6l8 MICROBIOLOGY OF ALCOHOLIC FERMENTATION 



difficulty in controlling the temperature. It may also have a dele- 

 terious effect on the color, especially if sulphur dioxide has not been 

 used. 



In any case, the main part of the fermentation should be over in 

 from three to five days in the case of red and in from seven to fourteen 

 days in the case of white wine. With heavy musts, however, there will 

 still remain from 0.5 to i or 2 per cent of sugar. With certain special 

 wines such as Sauternes it is desirable to retain the slight sweetness due 

 to this small amount of unfermented sugar. This is accomplished by 

 the judicious use of sulphurous acid, prompt clarification by filtration or 

 fining and when necessary by pasteurization. The pasteurization tends 

 to remove those proteins which are coagulated by heat and which are 

 the preferred food of bacteria. 



In the case of dry wines, protection from bacteria is best obtained 

 by prompt and complete attenuation. Fermentation should not be al- 

 lowed to cease until all the sugar has disappeared. For this purpose, 

 one, two or more aerations by pumping over are usually necessary im- 

 mediately after the end of the tumultuous fermentation. The tem- 

 perature of the wine should not be allowed to fall sufficiently to check 

 the action of the yeast until all the sugar has disappeared. 



AFTER FERMENTATION. As soon as all the sugar has been destroyed 

 in the case of dry wines, or the desired degree of attenuation has been 

 obtained in the case of sweet wines, all the useful work of microorgan- 

 isms has been accomplished. The quality and safety of the wine then 

 depend on freeing it from all organisms present and preventing the 

 entrance and action of all others. 



As soon as bubbles of carbon dioxide cease to be given off, the yeast 

 and other solid matters will settle to the bottom and the liquid become 

 clear. This often occurs before the fermentation is complete. In this 

 case the yeast should be stimulated by aeration as described above. 



If the wine is dry, it should be racked (drawn off, decanted) from the sediment 

 into clean casks. The first racking is usually done while the wine is still slightly cloudy 

 during the first month or six weeks to remove the more bulky sediment. If left too 

 long on the yeast the autophagy or degeneration of the latter may produce substances 

 which injure, the brightness and flavor of the wine. 



A second racking is necessary at the end of winter before the spring rise of tem- 

 perature tends to renew the activity of the microorganisms which always remain 

 in the wine. A well made wine at this time should be perfectly bright and all solid 



