APPLICATIONS OF LACTIC ACID FERMENTATION 113 



the older and dirtier the milk, the lower should be the temperature, 

 and the higher the percentage of starter. If the souring is carried 

 out at 18 to 20 C., the amount of starter should, nevertheless, not 

 be below 4 to 5 per cent., for the cream should in any case be ripe 

 before nine p.m., so that there will be time to cool it in order 

 that it may stand at a low temperature overnight. The com- 

 pleter the cooling, the less will be the likelihood of oversouring, 

 the smaller the loss on churning, and the firmer the butter. 

 Cooling is best accomplished by the addition of crushed ice which 

 is stirred into the cream. If ice is not available, it is necessary to 

 use ripening tanks, furnished with cooling jackets or pipes, through 

 which cold water can be circulated. Ahlborn's cream-ripening 

 tank 1 is of a type which can specially be recommended ; several 

 imitations of it are made in Denmark, some of them being im- 

 provements on the original type. It is doubtful whether any 

 advantage is to be gained by covering the ripening tank with a 

 lid if it stands in a clean and dry room ; the air between the 

 cream surface and the lid will be saturated with moisture, thus 

 affording ideal conditions for the development of moulds. For 

 example, if two plates of milk are left to sour, one of them being 

 covered with another plate, a vigorous growth of mould will be 

 found on the surface of the covered milk after twenty -four hours, 

 while none will be seen on the uncovered milk. 



We may now discuss the starter, which influences the quality of 

 the butter more than anything else. It is of great importance to 

 keep the starter pure, cleaning and sterilising everything with 

 which it comes into contact with the greatest care. Only per- 

 fectly fresh morning milk must be used in its preparation. By 

 the systematic application of the reductase and fermenting test 

 (see p. 166), it will be easy to pick out the farmers who supply the 

 cleanest milk. By passing through a clean separator, the milk 

 will be further purified, for, as was mentioned above, a large 

 proportion of the bacteria pass into the cream, and particularly 

 into the separator slime. As regards the pasteurisation of the 

 starter milk, the chief point is to carry it out in such a manner as 

 to secure the best conditions for the development of the lactic 

 acid bacteria which are to be added to the milk. Formerly the 

 lactic acid bacteria were supposed to thrive best in raw milk ; as 

 this contains dissolved albumin, and from this point of view, low 

 temperature pasteurisation would be the best. On the other 

 hand, there is a good deal to be said in favour of high temperature 

 pasteurisation, which destroys the foreign bacteria more effectively 

 and also destroys the bactericidal constituents of the milk, which 



1 " Maelkeritidende," 1910, p. 1094. 



