353 
According to this author the action of rennin consists not only in 
the splitting of casein ogen into paracasein and whey proteid, but also 
in the conversion of other milk proteids into more soluble form. He 
seems to think that under favorable conditions paracasein may be 
formed from the albumin as well as from the caseinogen. 
PART II (4a) CHEMICAL CHANGES IN MILK PRODUCED BY BACTERIA 
AND VARIOUS OTHER MICRO-ORGANISMS. 
The more obvious changes in milk with which we are familiar 
are those that are brought about by bacteria and various other 
micro-organisms. Among these changes may be mentioned: The 
ordinary souring and curdling of milk, with the production of lactic 
acid as the chief product; the production in milk of various odor- 
iferous substances, or highly flavored substances, many of a some- 
what disagreeable character, good examples being met with in the 
ripening of cream and cheese; the production of colored substances 
which impart to the milk unusual colors, such as the formation of 
blue milk; the formation of mucilaginous, or mucin-like substances, 
which serve to impart to the milk a characteristic ropiness, known 
as ropy milk, and finally we must include under this head those 
bacterial changes in milk which result in the formation of poisonous 
substances, such as tyrotoxicon, toxins, etc. 
The lactic acid fermentation of milk . — The lactic acid fermentation 
is the commonest and best known of all the many bacterial changes 
that occur in milk. The fact that on standing at ordinary tempera- 
tures milk gradually turns sour and finally curdles has been known 
ever since milk was first used as a food by man. In early times the 
acid of milk was / supposed to be acetic acid, the same as is present 
in vinegar, and as has already been pointed out this acid does, accord- 
ing to Bechamp (1), occur in even freshly drawn milk in small quan- 
tities. The substance really responsible for the souring of milk, viz, 
lactic acid, was first discovered in milk by Scheele in 1780. The 
new acid was also studied by Berzelius and its composition definitely 
established through the work of Mitscherlich and Liebig in 1832. 
Its chemical constitution and its relation to other varieties of lactic 
acid, occurring in nature or the products of chemical synthesis, 
were first established as the result of the labors of Strecker, Erlen- 
meyer, and Wislecenus. 
In 1847, Blondeau (2) discovered micro-organisms is sour milk, but 
attached to these no particular significance so far as the souring of milk 
is concerned. It remained for Pasteur (3), in 1857, to definitely and 
conclusively show as one of the results of his classic investigations on 
fermentation that the souring of milk is really a kind of fermentation, 
24907— Bull. 41—08 23 
