XV. MILK AND MILK PEODUCTS 337 



tion. At first, the increase in size of the fat particles and their 

 irregular shape give rise to increased viscosity (this phenomenon is 

 sometimes known as " going to sleep ") ; but as the particles grow larger 

 they tend to separate more completely from the butter-milk and float, 

 the contents of the churn becoming mobile. The butter grains are then, 

 by working, pressed together, and more and more of the butter-milk is 

 separated from the fat. In order that the amount of liquid retained by 

 the butter may be small, it is necessary that the temperature should be 

 carefully adjusted. The optimum temperature, however, depends 

 partly upon the temperature at which the cream has been for some 

 time prior to the churning and the rapidity with which it has been 

 raised or lowered. Eichmond gives the following as most suitable : 



Recently separated cream (quick churning) . . . 8 C. 



(slow churning) . . . 13 C. 



Spur cream (in summer) ...... 13 C. 



,, (in winter) 18 C. 



Fleischmann recommends 13 for sweet'cream, 16 for sour cream. 



If churned at too high or too low a temperature, the butter contains 

 a higher proportion of water. According, however, to American ex- 

 periments, the optimum temperature for churning varies with the breed 

 of cows and also with their food, being higher when cotton seed or 

 cotton-seed meal is used. It seems to be generally admitted that 

 "ripened," i.e., sour, cream, gives a higher yield of butter and churns 

 more readily than sweet cream. 



Opinions greatly differ as to the relative quality of butter from 

 sweet and from ripened cream, but it is generally agreed that the best 

 flavour and aroma in butter can only be obtained from the use of 

 properly ripened cream (vide infra), though disagreeable flavours are 

 also liable to be produced owing to the products of undesirable micro- 

 organisms. Moreover, if the cream be very sour, and especially if it 

 be sour before removal from the milk, the resulting butter will prob- 

 ably contain a large amount of casein, and, on this account, will more 

 readily become rancid. 



Salt is usually added to butter, both as a condiment and also to 

 check decomposition. The amount used varies greatly, from a mere 

 trace up to 7 per cent being found. 



It is difficult to give any average composition of such a variable 

 product ; usually it varies between the following limits : 



Per cent. 

 Fat , 78-0 to 94-0 



Water 

 Casein 

 Ash . 

 Sugar 



5-0 ,, 16-0 



0-5 ,, 3-0 



0-1 4-0 



0-2 ,, 0-7 



The butter from ripened cream is usually richer in casein and 

 water than that from fresh cream. It is generally stated that salt 

 butter contains a higher proportion of water than fresh, but according 

 to Eichmond this is not so ; although salt butter appears to be wetter 

 and, on being cut, allows brine to flow out, thus giving it a wet appear- 



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