YEAST. 



ing. Its action is arrested also by the tempera- 

 tore of boiling water, by alcohol, common salt, 

 o excess of sugar, oxide of mercury, corro- 

 tive sublimate, pyroligneous acid, sulphurous 

 nitrate of silver, volatile oils, and, in 

 short, by all antiseptic substances. 



Tht intolublt part of the subttance called fer- 

 mtHt duet tot cauie fermentation. For when the 

 yeast from wine or beer is carefully wa- hed 

 with water, care being taken that it is always 

 covered with this fluid, the residue does not 

 produce fermentation. 



* The iolttble part of ferment likewise does not ex- 

 dttfrrnnntatinn. An aqueous infusion of yeast 

 may be mixed with a solution of sugar, and 

 preserved in vessels from which the air is ex- 

 cluded, without either experiencing the slight- 

 est change. What then, we may ask, is the mat- 

 :it which excites fermentation, if nei- 

 ther the soluble 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. 

 Before it obtains this power, the decanted in- 

 fuftion must be allowed to cool in contact with 

 (he air, and to remain some time exposed to 

 its action. When introduced into a solution 

 of sugar in this state, it produces a brisk fer- 

 mentation ; but without a previous exposure to 

 the air it manifests no such property. 



* During the fermentation of sugar by yeast, 

 both of these substances suffer decomposition 

 at the same time, and disappear in conse- 

 quence. But if yeast be a body which excites 

 fermentation by being itself in a state of de- 

 conpoMtinn. all other matters in the same con- 

 h<>uld have a similar action upon sugar; 

 and this i> in reality the case. Muscle, urine, 

 iaaaglas?, osmazome,* albumen, cheese, glia- 

 dioe, gluten, legumin, and blood, when in a 

 state of putrefaction, have all the power of 

 producing the putrefaction or fermentation of 

 a solution of sugar. Yeast, which by con- 

 tinued washing has entirely lost the property 

 of inducing fermentation, regains it when its 

 putrefaction has recommenced, in consequence 

 of its being kept in a warm situation for some 

 lime. 



44 If we consider the process of the fermenta- 

 tion of pure sugar, in a practical point of view, 

 we meet with two facts of constant occurrence. 

 When ihe quantity of ferment is too small in 

 proportion to that of the sugar, its putrefaction 

 will be completed before the transformation of 

 all the sugar is effected. Some sugar here re- 

 mains tir > decomposed, because the cause of its 

 transformation is absent, viz., contact with a 

 body in a state of decomposition. 



I But when the quantity of ferment predomi- 

 nates, a certain quantity of it remains after all 

 the sugar has fermented, its decomposition pro- 

 c.eedin^' very slowly, on account of its insolu- 

 bility m wa residue of ferment is 



II able to induce fermentation, when intro- 

 uced into a fresh solution of sugar, and re- 1 



tains the same power until it has passed I 

 * all the stages of its own transformation. 

 tence & certain quantity of yeast is neces- 



ich the peculiar 



J hence il8 name ' 



1/76 



YELLOW-WOOD. 



sary in order to effect the transformation of ft 

 certain portion of sugar, not because it acts by 

 its quantity increasing any affinity, but because 

 its influence depends solely on its presence, 

 and its presence is necessary, until the last 

 atom of sugar is decomposed. 



" We have seen that ferment or yeast is a 

 body in the state of decomposition, the atoms 

 of which, consequently, are in a state of mo- 

 tion or transposition. Yeast, placed in contact 

 with sugar, communicates to the elements of 

 that compound the same state, in consequence 

 of which, the constituents of the sugar arrange 

 themselves into new and simpler forms, name- 

 ly, into alcohol and carbonic acid. In these 

 new compounds, the elements are united toge- 

 ther by stronger affinities than they were in 

 the sugar, and therefore under the conditions 

 in which they were produced further decompo- 

 sition is arrested. 



" We know, also, that the elements of sugar 

 assume totally different arrangements, when 

 the substances which excite their transposition 

 are in a different state of decomposition from 

 the yeast just mentioned. Thus, when sugar 

 is acted on by rennet or putrefying vegetable 

 juices, it is not converted into alcohol and 

 carbonic acid, but into lactic acid, mannite, 

 and gum. 



" Again, it has been shown, that yeast added 

 to a solution of pure sugar gradually disap- 

 pears, but that when added to vegetable juices 

 which contain gluten as well as sugar, it is re- 

 produced by the decomposition of the former 

 substance. 



" The yeast with which these liquids are made 

 to ferment, has itself been originally produced 

 from gluten. 



" The conversion of gluten into yeast in these 

 vegetable juices is dependent on the decompo- 

 sition (fermentation) of sugar; for, when the 

 sugar has completely disappeared, any gluten, 

 which may still remain in the liquid does not 

 suffer change from contact with the newly de- 

 posited yeast, but retains all the characters of 

 gluten. 



"Yeast is a produ.ct of the decomposition 

 of gluten ; but it passes into a second stage 

 of decomposition when in contact with water. 

 On account of its being in this state of further 

 change, yeast excites fermentation in a fresh 

 solution of sugar, and if this second saccha- 

 rine fluid should contain gluten, (should it be 

 wort, for example,) yeast is again generated in 

 consequence of the transposition of the ele- 

 ments of the sugar exciting a similar change 

 in this gluten. 



" After this explanation, the idea that yeast 

 reproduces itself as seeds reproduce seeds, 

 cannot for a moment be entertained." (Liebig.) 



YELLOW-BEAR. See CATERPILLAR. 



YELLOW-TOP (Agrostis alba}. A variety 

 of Herds. Called also, in the New England 

 states, White-top. 



YELLOW-WEED. See WELD. 



YELLOW-WOOD (Virgilia lutea). This 

 tree, says Michaux, is confined to that part of 

 West Tennessee which lies between the 35th 

 and 37th degrees of latitude, where it is com- 

 monly designated by the name which is here 

 adopted. 



