_ of putrefaction and eremacausis. 
| a state of eremacausis, it converts the oxygen of 
YEARLING. 
YARR. See Spurrey. 
YARROW. See Mizrorn. 
YEARLING. A colt, calf, or other young farm 
animal, which has completed its first year. 
YEAST. A peculiar froth or scum, closely al- 
lied in chemical composition to gluten, and pos- 
sessing the property of easily exciting fermenta- 
tion. It is always generated during the vinous 
fermentation of vegetable juices and decoctions; 
and rises to the surface in the condition of a 
frothy, flocculent, and somewhat viscid substance. 
The variety of it most commonly in use is col- 
lected from the surface of fermenting or ferment- 
ed malt liquors; and many other varieties are 
either incidentally obtained from other fermen- 
tations or expressly gathered from decoctions 
made for the very purpose of yielding them. 
is insoluble in water and in alcohol; and when 
exposed to ordinary atmospheric heat and mois- 
ture, ib soon and gradually putrefies,—when sub- 
mitted to a moderate heat, considerably above the 
ordinary atmospheric temperatures, it becomes a 
dry, hard, slightly cohesive paste, which may be 
preserved for a long time without undergoing any 
change,—when heated to redness in close vessels, 
it is entirely decomposed, and passes into products 
similar to those which are obtained in the same 
circumstances from gluten,—and when exposed 
for a few minutes to the heat of boiling water, it 
loses its characteristic property of exciting fer- 
mentation, yet suffers little apparent change, and 
eventually reacquires its characteristic property. 
Is seems to owe its fermentative power to a nitro- 
genous principle either quite or very nearly iden- 
tical with gluten; but, in other respects, it has 
a variable composition, and is much controlled 
in character by the kind of juice or decoction 
from which it is obtained, and by the particular 
circumstances in which it is collected. A speci- 
men of it analysed by Westrumb yielded potash, 
lime, carbonic acid, acetic acid, malic acid, alco- 
hol, extractive, mucilage, sugar, gluten, and 
water; and all the kinds of it proper to be used 
in medicine, or fresh specimens obtained from 
fermenting beer, probably owe their therapeutic 
powers entirely to carbonic acid, alcohol, and the 
bitter extractive of hops. Good beer yeast has 
a sour vinous odour and a bitter taste, and red- 
dens vegetable blues; and when filtered, the 
portion of it which remains on the filter is 
glutinous, and the portion which passes the fil- 
ter has no power to excite fermentation unless 
the glutinous matter is restored to it. 
“ Yeast,” says Liebig, “ possesses all the cha- 
racters of a compound of nitrogen in the state 
Like wood in 
the surrounding air into carbonic acid; but it 
also evolves this gas from its own mass, like bo- 
dies in the state of putrefaction. When kept 
under water, it emits carbonic acid, accompanied 
by gases of an offensive smell, and is at last con- 
verted into a substance resembling old cheese. 
It. 
YEAST. 
But when its own putrefaction is completed, it 
it has no longer the power of inducing fermen- 
tation in other bodies. The presence of water is 
quite necessary for sustaining its properties; for 
by simple pressure its power to excite fermenta- 
tion is much diminished, and is completely de- 
stroyed by drying. Its action is arrested also by 
the temperature of boiling water, by alcohol, 
common salt, an excess of sugar, oxide of mer- 
cury, corrosive sublimate, pyroligneous acid, sul- 
phurous acid, nitrate of silver, volatile oils, and 
in short substances, all of which possess antisep- 
tic properties. The insoluble part of it does not 
cause fermentation; for when the yeast from 
wine or beer is carefully washed with water, 
care being taken that it is always covered with 
this fluid, the residue does not produce fermen- 
tation. Nor does the soluble part of it excite 
fermentation ; for an aqueous infusion of yeast 
may be mixed with a solution of sugar, and pre- 
served in vessels from which the air is excluded, 
without either experiencing the slightest change. 
What then, we may ask, is the matter in yeast 
which excites fermentation, if neither the solu- 
ble nor insoluble parts possess the power? ‘This 
question has been answered by Colin in the most 
satisfactory manner. He has shown that in 
reality it is the soluble part, but that before it 
obtains this power, the decanted infusion must 
be allowed to cool in contact with the air, and to 
remain some time exposed to its action. When 
yeast is introduced into a solution of sugar in 
this state, it produces a brisk fermentation; but 
without previous exposure to the air, it manifests 
no such property. The infusion absorbs oxygen 
during its exposure to the air, and carbonic acid 
may be found in it after a short time. Yeast 
produces fermentation in consequence of the pro- 
gressive decomposition which it suffers from the 
action of air and water. Now when yeast is 
made to act on sugar, it is found that after the 
completion of the transformation of the latter 
substance into carbonic acid and alcohol, part of 
the yeast itself has disappeared. From 20 parts 
of fresh yeast from beer, and 100 parts of sugar, 
Thénard obtained, after the fermentation was 
completed, 13'7 parts of an insoluble residue, 
which diminished to ten parts when employed 
in the same way with a fresh portion of sugar. 
These ten parts were white, possessed of the pro- 
perties of woody fibre, and had no further action 
on sugar. It is evident, therefore, that, during 
the fermentation of sugar by yeast, both of these 
substances suffer decomposition at the same time, 
and disappear in consequence. But if yeast be 
a body which excites fermentation by being itself | 
in a state of decomposition, all other matters in 
the same condition should have a similar action 
upon sugar; and this is in reality the case. 
Muscle, urine, isinglass, osmazome, albumen, 
cheese, gliadine, gluten, legumin, and_ blood, 
when in a state of putrefaction, all have the 
power of producing the putrefaction or fermen- 
