It is with some hesitancy that I attempt to give a brief summary of the results of scientific investigation into this subject, for fear of going beyond the legitimate limits of a practical work, as this book is intended preëminently for practical men. But as the work would be incomplete without it, and as a knowledge of the general phenomena of fermentation, and of the different influences to which it is subject, are of vast importance to those who will intelligently apply their principles, I give the following as but a brief resumé, and will put it as plainly as the subject will permit. Most of the ideas given below are extracted from Schutzenberger’s work on fermentation.

There are several different kinds of Fermentation, as (1) vinous, alcoholic or spirituous fermentation; (2) mucous or viscous fermentation; (3) lactic fermentation; (4) ammoniacal fermentation; (5) butyric fermentation; (6) putrifaction; and (7) acetic fermentation, or fermentation by oxidation, and others.

Alcoholic Fermentation is that which sugar undergoes under the influence of the ferment or yeast; and it is now agreed that this ferment consists principally of an aggregation of living organisms, or an assemblage of microscopic cells.

The Yeast Plant.—Our author gives them the name of saccharomyces cerevisiæ, following those who consider it to be a species of fungus, and states that it is now very generally admitted that ferments are fungi, although by some they have been considered animal in their nature. These cells are round or oval, and are from .00031 to .00035 of an inch in their greatest diameter. “They are formed of a thin and elastic membrane of colorless cellulose, and of a protoplasm, also colorless, sometimes homogeneous, sometimes composed of small granulations.” The cells are separate or united two by two. When they are deposited in a fermentable liquid, as a sugar solution or a must, small prominences are seen to arise at one or rarely two points, the interior of which is filled with protoplasm from the mother cell; these prominences grow until they have attained the size of the original cell, when the base contracts, forming a kind of neck, and immediately they separate from the mother cell, and under favorable conditions one cell produces several generations, but by degrees it loses all its protoplasm, which at last unites in granules swimming in super-abundant cellular juice. The cell ceases to produce, and dies; the membrane is ruptured, and the granular contents are diffused in the liquid. In the manufacture of beer the fermentation is of two kinds: surface fermentation and sedimentary fermentation, depending upon a high or a low degree of heat. The surface saccharomyces develop more rapidly than the others, are larger, and they bud so rapidly that the cells which issue from each other do not separate, but remain attached, forming ramified chains of from six to twelve or more buds. The bubbles of rising gas have a greater hold on these chaplets than on single cells, which causes the newly formed yeast to rise to the surface during active fermentation. These organisms or fungi produce spores which are sown on the surface of fruits, and get into the juice by crushing, when they commence their reproduction by budding. So that the basis or cause of the phenomena which we call fermentation is the growth and reproduction of yeast or ferment, which is made up principally of the minute organisms just described.

Functions of Yeast.—Yeast is a living organism, belonging to the family of fungi, genus Saccharomyces, destitute of mycelium, capable of reproduction, like all the elementary fungi, by buds and spores. Its composition singularly resembles that of other vegetable tissues, and especially the plants of the same family. It does not differ essentially from other elementary cells, unprovided with chlorophyll.

Normal Conditions of the Life of Yeast.—The conditions which our author calls normal in the life-history of yeast, are those in which it develops itself and increases with the greatest activity and energy. They are of two orders, physical and chemical.

With respect to physical conditions, it is only necessary to notice the temperature. That most favorable to the nutrition of yeast, and that which is found advantageous to other cellular vegetable organisms, is between 25° C. and 35° C. (77° and 95° F.) Above and below these limits, the vital manifestations do not cease until we descend below 9° C. (48.2° F.), or rise above 60° C. (140° F.), the temperature at which albuminoid principles begin to coagulate.

