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 i.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.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 



