172 



FERMENTATION. 



water, yeast, and a certain temperature. To ob- 

 serve the chemical changes which occur, we must 

 dissolve five parts of sugar in about twenty of water, 

 adding a little yeast, uiul, introducing the mixture 

 into a glass flask, furnished with a bent tube, the 

 extremity of which opens under an inverted jar full 

 of water or mercury, apply a temperature of 60 or 

 70 Fahr. to the materials. In a short time, we 

 shall observe the syrup to become muddy, and a mul- 

 titude of air bubbles to form around the ferment ; 

 these unite, and, attaching themselves to particles of 

 the yeast, rise along with it to the surface, forming 

 a stratum of froth. The yeasty matter will then dis- 

 engage itself from the air, fall to the bottom of the 

 vessel, to acquire buoyancy a second time, and so on. 

 The fermentation will continue for two or three days, 

 when it will terminate, leaving the impurities to sub- 

 side, and the liquor clear and transparent. The only 

 appreciable changes which are found to have occurred 

 during the process, are the disappearance of the 

 sugar, and the formation of alcohol which remains in 

 the flask, and of carbonic acid which is collected in 

 the inverted jar. The yeast appears to have oper- 

 ated only by bringing on the fermentation, without 

 further contributing to the products. The atmo- 

 spheric air, having been excluded by the nature of 

 the apparatus, can have exercised no effect upon the 

 result. The true theory of the process is founded 

 on the fact, that the sugar, which disappears, is 

 almost precisely equal to the united weights of the 

 alcohol and carbonic acid ; and hence the former is 

 supposed to be resolved, during the process, into the 

 two latter. Though a solution of pure sugar is not 

 susceptible of the vinous fermentation, without being 

 mixed with yeast, yet the saccharine juices of plants 

 do not require the addition of that substance ; or, in 

 other words, they contain some principle, which, like 

 yeast, excites the fermentative process. Thus the 

 juice of the grape, of the apple, &c., ferments spon- 

 taneously, but not without enjoying access to the air ; 

 from which it would appear, that it must contain 

 a principle which is convertible into yeast, or, at 

 least, into a compound, which acquires the charac- 

 teristic property of that substance, by absorbing 

 oxygen. 



The various kinds of stimulating fluids, prepared 

 by means of the vinous fermentation, are divisible 

 into wines, which are formed from the juices of sac- 

 charine fruits, and the various kinds of ale and beer 

 produced from a decoction of the nutritive grains 

 previously malted. The juice of the grape is superior, 

 for the purpose of making wine, to that of all other 

 fruits, not merely in containing a larger proportion 

 of saccharine matter, since this deficiency may be. 

 supplied artificially, but in the nature of its acid. 

 The chief or only acidulous principle of the mature 

 grape, ripened in a warm climate, such as Spain, 

 Portugal, or Madeira, is the bitartrate of potash. 

 As this salt is insoluble in alcohol, the greater part 

 of it is deposited during the vinous fermentation ; 

 and an additional quantity subsides, constituting the 

 crust, during the progress of wine towards its point 

 of highest perfection. The juices of other fruits, on 

 the contrary, such as the gooseberry or currant, con- 

 tain the malic or citric acids, which are soluble both 

 in water and alcohol, and of which, therefore, they 

 can never be deprived. Consequently, these wines 

 are only rendered palatable by the presence of free 

 sugar, which conceals the taste of the acid ; and 

 hence it is necessary to arrest the progress of the 

 fermentation long before the whole of the saccharine 

 matter is consumed. For the same reason, these 

 wines do not admit of being long kept ; for as soon 

 as the free sugar is converted into alcohol, by the 

 slow fermentative process, which may be retarded by 



the addition of brandy, but cannot be prevented, the 

 liquor acquires a strong, sour taste. 



Ale and beer differ from wines, in containing a 

 large quantity of mucilaginous find extractive mat- 

 ters, derived from the malt with which they are 

 made. From the presence of these substances, they 

 always contain a free acid, and are greatly disposed 

 to pass into the acetous fermentation. The sour 

 taste is concealed partly by free sugar, and part- 

 ly by the bitter flavour of the hop, the presence 

 of which diminishes the tendency to the formation 

 of an acid. The fermentative process, which takes 

 place in dough mixed with yeast, and on which de- 

 pends the formation of good bread, has been sup- 

 posed, by some, to be of a peculiar kind, and accord- 

 ingly designated by the name of the panary fermen- 

 tation. ' More recent researches upon this subject, 

 however, leave little doubt that the phenomena are 

 to be ascribed to the saccharine matter of the flour 

 undergoing the vinous fermentation, by which it is 

 resolved into alcohol and carbonic acid. When any 

 liquid lias undergone the vinous fermentation, or even 

 pure alcohol, diluted with water, is mixed with 

 yeast, and exposed in a warm pkce to the open air, 

 the acetous fermentation takes place. This change 

 is attended with an intestine movement, and the deve- 

 lopment of heat and carbonic acid gas ; the fluid, 

 at the same time, becoming turbid, from the deposi- 

 tion of a peculiar filamentous matter. This process 

 goes on tardily below 60 Fahr.; at 50, is very 

 sluggish ; and at 32, is wholly arrested. On the 

 contrary, when the temperature is as high as 80, it 

 proceeds with vigour. It is necessary to distin- 

 guish between the mere formation of acetic acid, 

 and the acetous fermentation. Most vegetable sub- 

 stances yield acetic acid, when they undergo spon- 

 taneous decomposition ; and inferior kinds of ale and 

 beer are known to acquire acidity in a short time, 

 even when confined in well corked bottles. These 

 processes, and a variety of others, however, are 

 quite different from the proper acetous fermentation, 

 above described, being unattended with visible 

 movement in the liquid, with the absorption of 

 oxygen from the air, or the evolution of carbonic 

 acid. The true acetous fermentation consists in the 

 conversion of alcohol into acetic acid, the quantity 

 of the latter being precisely proportional to that of 

 the former. The nature of the chemical action is, 

 however, at present, obscure. It has been imagined 

 that pure alcohol contains a greater proportional 

 quantity of carbon and hydrogen than acetic acid ; 

 that the oxygen of the atmosphere, the presence of 

 which is indispensable, abstracts so much of those 

 elements, by giving rise to the formation of carbonic 

 acid and water, as to leave the remaining carbon, 

 hydrogen and oxygen of the alcohol, in the precise 

 ratio for forming acetic acid. 



The acetous fermentation is conducted on a large 

 scale, for yielding the common vinegar of commerce. 

 In France, it is prepared by exposing weak wines to 

 the air during warm weather. In England, it is 

 made from a solution of brown sugar or molasses, or 

 an infusion of malt. The vinegar thus obtained, 

 however, always contains a large quantity of muci- 

 laginous and other vegetable matters, the presence 

 of which renders it liable to several ulterior changes. 

 In America, it is more generally the product of 

 cider. The putrefactive fermentation is confined to 

 those vegetable substances, in which the oxygen 

 and hydrogen exist, in a proportion to form water ; 

 and in such, particularly, as contain nitrogen. Those 

 proximate principles, in which carbon and hydrogen 

 prevail, such as the oils, resins, and alcohol, do not 

 undergo the putrefactive fermentation ; nor do acids, 

 which contain a considerable excess of oxygen, mani- 



