244 BACTERIA IN MILK AND MILK PRODUCTS 



work and influence of Professor Storch in Denmark, which is based 

 upon our new knowledge respecting bacterial action in cream- 

 ripening. We refer to the artificial processes of ripening set up by 

 the addition of pure cultures of favourable germs* If a culture of 

 organisms possessing the faculty of producing in cream a good flavour 

 be added to the sweet cream, it is clear that advantage will accrue. 

 This simple plan of starting any special or desired flavour by intro- 

 ducing the specific micro-organism of that flavour, may be adopted 

 in two or three different ways. If cream be inoculated with a large, 

 pure culture of some particular kind of bacteria, this species will 

 frequently grow so well and so rapidly that it will check the growth 

 of the other bacteria which were present in the cream at the com- 

 mencement and before the "starter" was added. That is, perhaps, 

 the simplest method of adding an artificial culture. But secondly, 

 it will be apparent to those who have followed us thus far, that if 

 the cream is i[)veYio\islj pasteurised at 70° C, these competing bacteria 

 will have been mostly or entirely destroyed, and the pure culture, 

 or " starter," will have the field to itself. There is a third modifica- 

 tion, which is sometimes termed ripening by natural starters. A 

 " natural starter " is a certain small quantity of cream taken from a 

 favourable ripening — from a clean dairy or a good herd — and placed 

 aside to sour for two days until it is heavily impregnated with the 

 specific organism which was present in the whole favourable stock 

 of which the " natural starter " is but a part. It is then added to 

 the new cream, the favourable ripening of which is desired. Of the 

 species which produce good flavours in butter, the majority are found 

 to be members of the acid-producing class; but probably the flavour 

 is not dependent upon the acid. The aroma of good ripening is 

 also probably independent of the acid production. 



Artificial Ripening. — Of all the methods of ripening — natural 

 ripening, the addition of "natural starters," the addition of pure 

 cultures with or without pasteurisation — there can be no doubt that 

 pure culture after pasteurisation is the most accurate and reliable. 

 The use of " natural starters " is a method in the right direction ; 

 yet it is, after all, a mixed culture, and therefore not uniform in 

 action. In order to obtain the best results with the addition of 

 pure cultures. Professor Eussell has made the following recom- 

 mendations : — 



1. The dry powder of the pure culture must be added to a small 

 amount of milk that has been first pasteurised, in order to develop 

 an active growth from the dried material. 



2. The cream to be ripened must first be pasteurised, in order to 



* Such pure cultures for such purposes are in the United States termed 

 " starters," because they start the process of special ripening. For the sake of 

 convenience the term will be used here. 



