Taking account of the gradual accumulation of fact and theory we find at the time of Lavoisier, from which the modern aspect of the problem dates, that Stahl's theoretical views were generally accepted. Alcoholic fermentation was known to require the presence of sugar and was thought to lead to the production of carbon dioxide, acetic acid, and alcohol.
The composition of organic compounds was at that time not understood, and it was Lavoisier who established the fact that they consisted of carbon, hydrogen, and oxygen, and who made systematic analyses of the substances concerned in fermentation (1784–1789). Lavoisier [[1789]] applied the results of these analyses to the study of alcoholic fermentation, and by employing the principle which he regarded as the foundation of experimental chemistry, "that there is the same quantity of matter before and after the operation," he drew up an equation between the quantities of carbon, hydrogen, and oxygen in the original sugar and in the resulting substances, alcohol, carbon dioxide, and acetic acid, showing that the products contained the whole matter of the sugar, and thus for the first time giving a clear view of the chemical [p003] change which occurs in fermentation. The conclusion to which he came was, we now know, very nearly accurate, but the research must be regarded as one of those remarkable instances in which the genius of the investigator triumphs over experimental deficiencies, for the analytical numbers employed contained grave errors, and it was only by a fortunate compensation of these that a result so near the truth was attained.
Lavoisier's equation or balance sheet was as follows:—
| Carbon. | Hydrogen. | Oxygen. | |
|---|---|---|---|
| 95·9 pounds of sugar (cane sugar) consist of | 26·8 | 7·7 | 61·4 |
| These yield:— | |||
| 57·7 pounds of alcohol containing | 16·7 | 9·6 | 31·4 |
| 35·3 pounds of carbon dioxide containing | 9·9 | — | 25·4 |
| 2·5 pounds of acetic acid containing | 0·6 | 0·2 | 1·7 |
| ——— | ——— | ——— | |
| Total contained in products | 27·2 | 9·8 | 58·5 |
The true composition of the sugar used was carbon 40·4, hydrogen 6·1, oxygen 49·4.
Lavoisier expressed no view as to the agency by which fermentation was brought about, but came to a very definite and characteristic conclusion as to the chemical nature of the change. The sugar, which he regarded in harmony with his general views as an oxide, was split into two parts, one of which was oxidised at the expense of the other to form carbonic acid, whilst the other was deoxygenised in favour of the former to produce the combustible substance alcohol, "so that if it were possible to recombine these two substances, alcohol and carbonic acid, sugar would result".
From this point commences the modern study of the problem. Provided by the genius of Lavoisier with the assurance that the hitherto mysterious process of fermentation was to be ranked along with familiar chemical changes, and that it proceeded in harmony with the same quantitative laws as these simpler reactions, chemists were stimulated in their desire to penetrate further into the mysteries of the phenomenon, and the importance and interest of the problem attracted many workers.
So important indeed did the matter appear to Lavoisier's countrymen that in the year 8 of the French Republic (1800) a prize—consisting of a gold medal, the value of which, expressed in terms of the newly introduced metric system, was that of one kilogram of gold—was offered by the Institute for the best answer to the question: "What are the characteristics by which animal and vegetable substances which act as ferments can be distinguished from those which they are capable of fermenting?" [p004]
This valuable prize was again offered in 1802 but was never awarded, as the fund from which it was to be drawn was sequestrated from the Institute in 1804. The first response to this stimulating offer was an important memoir by citizen Thenard [[1803]], which provided many of the facts upon which Liebig subsequently based his views. Thenard combats the prevailing idea, first expressed by Fabroni (1787–1799), that fermentation is caused by the action of gluten derived from grain on starch and sugar, but is himself uncertain as to the actual nature of the ferment. He points out that all fermenting liquids deposit a material resembling brewer's yeast, and he shows that this contains nitrogen, much of which is evolved as ammonia on distillation. His most important result is, however, that when yeast is used to ferment pure sugar, it undergoes a gradual change and is finally left as a white mass, much reduced in weight, which contains no nitrogen and is without action on sugar. Thenard, moreover, it is interesting to note, differs from Lavoisier, inasmuch as he ascribes the origin of some of the carbonic acid to the carbon of the ferment, an opinion which was still held in various degrees by many investigators (see Seguin, quoted by Thenard).
Thenard's memoir was followed by a communication of fundamental importance from Gay-Lussac [[1810]]. A process for preserving food had been introduced by Appert, which consisted in placing the material in bottles, closing these very carefully and exposing them to the temperature of boiling water for some time. Gay-Lussac was struck by the fact that when such a bottle was opened fermentation or putrefaction set in rapidly. Analysis of the air left in such a sealed bottle showed that all the oxygen had been absorbed, and these facts led to the view that fermentation was set up by the action of oxygen on the fermentable material. Experiment appeared to confirm this in the most striking way. A bottle of preserved grape-juice was opened over mercury and part of its contents passed through the mercury into a bell-jar containing air, the remainder into a similar vessel free from air. In the presence of air fermentation set in at once, in the absence of air no fermentation whatever occurred. This connection between fermentation and the presence of air was established by numerous experiments and appeared incontestable. Fermentation, it was found, could be checked by boiling even after the addition of oxygen, and hence food could be preserved in free contact with the air, provided only that it was raised to the temperature of boiling water at short intervals of time. Gay-Lussac's opinion was that the ferment was formed by the action of the oxygen on the [p005] liquid, and that the product of this action was altered by heat and rendered incapable of producing fermentation, as was also brewer's yeast, which, however, he regarded, on account of its insolubility, as different from the soluble ferment which initiated the change in the limpid grape-juice. Colin, on the other hand [[1825]], recognised that alcoholic fermentation by whatever substance it was started, resulted in the formation of an insoluble deposit more active than the original substance, and he suggested that this deposit might possibly in every case be of the same nature.