359 
exercised toward the ferment itself or whether they react on the milk 
or participate only in the second phase of the rennin coagulation. 
The further consideration of such agents is beyond the scope of this 
j communication. Before leaving the subject, however, it should be 
j observed that Hillman (41) has studied the rennin coagulation of 
I milk in its practical aspects. This author has found that the milk 
I of fresh cows is better suited to the rennin coagulation than the milk 
of cows which are nearly dry. In his opinion this is probably to be 
explained by the diminution in the calcium content of milk during 
• the period of lactation. He found, further, that the degree of acidity 
j of milk in relation to the calcium content is an important factor. 
According to this author a high calcium content and high acidity 
; prevail at the beginning of lactation and are usually accompanied 
[ by high total albumin and a high caseinogen content, all of which 
5 conduce to a large yield of paracasein. He also finds that the time 
i of coagulation and the yield of paracasein are independent of one 
another; generally, however, a short coagulation time and a large 
yield of paracasein are associated. Strong dilution of the milk 
with water tends to diminish the yield of paracasein, whereas the 
addition of soluble calcium salts tends to increase it. 
i According to this author the action or rennin consists not only in 
' the splitting of caseinogen into paracasein and whey proteid, but also 
j' 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. 
j ^ 
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 or highly flavored substances, many of a somewhat dis- 
agreeable character, good examples being met with in the ripening of 
cream and cheese; the production of colored substances which im- 
part 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 
i 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 
I ti 
I 
