520 MICROBIOLOGY OF SPECIAL INDUSTRIES , 



therefore a deterring, effect on some of the most dangerous forms. It 

 is seldom necessary, however, to modify the composition for this pur- 

 pose if the other means of control are used. The addition of acid or 

 its decrease by dilution or neutralization should be solely for the direct 

 improvement of the taste. 



The quaUty and character of the wine depends greatly on the tem- 

 perature of fermentation. If too low, the fermentation may be unduly 

 prolonged, the wine yeast may have difficulty in overcoming its com- 

 petitors and the wine may remain inferior and cloudy. With red wine, 

 the desired color, tannin and body may not be secured. On the other 

 hand, if the temperature is too high the results are worse. The growth 

 of bacteria is promoted, injuring the wine by the volatile acid and dis- 

 pleasing flavors produced and preventing the proper action of the yeast. 

 Such wines may remain sweet on account of the failure of the yeast to 

 do its work and become unpleasantly acid owing to the volatile acids 

 produced by the bacteria. 



Some means of controlling the temperature is therefore always 

 needed. Where heat is deficient it may be supplied by direct heating 

 of the must or part of it, or by heating the cellar. Where the heat is 

 excessive, it may be diminished by crushing only cold grapes, using 

 small fermenting vats to promote radiation and finally by the use of 

 cooling machines applied directly to the fermenting wine. 



The best temperature for fermentation depends on the kind of wine. 

 For light white wines, the maximum should not exceed 25°, for heavier 

 wines 30°, while for heavy red wines where high extract and tannin are 

 required, it may be allowed to reach 35°. Sound wines can be made at 

 all these temperatures. 



As already explained, the ordinary processes of treatment of grapes 

 result in sufficient aeration for the multiplication of the yeast. With 

 grapes containing little sugar, this may suffice to complete fermentation. 

 With sweeter grapes, the fermentation usually slackens when the 

 alcohol reaches 11 or 12 per cent by volume or sooner, unless some 

 supplementary aeration is given. With white wine this is seldom done, 

 with the result that the time of fermentation is prolonged. With red 

 wine, the necessary stirring of the pomace to promote color extraction 

 or the pumping over of the must in the cooling process usually gives a 

 large amount of aeration which is sometimes excessive. Too much 

 aeration results in extremely rapid fermentation and consequent 



