Elements of Chemistry, by Antoine Lavoisier

Chapter XIII.

Of the Decomposition of Vegetable Oxyds by the Vinous Fermentation.

The manner in which wine, cyder, mead, and all the liquors formed by the spiritous fermentation, are produced, is well known to every one. The juice of grapes or of apples being expressed, and the latter being diluted with water, they are put into large vats, which are kept in a temperature of at least 10° (54.5°) of the thermometer. A rapid intestine motion, or fermentation, very soon takes place, numerous globules of gas form in the liquid and burst at the surface; when the fermentation is at its height, the quantity of gas disengaged is so great as to make the liquor appear as if boiling violently over a fire. When this gas is carefully gathered, it is found to be carbonic acid perfectly pure, and free from admixture with any other species of air or gas whatever.

When the fermentation is completed, the juice of grapes is changed from being sweet, and full of sugar, into a vinous liquor which no longer contains any sugar, and from which we procure, by distillation, an inflammable liquor, known in commerce under the name of Spirit of Wine. As this liquor is produced by the fermentation of any saccharine matter whatever diluted with water, it must have been contrary to the principles of our nomenclature to call it spirit of wine rather than spirit of cyder, or of fermented sugar; wherefore, we have adopted a more general term, and the Arabic word alkohol seems extremely proper for the purpose.

This operation is one of the most extraordinary in chemistry: We must examine whence proceed the disengaged carbonic acid and the inflammable liquor produced, and in what manner a sweet vegetable oxyd becomes thus converted into two such opposite substances, whereof one is combustible, and the other eminently the contrary. To solve these two questions, it is necessary to be previously acquainted with the analysis of the fermentable substance, and of the products of the fermentation. We may lay it down as an incontestible axiom, that, in all the operations of art and nature, nothing is created; an equal quantity of matter exists both before and after the experiment; the quality and quantity of the elements remain precisely the same; and nothing takes place beyond changes and modifications in the combination of these elements. Upon this principle the whole art of performing chemical experiments depends: We must always suppose an exact equality between the elements of the body examined and those of the products of its analysis.

Hence, since from must of grapes we procure alkohol and carbonic acid, I have an undoubted right to suppose that must consists of carbonic acid and alkohol. From these premises, we have two methods of ascertaining what passes during vinous fermentation, by determining the nature of, and the elements which compose, the fermentable substances, or by accurately examining the produces resulting from fermentation; and it is evident that the knowledge of either of these must lead to accurate conclusions concerning the nature and composition of the other. From these considerations, it became necessary accurately to determine the constituent elements of the fermentable substances; and, for this purpose, I did not make use of the compound juices of fruits, the rigorous analysis of which is perhaps impossible, but made choice of sugar, which is easily analysed, and the nature of which I have already explained. This substance is a true vegetable oxyd with two bases, composed of hydrogen and charcoal brought to the state of an oxyd, by a certain proportion of oxygen; and these three elements are combined in such a way, that a very slight force is sufficient to destroy the equilibrium of their connection. By a long train of experiments, made in various ways, and often repeated, I ascertained that the proportion in which these ingredients exist in sugar, are nearly eight parts of hydrogen, 64 parts of oxygen, and 28 parts of charcoal, all by weight, forming 100 parts of sugar.

Sugar must be mixed with about four times its weight of water, to render it susceptible of fermentation; and even then the equilibrium of its elements would remain undisturbed, without the assistance of some substance, to give a commencement to the fermentation. This is accomplished by means of a little yeast from beer; and, when the fermentation is once excited, it continues of itself until completed. I shall, in another place, give an account of the effects of yeast, and other ferments, upon fermentable substances. I have usually employed 10 libs. of yeast, in the state of paste, for each 100 libs. of sugar, with as much water as is four times the weight of the sugar. I shall give the results of my experiments exactly as they were obtained, preserving even the fractions produced by calculation.

