The application of our knowledge respecting the phenomena attendant upon decay, to the manufacture of beer and wine, is easy and obvious. The property of beer and wine to be converted into vinegar when in contact with the air, depends invariably upon the presence of foreign matters which transmit their own inherent aptitude to absorb oxygen to the particles of alcohol in contact with them. By removing completely all such substances from wine and beer, these lose altogether the property of acidifying, or of being converted into vinegar. In the juice of grapes poor in sugar there remains, after the completion of the process of fermentation—that is, after the resolution of the sugar into carbonic acid and alcohol—a considerable amount of nitrogenous constituents retaining the same properties which they possessed in the juice previous to fermentation. This does not happen with the juice of the grapes of southern climates. These grapes are rich in sugar, and a considerable amount of this substance remains undecom-posed after all nitrogenous matters have completely separated in an insoluble state, as yeast. Such wines alter very little when exposed to the air; the red wines of this kind^ hnTTHTTi 1irH:fr W-iiiTn tH;- --Inriirg Tit ter is of ready mutability, and performs, when, in contact with the air, the part of the nitrogenous constituents. The nitrogenous constituents of the grape-juice which remain in wine, after fermentation, are those ferments or exciters of fermentation in the sugar, of which I have already spoken in other letters. After the complete transformation of the sugar, they exercise upon the alcohol exactly the same effect as the decaying wood—they are the exciting causes of the ensuing process of acidification. The affinity of these substances for oxygen is very powerful; during the short space of time necessary to transfer wine from one cask into another, they absorb oxygen trom the air, and induce a state of acidity in the wine, which goes on irresistibly if it be not checked by artificial means, it is well known that this check is practically effected by sulphura-tion. A piece ot sulphur is burned in the cask destined to receive the wine, the contained air is thus deprived of its oxygen, and an amount of sulphurous acid is formed equal to the volume of the oxygen. This newly formed sulphurous acid is rapidly absorbed by the moist internal surface of the cask. Sulphurous acid possesses a stronger affinity tor oxygen than the exciters of acidification in the wine. The acid is gradually diffused from the internal surface of the cask through the wine, and withdraws from those substances, as well as from the wine itself, all the oxygen they have absorbed from the atmosphere, and thus reconverts the wine into the state in which it existed previously to being transferred into the new cask. The sulphurous acid in this process becomes converted into sulphuric acid, and exists as such in the wine. When the wine is stored up in casks to ripen, a constant, although very slow, diffu^ sion of air takes place through the pores 'of the wood, or, what comes to the same thing, the wine is incessantly in contact with a minute amount of oxygen; by means of which, after the lapse of a certain time, the entire quantity of the exciters of acidification, that s, the nitrogenous substances present in the wine, oxidize and separate in the form of a sediment or dregs, termed under yeast or sedimentary yeast. The separation of yeast from wine or beer, during the fermentation of grape-juice, or oi worts, takes place in consequence of the absorption of oxygen", or in other words, is a process of oxidation, occurring in the fermenting liquid. The nitrogenous constituent of barley is in its primary state insoluble in water, but in the process of malting, or whilst the grain is germinating, it becomes soluble in water, it assumes the same condition or nature which belongs to the nitrogenous constituents of grape-juice originally. Both these substances lose their solubility in wine or in beer, by absorbing oxygen.— According to analyses in which we may confide, made with regard to this point, wine-yeast andfbeer-yeast are far richer in oxygen than the nitrogenous substances from which they are derived. As long as any particles of sugar, in a state of fermentation, are present in the fluid together with these nitrogenous matters, the fluid itself supplies the oxygen required for their transformation into yeast by the decomposition of a small amount ot the sugar or of water. This oxidizing process within the fluid itself, which causes the nitrogenous constituents to become insoluble, ceases with the disappearance of the sugar; but it is renewed if the fluid is reconverted into a fermenting state, by the addition ot new portions of sugar, and it ensues also when the surface of thfe fluid is exposed to the free access of the atmosphere. In the latter case the separation of the nitrogenous constituents is effected by the atmospheric oxygen, and is thus a consequence of their decay or slow