110 DAIRY BACTERIOLOGY 



last stage by the use of commercial starters ; naturally both of 

 these factors have an influence on the aroma of the butter. The 

 most aromatic butter, that from Isigny, is still made by the old- 

 fashioned methods from very over-ripe cream in which aroma - 

 producing yeasts have been found. At the same time, the aroma 

 depends on the nature of the feed as well as on the action of micro- 

 organisms ; very probably it is the resultant of the combined 

 influence of both factors. Thus it is well known that grass butter 

 has a far more pronounced aroma than winter butter, and there 

 can hardly be any doubt that the fine aroma of Isigny butter is 

 largely attributable to the excellent Norman pastures. In this 

 connection, however, it should be pointed out that the aromatic 

 substances originating from the feed do not always pass unchanged 

 into the milk (whether through the udder or by way of the 

 manure) ; the most usual course of events is the secretion of 

 complex substances, which only give rise to aromatic products on 

 hydrolysis. Microorganisms which can effect these hydrolyses 

 will thus be aroma-producing in milk or cream which contains the 

 substances in question, but not necessarily in any milk or cream. 

 Several butler aroma bacteria (mostly belonging to the. Coli group) 

 have already been isolated and recommended for use in con- 

 junction with lactic acid bacteria in the ripening of cream, and 

 the fact that none of them have attained to any practical signi- 

 ficance is probably to be attributed, to the circumstances just 

 mentioned. At present we have only certain lactic acid bacteria, 

 producing a faint aroma, which can invariably be used with 

 advantage ; the above-mentioned discovery of an organism which 

 might be used in conjunction with these in order to increase the 

 aroma may be of great practical value. Experience shows, 

 however, that butter which has a pronounced aroma does not 

 generally keep well, and it will always be more to the advantage 

 of the producer to make sure of the keeping qualities than to 

 concentrate attention on the aroma, which, after all, is a matter of 

 local or even only temporary preference as far as the public is 

 concerned, and this can be influenced to a certain extent by the 

 large producers. 



We will now pass on to the detailed discussion of the ripening 

 of cream by modern methods. The first question of importance 

 is the fat percentage of the cream. As the lactic acid bacteria do 

 not act on the butter -fat, they must necessarily produce the 

 characteristic butter aroma and acid from the other constituents 

 of the milk. Seeing that the aroma is produced outside the fat 

 globules, one might suppose that it would be washed out as easily 

 as the lactic acid, but this is not the case. Thus Isigny butter 

 has a stronger aroma than any other make, but its process of 



