246 LACTIC ACID BACTERIA IN DISTILLING, ETC. 



acidity, their development may be hindered by quickly making 

 the fresh mash decidedly acid. To attain this end the lactic acid 

 bacteria are called in aid. 



The question now arises, How can the latter be cultivated 

 without allowing the butyric ferments to gain the upper hand? 

 This can be secured by maintaining the optimum temperature, 

 which for the lactic acid ferments now under consideration is 

 between 47 and 52 C., whereas the butyric ferments thrive best 

 at about 40 C. The sweet yeast-mash is therefore kept at 

 about 50 C. ; consequently the lactic acid bacteria develop with 

 vigour, and the increase in their activity can be determined by 

 titration. 



When the operation progresses satisfactorily, the acid content 

 rises with increasing rapidity and attains 2.2-2.5 degrees of acidity ; 

 i.e. 20 c.c. of the filtered sour mash require 2.2 to 2.5 c.c. of 

 normal alkali for complete neutralisation, a quantity corresponding 

 to i.o-i.i per cent, of lactic acid. When this point is reached, the 

 mash is heated to 70 C. in order to kill the lactic acid bacteria, and 

 is immediately re-cooled to 1 7-2o C. and pitched with yeast. For 

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

 hectolitre 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 fiir 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 infection. This explains the old 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 per cent, of lactic acid, and this increases during fermenta- 

 tion 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. 



