for 10 minutes in small volumes, and that it survived heating at 90 C for 5 minutes, in large volumes of whey. Nelson, Harriman and Hammer (1939) reported that 15 minutes at 70 C was required to inactivate the most resistant bacteriophage strain active against S. lactis. The most heat labile strains were inactivated in less than 5 minutes at 65 C. These studies were carried out at neutral reaction. Whitehead (1944) showed that heating cream containing 50 per cent fat at 170 F for 30 minutes in hot water failed to destroy bacteriophage, but heating to 220 F with live steam and holding for 30 minutes was adequate for bacteriophage destruction. Nichols and Wolf (1945) reported that 26 bacteriophages of lactic streptococci were not destroyed by heating for 50-60 minutes at 65-67 C and most of them survived heating at 70 C for 10-15 minutes. All determinations were made in milk at pH 6.42 to 6.28,