338 MICROBIOLOGY OF MILK AND MILK PRODUCTS. 



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

 use of the separator. 



SPONTANEOUS RIPENING OF 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 is often 

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

 taneously 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 bacteria, the 

 product was likely to be of good quality, but such ideal conditions did 

 not always obtain. Cream containing a large proportion of harmful 

 bacteria, or in an advanced state of fermentation, or possessing an unde- 

 sirable 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 microbiology 

 progressed and the role of microorganisms in all kinds of fermentation 

 became known, it was evident that the control of the causal organism is 

 an important factor in determining the quality of any product 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 fer- 

 mentation went on more rapidly and in more definite directions than 

 without such additions, as the bacteria added were largely of the desirable 

 group, Bact. lactis acidi. The addition of fermented milk to accelerate 

 the souring of cream antedates by many hundred years the science of 

 bacteriology. 



The next logical step in the development of the process was the use of 



