26 PRINCIPLES AND PRACTICE OF MILK HYGIENE 
The standard of the United States Department of 
Agriculture calls for 3.25 per cent. of fat and 8.5 per 
cent. solids not fat for milk; 9.25 per cent. solids for skim 
milk, and 18 per cent. fat for cream. The standards for 
milk and other dairy products adopted by the various 
states and territories, as reported by the United States 
Bureau of Animal Industry, will be found on pages 
24 and 25. 
Reaction.—The reaction of milk is amphoteric to lit- 
mus—t.e., it turns blue litmus red (acid monobasic phos- 
phates) and changes red litmus to blue (alkaline di- 
basic phosphates). To phenolphthalein solution it is 
acid. When phenolphthalein solution is added to milk, 
no color reaction occurs, because the color of phenol- 
phthalein solution is not changed by acids. But if, after 
the addition of phenolphthalein solution, sodium hydrox- 
ide solution is added to the milk in excess of the amount 
necessary to neutralize the acidity the fluid assumes a 
pink color, which is permanent. This reaction is made 
use of in determining the degree of acidity of milk and 
cream. 
To neutralize the acidity in 100 c.c. of normal, fresh 
market milk, 18 to 19 ¢.c. of a one-tenth normal solution 
of sodium hydroxide are required. This represents an 
acidity of 0.16 to 0.17 per cent. The acidity of milk 
when it is drawn from the udder is less than 0.1 per cent. 
This original or native acidity is due to the casein and 
acid phosphates. The fermentation of the milk sugar 
by bacteria increases the acidity. Lactic or other acids 
formed in this manner are present whenever the acidity 
is over 0.1 per cent. Market milk with an acidity of 0.1 
to 0.2 per cent. is considered fresh and good. In some 
