MILK 203 



ary causes of milk souring are known except that of micro-organ- 

 isms. Experiments on the relation of electricity to milk have verified 

 the conclusion that electricity has no direct influence in producing 

 the souring of milk. The only recent work on this subject has been 

 that of Liebig, whose conclusions are that the souring of milk dur- 

 ing a thunderstorm is simply due to the rapid growth of bacteria, 

 produced by the same conditions which produce the thunderstorm. 

 (See Effect of Thunderstorms on the Souring of Milk.) 



Butyric Acid. We know little in regard to this type of fer- 

 mentation. Butyric acid fermentation is a very common type, At 

 the present time about a dozen organisms are known to produce this 

 fermentation and have been described. There are many others 

 which have been less thoroughly studied, but which produce butyric 

 acid as a by-product. They have been found in various milk products 

 (milk, butter, and cheese) and in other places in nature as well. 



The butyric-acid fermentation does not appear to be so simple 

 a one as the lactic fermentation. In the lactic fermentation we have 

 what we regard as nearly a smooth splitting of the milk-sugar mole- 

 cule into carbonic acid and lactic acid. It is true that there are 

 always produced other by-products, and these sometimes in consid- 

 erable amount, but these secondary products are not very important 

 in the lactic type of fermentation. In the case of butyric acid, how- 

 ever, the reverse is the case. Butyric acid is to be regarded prob- 

 ably in all cases as an end product of a long series of fermentative 

 changes. It is always accompanied by many by-products, and 

 usually the butyric acid is slow in its appearance and is not the chief 

 product in the fermentation. (U. S. Dep. Agr. 0. E. S. B. 25.) 



Flavors in Milk. Milk may acquire abnormal flavors and odors 

 in various ways: 



(1) The cow may, through some pathological condition, pro- 

 duce milk with an unusual flavor. This may occur when the cow 

 shows no outward sign of disorder and usually last for a short time 

 only. 



(2) Highly flavored foods may impart their peculiar flavors 

 to the milk. The disagreeable results of feeding even small quan- 

 tities of wild onion, turnips, and similar feeds are unfortunately too 

 familiar to need comment. Other feeds with a less pungent taste 

 no doubt affect the flavor of the milk to a less degree. 



(3) Milk, especially warm milk, takes up the odors and flavors 

 of the surrounding air with great rapidity. The flavor thus ac- 

 quired may be so slight that it ordinarily passes unnoticed, or it may 

 be so pronounced that anyone may recognize its source. 



(4) The flavor of milk may be materially changed by the 

 growth of bacteria, with the infinite variety of by-products which 

 result from their development. If milk is sterilized and then inocu- 

 lated with some one kind of bacteria, a certain flavor, frequently 

 very pronounced, will result; and under the same conditions this 

 particular variety will always produce the same flavor. Another 

 kind may produce an equally pronounced but entirely different 

 flavor, while some species may grow for a long time without causing 



