196 BACTERIA IN MILK AND MILK PRODUCTS 



1. The Peementations of Milk 



(1) Lactic Acid Fermentation: the Souring of Milk. — If milk is 

 left undisturbed, it is well known that eventually it becomes sour. 

 The casein is coagulated, and falls to the bottom of the vessel ; the 

 whey or serum rises, carrying to the surface flakes or lumps of fat. 

 In fact, a coagulation analogous to the clotting of blood has taken 

 place. In addition to this, the whole has acquired an acid taste. 

 Now this double change is not due to any one of the constituents we 

 have named above. It is, in short, a fermentation set up by a living 

 ferment introduced from without. The constituents most affected 

 by the fermentation are (a) the milk-sugar, which is broken down 

 into lactic acid, carbonic acid gas, and other products, and (b) the 

 casein, which is curdled and becomes suspended in a semi-coUoidal 

 form. 



For many years it has been known that sour milk contained 

 bacteria. Pasteur first described the Bacillus acidi lactici, which 

 Lister isolated in 1877, and obtained in pure culture by the dilution 

 method. In 1884 Hueppe contributed still further to what was 

 known of this bacillus, and pointed out that there were a large 

 number of varieties, rather than one species, to be included under 

 the term B. acidi lactici. We have already dealt with the chief 

 characters of this family of organisms. When a certain quantity of 

 lactic acid has been formed, the fermentation ceases. It will 

 recommence if the liquid be neutralised with carbonate of lime, or 

 if pepsine be added. Since Pasteur's discovery of a causal bacillus 

 for this fermentation, other investigators have added a number of 

 bacteria to the lactic acid family. Some of these in pure culture 

 have been used in dairy industry, as we shall subsequently have 

 occasion to notice. 



We have already seen that milk as it leaves the healthy udder 

 is generally sterile, and immediately gains bacteria from air, dust, 

 etc. Whilst the exact origin of lactic acid bacilli is not known, 

 many bacteriologists hold that they gain entrance to the milk from 

 the surrounding air of byre or dairy. Others maintain that some 

 species, at any rate, are soil bacteria, and associated with certain 

 geographical localities. Eussell states that, under ordinary con- 

 ditions, the organisms found in the teat of the udder are those which 

 produce lactic fermentation. He quotes BoUey and Hall as finding 

 twelve out of sixteen species in the teat of the udder to be lactic acid 

 producers.* Veranus Moore has arrived at very similar results.^ 

 EoUin Burr has recently investigated this subject with a different 



* Outlines of Dairy Bacteriology, H. L. Russell, 1898, p. 43. 

 t Twelfth and Thirteenth Reports of the Bureau of Animal Industry, U.S.A., 1895 

 and 1896, p. 265. 



