AGRICULTURAL CHEMISTRY. 



possessing an offensive smell are evolved in 

 considerable quantity., and when the liquor 

 is examined after the decomposition is com- 

 pleted,, no alcohol can be detected. The 

 sugar has also disappeared, and with it all 

 the azotised compounds which existed in the 

 juice previously to its fermentation. Both 

 were decomposed at the same time ; the nitro- 

 gen of the azotised compounds remains in the 

 liquid as ammoina, and, in addition to it, 

 there are three new products, formed from 

 the component parts of the juice. One of 

 these is lactic acid, the slightly volatile com- 

 pound found in the animal organism ; the 

 other is the crystalline body which forms 

 the principal constituent of manna ; and the 

 third is a mass resembling gum-arabic, which 

 forms a thick viscous solution with water. 

 These three products weigh more than the 

 sugar contained in the juice, even without 

 calculating the weight of the gaseous pro- 

 ducts. Hence they are not produced from 

 the elements of the sugar alone. None of 

 these three substances could be detected in 

 the juice before fermentation. They must, 

 therefore, have been formed by the inter- 

 change of the elements of the sugar with 

 those of the foreign substances also present. 

 It is this mixed transformation of two or 

 --inore compounds which receives the special 

 lame of putrefaction. 



YEAST OR FERMENT. 



When attention is directed to the condi- 

 tion of those substances which possess the 

 power of inducing fermentation and putre- 

 faction in other bodies, evidences are found 

 m their general characters, and in the man- 

 ner in which they combine, that they all are 

 bodies, the atoms of which are in the act of 

 transposition. 



The characters of the remarkable matter 

 which is deposited in an insoluble state 

 during the fermentation of beer, wine, and 

 vegetable juices, may first be studied. 



This substance, which has been called 

 yeast or ferment, from the power which it 

 possesses of causing fermentation in sugar, 

 or saccharine vegetable juices, possesses all 

 the characters of a compound of nitrogen in 

 the state of putrefaction and eremacausis. 



Like wood in the state of eremacausis, 

 yeast converts the oxygen of the surrounding 

 air into carbonic acid, but it also evolves this 

 gas from its own mass, like bodies in the 

 state of putrefaction. (Colin.) When kept 

 underwater, it emits carbonic acid, accompa- 

 nied by gases of an offensive smell, (The- 

 nard,) and is at last converted into a sub- 

 stance resembling old cheese. (Proust.) 

 But when its own putrefaction is completed, 

 it has no longer the power of inducing fer- 

 mentation in other bodies. The presence 

 of water is quite necessary for sustaining 

 the properties of ferment, for by simple pres- 

 sure its power to excite fermentation is 

 much diminished, and is completely de- 

 stroved by drying. Its action is arrested also 



by the temperature of boiling water, by al- 

 cohol, common salt, an excess of sugar, 

 oxide of mercury, corrosive sublimate, pyro- 

 ligneous acid, sulphurous acid, nitrate of 

 silver, volatile oils, and in short by ah 1 anti- 

 i septic substances. 



The insoluble part of the substance called 

 ferment does not cause fermentation. For 

 when the yeast from wine or beer is care- 

 fully washed with water, care being taken 

 that it is always covered with this fluid, the 

 residue does not produce fermentation. 



The soluble part of ferment likewise does 

 not excite fermentation. An aqueous infu- 

 sion of yeast may be mixed with a solution 

 of sugar, and preserved in vessels from which 

 the air is excluded, without either experi- 

 encing the slightest change. What then, we 

 may ask, is the matter in ferment which ex- 

 cites 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 

 before it obtains this power, the decanted 

 infusion must be allowed to cool in contact 

 with the air, and to remain some time ex- 

 j posed to its action. When 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 conse- 

 quence of the progressive 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 transformation of 

 the latter substance into carbonic acid and 

 alcohol is completed, part of the yeast itself 

 has disappeared. 



From 20 parts of fresh yeast from beer, 

 and 100 parts of sugar, Thenard obtained, 

 after the fermentation was completed, 13'7 

 parts of an insoluble residue, which dimi- 

 nished to 10 parts when employed in the 

 same way with a fresh portion of sugar. 

 These ten parts were white, possessed of the 

 properties of woody fibre, and had no farther 

 action on sugar. 



It is evident, therefore, that during the fer- 

 mentation 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 fermenta- 

 tion by being itself in a state of decomposi- 

 tion, all other matters in the same condition 

 should have a similar action upon sugar; and 

 I this is in reality the case. Muscle, urine, 

 isinglass, osmazome, albumen, cheese, glia- 

 dine, 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 

 continued washing has entirely lost the pro- 

 perty of inducing fermentation, regains it 



