CHAPTER XXV. 



LACTIC ACID BACTERIA IN DISTILLING, BREWING, 

 AND VINDICATION. 



148. The Spontaneous Acidification of Distillery Yeast-Mash. 



THE preparation of the pitching yeast for distillery work is not such an easy 

 matter as in the sister industry of brewing. In the regular course of the latter 

 no special labour is required for the production of the necessary quantity of 

 yeast, since in this case the yeast settles down, as soon as the fermentation is at 

 an end, to the bottom of the tun, and can then after the immature beer is racked 

 off be used at once for " pitching " (i.e. inducing fermentation in) a fresh 

 quantity of wort. The case is different in distillery work, where the liquid to 

 be fermented, instead of being thin and self-clarifying like wort, is a thick mash, 

 in which the yeast cannot settle down. For this reason the distiller is obliged 

 to prepare his pitching yeast in another way. He grows it artificially in special 

 vats, and, on this account, terms it "artificial yeast." For this purpose a sweet 

 mash is prepared in a small tun, the quantity amounting to about 10 per cent, 

 of the principal mash to be fermented. A more detailed description of the pre- 

 paration of this yeast-mash belongs to the domain of Chemical Technology, and 

 we will here content ourselves with briefly mentioning that crushed green malt 

 is mixed with water and gradually warmed to 6-j-To C., then mixed with a 

 variable) amount of sweet "goods" from the principal mash tun, and the mix- 

 ture left to saccharify for two hours at 70 C. Before this medium is pitched 

 with the yeast to be reproduced, it must, however, be subjected to another 

 preliminary treatment known as " souring." 



The green malt is infested with a copious flora of various kinds of bacteria, 

 chief among which are the species of the hardy organisms causing butyric fer- 

 mentation. These spores are not killed by the aforesaid mashing temperature 

 which, moreover, for reasons connected with the preservation of the diastase, 

 must not be exceeded and therefore they afterwards germinate and increase, 

 and produce butyric acid. Now this acid, being a powerful yeast poison, would 

 injure the development of the pitching yeast ; but since the injurious bacteria 

 are themselves very sensitive to high degrees of 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 main- 

 taining 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 neutralisa- 

 tion, a quantity corresponding to i.o-i.i per cent, of lactic acid. When this 



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