Alcoholic fermentation

“ Fermentation.—The fermentation of grape juice, i.e. the must, is, as we have seen, a relatively simple operation, consisting as it does in exposing it to the spontaneous action of the micro-organisms contained in it. The main products formed are, as in all cases of alcoholic fermentation, ethylic alcohol, water and carbonic acid. At the same time various subsidiary products such as glycerin, succinic acid, small quantities of higher alcohols, volatile acids and compound esters are produced. ”

“ The species of bacteria which infect and injure wine are different from those which infect and injure beer. They are ever present as possibilities in the great alcoholic fermentations. They are dangers which must be guarded against. In former years the troubles from these sources were much greater than they are at present. Since it has been demonstrated that the different imperfections in the fermentative process are due to bacterial impurities, commonly in the yeasts which are used to produce the fermentation, methods of avoiding them are readily devised. ”

“ The study of cell-free fermentation by means of yeast-juice or zymin has, however, revealed the facts that certain of these substances are not formed in the absence of living cells, and that their origin is to be sought in the metabolic processes which accompany the life of the cell. Their source, moreover, has been traced not to the sugar but to the amino-acids, formed by the hydrolysis of the proteins, which occur in all such liquids as beer wort, grape juice, etc., which are usually submitted to alcoholic fermentation. ”

“ Pasteur was therefore led to consider fermentation as a physiological process accompanying the life of the yeast. His conclusions were couched in unmistakable words: "The chemical act of fermentation is essentially a phenomenon correlative with a vital act, commencing and ceasing with the latter. I am of opinion that alcoholic fermentation never occurs without simultaneous organisation, development, multiplication of cells, or the continued life of cells already formed. The results expressed in this memoir seem to me to be completely opposed to the opinions of Liebig and Berzelius. ”

“ In view of the fact that fresh grape juice contains innumerable bacteria and moulds, in addition to the yeast cells which bring about the alcoholic fermentation, and that the means which are adopted by the brewer and the distiller for checking the action of these undesirable organisms cannot be employed by the wine-maker, it is no doubt remarkable that the natural wine yeast so seldom fails to assert a preponderating action, particularly as the number of yeast cells at the beginning of fermentation is relatively small. ”

“ Alcoholic fermentation depends on the formation of yeast, which is produced when the necessary elements and the proper conditions are present and then propagates itself. The action on the liquid thus increases and the constituents not required to form the organism combine to form unorganised substances, the carbonic acid and alcohol. "It is obvious," says Kützing, in a passage which roused the sarcasm of Berzelius, "that chemists must now strike yeast off the roll of chemical compounds, since it is not a compound but an organised body, an organism." ”

“ The effect, therefore, of fermentation is to change entirely the character of the substance upon which it acts; hence it is an error to assume that fermented liquors, as beer, wine, and cider, are safe drinks because the grains or fruits from which they are produced are healthful foods.
YEAST is a ferment which causes alcoholic fermentation. It consists of microscopic plants, which increase by the formation of multitudes of tiny cells not more than 1/2400 of an inch in diameter. In the brewing of beer they grow in great abundance, making common brewer's yeast. ”

“ The problem of alcoholic fermentation, of the origin and nature of that mysterious and apparently spontaneous change which converted the insipid juice of the grape into stimulating wine, seems to have exerted a fascination over the minds of natural philosophers from the very earliest times. ”

“ In his first papers [1897, 1, 2; 1898] , Buchner established the following facts: (1) yeast-juice free from cells is capable of producing the alcoholic fermentation of glucose, fructose, cane sugar, and maltose; (2) the fermenting power of the juice is neither destroyed by the addition of chloroform, benzene, or sodium arsenite [Hans Buchner, 1897] , by filtration through a Berkefeld filter, by evaporation to dryness at 30° to 35°, nor by precipitation with alcohol; (3) the fermenting power is completely destroyed when the liquid is heated to 50°. ”

