ARTIFICIAL SOURING WITH LACTIC ACID BACTERIA 189 



point is reached, the mash is heated to 70 C. in order to kill the lactic acid 

 bacteria, and is immediately re-cooled to i7-2O C. and pitched with yeast. 

 For the first yeast-mash of a new season a sufficient quantity (i kilo, per hecto- 

 litre of mash, i.e. at the rate of i Ib. per 10 gallons) of a pure culture of a 

 selected race of distillery yeast is employed. Such yeast can be obtained from 

 the Berlin Experimental Distillery Station (Versuchsstation fur Brennerei). At 

 the expiration of some fourteen to sixteen hours the development of the yeast 

 has so far progressed that the contents of the vat can be applied to their destined 

 purpose, a portion (about one-tenth) being, however, reserved, under the name 

 of mother-yeast, for pitching the soured yeast-mash on the following day. The 

 remaining nine-tenths of the prepared yeast are then transferred to the principal 

 mash, whereby the latter not only receives the requisite amount of active yeast, 

 but is also rendered acid, and is thereby better enabled to resist bacterial infec- 

 tion. This explains the whole dictum of the distiller, " The more acid in the 

 yeast, the less in the fermenting tun," because the greater the acidity of the 

 mature yeast-mash, the lower the possibility of injurious (acid-producing) germs 

 developing in the principal mash during fermentation. The reason for this is 

 that lactic acid reduces the vital activity of the microbes (butyric acid and acetic 

 acid bacteria) now under consideration. The increase of acidity in the mash is 

 employed as a measure of the progress of the fermentation. When the yeast is 

 first added, the sweet mash exhibits an acidity of o.5-o.7, corresponding to 

 0.2-0.3 P er cent, of lactic acid, and this increases during fermentation by 0.2 

 when the management is first-class, 0.3 when good, and by as much as 0.4 and 

 more when the process is not properly carried out. 



149. Artificial Souring- by the Aid of Pure Cultures 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 SCHULTE IM HOFE (I.), viz., that lactic acid is necessary, or at any 

 rate favourable, to the conversion of the (insoluble and undiffusible) albumi- 

 noids 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 indicated 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 fer- 

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

 restrictive temperature of 50 C. maintained during the souring process. 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 operation frequently miscarries, failures being, under such circum- 

 stances, inevitable. 



