PURITY OF MILK SUPPLY

Southland Times, Issue 23664, 12 November 1938, Page 19

PURITY OF MILK SUPPLY

Effect On Openness In Cheese CONTROL OF FLAVOUR IN PRODUCT The need for the maintenance of purity in the milk supply in order to avoid the introduction and development of undesirable forms of bacteria producing openness and unpleasant flavours in cheese is emphasized in the annual report of the New Zealand Dairy Board in a review of the work of the Dairy Research Institute at Palmerston North. Patient research on the cause of open texture in cheddar cheese has been rewarded by the observation that slit openness, the commonest of. all types, is due to the action of certain bacteria which produce gas while the cheese is ripening. These bacteria belong to a broad classification containing those which impart to cheese its characteristic flavour. The desirable types seldom produce gas, while the undesirable types frequently produce both gas and discolouration. It is obvious that the extent of openness depends upon the number of gas-producing organisms in the cheese, and also upon their virulence. It is also influenced by the activity of the starter. The practical solution for avoiding trouble is made exceedingly difficult by the fact that both the desirable and undesirable types of bacteria originate from the same source, namely, the milk supply. There is no known method of treating milk which will prevent the entrance of undesirable types and simultaneously encourage the growth of the desirable ones. It would appear, therefore, that one key to the solution of the problem is to make every endeavour to improve the purity of the milk supply, thereby keeping all types from gaining access to it. The desirable types can readily be added in the form of a special starter. There is as yet no simple explanation as to why more difficulty is experienced in'some districts than in others, but this matter is receiving close attention. Unfortunately, ordinary methods of pasteurization do not destroy all the undesirable types of bacteria, though these methods play an important part in reducing the intensity of their effects. Investigations are in progress to determine whether special methods of pasteurization can destroy undesirable types without injuring the quality of the cheese. ACTIVITY OF STARTER It has long been known that there is a correlation. between the activity of the starter and the incidence of openness, inactive starters fending to accentuate openness while active starters mitigate it. In recent annual reports it has been pointed out that the loss of vigour in starters is frequently due to the occurrence of bacteriophage, a condition which is similar in effect to the action of viruses on plants. Mixed starters, such as are commonly supplied in dried form, are frequently sensitive to phages, and although the whole population of the starter is not destroyed by any one phage, the vitality of the starter as a whole is subject to considerable variation from day to day. Accordingly, the institute has lately been concentrating upon the development of single-culture starters with the object of developing types which are constant in activity all the while. These single-strain types are also sensitive to phage. ’ There has been devised a method of propagating these starters which, in most cases has been successful in preventing the starter from being affected by phage. This method entails the careful observation of certain details in the preparation and care of the starter; among other points, special care has to be taken to avoid contamination of the starter while it is being prepared, to use a heavy inoculation with mother culture at the time of preparing a fresh batch of starter and to maintain cultures at certain temperatures after they have been propag“4ed. This method of propagating starters has given every satisfaction at the institute factory over the past two years, and has been employed with success in several commercial factories, but some managers have experienced difficulty in spite of observing all prescribed precautions.

It has been shown that some other factor makes particular milk supplies sensitive to phage. This matter is receiving very careful investigation. The use of single-strain starters has resulted, on the average, in production of cheese uniform in quality from day to day, and in the adoption of a method of manufacture which can be comparatively standardized. Good quality cheeses have been made from these starters, but certain departures have to be made from the ordinary cheese-making process in the manufacture of cheese from milk to which they have been added. An extensive series of experiments involving much detailed chemical work has been carried out in the latter question, with a view to defining the best manufacturing procedure that should be adopted for particular strains of starter cultures. Considerable progress has been made with the development of characteristic and desirable cheese flavour by the addition to milk, in combination with ordinary starter cultures, of certain selected flavour-producing bacteria.