194 ECONOMIC BACTERIA IN MILK AND MILK PRODUCTS 



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 oi 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 introducing 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 commencement, and before the 

 " starter " was added. 



The usual method adopted is to take a small amount of milk, 

 cool it, and add the commercial culture provided. Allow the 

 mixture to stand, protected from dust, for twenty-four hours, and 

 then pour into a larger volume of milk or cream to increase the 

 number of bacteria. The whole " starter " is now ready for use. 

 That is, perhaps, the simplest method of adding an artificial cul- 

 ture. But secondly, it will be apparent that if the cream is 

 previously 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 This is the method as 

 introduced by Storch, and as now practised in Denmark, where 

 97 per cent, of the butter is made on this principle. Pasteurisa- 

 tion in Denmark is required by law, with the view of extermina- 

 ting tuberculosis.^ 



There is a third modification, which is sometimes termed ripen- 

 ing by natural starters. A natural " starter " is a certain small 

 quantity of cream taken from a favourable ripening — from a clean 

 dairy and a good herd, and under conditions of strict cleanliness — 

 and placed aside in a covered sterilised vessel 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. A " natural starter " is 

 therefore not a pure culture, but a mixture of which 95 per cent, 

 of the organisms are of two or three species of lactic acid 

 bacteria. 



There are at the present time more than a score of different 



1 See also " Pasteurisation as applied to Butter-making," Wisconsin Agri- 

 cultural Expt. Sta.^ Bulletin 69, 1898. 



