242 BACTERIA IN MILK AND MILK PRODUCTS 



in a soft household cheese. Only a small percentage of these are 

 of the peptonising species. Tides of organisms occur in cheese, as 

 in butter and milk. 



Method of Examination of Butter and Cheese. Several grains of 

 the butter or cheese should be placed in a large test-tube, which is 

 then two-thirds filled with sterilised water and placed in a water- 

 bath at about 45 C. until the butter or cheese is melted or " washed." 

 A small quantity may then be added to gelatine or agar and plated 

 out on Petri dishes, or in flat-bottomed flasks, in the usual way. 

 After which the tube may be well shaken and returned to the bath 

 inverted. In the space of twenty or thirty minutes the butter or 

 cheese has separated from the water with which it has been 

 emulsified. It is then placed in the cold to set. The water may be 

 now either centrifugalised or placed in sedimentation flasks, and the 

 deposit examined for bacteria. 



The Uses of Bacteria in Dairy Produce 



In considering the relation of bacteria to milk, we found that 

 many of the species present were injurious rather than otherwise, 

 and when we come to consider bacteria in dairy products, like butter 

 and cheese, we find that the dairyman possesses in them very powerful 

 allies. "Within recent years almost a new industry has arisen owing 

 to the scientific application of bacteriology to butter and cheese 

 making. 



1. Bacteria in Butter-Making 



As a preliminary to butter-making, the general custom in most 

 countries is to subject the cream to a process of " ripening." As we 

 have seen, cream in ordinary dairies and creameries invariably 

 contains some bacteria, a large number of which are in no sense 

 injurious. Indeed, it is to these bacteria that the ripening and 

 flavouring processes are due. They are perfectly consistent with 

 the production of the best quality of butter. The aroma of butter, 

 as we know, controls in a large measure its price in the market. 

 This aroma is due to the decomposing effect upon the constituents 

 of the butter of the bacteria contained in the cream. In the months 

 of May and June the variety and number of these types of bacteria 

 are decidedly greater than in the winter months, and this explains 

 in part the better quality of the butter at these seasons. As a 

 result of these ripening bacteria the milk becomes changed and 

 soured, and slightly curdled. Thus it is rendered more fit for butter- 

 making, and acquires its pleasant taste and aroma. It is then 

 churned, after which bacterial action is reduced to a minimum or 

 absent altogether. Sweet-cream butter lacks the flavour of ripened 



