RELATION OF MICROORGANISMS TO BUTTER 477 



milk, since the immense number of fat globules passing through the 

 milk serum carry mechanically a considerable proportion of the bacteria 

 of the milk into the cream. This phenomenon is to be noted in gravity 

 creaming, but to a much greater extent in the removal of the cream by 

 use of the separator. 



SPONTANEOUS RIPENING or CREAM. By this expression is meant 

 the fermentation of the cream by those acid-forming bacteria that have, 

 from one source and another, gained entrance to it, but which have not 

 been intentionally added. Under these conditions the butter-maker can 

 exert but little control over the fermentation. A very considerable part 

 of the butter made from such cream has an excellent flavor, because at 

 the temperature at which cream is usually kept, Bact. lactis acidi and 

 related organisms are the primary factors concerned in its fermentation 

 and their by-products produce desirable flavors in butter. It has often 

 been asserted that the highest type of butter can be made only from 

 spontaneously ripened cream. 



As the cream from many farms was assembled at a creamery for the 

 manufacture of butter, it became evident that some means of controlling 

 the type of fermentation in the cream was needed. If the milk had been 

 produced under clean conditions, and had been received at the creamery 

 before the acid fermentation had gone on to any extent, and if the 

 cream was then kept at temperatures most favorable for the lactic bac- 

 teria, the product was likely to be of good quality, but such ideal condi- 

 tions did not always obtain. Cream containing a large proportion of 

 harmful bacteria, or in an advanced state of fermentation, or possessing 

 an undesirable flavor was often received, and the butter-maker could 

 not control the quality of the product under such conditions. 



USE OF CULTURES IN BUTTER MAKING. As the science of micro- 

 biology progressed and the role of microorganisms in all kinds of fermen- 

 tation became known, it became evident that the control of the causal 

 organism is an important factor in determining the quality of any prod- 

 uct of the fermentation industries. In the manufacture of butter, the 

 first step in this direction was the addition of some fermented milk, 

 cream, or of buttermilk to the cream to be ripened. In this manner the 

 number of acid-forming organisms in the cream was greatly increased, 

 and the fermentation went on more rapidly and in a more definite direc- 

 tion than without such additions, as the bacteria added were largely of 

 the desirable group. Bad. lactis acidi. The addition of fermented milk 



