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slightly increased. It was further observed by this investigator 
that work exerts a harmful influence on the quality of the milk, 
especially on its keeping qualities. For example it was found in 
the case of a certain cow that ordinarily her milk curdled when the 
acidity reached T0°-T5°, whereas the milk of the same cow when 
fatigued by work, curdled when the acidity reached 45°. Accord- 
ing to Moerman (78) also, work lessens the amount of milk secreted 
and raises the proportion of solids. The differences, however, in 
the quantity and quality of the milk in all of these investigations 
were only slight, indeed in some instances the results obtained were 
not very definite. 
PART II.— CHANGES IN THE COMPOSITION OF MILK. 
On account of the milk sugar and proteids which milk contains, it 
is an exceedingly unstable liquid. When first drawn from the cow, it 
has a characteristic odor and a sweet taste. Even in the perfectly 
fresh state, it reacts acid to phenolphthalein. The acidity of fresh 
milk is due primarily to carbonic acid and acid phosphates and also 
in part to dicalcium caseinogenate. According to Thorner (1) the 
acidity of fresh milk varies between 12 and 16 degrees. According to 
Richmond (2) it has an acidity of 20 degrees. On standing exposed 
to the air for some time it gradually loses its sweet taste. The sugar 
of milk is gradually transformed into lactic acid through the action 
of bacteria. The milk becomes sour to the taste and ultimately clots 
or curdles as the result of the precipitation of the caseinogen by the 
combined action of acids and soluble calcium salts. Stokes (3) gives 
figures and tests to show that milk having an average acidity of 44 
degrees, corresponding to 0.396 per cent of lactic acid, tastes sour. 
According to Richmond (2) milk tastes sour when the acidity reaches 
45 degrees, corresponding to 0.405 per cent lactic acid, and when it 
has an acidity of 85 degrees, equivalent to 0.765 per cent of lactic 
acid, it curdles at ordinary temperatures. 
Under certain conditions, milk may also develop rancid and cheesy 
odors which render it quite disagreeable. 
The principal changes occurring in milk are those produced by — 
(1) The action of heat and acids. 
(2) The action of milk enzymes. 
(3) The action of the digestive enzymes. 
(4) Bacteria and various other micro-organisms. 
Paet II. — (1) Changes in Milk Produced by the Action of Heat and Acids. 
When milk is heated a film or skin forms on the surface, which, 
according to Jamison and Hertz (4), is due to the drying and coagu- 
lation of a part of the proteids which the milk contains. They 
have shown that such a skin may be formed on the surface of any 
