CHAPTER XXIII. 



THE ARTIFICIAL SOURING OF CREAM. 



141. The Acid Generator. 



THE preferences exhibited by the various families of the human race for different 

 kinds of butter are very marked. China and Japan, for instance to which 

 countries Denmark ships large quantities of this food-stuff prefer sweet-cream 

 butter, i.e. that prepared from fresh, sweet cream ; whereas, in Scandinavian 

 countries, Denmark itself, North Germany, and England, a preference for sour- 

 cream butter prevails. 



In order to obtain the latter product, the cream is allowed to turn sour 

 and undergo a fermentation, principally of a lactic character. Until within the 

 last few years the general practice was simply to leave the cream to become sour 

 spontaneously ; hence, in view of the fluctuation to which the bacterial flora of 

 milk and cream is exposed, it is not surprising that such a method of procedure 

 frequently resulted in the production of defective butter. 



A reliable means of combating this adverse tendency is, however, now avail- 

 able, namely, the process introduced into the dairy industry in 1890 by H. 

 WEIGMANN (III.-V.) of artificially souring cream by the aid of pure cultures 

 of selected races of lactic acid bacteria. This process is divided into two 

 manipulations: the preparation of the acid generator and the preliminary 

 treatment of the cream to be soured. 



The acid generator (or starter) brings the cream quickly into a state of 

 fermentation. According to Weigmann's recipe, it is prepared as follows: 

 Separated or skimmed milk in the proportion of 2-3 per cent, of the cream to 

 be acidified is warmed up to about 60 C. and then immediately re-cooled as 

 quickly and as much as possible. This treatment kills some of the bacteria in 

 the milk and weakens others to such an extent that they cannot offer more than 

 a feeble opposition to the development of the lactic acid bacteria, which are then 

 added to the treated skim-milk. For this purpose a pure culture of lactic fer- 

 ment, obtained from a Dairy Experimental Station, is employed. The vessel is 

 kept for twenty-four hours at a medium temperature (15 C.), by which time 

 its contents will be converted into acid generator ready for use. 



The cream, also, requires a preliminary treatment to prepare the way for the 

 action of the acid generator. Sterilisation, or at least Pasteurisation, would 

 afford the best results, but as these are generally difficult to effect in a reliable 

 manner, a method of weakening the " wild " bacteria present, by cooling the 

 cream down to a low temperature and then quickly warming it up again to 

 i6-2o C., has to be resorted to. The acid generator is then added and well 

 incorporated by stirring, and the cream vat is left, at about 15-2O C., until 

 the following day, by which time the cream will be ripe for churning. 



If it be desired to cultivate the acid generator further, a small portion is 

 taken from the bulk before use and employed in the same way as a pure 

 culture. 



In spite of the adverse opinion of many practical men, the possibility of 

 producing good butter from Pasteurised, or even sterilised cream, has been 



180 



