228 AGRICULTURAL BACTERIOLOGY. 



chief purpose of ripening is to obtain the butter flavor. It is 

 evident that the flavor is due to the products of the bacterial 

 growth during ripening. The enormous multiplication of bac- 

 teria must produce a considerable number of chemical changes 

 in the cream, the nature of which has been only partly deter- 

 mined. One change is, of course, the formation of lactic acid. 

 A second class of changes produces products with high flavors 

 and odors. : Of the nature of these flavoring products we 

 know at present practically nothing. They are certainly 

 something different from the acid, since lactic acid has no 

 odor, and its flavor, being simply sour, is not the flavor of 

 ripened cream or butter. We do not even know whether they 

 are the by-products of chemical decomposition or are excretions 

 from the bacteria. But whatever be their nature, and whether 

 they be by-products or excretions, they are present in small 

 amount only, though strong enough to impregnate the butter 

 with the characteristic butter flavor. 



THE EFFECT OF DIFFERENT SPECIES OF BACTERIA. 



We have already noticed that different species of bacteria 

 produce very different types of chemical decomposition in 

 milk. All of the varieties of bacteria which get into the milk 

 are likely to get into the cream. When cream is collected for 

 a large creamery and comes from many patrons, there are sure 

 to be in it large numbers of different varieties of dairy bacteria. 

 Each patron contributes his quota, and the resultant mixed 

 cream is found to contain many different species. Each of 

 these may be expected to have its effect upon the cream dur- 

 ing the ripening, and the resulting butter will have a different 

 character where the cream is ripened with different bacteria. 

 Indeed, actual study shows that different species of bacteria, 

 when allowed to grow in the ripening cream, produce very dif- 

 ferent types of butter. Some species produce bitter butter, 

 others tainted butter, others insipid butter and others a strong 



