BACTERIA IN MILK AND DAIRY PRODUCE It has been stated already that milk is a perfect food for the development of bacteria. Without bacteria or other micro organisms milk undergoes no change which can be detected by the senses and it may be kept in its original state almost indefinitely. The length of time during which milk will remain wholesome de pends very largely upon its initial bacterial content. Appreciation of this fact has led the progressive dairyman to adopt methods to secure milk as free from bacteria as is consistent with its eco nomical production; by steam sterilization of all vessels used, by washing the udders and cleaning the cows, by sterilizing the hands of all milkers, and by absolute cleanliness of the byre, the production of "Certified" milk guaranteed to contain fewer than 30,000 organisms per cu.cm. has been rendered possible.
Milk that has become sour is considered undesirable for drink ing purposes in this country, but in different parts of Europe and Asia natural beverages are prepared by submitting milk to condi tions which favour the development of certain acid-producing bacteria. The Bulgars, for instance, drink a great deal of milk soured by E. Metchnikoff's organism Bacillus bulgaricus. In other parts alcoholic beverages, kefir, kumiss, matzoon, etc., are brewed in the homes by the combined action of various yeasts with Bacillus acidi lactici and other acid-producing bacteria.
Much use is made of the acid-forming bac teria in the preparation of cream for butter-making. It is possible to produce butter from fresh cream and in some of the European countries there is a strong demand for such butter. The keeping qualities of this fresh cream butter, however, are very poor and it therefore must be consumed within a short time of its produc tion. This defect is to a large extent disposed of by allowing cream to sour before it is churned. In this bacterial "ripening" of cream the globules of fat are altered in that the protein en velope of the fat is practically digested by the bacteria and the enclosed fat is more readily coalesced to form butter. Further, since acids prevent the development of most bacteria, the high degree of acidity reached at the time of churning eliminates many of the bacterial kinds responsible for the rapid spoiling of butter made from fresh cream. In ordinary farm practice the cream is allowed to ripen in an almost uncontrolled manner by any or ganisms falling from the air into the cream accumulated from day to day until the quantity is sufficient for the churn. In this way the butter produced varies considerably in flavour and texture in accordance with the character of the bacterial flora. In modern dairy practice, on the other hand, the ripening process is scien tifically controlled; mixed cultures of organisms (starters) chosen for their power to produce quickly a high degree of acidity and to enhance the flavour of the final product are employed. The tem perature of the inoculated cream is kept practically constant so that when it enters the churn bacterially and chemically it is in a uniform condition. The result is a uniform butter free from un desirable taints and possessed of excellent keeping qualities. Such butter will be good for two or three weeks but sooner or later un desirable flavours will develop. Chief among these is the familiar rancidity produced by the conversion of lactic acid into butyric acid by Bacillus butyricus and a number of other organisms. Mottled, putrid, bitter and tallowy butter are faults that can usual ly be traced, sometimes with considerable difficulty, to certain contaminating bacteria and these are not always in the ripened cream but are intro duced into the butter with the wash water.
Even creameries with an established repu tation for excellent butter are sometimes faced with the problem of eradicating some micro-organism causing rapid deterioration of their product. Similar faults arise in milk through certain bacteria gaining the upper hand and producing taints and colours ; for example, blue milk, red milk, ropy milk and bitter milk.
The starting ma terial for all cheeses is practically the same, namely a curd produced by the action of rennet upon milk. The differences which characterize the various cheeses arise dur ing the ripening process and are the result of the temperature and moisture conditions under which the curd is maintained during the ripening process. For the manufacture of hard cheese the curd is pressed to greater or less degree and more or less fat is retained in the curd according to the kind of cheese it is desired to produce. For the manufacture of soft cheeses the curd is merely drained or sub mitted to slight pressure only. The curd for hard cheese comes from the press in a hard, partly dry form whereas for a soft cheese the curd is left very wet. The latter condition is much more favourable to the growth of organisms, one therefore finds that soft cheeses ripen much more quickly than hard cheeses. The whole story of the ripening of cheese has not yet been revealed, but it is known that it is not entirely due to bacterial action. Some of the softening which the curd undergoes is a chemical process slowly developed under the influence of enzymes (catalytic agents or chemical stimuli). These agents probably play a much larger part in hard than in soft cheese, but the flavours and texture which distinguish the different cheeses are undoubtedly the result of the growth of organisms, bacteria, yeasts and moulds. Most of the bacteria in cheese ripening are of the acid-forming type and these are not all equally suitable for the purpose. Artificial cultures or "cheese starters" are therefore sometimes employed, though their use is by no means so general as is the use of "butter starters." The famous Edam cheese of Holland is started by adding to the milk to be curdled some slimy milk formed by a pure culture of the "ropy-milk" organism. From the nature of the case it will readily be seen that bacteria of the wrong kind may dominate the situation and may seriously affect the flavour and appearance of the cheese. Many faults may arise in this way; chief among these is a gassy condition, caused by Bacillus lactis aerogenes; other faults are sweet flavour, putrefaction, and variously coloured spots. These last are sometimes due to yeasts and sometimes to moulds.
