336 ON THE NATURE OF FERMENTATION 
I witnessed in the north of Italy the time-honoured practice of treading grapes 
in the wine-vat. I was told that the juice would within twenty-four hours 
boil, as it was said, over the vats into which it was introduced ; in other words, 
that the sugar of the grape-juice would within that short time be so converted 
into alcohol and carbonic acid that the carbonic-acid gas, by its evolution, 
would cause sufficient frothing to produce the effect to which I have referred. 
This conversion of the sugar of the grape into alcohol and carbonic acid is 
accompanied by the development of a microscopic organism, the yeast plant, or, 
to continue the old nomenclature, Torula Cerevisiae, consisting of microscopic cells 
multiplying by pullulation, as indicated in this diagram (not here represented) 
Now, it is, I believe, universally admitted that the alcoholic fermentation o 
grape-sugar is due to the growth of the yeast plant. M. Pasteur thinks that 
he has traced the origin of the yeast plant in the juice of the grape to a minute 
fungus adhering to the outside of the skin of the grape.t. Be this as it may, 
it is admitted on all hands that the alcoholic fermentation is caused by the 
growth of the yeast plant. So long as the juice of the grape is protected by the 
skin of the berry, no fermentation occurs ; but, as soon as it escapes from that 
protection, the organism, by its development, induces the fermentation. Nor is 
it by any means exclusively in the natural juices of fruits that such fermentation 
occurs. Any sugary solution, provided it contains, besides the sugar, other 
ingredients requisite for the nutrition of the yeast plant, will serve as pabulum 
for the organism, and in that case the yeast plant will give rise to the fermenta- 
tion. Here is a glass containing such a liquid, termed Pasteur’s solution, because 
it was devised by M. Pasteur for the very purpose of affording nourishment to the 
yeast plant and other minute organisms. This was prepared on the 7th of August 
in a flask purified by heat, covered over with a cap of pure cotton-wool,? which 
allows the entrance of air, but does not permit the access either of the yeast 
plant or of any other form of dust. The Pasteur’s solution, containing, besides 
sugar, ammoniacal and earthy salts for the nutrition of the fungus, was heated 
to about the temperature of boiling water, so as to destroy any organisms that 
might exist in the water. The result is, that it continues perfectly unchanged, 
just as it was on the 7th of August; but, if we were to add to it a little of the 
yeast plant from fermenting grape-juice, we should find that, at the temperature 
of summer weather, it would very soon be in a state of free fermentation at 
the same time that the yeast plant would multiply. This, then, is a typical 
instance of fermentation. We have an active agent termed the ferment, which 
1 Vide Pasteur, Etudes sur la Biére, pp. 150 sqq. 
* The cotton-wool was rendered free from living organisms by soaking it with a solution of carbolic 
acid in one hundred parts of anhydrous ether and allowing the ether to evaporate, leaving the carbolic 
acid behind in the cotton. 
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