264 MILK 



since diseases have also been spread by milk products, and by- 

 preventing infectious material from entering milk, or destroying 

 disease germs by application of heat, the menace from dairy prod- 

 ucts is reduced. 



Micro-organisms in milk produce profound changes in its 

 constituents, and these changes become manifest, shortly after 

 milking; the larger the number of organisms present, the sooner, 

 as a rule, does decomposition commence. Milk proteins are 

 broken down by proteolytic bacteria and by molds, the products 

 being similar to the proteolytic changes of other proteins. 

 Caseoses, caseones, amino-acids, ammonia, hydrogen sulphid, 

 phosphoric acid, and gases are some of the bacterial products. 

 Lactalbumin is broken down by bacteria into albimaoses, peptones, 

 amino-acids, etc. Milk-sugar yields lactic acid, volatile acids, 

 alcohol, gases, etc. Milk-fat is not readily decomposed, but a 

 few types of bacteria that have the ability to decompose fat oc- 

 cur quite commonly in milk. However, these fat-sphtting organ- 

 isms are usually held in check by other ones, chiefly the lactic- 

 acid-producing bacteria, so that decomposition of milk-fat is a 

 slow process, and does not become noticeable, as a rule, for many 

 days. 



The nature of decomposition of milk depends largely upon the 

 predominant kinds of organisms present and the temperature at 

 which the milk is kept. At low temperature a different result 

 obtains from room temperature, and at 37° C, or higher still, 

 other products appear. At room temperature milk undergoes 

 changes which are fairly uniform, although exceptions are not 

 wanting. These changes are sometimes called "spontaneous" 

 or "normal." Both these terms are not strictly correct and are 

 apt to give misleading impressions. The term "spontaneous" 

 is not correct, because milk will not decompose unless micro-organ- 

 isms are present. It is true that market milk without micro- 

 organisms does not exist, but in its true meaning "spontaneous" 

 suggests decomposition without the presence of a foreign agency. 

 The term "normal" is perhaps more appropriate, since the bac- 

 terial flora of milk almost invariably includes lactic acid bacteria, 

 and these multiply in milk at such a rate as to outstrip other 

 organisms in the large majority of instances. 



The "normal" changes taking place in milk when it stands 

 at room temperature take a fairly definite course and represent 

 periods which can be observed with regularity. These periods 

 are, of course, not distinctly separated from each other, but for 

 convenience may be divided into five phases. 



The first phase is the so-called incubation period of milk. 

 The changes in milk during this period are relatively insignificant. 