combustion. I have already stated that the presence of nitrogenous matters in alcohol, causes the transformation of the alcohol into acetic acid when there is a sufficient supply of air; now it is owing to their inequalities in their relative affinities for oxygen, that during the maturation of wine in the storehouse, when the access of air is extremely limited, that the nitrogenous snhstaruys alone oxidize, and not the alcohol. In open" vessels, under these circumstances, the wine would become converted into vinegar. The preceding remarka render it; obvious that if we possessed any means ot preventing the transformation of alcohol into acetic acid we should be able to preserve wine and beer for an unlimited period, and to bring those liquors into a state of perfect maturity; for, under such circumstances, all those substances which cause wine and beer to acidify would become insoluble by combining with oxygen, and separate from the liquid, and with their perfect removal the alcohol present would altogether lose the property of absorbing oxygen. Experimental art has discovered a means of accomplishing this purpose perfectly. It consists in keeping the fluid at a low temperature when undergoing fermentation. The rcethod, based upon this principle, and employed in Bavaria, is one which the most perfect theory could scarcely have surpassed in certainty and simplicity, and it seems impossible to devise one more in accordance with science. The transformation of alcohol into acetic acid by contact with a substance in a state of decay occurs most rapidly at a temperature of 95 Fahrenheit. At lower temperatures the affinity of alcohol for oxygen decreases, and at from 46 to 50 Fahrenheit no combination with oxygen takes place under these circumstances, whilst the tendency of nitrogenous substances to absorb oxygen at this low temperature is scarcely diminished in any perceptible degree. It is, therefore, obvious, that it wort be fermented in wide, open, and shallow vessels, as is done in Bavaria, which afford free and unlimited access to the" atmospheric oxygen, and this in a situation where the temperature does not exceed 46 to 50 Fahrenheit, a separation of the nitrogenous constituents, i.e., the exciters of acidification, takes place simultaneously on the surface, and within the whole body of the liquid. The clearing of the beer is the sign by which it is known that these matters are separated. A more or less perfectly complete removal of these nitrogenous substances, however, according to this method of fermentation, depends upon the skill and experience of the brewer. It may be easily conceived that an absolutely perfect separation oi them is attained only in rare and extremely happy instances. Nevertheless, the beer obtained in this manner is invariably far superior in quality and stability to that brewed according to the common method. The exceedingly favorable influence which* the adoption of this principle must exercise upon the manufacture of wine is indisputable. It is too evident to admit of a'doubt that it will lead to the adoption of a more rational method than has hitherto been employed. Wine prepared by this method will, of course, bear the same relation to the wine prepared in the ordinary way, that Bavarian beer bears to common beer, in the fabrication of which the same amount of malt and hops has been employed. In the shortest possible time, the same quality, the same maturity may be attained by the wine which, under ordinary circumstances would result, only after long and protracted storing. It it be borne in mind that the period for the manufacture of wine is the end of October, just at the cool season which is peculiarly favorable to the fermentation of beer, and that no other conditions are necessary to the vinous fermentation than a cool cellar, and open, wide fermenting vessels, and further, that under all circumstances the danger of acidification is much less with wine than with beer, it is evident that the best success may confidently be expected trom the application of this method. It must not be forgotten, that wine contains a much smaller proportion of nitrogenous matters after fermentation, than beer-worts, and that a much more limited access of air is required for its complete oxidation and separation in an insoluble form. The method employed at most places on the Rhine proceeds upon principles the very reverse of this.— The wine is left to ferment, not in cool cellars, but in rooms, situate much too high and too warm; the access of air is completely precluded during the process of fermentation by fem-pIaU. lubua, wfttjud_with water. These tubes certainly exercise an injurious effect upon the quality of the wine; they are, in every respect, futile—the invention of some idle brain ; they serve no object, and yet they are used by people who imitate others, without assigning any reason for doing so.
This article was originally published with the title "Liebig on the Fermentation of Wine and Beer" in Scientific American 8, 5, 34 (October 1852)