“ There is not the least authority for such a supposition. When we come to exact quantitative estimations—and these are to be found in the figures supplied by Messrs. Lechartier and Bellamy—it will be seen that the proportions of alcohol and carbonic acid gas produced in the fermentation of fruits differ widely from those that we find in alcoholic fermentations properly so called, as must necessarily be the case since in the former the fermentaction is effected by the cells of a fruit, but in the latter by cells of ordinary alcoholic ferment. ”

“ By a rare accident we might get the true alcoholic fermentation, but the odds against obtaining it would be enormous. Pure air acting upon a lifeless liquid will never provoke fermentation; but our ordinary air is the vehicle of numberless germs which act as ferments when they fall into appropriate infusions. Some of them produce acidity, some putrefaction. The germs of our yeast-plant are also in the air; but so sparingly distributed that an infusion like beer-wort, exposed to the air, is almost sure to be taken possession of by foreign organisms. ”

“ Thus the alcoholic fermentation of a saccharine liquid was found to be due to a torula (Torula cerevisiæ) , the acetic fermentation of an alcoholic liquid to a bacterial ferment (Pasteur's Mycoderma aceti) , etc. Subsequent researches show that alcoholic fermentation may be induced by several species of torula, and even by certain bacteria; while the number of bacterial ferments now known to science is very considerable and is constantly being added to. Among the most important of these we may mention the Bacillus acidi lactici, which is the usual cause of the acid fermentation of milk ”

“ In such experiments we encounter another difficulty. If the yeast sown in the non-aerated fermentable liquid is in the least degree impure, especially if we use sweetened yeast-water, we may be sure that alcoholic fermentation will soon cease, if, indeed, it ever commences, and that accessory fermentations will go on. The vibrios of butyric fermentation, for instance, will propagate with remarkable facility under these circumstances. Clearly then, the purity of the yeast at the moment of impregnation, and the purity of the liquid in the funnel, are conditions indispensable to success. ”

“ CH3·CH (C2H5) ·CH2·OH, in precisely the same relation as leucine to isoamyl alcohol. This suggestive fact at once directed his attention to the problem of the origin of the amyl alcohols in alcoholic fermentation. Using a pure culture of yeast, and thus excluding the participation of bacteria in the change, he found that leucine readily yielded isoamyl alcohol, and isoleucine d-amyl alcohol when these amino-acids were added in the pure state [p087] to a solution of sugar and treated with a considerable proportion of yeast [1905; 1906, 2, 3; 1907, 1, 3] . ”

“ The alcoholic ferments, therefore, present themselves to us as plants which possess at least two singular properties: they can live without air, that is without oxygen, and they can cause decomposition to an amount which, though variable, yet, as estimated by weight of product formed, is out of all proportion to the weight of their own substance. These are facts of so great importance, and so intimately connected with the theory of fermentation, that it is indispensable to endeavour to establish them experimentally, with all the exactness of which they will admit. ”

“ It is well known that alcoholic fermentation is due to the presence of a minute fungus, the yeast-fungus, the living protoplasm of whose cells has the property of separating solutions of sugar into alcohol, which remains in the liquid, and carbonic acid, which escapes into the air. ”

“ It is now put into casks, and the sweet or sugar stage of fermentation, which is already begun, soon passes into the vinous or alcoholic stage, as it is called, and alcohol is formed. The prudent farmer, at this point, when the juice is done working, or fermenting, immediately bungs his casks, and does such other things as his skill and experience may suggest, to prevent his cider becoming sour, which it will do if the third stage of fermentation is permitted to succeed. Here, then, he has perfect alcohol, though in small proportions; as perfect as it is in brandy, gin, rum, and whiskey. ”

“ It is true that the chemical changes induced by organised ferments depend on the life processes of micro-organisms which feed upon the sugar or other substance in solution, and excrete the product of the fermentation. Fermentation nearly always consists of a process of breaking down of complex bodies, like sugar, into simpler ones, like alcohol and carbonic acid. Of such fermentation we may mention at least five: the alcoholic, by which alcohol is produced; the acetous, by which wine absorbs oxygen from the air and becomes vinegar; the lactic, which sours milk ”

