IT is a curious fact that although cultures of lactic streptococci have been used as starters in the manufacture of cheese during the past thirty years, there are no published accounts (so far as we are aware) of attempts to explain the dramatic changes in activity which such cultures exhibit from time to time even when they are subcultured daily under apparently constant conditions. A large mass of work has been published dealing with the influence of the lactic streptococci on the ripening of and development of flavour in cheese, but the very important part played by the bacteria in the formation of acid during the process of manufacture has either not attracted the attention of research workers or has proved a subject of exceptional difficulty. In the manufacture of many types of cheese, particularly those of soft texture, the demands for acid production made upon the starter are not very heavy and consequently slight variations of activity do not obtrude themselves unduly on the attention of the cheese maker. With Cheddar cheese, however, the case is different. The particles of curd in which the starter is expected to develop are relatively dry; and the temperature to which the curd is raised (normally 100° F., in extreme cases 105° F.) in order to promote the expulsion of moisture is often high enough to retard quite markedly the growth of the bacteria. Thus in the manufacture of Cheddar cheese it is necessary that the starter should produce, within six hours of the start of the process, the equivalent of 1 per cent, of lactic acid in the whey under adverse conditions both of moisture supply and temperature. It is not to be wondered at, therefore, that starters frequently fail to attain this performance. Apart, however, from the cases where the starter is rather slow in producing the required amount of acid there are those instances where a starter, having functioned satisfactorily for some time, suddenly undergoes such a sudden loss in vitality that it fails completely to approach the required standard of performance. In the large Cheddar cheese factories in New Zealand failure of starter cultures even for a single day is a serious matter from the economic point of view; therefore attention has been focused for some time at this Institute on the problem presented by the apparently uncontrollable variations in starter activity. The present paper
