226 OUTLINES OF BACTERIOLOGY 



added, after which the cream is kept for 24 hours at about 15° C. This 

 secures the multiplication of the inoculated organism, and in 24 hours 

 the starter, as the cream is now called, contains many millions of the 

 inoculated bacteria, and is ready to be added to the bulk of the cream. 

 The latter is prepared for the inception of the starter by having been 

 previously cooled to a low temperature to weaken the bacteria con- 

 tained in it. Just before the inception of the starter, the bulk of the 

 cream is quickly heated up to 15°-20°C. After adding the starter, 

 fermentation is allowed to proceed at 15°-20°C. till the following day, 

 when the cream is ready for churning. 



Several modifications of this method are in use, but in principle they 

 are all the same. They differ only in the methods employed for the 

 weakening of the undesirable bacteria before the introduction of the 

 pure culture. 



Centres like the Kilmarnock Dairy School distribute starters to 

 various parts of the country in the form of a dry powder, containing 

 the microbe which it is desired to introduce into the cream. 



III. Pure Culture Method regulating the Aroma of Butter. The 

 commercial value of butter chiefly depends on its aroma. In this 

 method the souring is left more or less to chance, but care is taken to 

 introduce into the ripening cream an organism which is known to act 

 on the albuminoid constituents of cream, producing a substance which 

 gives the butter a fine aroma. One of these organisms has been 

 isolated by Conn and is known as Bacillus No. 41. The method of 

 ripening when this bacillus is employed is as follows : Six quarts of 

 cream are pasteurised (by heating at 155° F.), and then cooled. A 

 pellet containing Bacillus No. 41 is thrown in, the cream being then 

 set aside in a warm place (70° F.). After being allowed to ripen for 

 two days, the cream, containing now many millions of the progeny of 

 Bacillus No. 41, is added to 25 gallons of ordinary cream, which in its 

 turn is allowed to ripen in the same way. This ripened cream is used 

 as a starter to the large cream vats in the proportion of one gallon of 

 starter to 25 gallons of unripened cream. The essential point to note 

 in connection with this method is, that the bulk of the cream is not 

 pasteurised or treated in any way so as to weaken the bacteria 

 contained in it. As the starter does not contain a lactic-acid pro- 

 ducing microbe, the souring must be left to the bacteria contained 

 in the main bulk of the cream. This method naturally increases the 

 risk of a bad ripening, but seems to work well in practice. 



