38 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
Peroxydase.—If paraphenyldiamin or tincture of 
guaiac is added to milk with a little hydrogen peroxide, 
the milk at once assumes a blue color. This change 
occurs because the ferment contained in the milk called 
“ peroxydase” splits off oxygen from the hydrogen 
peroxide and this free oxygen oxidizes the paraphenyl- 
diamin or the guaiac to a colored compound. If the milk 
is heated to 80° C. (176° F), the reaction does not occur 
because the ferment is destroyed. The reaction occurs 
best at 40 to 50° C. (104 to 122° F.). Peroxydase is 
present in milk when it is formed in the udder and it is 
not secreted by bacteria. It is therefore an original 
ferment. (See tests for heated milk on page 298.) 
Catalase-—This ferment, which is also known as 
superoxidase, possesses the specific property of splitting 
up hydrogen peroxide into water and oxygen. The re- 
action which occurs is as follows: 2H,O2. = 2H.O + O2. 
Catalase is both an original and a bacterial ferment. It 
is secreted with the milk and is contained in leucocytes 
and in blood. It is also secreted by many of the bacteria 
found in milk, but the various species differ in their 
capacity to produce the ferment. The putrefactive or- 
ganisms appear to produce it in the greatest quantity. 
The ability of the lactic acid bacteria to produce catalase 
is in dispute. 
The amount of catalase in milk as it comes from the 
udder varies at different stages of lactation. The cata- 
lase content is high during the colostral period and this 
condition usually continues for three weeks, although in 
exceptional cases it falls to the amount normal for milk 
by the fourth or fifth day after parturition. Near the 
end of lactation, when the milk has fallen to about a 
quart per day or less, the catalase again increases. Some 
