108 DAIRY BACTERIOLOGY 



of acidity is inversely proportional to the fat percentage. The 

 consistency is a far better guide than the acidity, especially in 

 view of the difficulty in measuring an exact amount of the thick 

 cream, and in determining the end point in the titration. As soon 

 as the cream has the proper consistency, the souring should be 

 stopped by churning and washing out the bulk of the lactose, or, 

 if churning cannot be proceeded with immediately, by thorough 

 cooling. 



In order to promote the souring, a Lactic Acid Culture or Starter 

 may be added. In this way the first stage of the souring may be 

 shortened, i.e., the stage during which many different kinds of 

 milk bacteria develop ; although some of these may tend to 

 improve the aroma of the butter, most of them will in all proba- 

 bility do harm. The final stage of the souring will also be 

 shortened, especially if an over-sour starter, such as buttermilk, is 

 used. Buttermilk is probably the oldest form of starter used, 

 and if it originates from butter of good aroma, and has not been 

 diluted with bad water, it will impart a good aroma to the product ; 

 as a rule, however, the development of harmful organisms will be 

 encouraged and butter defects will be perpetuated by this method. 

 A safer method is to let extra good milk sour spontaneously, using 

 it as a starter when it has reached the right stage ; this method 

 became prevalent before the so-called pure cultures were put on 

 the market. The introduction of these cultures was due to the 

 initiative of Storch, who found that different species or strains of 

 lactic acid bacteria produced very different odours or tastes in 

 the soured products 1 . Among other lactic acid bacteria, Storch 

 isolated a stout streptococcus, " No. 18 " (Fig. 38), which produced 

 a particularly fine aroma. This particular strain was not used in 

 actual practice to any extent, but already in the same year the 

 firm Blauenfeldt and Tvede put cultures on the market, and others 

 soon followed their example. All these cultures which are pro- 

 pagated in the best pasteurised milk, generally contain several 

 different species of lactic acid bacteria, and very probably they 

 owe their good qualities to this very fact, for a higher degree of 

 acidity is obtained by the co-operation of several species than by 

 the action of one species alone, while lactic acid bacteria in pure 

 cultures are apt to degenerate or become slimy. As a rule, 

 diplococci are met with among the stout streptococci (Strepto- 

 coccus cremoris). Boekhout 2 and Storch 3 have described anon- 

 souring or slightly souring streptococcus which appears to con- 



1 " Forsogslaboratoriets," 18 de Beretning, 1890. 



2 " Vereeniging tot Exploitatie eener Proefzuivelboerderij te Hoorn," 

 Verslag over det jaar, 1917. 



3 Researches carried out some years before his death, still unpublished. 



