288 MILK, BUTTER, AND CHEESE [chap. 



can only be avoided by careful attention to cleanliness 

 and to the purity of the materials employed. The 

 wrong kind of organisms may have already obtained a 

 hold in the milk itself at starting, generally through 

 want of cleanliness in the milking. Next, they may be 

 introduced by imperfect cleaning of the utensils in which 

 the milk stands ; the water used is also often a source 

 of danger, and has frequently been found to be a cause 

 of various troubles in the final product. When natural 

 souring is trusted to, a not very satisfactory flora of the 

 various lactic acid making. bacteria may become estab- 

 lished in the dairy. For this reason it is sometimes 

 customary both in cheese- and butter-making on a large 

 scale to begin by pasteurising the milk ; that is, by heating 

 it up to a temperature of about 170° R, which kills 

 practically all the bacteria present. After cooling, the 

 pasteurised milk is then mixed with a "starter," con- 

 sisting either of a pure culture of the desired bacteria or 

 of the good but mixed culture contained in the butter- 

 milk or the sour whey derived from a previous churning, 

 or from another dairy turning out products of an 

 appropriate flavour. In this way the development of 

 acidity in the milk can be controlled, and this is the 

 first step towards obtaining a standard product. 



In all the farmer's dealings with milk and its products it 

 is important to remember that the milk itself provides an 

 almost ideal feeding-ground for bacteria ; and as bacteria 

 of all kinds are particularly prevalent in dust and dirt, 

 specially in the organic dirt that collects in cow stalls 

 and cattle sheds, or in a dairy which is not kept most 

 scrupulously clean, milk will deteriorate more readily 

 than almost any other natural product. The only way 

 to overcome this difficulty is to exercise the most 

 scrupulous cleanliness, and to keep the temperature as 

 low as possible. In the case of milk and milk products, 



