323 
PART 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 an} r 
albuminous solution containing fat or paraffin. liettger (5) also 
has arrived at the conclusion that its formation is dependent on the 
presence of proteid. This proteid is caseinogen. Surface evapora- 
tion and the presence of fat facilitate its formation although neither 
is absolutely essential. According to Harris (6), also, the scum of 
boiling milk consists very largely of caseinogen. It is also well 
known that certain changes occur in the odor and taste of milk as 
the result of boiling. These changes seem to be due to the partial 
decomposition of certain of the proteids with the liberation of a 
volatile sulphide, probably hydrogen sulphide. That such is the case 
has been proven by Rettger (7), and also by Franz Utz (8). Accord- 
ing to the former, when milk is heated to 85° C., a volatile substance, 
probably hydrogen sulphide, is liberated. The amount of this, 
though small, suffices to blacken lead acetate paper and to decolorize 
dilute solutions of potassium permanganate. He found that alkalis 
and alkali phosphates accelerate the formation of the sulphide, 
whereas acids and acid phosphates retard this change. According 
to this author this change, is believed to indicate proteid decomposi- 
tion, and may partly account for what some observers describe as 
the injurious effect of heating milk. These observations have been 
confirmed by Utz (8), who was able to recognize the hydrogen 
sulphide resulting from the boiling of milk by lead acetate paper 
and also by Ganassini’s reagent. 
When milk is boiled there seems also to be a partial fixation of the 
calcium salts which it ordinarily contains. These are probably par- 
tially precipitated in the form of tricalcium phosphate. This would 
account for the fact that the coagulation of milk by rennin takes 
place more slowly in boiled milk than in unboiled milk. (See. p. 332.) 
In this connection Wassermann and Schiitze (9) have pointed out 
that cooked milk is not coagulated by lactoserum. According to 
P. T. Muller (10) the fact that cooked milk can not be coagulated by 
lactoserum is in some way associated with a diminution in the quan- 
tity of soluble calcium salts contained in the milk, this diminution 
having been caused by the action of heat. On the other hand, both 
Moro and Muller (10) have observed that certain milks do not show 
any diminution in coagulability by lactoserum after boiling. 
According to Muller (10) this is to be attributed to the large amount 
of soluble calcium salts present in the milk of certain particular 
