J. Z. Wolf

324. The heat resistance of the bacteriophages of cheese starter with observations on the estimation of phage concentration

The heat resistance, in milk, of bacteriophages attacking strains of lactic streptococci isolated from cheese starters has been determined. Active phages did not usually survive a temperature of 75°C. for 7½ min. but many phages were not destroyed by heating for 50–60 min. at 65–67°C. and most survived at 70°C. for 10–15 min. The practical implications of these findings in the ‘sterilization’ of milk for starter making have been discussed. Growth in yeast dextrose broth was found to be superior to methylene blue reduction for the demonstration of residual phages after heating.

281. Volatile acids of cheese: III. Application of the extraction method

1.Details are given of the extraction method for the estimation of the volatile acids in cheese, as used in the study of some white and blue-veined varieties. 2. On the basis of the distribution of valatile acids, hydrogen-ion concentration and nitrogen breakdown, distinctions could be drawn not only between the white and blue-veined varieties as a whole, but also between the Roquefort and Stilton groups of the latter varieties. 3. A high total quantity and a characteristic distribution of the volatile acids appeared to be related to the heavy pricking of cheese of the Roquefort group.

323. Observations on cheese starters with reference to bacteriophage and the phageorganism relationships of strains isolated

1. Frequent examination of the mother and bulk starters from several cheese-making creameries during the 1942 season has shown bacteriophage to be prevalent in some factories and apparently absent from others. Outbreaks of phage attack could not be correlated with heat treatment of the cheese milk, and were geographically widespread over the country. 2. The phage-organism relationships of these starter strains and their phage races were studied, and on this basis it was possible to arrive at the relationship and in some cases the identity of some of the strains circulating in commerce. It was found that for the isolation of some phaging strains, milk was a more sensitive medium than broth. Commercial starters appear to be composed of more than one distinct strain, and no starter can be judged by the reactions of one of its component strains. The nascent phage phenomenon was found not to be common in the selected starter strains. The reaction itself may, however, possibly be used to demonstrate fundamental relationships between strains.