ARTIFICIAL SOURING. 247 



149. Artificial Souring by the Aid of Pure Cul- 

 tures of Lactic Acid Bacteria. 



The credit of recognising the utility of souring the mash is due 

 to practical distillers themselves, their experience on this point 

 having been gained by repeated experiments. It is only in recent 

 years, however, that a closer insight into the characteristics and 

 actual value of this preliminary treatment of the yeast-mash has 

 been obtained. Until lately the generally accepted opinion was 

 that expressed by SCHULTK IM HOFE (I.), viz., that lactic acid is 

 necessary, or at any rate favourable, to the conversion of the 

 (insoluble and undiffusible) albuminoids of the wort into peptones 

 assimilable by yeast. Delbriick's researches on this point failed, 

 however, to reveal the presence of any appreciable quantity of 

 peptones in the soured yeast-mash, and it is now certain that the 

 favourable result is solely due to the relative toxic action of lactic 

 acid. This acid acts much more quickly and powerfully on the 

 development of the bacteria than on yeast, the latter being able 

 to stand a fairly large amount of the acid without appreciable 

 injury. 



The reader may well inquire from what source these lactic acid 

 bacteria which cause the souring of the yeast-mash are derived. 

 Until recently the answer was far from satisfactory, since it indi- 

 cated that the matter was left to chance. The initial temperature 

 of 70 C. in the yeast-mash kills the lactic acid bacteria already 

 present therein, but not the spores of the butyric ferment ; the 

 subsequent development of the latter is, however, prevented by the 

 restrictive temperature of 50 C. maintained during the souring pro- 

 cess. The active lactic acid bacteria must, therefore, make their 

 way into the mash from outside sources, e.g. the air, the vessels, and 

 utensils, &c., so that the inoculation of the mash is left entirely to 

 chance. Consequently it is not surprising to learn that the opera- 

 tion frequently miscarries, failures being, under such circumstances, 

 inevitable. 



We are indebted to Morawsky for the first improvement on 

 this point. Instead of waiting for the yeast-mash to become 

 infected spontaneously by lactic acid bacteria, as in the ordinary 

 course, he proposed to set aside about one-tenth of the soured 

 mash before applying heat, and to add this mother-acid to the 

 next day's mash as soon as the latter has been saccharified and 

 cooled down to 50 C. This modification, although constituting a 

 valuable improvement on the older method when once operations 

 are in full swing, nevertheless does not positively guarantee good 

 souring ; and its deficiencies are most apparent at a time when help 

 is most essential, viz., at the commencement of a new season. 

 During the first few days after work is resumed, it often becomes 

 apparent from the odour permeating the yeast-room that the sour- 



