BACTERIA AND ENZYMES IN AGRICULTURE 275 



the cream is separated from the milk by centrifugal action in 

 suitable machines, is the most rapid, and consequently the 

 least liable to infection. If, at the same time, a low tempera- 

 ture is maintained, the danger of uncertainty in the subsequent 

 souring of the cream is still further reduced. 



If cream is churned in a perfectly fresh state, sweet 

 butter is obtained, which is somewhat tasteless. By careful 

 souring of the cream previous to churning, butter of a more 

 defined flavour is produced ; it is in the control of this flavour 

 that bacteria play their part. 



In following the changes which they bring about, it must 

 be understood that the cream, as used for churning, will 

 contain not only the fat of the milk, but also a certain 

 quantity of its other constituents. The composition of the 

 butter is conditioned therefore by the method used for the 

 collection of the cream. 



The souring or ' ripening ' of the separated cream may be 

 effected by the addition of a small quantity of sour milk, 

 which will contain the necessary bacteria, notably the lactic 

 acid bacteria. Satisfactory ripening of the cream can gener- 

 ally be judged by practice ; chemical examination of such 

 cream should not discover any appreciable quantity of 

 casein. Its presence would point to the souring process having 

 gone too far, resulting in the production of a certain amount 

 of clotting of the buttermilk present. Working in this 

 manner, and with careful attention throughout to the avoid- 

 ance of infection by deleterious bacteria, excellent results are 

 obtainable so long as the conditions of milk supply and the 

 general control of the dairy remain unchanged. An element 

 of uncertainty, however, still exists, and the main reason for 

 the greater sale of Danish butter, compared with that produced 

 in England and Ireland, is its constancy of quality. 



This constant quality has been attained by still further 

 development in the control of the souring process. 



Such absolute control is obtained when, in the first place, 



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