6l2 MICROBIOLOGY OF ALCOHOLIC FERMENTATION 



mannit. The latter may reach over 2 per cent and the former 5 per 

 cent, giving a sweet-sour wine which is completely spoiled. 



The bacteria grow abundantly only at high temperatures approach- 

 ing 40 and can be controlled by cool fermentation, increase of acidity 

 and proper use of sulphurous acid. 



Butyric Bacteria. In the cooler climates, wines, especially old red 

 wines in bottles, often become bitter. This trouble is due to com- 

 paratively large rod-shaped bacteria, first described by Pasteur. 

 The cells remain united in angular filaments, short at first, but be- 

 coming longer and finally thicker with age by incrustations of coloring 

 matter. 



The tannin, coloring matter, and glycerin of the wine are attacked, 

 acetic and butyric acids being formed. In small amounts the bacteria 

 do little or no harm, in larger amounts they may spoil the wine. Means 

 which increase the alcohol, tannin and acidity diminish the liability to 

 the disease. Prompt attenuation and clarification and in extreme cases 

 pasteurization will cure wines not too badly affected. 



All the above anaerobic bacteria of wine diseases probably exist in 

 most wines. Which develop most, or whether any develop sufficiently 

 to injure the wine depends on conditions, chiefly the composition of the 

 must and the temperature at which the wine is fermented or stored. 

 Most diseased wines show a mixed infection of several forms. Re- 

 cently W. V. Cruess has found bacteria in wine containing twenty 

 per cent of alcohol. These bacteria were living and causing cloudi- 

 ness and increasing the volatile acids. 



CONTROL OF THE MICROORGANISMS 



Given grapes of suitable composition, the quality of the wine 

 depends on the work of microorganisms. The art of the wine-maker 

 consists almost entirely in the control of these microorganisms. His 

 success in facilitating the work of the useful form (true wine yeast) 

 and in preventing or hindering the work of injurious forms determines 

 the quality of his product. 



BEFORE FERMENTATION. On the skins of sound ripe grapes as they 

 hang in the vineyard, the microorganisms are comparatively few and in 

 an inactive condition. When proper methods are used they cannot 

 injure the wine. On broken or injured grapes, the number is greater 



