BACTERIOPHAGE IN CHEESE MANUFACTURE

In 1935 Whitehead and Cox (32) reported the observation that bacteriophage (bacterial virus) was responsible for the failure in acid producing activity of a culture of Streptococcus cremoris used as a cheese starter. The virus was shown to lyse the bacteria, thus terminating acid production in the culture as a whole. This discovery opened up a new and profitable line of research in dairy manufacturing technique, more particularly in connection with the manufacture of cheddar cheese where uncontrollable variations in rate of acid development from day to day had long been a cause of difficulty and spoilage. Workers in Iowa (11) had previously observed that filtrates from certain butter cultures, i.e., milk cultures containing a mixed flora of lactic streptococci and betacocci (Leuconostoc spp) sometimes contained an "inhibitory" factor which caused a check in acid production when added to other butter cultures. They had not, however, realized the true significance of their observation and only later agreed that the phenomenon was due to the presence of a bacterial virus in the "inhibitory" filtrate (19). Since 1935 evidence has gradually accumulated that phage is responsible all over the world for much of the irregular behavior observed in bacterial cultures used in the manufacture of dairy produce (2, 3, 12, 15, 18, 28, 30, 47). The effects are perhaps most evident in the manufacture of cheddar cheese where a vigorous growth of lactic streptococci sufficient to produce the equivalent of about 1 per cent of lactic acid is required within a period of five to six hours; but phage evidently is also capable of causing difficulties in the ripening of cream for butter manufacture and in the production of various fermented milk preparations. Characteristics of phage action in cheese manufacture. Until the present century cheese makers relied upon contaminant bacteria, present by chance in the milk, to bring about the fermentation of lactose and gradual production of acid which is an essential part of cheese manufacture. A gradual improvement in hygiene in the production and handling of milk led to the necessity for addition of a culture of lactic streptococci (starter) to the cheese milk since, in the absence of such an addition, the milk would not now sour quickly enough. Pasteurization of the cheese milk, practiced most widely perhaps in New Zealand, made the use of starter still more essential. Cultures containing a mixed flora of lactic streptococci and betacocci were similarly introduced early in this century for the ripening of cream for butter manufacture. The essential feature of this process was later shown to be the production of diacetyl, the butter-aroma substance, by the betacocci under the acid conditions produced by the streptococci. The preparation and supply of starter cultures to the dairy industry were for many years almost entirely in the hands of commercial interests who were secretive about the principles followed in selecting the strains of bacteria used in the mixed cultures. It appears also to have been assumed that the mixtures used in butter starters were equally suitable for cheese manufacture.