Table I. Materials of Fermentation.
  libs. oz. gros grs.
Water   400 0 0 0
Sugar   100 0 0 0
Yeast in paste, 10 libs. composed of { Water 7 3 6 44
  { Dry yeast 2 12 1 28
    —— —— —— ——
  Total 510
Table II. Constituent Elements of the Materials of Fermentation.
    libs. oz. gros grs.
407 libs, 3 oz. 6 gros 44 grs. of water, composed of {Hydrogen 61 1 2 71.40
  { Oxygen 346 2 3 44.60
  {Hydrogen 8 0 0 0
100 libs. sugar, composed of {Oxygen 64 0 0 0
  {Charcoal 28 0 0 0
  {Hydrogen 0 4 5 9.30
2 libs. 12 oz. 1 gros 28 grs. of dry yeast, composed of {Oxygen 1 10 2 28.76
  { Charcoal 0 12 4 59
  { Azote 0 0 5 2.94
  ——— ——— —— ———
  Total weight 510 0 0 0
Table III. Recapitulation of these Elements.
libs. oz. gros grs.
   of the water 340 0 0 0} libs. oz. gros grs.
   of the water in the yeast 6 2 3 44.60} 411 12 6 1.36
   of the sugar 64 0 0 0}
   of the dry yeast 1 10 2 28.76}
   of the water 60 0 0 0}
   of the water in the yeast 1 1 2 71.40} 69 6 0 8.70
   of the sugar 8 0 0 0}
   of the dry yeast 0 4 5 9.30}
   of the sugar 28 0 0 0}
   of the yeast 0 12 4 59.00} 28 12 4 59.00
Azote of the yeast - - - -    } 0 0 5 2.94
—— ——— ——— ———
        In all 510 0 0 0

Having thus accurately determined the nature and quantity of the constituent elements of the materials submitted to fermentation, we have next to examine the products resulting from that process. For this purpose, I placed the above 510 libs. of fermentable liquor in a proper26 apparatus, by means of which I could accurately determine the quantity and quality of gas disengaged during the fermentation, and could even weigh every one of the products separately, at any period of the process I judged proper. An hour or two after the substances are mixed together, especially if they are kept in a temperature of from 15° (65.75°) to 18° (72.5°) of the thermometer, the first marks of fermentation commence; the liquor turns thick and frothy, little globules of air are disengaged, which rise and burst at the surface; the quantity of these globules quickly increases, and there is a rapid and abundant production of very pure carbonic acid, accompanied with a scum, which is the yeast separating from the mixture. After some days, less or more according to the degree of heat, the intestine motion and disengagement of gas diminish; but these do not cease entirely, nor is the fermentation completed for a considerable time. During the process, 35 libs. 5 oz. 4 gros 19 grs. of dry carbonic acid are disengaged, which carry alongst with them 13 libs. 14 oz. 5 gros of water. There remains in the vessel 460 libs. 11 oz. 6 gros 53 grs. of vinous liquor, slightly acidulous. This is at first muddy, but clears of itself, and deposits a portion of yeast. When we separately analise all these substances, which is effected by very troublesome processes, we have the results as given in the following Tables. This process, with all the subordinate calculations and analyses, will be detailed at large in the Memoirs of the Academy.

Table IV. Product of Fermentation.
libs. oz. gros grs.
35 libs. 5 oz. 4 gros 19 grs. of carbonic acid, composed of {Oxygen 25 7 1 34
{Charcoal 9 14 2 57
408 libs. 15 oz. 5 gros 14 grs. of water, composed of {Oxygen 347 10 0 59
  {Hydrogen 61 5 4 27
  {Oxygen, combined with hydrogen 31 6 1 64
57 libs. 11 oz. 1 gros 58 grs. of dry alkohol, composed of {Hydrogen, combined with oxygen 5 8 5 3
  {Hydrogen, combined with charcoal 4 0 5 0
  {Charcoal, combined with hydrogen 16 11 5 63
2 libs. 8 oz. of dry acetous acid, composed of {Hydrogen 0 2 4 0
{Oxygen 1 11 4 0
{Charcoal 0 10 0 0
4 libs. 1 oz. 4 gros 3 grs. of residuum of sugar, composed of {Hydrogen 0 5 1 67
{Oxygen 2 9 7 27
{Charcoal 1 2 2 53
{Hydrogen 0 2 2 41
1 lib. 6 oz. 0 gros 5 grs. of dry yeast, composed of {Oxygen 0 13 1 14
{ Charcoal 0 6 2 30
{Azote 0 0 2 37
—— —— —— ——
510 libs. Total 510 0 0 0
Table V. Recapitulation of the Products.
  libs. oz. gros grs.
409 libs. 10 oz. 0 gros 54 grs. of oxygen contained in the Water 347 10 0 59
Carbonic acid 25 7 1 34
Alkohol 31 6 1 64
Acetous acid 1 11 4 0
Residuum of sugar 2 9 7 27
Yeast 0 13 1 14
28 libs. 12 oz. 5 gros 59 grs. of charcoal contained in the Carbonic acid 9 14 2 57
Alkohol 16 11 5 63
Acetous acid 0 10 0 0
Residuum of sugar 1 2 2 53
Yeast 0 6 2 30
71 libs. 8 oz. 6 gros 66 grs. of hydrogen contained in the Water 61 5 4 27
Water of the alkohol 5 8 5 3
Combined with the charcoal of the alko. 4 0 5 0
Acetous acid 0 2 4 0
Residuum of sugar 0 5 1 67
Yeast 0 2 2 41
  2 gros 37 grs. of azote in the yeast 0 0 2 37
——   ——— ——— ——— ———
510 libs. Total 510 0 0 0

