116 MICRO-ORGANISMS AND FERMENTATION. 



suitable degree of souring with lactic acid bacteria appears 

 to be a certain means of preventing these mishaps. 



The blue flecks which appear in certain kinds of cheese 

 may be produced, according to Beijerinck, in some cases by 

 Bac. cyanofuscus, which is derived from water. Black flecks 

 may be caused by growths of moulds, such as Cladosporium 

 and Fumago. Rusty specks, according to Connell, Harding, 

 and other American observers, are caused by a definite species, 

 Bac. rudensis. In the same way a reddish colour is produced 

 by red moulds, micrococci, etc. 



Lastly, a fault must be mentioned which may occur in all 

 kinds of cheese, the bitter taste, which is caused by certain 

 bacteria, as, for instance, by Micrococcus casei amari, described 

 by Freudenreich, and also by a species occurring in bitter 

 milk, and even by certain moulds and Torulce. 



Since 1890 methodically selected species of bacteria have 

 been applied in dairies, to bring about a regular and certain 

 souring of the cream used in the manufacture of butter, and 

 to avoid any taint in butter. The progress made in this 

 field is associated with the researches of Storch, Weigmann, 

 Quist, in the author's laboratory, and others. The pure 

 culture selected is added to skim milk, previously heated to 

 about 90 C., and the culture is allowed to develop at about 

 15 C. After standing 24 hours, this "starter" is fit for 

 use. In order to render the cream which is to receive the 

 culture as free from germs as circumstances permit, it is 

 pasteurised at about 85 C., and then quickly cooled. In the 

 course of ten hours or so, the starter is allowed to develop 

 in cream at about 16 C. It is then cooled below 10 C., and 

 churning is begun. 



Among the forms isolated by Storch of Copenhagen (1890) 

 from butter, sour cream, and butter-milk, the coccus form 

 of the group Streptococcus lacticus seems to be most frequent 

 and best suited to sour the cream. It occurs in a large number 

 of varieties, which, according to their main characteristics, 

 may be classed in two groups one including those which 

 give a specially pure and mildly sour taste and a fine aroma, 

 and another embracing those which yield a product possessing 

 great keeping powers. Morphologically the growths are dis- 



