3 2 4 THE CHEMISTRY OF DAIRYING 



To effect these changes is, of course, the object of churn- 

 ing. Provided the fats are solid, the softer they are the more 

 readily will they cohere. Within limits, therefore, the butter 

 " comes " more quickly when the temperature is raised. The 

 most suitable temperature for churning depends upon the size 

 of the globules, the acidity of the milk or cream, and other 

 conditions, but usually it is between 55 and 60 F. 



At higher temperatures (84 to 106 F.) the fats would 

 again turn liquid. When the temperature is too high, but 

 considerably below the melting-point of the fat, the butter is 

 formed very rapidly, but is apt to have a greasy as opposed 

 to a granular consistency. 



The rise of temperature in the churn is probably not 

 wholly accounted for by the conversion of the latent heat of 

 the fat into thermic energy. A certain amount of heat must 

 be produced by friction of the contents of the churn. Some 

 chemical changes, probably, also occur, and a considerable 

 quantity of carbon dioxide is evolved. The thorough aeration 

 of the liquid is doubtless also favourable to the action of the 

 lactic ferments, and the milk or cream, even if sweet to com- 

 mence with, becomes perceptibly acid. 



A certain degree of acidity of the milk or cream is found 

 lo be favourable to the rapid formation of butter. The acid 

 tends to coagulate the casein and so reduces its emulsifying 

 power. This condition, it has been said, retards the formation 

 of cream, but as the fat globules are disengaged by the agita- 

 tion of churning, it favours the formation of butter. For this 

 reason, the milk or cream is generally allowed to " ripen " 

 before churning. 



Butter Milk. It is impossible in practice to recover the 

 whole of the fat in the cream as butter. The amount of fat 

 which remains in the butter milk is a test of the efficiency with 

 which the process of churning has been carried out. Rich- 

 mond has found from o'i5 to 5*6 per cent, of fat in butter 

 milk ; with average good working it should not contain more 

 than about one per cent. As a matter of fact, it is generally 

 less when derived from ripe cream. The loss in churning 

 sweet cream is usually somewhat larger. 