In these results, I have been exact, even to grains; not that it is possible, in experiments of this nature, to carry our accuracy so far, but as the experiments were made only with a few pounds of sugar, and as, for the sake of comparison, I reduced the results of the actual experiments to the quintal or imaginary hundred pounds, I thought it necessary to leave the fractional parts precisely as produced by calculation.

When we consider the results presented by these tables with attention, it is easy to discover exactly what occurs during fermentation. In the first place, out of the 100 libs. of sugar employed, 4 libs. 1 oz. 4 gros 3 grs. remain, without having suffered decomposition; so that, in reality, we have only operated upon 95 libs. 14 oz. 3 gros 69 grs. of sugar; that is to say, upon 61 libs. 6 oz. 45 grs. of oxygen, 7 libs. 10 oz. 6 gros 6 grs. of hydrogen, and 26 libs. 13 oz. 5 gros 19 grs. of charcoal. By comparing these quantities, we find that they are fully sufficient for forming the whole of the alkohol, carbonic acid and acetous acid produced by the fermentation. It is not, therefore, necessary to suppose that any water has been decomposed during the experiment, unless it be pretended that the oxygen and hydrogen exist in the sugar in that state. On the contrary, I have already made it evident that hydrogen, oxygen and charcoal, the three constituent elements of vegetables, remain in a state of equilibrium or mutual union with each other which subsists so long as this union remains undisturbed by increased temperature, or by some new compound attraction; and that then only these elements combine, two and two together, to form water and carbonic acid.

The effects of the vinous fermentation upon sugar is thus reduced to the mere separation of its elements into two portions; one part is oxygenated at the expence of the other, so as to form carbonic acid, whilst the other part, being deoxygenated in favour of the former, is converted into the combustible substance alkohol; therefore, if it were possible to reunite alkohol and carbonic acid together, we ought to form sugar. It is evident that the charcoal and hydrogen in the alkohol do not exist in the state of oil, they are combined with a portion of oxygen, which renders them miscible with water; wherefore these three substances, oxygen, hydrogen, and charcoal, exist here likewise in a species of equilibrium or reciprocal combination; and in fact, when they are made to pass through a red hot tube of glass or porcelain, this union or equilibrium is destroyed, the elements become combined, two and two, and water and carbonic acid are formed.

I had formally advanced, in my first Memoirs upon the formation of water, that it was decomposed in a great number of chemical experiments, and particularly during the vinous fermentation. I then supposed that water existed ready formed in sugar, though I am now convinced that sugar only contains the elements proper for composing it. It may be readily conceived, that it must have cost me a good deal to abandon my first notions, but by several years reflection, and after a great number of experiments and observations upon vegetable substances, I have fixed my ideas as above.

I shall finish what I have to say upon vinous fermentation, by observing, that it furnishes us with the means of analysing sugar and every vegetable fermentable matter. We may consider the substances submitted to fermentation, and the products resulting from that operation, as forming an algebraic equation; and, by successively supposing each of the elements in this equation unknown, we can calculate their values in succession, and thus verify our experiments by calculation, and our calculation by experiment reciprocally. I have often successfully employed this method for correcting the first results of my experiments, and to direct me in the proper road for repeating them to advantage. I have explained myself at large upon this subject, in a Memoir upon vinous fermentation already presented to the Academy, and which will speedily be published.

26 The above apparatus is described in the Third Part. — A.


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