July 23, 1870.] 
THE PHARMACEUTICAL JOURNAL AND TRANSACTIONS. 
61 
ALCOHOLIC FERMENTATION. * * * § 
BY BARON VON LIEBIG. 
Some years ago Pasteur inferred, from a series of 
experiments as to the behaviour of yeast in vinous 
fermentation,f that the explanation given by me of 
the action of yeast on sugar was destitute of founda¬ 
tion. I assumed that the breaking up of the ferment¬ 
able substance into simpler compounds was to be re¬ 
ferred to a process of internal change obtaining in 
the ferment, and that the influence of the ferment 
upon the fermentable substance, would continue or 
cease just as the metamorphosis of the ferment con¬ 
tinued or ceased. 
The transposition of the sugar atoms in the sugar 
molecule would thus be a consequence of the decom¬ 
position or dislocation of one or more constituents of 
the ferment,—it would take place only while the two 
were in contact. 
Pasteur considers that “ the chemical change in 
fermentation is essentially a phenomenon accom¬ 
panying the vital activity of the yeast, beginning 
and ending with this: vinous fermentation never 
takes place without simultaneous organization, de¬ 
velopment, and reproduction, i.e. without continued 
vitality. ”t He regards fermentation as a chemical 
process, accompanying and dependent upon a phy¬ 
siological process. This view was entertained more 
than twenty years ago, and, although the nature of 
yeast was known to me when I put forward my view 
as to fermentation, § the physiological process did not 
come within my province ; my endeavour was to refer 
the chemical change of the sugar to some simple ex¬ 
pression comprising all similar processes. 
Pasteur has not gone into that wfliich I sought to 
explain, viz. the disintegration of the fermentable 
substance in contact with yeast-cells, and, inasmuch 
as he refers us to “vital activity” as the cause of 
fermentation, he substitutes, for an explanation, a 
fact which requires explanation itself. 
From the chemical point of view, which I cannot 
abandon, “vital activity” is a “state of motion,” and, 
in this sense, Pasteur’s view is neither inconsistent 
with nor contradictory of mine. It is matter of obser¬ 
vation, I say, that yeast undergoes alteration when 
kept under water, and ultimately putrefies like annual 
substances. The commencement and termination of 
this process indicate that the parts of the yeast are 
in a state of transposition or motion, terminating 
with conversion into other compounds that are more 
simple and do not change further while air is ex¬ 
cluded. In tliis case equilibrium is established when 
the motion ceases. The state of motion is quite in- 
dependent of joint action in.other substances. It is 
also observed that a great number of substances 
undergo alteration in the arrangement of their atoms 
when in contact with yeast, new substances being 
produced. Thus, for instance, sugar behaves as if it 
were a part or constituent of the yeast-cells; there is 
a transposition or dislocation of the sugar atoms. 
Comparing, as I did, the action of the ferment on 
fermentable substances, with the action of heat oh 
organic molecules, motion of the atoms is evident in 
both cases. Acetic acid is separated by heat into car¬ 
bonic acid and acetone, just as sugar is separated by 
yeast into carbonic acid and alcohol. In the former 
case the carbonic acid contains two-tliirds of the 
oxygen, the acetone all the hydrogen of the acetic acid 
while, in the latter case, the carbonic acid contains 
two-thirds of the oxygen, and the alcohol all the 
hydrogen of the sugar. 
The development of a plant—the formation and 
increase of yeast—is dependent on the assimila¬ 
tion of food which is internally converted into parts 
of the living organism; but in fermentation there 
is, so to speak, an action outwards upon substances 
that are resolved into products not serviceable for 
the living organism. Obviously, vital action and 
chemical action are phenomena which must be con¬ 
sidered separately in seeking to explain fermentation. 
The view that the decomposition of sugar in fer¬ 
mentation is due to the development and increase of 
the yeast-cells, is opposed by the fact that yeast 
causes fermentation in a pure solution of sugar, for 
yeast consists chiefly of a substance containing 
nitrogen and sulphur, besides phosphates, which 
could not be furnished by the sugar. Moreover, beer- 
}mast causes a similar decomposition of other sub¬ 
stances besides sugar; malate of lime is thus con¬ 
verted into carbonic acid, acetate, carbonate, and 
succinate of lime. 
Hitherto sugar fermentation is the only case in 
which the formation of yeast, capable of separa¬ 
ting sugar into alcohol and carbonic acid, has been 
observed. Malic, citric, and other acids contain no 
sugar, but they are decomposed like sugar by beer- 
yeast, and if its influence were due to the physiolo¬ 
gical process, it should have been increased and de¬ 
veloped in these cases also. 
Salicin also is decomposed by yeast into saligenin 
and salicylic acid, and a similar separation of sa¬ 
licin is caused by emulsin, without any recogniz¬ 
able physiological process being concerned in the 
change. 
Emulsin acts upon salicin and amygdalin in like 
manner, its effects being recognizable in a few 
min utes by the new products,—in the case of salicin, 
by the violet coloration with percliloride of iron ; in 
the case of amygdalin, by the formation of Prussian 
blue. 
In the decomposition of amygdalin by emulsin, it 
is known that water takes part; only so much amyg- 
dalin is decomposed as corresponds to the water ne¬ 
cessary for dissolving the bitter almond oil produced; 
any excess remains intact, but, if more water be 
added, there is further decomposition. Emulsion of 
sweet almonds, which may be regarded as a strong 
solution of emulsin, undergoes active vinous fermen¬ 
tation when mixed with grape-sugar. 
If the decomposition of salicin by yeast be ascribed 
to the physiological process of growth and develop¬ 
ment of yeast, the action of emulsin upon salicin 
has still to be explained, and, if it be assumed that 
in this case the readily alterable sulphuretted and 
nitrogenous constituent of the sweet almond has 
caused the change, there is also a similar substance 
in yeast. Those substances are also alike in losing 
tlierr power to excite fermentation when boiled with 
water. 
But if substances containing sulphur and nitrogen, 
like emulsin, are, by reason of alteration in the ar¬ 
rangement of their atoms, capable of inducing change 
in other organic molecules, so that they separate into 
new products,—there is reason for suspecting that in 
the action which yeast exercises upon sugar, its sul¬ 
phuretted and nitrogenous constituent plays a similar 
* Read at a meeting of the Bavarian Academy of Sciences, 
f Ann. de Ch. et de Phys. (3) vol. lviii. p. 323. 
j Ibid. p. 359. 
§ See Liebig’s ‘ Letters on Chemistry.’ 
Third Series, No. 4. 
