DISTILLERY MASH. 119 



souring of the succeeding acid mash. About one-tenth of the 

 fermented acid mash is used for pitching the following mash. 



The acid thus introduced into the principal mash, together 

 with the surviving lactic acid bacteria; act as disinfectants, 

 besides exerting an influence on the yeast cells, both directly 

 and by reacting on the nutritive substances. 



The lactic acid bacteria occurring in the mash can be 

 distinguished in many ways from those occurring in milk. 

 Zopf was the first to prepare and investigate a culture of a 

 species belonging to this class, from a mash obtained from 

 dry malt and water at 50 C., according to Delbruck's process 

 (1881), following up an observation of Delbruck's that at this 

 temperature a vigorous lactic souring took place. A growth 

 of threads, rods, and cocci was developed. 



Pediococcus acidi lactici, examined by Lindner, gives a 

 strong acid reaction when cultivated in a neutral malt-extract 

 solution at 41 C. Both in a solution of this kind and in a 

 hay decoction, which has not been sterilised, this bacterium 

 develops so vigorously that, according to Lindner, all other 

 organisms are suppressed at this temperature. It has been 

 proved chemically that the acid, which is abundantly produced, 

 consists for the most part of lactic acid. When a malt mash 

 or malt-rye mash is maintained at 41 C., the Pediococcus 

 develops vigorously, and the rod-shaped lactic acid bacteria 

 are suppressed. According to Henneberg the optimum for 

 the formation of acid is 38 C. The optimum for growth on 

 beer and wort agar lies between 36 and 40 C. In a neutral 

 malt-extract solution the Pediococcus is killed after five minutes' 

 exposure to 62 C. In gelatine it does not thrive well ; it is 

 only in stab-cultures in neutral malt-extract gelatine that very 

 vigorous white colonies are formed below the surface. This 

 species appears, on the whole, to thrive better when air is 

 excluded. 



In 1893, Kruis and Rayman isolated a vigorous lactic acid 

 bacterium from yeast mash consisting of long and short rods, 

 which produced 0-9 per cent, of lactic acid at 40 C. in a clear 

 malt wort. It is of special interest to note that Kruis and 

 Rayman in studying this species proved, for the first time, that 

 lactic acid bacteria are capable of forming volatile fatty acids. 



