536 MICROBIOLOGY OF SPECIAL INDUSTRIES 



quantity of free mineral acid remaining in the completed solution pre- 

 vents any considerable growth of bacteria. In the malting process the 

 injurious bacteria are restrained by lactic acid produced by lactic 

 bacteria, originating in the malt or in the yeast starter. The requisite 

 bacteria are obtained by keeping the starter or mother yeast at 50° 

 to 58° for a certain time. This is a favorable temperature for lactic 

 and too high for the development of acetic or other injmrious bacteria 

 "When the acidity of the solution reaches 3-5 g- to 5 g. per liter the danger- 

 ous butyric bacteria cannot develop. 



Pure lactic acid may be added immediately after saccharification and 

 the loss of sugar, due to the action of the lactic bacteria avoided, but 

 the high cost of the pure acid prevents the practice. 



Yeast being much less sensitive to the presence of certain antiseptics 

 than bacteria it is possible to control the latter by the addition of 

 suitable amounts of an antiseptic to the sugar solution. In certain cases 

 moreover by gradually increasing the amount, yeast can be accustomed 

 to concentrations of antiseptics which render the growth of bacteria 

 impossible. In Effront's method for the preparation of distillation 

 material, hydrofluoric acid is used. This acid is added to the mother 

 yeast at the rate of 10 g. per hectoliter and to the sugar solution in 

 somewhat smaller amounts. This results in the inhibition of lactic, 

 butyric and other bacteria and an increase in the fermentative power of 

 the trained yeast. 



Fermentation. — The sugar solution properly diluted and acetified 

 or sterilized is fermented by the addition of a mother yeast, usually 

 taken from a previous fermentation. 



The original yeast may be obtained by a spontaneous fermentation 

 as is usual in the manufacture of rum. Such a yeast is always impure, 

 containing various yeasts, molds and bacteria, and is therefore very 

 variable and uncertain in its results. 



In the fermentation of beet juice and beet molasses, beer yeast of the 

 Frohberg type or special distillers yeasts are used. A starter or mother 

 yeast is prepared for each vat or the process is made continuous by 

 leaving one-third to one-half of the contents of a fermented vat to start 

 a fresh addition of the sugar solution. With the latter method the yeast 

 in time becomes weak and badly contaminated and a new start must 

 be m^de with fresh yeast. 



In the fermentation of solutions made from potatoes, corn or other 