“ The most noteworthy effect is that the production of acetaldehyde is increased both in autofermentation and [p112] in sugar fermentation. The course of the reaction is further modified in the sense that the percentage of sugar used up which can be accounted for in the products decreases, in other words the "disappearing sugar" (p. 31) increases. In long continued fermentations moreover and particularly with high concentrations of zinc chloride less alcohol is produced than is equivalent to the carbon dioxide evolved. ”

“ Man, seeking ways of producing alcohol from substances other than wine, early made the important observation that fermentation and the production of alcoholic liquids go hand in hand. This discovery, as time passed, became common knowledge, with the result that fermented liquids from different sources came to be looked upon as characteristic national drinks—thus in France wine from grapes, in Jamaica rum from cane, in Russia vodka from rye, in Japan saki from rice, in Germany beer from barley and in America whiskey from Indian corn. ”

“ The amount of fermentation produced is almost unaffected by the presence of such antiseptics as chloroform or toluene, although some others, such as arsenites and fluorides, decrease it when added in comparatively high concentrations, and it is only slightly diminished by dilution with three or four volumes of sugar solution, somewhat more considerably by dilution with water. ”

“ Fermentation is an obscure and seemingly spontaneous change or decomposition which takes place in most vegetable and animal substances when exposed at ordinary temperatures to air and moisture. While fermentation broadly covers decay or putrifaction, yet it is limited in ordinary use to the process for producing alcoholic liquors from sacchariferous mashes. ”

“ It was the only “natural” alcohol known at that time. That this alcohol would combine with phosphoric acid could be predicted, but that the ester, as obtained by Pelouze, still contained free acidic functions and formed a water-soluble barium salt was a new experience.
[190]
Figure 13.—Survey of alcoholic fermentation, 1951. The “well-known scheme of alcoholic fermentation” according to Albert Jan Kluyver (1888-1956) , presented before the Society of Chemical Industry in the Royal Institution, March 7, 1951. In Chemistry & Industry, 1952, page 136 ff., Kluyver restates that “... ”

“ What we call spontaneous fermentation often occurs, as when apple juice turns to hard cider by simple exposure to the air. Science teaches us, however, that this change is always effected by the action of the busy little ferments which, wandering about, drop into the liquid, begin their rapid propagation, and, in the act of growing, evolve the products of the fermentation. "If the above liquids be left only in contact with air which has been passed through a red-hot platinum tube, and thus the living sporules destroyed ”

“ In the selecting of cultures for the fermenting, the manufacturer should use only the purest, otherwise acetic acid and other foreign substances will be formed during fermentation, thus decreasing the yield of the alcohol as well as lowering its purity. ”

“ It results when the fermenting liquor is exposed to the air. When this is the case, the liquor absorbs a portion of the oxygen, which unites with the alcohol, thus converting it into acetic acid as rapidly as it is formed. When acetous fermentation begins, the liquor becomes turbid, and a long, stringy substance appears, which after a time settles down to the bottom of the vat. ”

“ No fermentation followed, consequently no alcohol resulted. From this it was learned that the yeasts necessary for the production of the alcohol of wine live in nature in the air and are found in abundance on the outside of the grape. If the grapes be crushed the sweet juices serve as food for the yeast plants. These when well fed grow rapidly and, by a simple process of budding, produce myriads of yeast plants. These, like their parents, give rise to ferments which break down the sugar into and alcohol. ”

“ The weak alcohol resulting is subjected to repeated distillation until sufficiently pure and concentrated. In the natural fermentation of fruit sugar in grape juice, during the formation of wine, the amount of alcohol is self-limited to 15, rarely 20 per cent, since the ferment is killed by a larger amount of alcohol than this.
Derivation.—The official alcohol is derived from rectified spirits, by maceration, first with anhydrous potassium carbonate, then freshly fused calcium chloride, and finally by distillation. ”