FERMENTATION. 



admixture. If a considerable quantity of 

 water, holding in solution about one 

 third of its weight of sugar, be exposed to 

 the air, at the temperature of about se- 

 venty degrees, after the addition of a 

 small quantity of yeast, it soon under- 

 goes a remarkable change. In the course 

 of a few hours the fluid becomes turbid 

 and frothy ; oxygen is absorbed, bubbles 

 of carbonic acid gas are disengaged, 

 which rise to the surface and break. The 

 disengagement becomes more and more 

 abundant ; mucilage is separated, part of 

 which subsides to the bottom ; and part 

 being expanded into froth by the elastic 

 fluid, forms yeast. During the course of 

 several days, these effects gradually come 

 to their height, and diminish again ; after 

 which they proceed very slowly, but are 

 long before they entirely cease. The 

 fermented liquor has then no longer the 

 sweet taste it had before, but becomes 

 brisk and lively, with a pungent, spiritu- 

 ous flavour. Its specific gravity is also 

 considerably less than before ; and when 

 exposed to distillation it affords a light 

 inflammable spirit. The quantity of this 

 spirit, or alcohol, any fermented liquor 

 will produce, is thought to follow some 

 proportion of the change its specific gra- 

 vity undergoes in fermentation ; but the 

 truth of this has not been clearly ascer- 

 tained. Wine, cider, and beer, are well 

 known liquors of this kind. 



It is usual to put fermented liquors in- 

 to casks before the vinous fermentation is 

 completely ended ; and in these closed 

 vessels, the change goes on for many 

 months. But if the fermentative process 

 be suffered to proceed in open vessels, 

 more especially if the temperature be 

 raised to ninety degrees, the acetous fer- 

 mentation comes on. In this a still great- 

 er portion of oxygen from the air is gra- 

 dually absorbed ; and this more especially 

 as the surfaces of the liquor are oftener 

 changed by lading it from one vessel to 

 another. The usual method of doing 

 this consists in exposing the fermented 

 liquor to the air, in casks placed so that 

 the sun may shine on them ; which seems 

 to be of advantage by raising the tempe- 

 rature of the liquor. By this absorption 

 of oxygen the inflammable substance be- 

 comes converted into an acid. If the 

 liquor be then exposed to distillation, 

 vinegar comes over instead of alcohol or 

 spirit. 



When the spontaneous decomposition 

 is suffered to proceed beyond the acetous 

 process, the vinegar gradually becomes 



viscid and foul ; a gas is emitted with an 

 offensive smell ; ammonia flies off, an 

 earthy sediment is deposited, and the re- 

 maining liquid, if any, is mere water. 

 This is the putrefactive process. 



Though fermentation is much better 

 understood at present, in consequence of 

 modern researches into the nature of the 

 gases, than it formerly was, it still re- 

 mains an interesting object of research. 

 It is not clearly ascertained what the 

 yeast or fermented matter performs in 

 this operation. It seems probable that 

 the fermentative process in considerable 

 masses would be carried on in succes- 

 sion from the surface downwards; and 

 would perhaps be completed in one part 

 of the fluid before it was perfectly begun 

 in another part, if the yeast, which is al- 

 ready in a state of fermentation, did not 

 occasion the process to begin in every 

 part of the fluid at once. Experiments 

 yet remain to be made towards ascertain- 

 ing the arrangements and quantity of the 

 component parts of alcohol. It appears 

 that hydrogen in combination with car- 

 bon and water, in certain proportions, 

 form this compound; that a greater pro- 

 portion of oxygen converts it into vine- 

 gar ; and that in the putrefactive pro- 

 cess the hydrogen, carbon, and oxygen, 

 are separated from each other, and fly off 

 in the elastic state. 



In the fermentation of wine, the tartar, 

 which probably existed for the most part 

 already formed in the juice of the grape, 

 is separated, and exhibits the properties 

 which are described in treating of that 

 substance. 



The fermentation of bread, by leaven, 

 is thought to be of a different nature 

 from the vinous fermentation. In this, 

 the mucilage of the corn is not previously 

 brought into the saccharine state. It 

 quickly becomes sour, if the process be 

 not stopped by baking ; in which parti- 

 cular the fermentation seems to be of 

 the acetous kind. The developement 

 of carbonic acid divides the dough into 

 thin parts, which are more effectually 

 and better baked than they could have 

 been in the solid consistent mass. When 

 bread is fermented by means of yeast, 

 the process seems to be of a saccha- 

 rine or vinous nature. A very minute 

 proportion of alum renders bread whi- 

 ter, and its pores more small and nu- 

 merous, but how it acts has not been 

 ascertained. It does not seem, either 

 from its quantity or quality, to be un- 

 wholesome. 



