342 BIOLOGICAL ACTIVITIES OF YEASTS 



mum sugar concentration is 22° Balling. Higher sugar concentra- 

 tions may favor the production of more than 13 per cent of alcohol 

 by volume but higher concentrations of alcohol also inhibit the fer- 

 mentation. Actually, the tolerance of yeast to alcohol is dependent 

 in part on the temperature (the higher the temperature, the lower the 

 tolerance) as well as on the strain of yeast. The sugar concentration 

 can be adjusted either by the addition of water or sugar, or by the 

 mixing of a must high in sugar with one low in sugar content. 



Initially, mixing or stirring is resorted to in order to aerate the 

 inoculated crushed grapes. Oxygen is required for the rapid growth 

 of the yeast which carries out the fermentation. A "cap" of grape 

 skins, seeds, and so on forms on the surface of the fermenting mix- 

 ture, necessitating the punching of holes in the cap, or pumping juice 

 from the bottom of the vat over it. The temperature is kept at 20° 

 to 24° C. If it rises too high a "stuck" wine may result; the high 

 temperature and alcohol inhibit the yeast and undesirable bacterial 

 contaminants may develop. After three to five days of fermentation, 

 the "free-run wine" is drawn off. The free-run wine is transferred 

 to closed storage tanks and allowed to ferment still further, bungs 

 being provided to allow the excess carbon dioxide to escape. The 

 residual fermentable sugar is utilized to the desired extent in seven 

 to eleven days at a temperature of about 21° to 29° C. 



The new wine is now transferred to storage tanks made of oak, 

 preferably, or redwood where it is stored for the aging process. The 

 tanks are filled to the top and sealed to prevent the access of excess 

 air. Undesirable organisms such as Acetobacter and "wine flowers," 

 Mycoderma vini, may develop under strictly aerobic condition. 

 Slight amounts of air are desirable, however, to oxidize the alcohol 

 to acids. These in turn combine with alcohol to produce the esters 

 which are necessary for the development of aroma and bouquet. The 

 wine is periodically racked (drawn off) , to separate it from the lees, 

 the settleable solids. The aging process may require several years. 

 Newer processes to hasten aging include such steps as flash pasteur- 

 ization and filtration. 



Wine defects may be caused by undesirable organisms such as 

 mycodermas, tourne bacilh (lactobacilli), and mannitol-producing or- 

 ganisms. Mrak ^* has studied the wine defects caused by iron or tin 

 salts, copper, zinc, aluminum,* and so on. Sometimes these casses 

 (turbidities) may be remedied by treatment of the wine with gelatin, 

 bentonite, or casein preparations. The defects caused by bacteria 

 may often be corrected by treatment with bentonite or filtration 

 through infusorial earths, followed by a subsequent passage through 



