316 
Book IV. 
An acid, 
And ex 
tract, are 
necessary, 
FERMENTED SUBSTANCES. 
All those juices of fruits which undergo the vinous 
fermentation, either with or without the addition of 
sugar, contain an acid. We have seen already in the 
First Chapter that the vegetable acids are obtained chief 
ly from fruits. The apple, for instance, contains ma 
lic acid ; the lemon, citric acid ; the grape, tartarous 
and malic acids. The Marquis de Bullion has ascertained 
that must will not ferment if all the tartarous acid which 
it contains be separated from it ; but it ferments per 
fectly well on restoring that acid. The same chemist 
ascertained that the strength of wine is considerably 
increased by adding tartar and sugar to the must *. We 
may conclude from these facts, that the presence of a 
vegetable acid is absolutely necessary for the commence 
ment of the vinous fermentation. This renders it pro 
bable that the essential part of barm is a vegetable acid, 
or something equivalent ; for if sugar be dissolved in 
four times its weight of water, mixed with the yeast of 
beer, and placed in a proper temperature, it undergoes 
the vinous fermentation f. 
All the juices of fruits which undergo the vinous fer 
mentation contain an extractive matter, composed of 
what Deyeux has called the sweet principle. This sub 
stance has not been examined with much precision; 
but it seems to consist of mucilage, jelly, and extract. 
Now the presence of this substance is also necessary for 
the commencement of fermentation. For sugar, though 
diluted with water, and mixed with a vegetable acid, 
refuses to ferment unless so memucilaginous matter be 
added. 
Thus we see, that for the production of wine a cer 
* Ann. de Chim. XXXVi. 20. 
Bergman.