in] THE CONSTITUENTS OF MILK 25 



melting-point of butter, are liquid at ordinary 

 temperatures. Olein, indeed, only becomes solid at 

 the freezing-point of water. Since the proportion in 

 which the nine fats are present in milk-fat is a vari- 

 able one, its melting-point is also variable, and may 

 be stated at between 29 and 41 C. (84 to 106 

 Fahr.) The nature of the fat is largely determined, 

 however, by the relative percentage of stearin, 

 palmitin, and olein. 



The fatty acids which go to form the above fats 

 may be divided into two classes, viz. those which 

 are insoluble and non- volatile and this class is by 

 far the largest in amount, and includes palmitic, 

 stearic, oleic, butic, and myristic acids and those 

 which are soluble and volatile. The respective 

 amounts of these two classes of fatty acids in milk- 

 fat, as we have just indicated, varies at different 

 times. Among the conditions which influence their 

 amount is the stage of lactation, the age, breed, and 

 feeding of the cow. The percentage of volatile fatty 

 acids was found by Duclaux in nineteen samples of 

 butter-fat to vary between 5*77 and 7*95 per cent, 

 the average being 7*3. They consisted almost 

 entirely of butyric (4'58 per cent) and capronic 

 (2'7 per cent) acids. The presence of these volatile 

 acids is of importance, as it affects the quality of the 

 milk-fat and influences its flavour. It may be said 

 that the higher the percentage of fatty acids in the 