With regard to the chemical conditions, our author says that the most favorable medium is that which contains the most appropriate nutritive elements. And as yeast contains water, mineral salts, especially potassium, magnesium, and calcium phosphates, therefore water and the alkaline and alkaline-earthy phosphates will be necessary. We find, besides, a great proportion of nitrogenous substances, either albuminous or otherwise; and therefore the food of yeast must contain nitrogen. It is supposed, however, that the cells are not directly nourished by albuminoids in the juices of fruits, the wort of beer, or yeast water, but by analogous compounds contained in them, which have the property of passing by osmose through the membranes; for the albuminoids themselves, it is said, cannot pass through. Pasteur has shown by his experiments, that mineral salts are absolutely necessary to the development and nutrition of the yeast cell; and Mayer follows him with details as follows: Preparations of iron, in small quantities, seem to have no influence; in larger proportions, they are injurious. Potassium phosphate is indispensable, and the absence of lime has little effect. Magnesium, on the contrary, appeared to be very useful, if not indispensable. The combinations of sodium present no material effects.

Sugar is one of the most important elements in the nourishment of the yeast cells, and Pasteur has shown that, in alcoholic fermentation, a part of the sugar is fixed in the yeast, in the state of cellulose or some analogous body, for, when the fermentation is completed, it is found that more yeast is present than at the commencement. Water is necessary, and the yeast cell manifests its activity, develops and is nourished within the limits of 40 and 80 per cent. of water, though yeast, dried with precaution, may regain its power when moistened. And the fact that a solution containing over 35 per cent. of sugar will not ferment, is explained on the theory that such a solution takes from the cells by osmose a sufficient quantity of water to lower their hydration below 40 per cent. The cells of the Saccharomyces cerevisiæ, introduced into a liquid medium, absorb oxygen with great rapidity, and develop a corresponding quantity of carbon dioxide. This constitutes respiration, comparable to that of animals. By careful experiments it has been shown that yeast breathes when placed in contact with dissolved oxygen, and the respiration is more active than that of fishes, and it plays as important a part in the life of those minute vegetable cells as in the higher forms of vegetable and animal life. Oxygen is furnished by atmospheric air, and fermentation is more rapid when a large surface of the liquid is exposed, and then the budding is more active.

Action of various Chemical and Physical Agents.—“It has long been known that certain chemical compounds, especially those which coagulate albuminous substances, and disorganize the tissues, or which, by their presence in sufficient quantities, are incompatible with life, are opposed to fermentation; such are the acids and alkalies in suitable proportions, silver nitrate, chlorine, iodine, the soluble iron, copper, and lead salts, tannin, phenol, creosote, chloroform, essence of mustard, alcohol when its strength is above 20 per cent., hydrocyanic and oxalic acids, even in very small quantities.

Experiments have shown that sparks of electricity passing through yeast do not modify its power of changing cane sugar into glucose, nor its activity as an alcoholic ferment. Fermentation is slower in the dark, and also in a vacuum. Flour of sulphur did not sensibly affect fermentation, but the carbonic acid evolved contained sulphuretted hydrogen. Sulphurous acid, however, arrests fermentation. Yeast is always acid, but an addition of an excess of different acids arrests the decomposition of sugar. If one hundred times the amount of acid contained in the yeast is added, fermentation does not take place.

M. Dumas has shown the action of various salts on yeast, but the subject has little if any interest for the wine maker.

Viscous or Mannitic Fermentation is also excited, according to Pasteur, by special ferment acting on glucose, transforming it into a kind of gum or dextrin, mannite, and carbon dioxide. This ferment is also formed of small globules united as in a necklace, whose diameter varies from .000047 to .000055 of an inch. These globules, sown in a saccharine liquid containing nutritive nitrogenous matter and mineral substances, always give rise to viscous fermentation. One hundred parts of cane sugar give: mannite, 51.09; gum, 45.48; and carbon dioxide, 6.18. The liquids which are most apt to produce viscous fermentation can also undergo lactic and butyric fermentation, but in this case the organized forms of life which are developed in the liquid are of a different nature. The conditions of action necessary to these gummy and mannitic ferments are the same as those which suit alcoholic ferment. The most favorable temperature is 30° C. (86° F.) This fermentation is what gives rise to the disease of wines, called by the French la graisse, or ropiness. White wine is more subject to it than red, and it is generally due to the want of tannin. (See Ropiness.)

Lactic Fermentation is the transformation which certain sugars, as sugar of milk and grape sugar, undergo, and by which they are changed into lactic acid. This takes place in the souring of milk. The most favorable temperature for it seems to be about 95° F. This also depends on a special ferment. Sugar solutions are also capable of butyric fermentation and putrefaction, and we generally see viscous, lactic, and butyric fermentation appear in succession.

Acetic Fermentation is to the wine maker and wine dealer, after alcoholic fermentation, the most important.

Fermentable matter and ferment are also concerned in it, but oxygen also is necessary.

It has long been known that the alcohol contained in fermented liquids, such as wine, beer, etc., will disappear under certain circumstances, and give place to vinegar or acetic acid, and that the air, or rather its oxygen, plays a part in this reaction.

To the chemist the reaction is simple, and is formulated thus:

or the oxidation may take place by two reactions, with the production of an intermediate product, aldehyde:

According to Pasteur, the oxidation of alcohol is the consequence of the action of a ferment or cryptogam, Mycoderma aceti, and it makes its appearance on the surface of liquids, while in acetic fermentation, in the form of a continuous membrane, mother of vinegar, either wrinkled or smooth, which is generally formed of very minute elongated cells, whose greater diameter varies from .000059 to .000118 of an inch; these cells are united in chains, or in the form of curved rods. Multiplication seems to be effected by the transverse division of the fully developed cells. The conditions of nutrition are similar to those suitable to the alcoholic ferment, the hydro-carbon matter being supplied by dilute alcohol. It may, however, be supplemented by the acetic acid itself; for if the process is left too long to itself, the vinegar loses its strength by being consumed. The most favorable temperature is between 76° and 82° F.

Antiseptic agents, which arrest the development of beer yeast, act in the same manner on the Mycoderma aceti. Sulphurous acid is especially active in this manner; hence the use of the sulphur match in sulphuring wine casks.

There is another ferment, Mycoderma vini, or flowers of wine, which is found in wine and other alcoholic liquids exposed to the air when fermentation is over or has become languid, which resembles in many respects the acetic ferment. It has the power of producing alcoholic fermentation, and is supposed by some to be derived from the Saccharomyces. Like the Mycoderma aceti, it is developed on the surface of fermented alcoholic liquors, in the form of smooth or wrinkled films or membranes, but thicker and more compact. It grows with great rapidity, and it has been calculated that one cell would, in forty-eight hours, produce about 35,378 cells. These cells are of various forms, ovoid, ellipsoidal, and cylindrical, with rounded extremities. The ovoid cells have their greater diameter about .000236, and their smaller one, .000157 of an inch. The cylinders have their diameters .00047 × .000118 in. The nutritive principles are the same as those of the mother of vinegar: alcohol, salts and nitrogenous compounds. It also appears capable of utilizing for nutrition the secondary products of alcoholic fermentation, such as succinic acid and glycerine. Its development is most active between 61° and 86° F. (See Sherry.)

Origin of Ferments.—In order to produce the different kinds of fermentation, the necessary ferment must be added, unless it is already contained in the fermentable matter or in the air. In the manufacture of beer and bread, yeast must be used; the other kinds of fermentation, except alcoholic, can generally be produced by the ferments or their spores furnished by the atmosphere; but Pasteur, in the course of his investigations, never produced alcoholic fermentation from spores found in the air. But the germs of the Saccharomyces cerevisiæ and of Mycoderma vini seem to be found only on the surface of fruits, and their stems.[2]

These different germs, however, are all found in the must of grapes, and in wine, and are ready to develop whenever favorable conditions offer themselves, and produce diseases in the wine. It is found that these germs are killed by raising the temperature of the liquid to 140° F., and hence the process of heating wines to preserve them (which see).

Leaving the germ theory of fermentation, we will pass to what is of more practical importance.

ALCOHOLIC FERMENTATION
IN WINE MAKING.

Vinous or Alcoholic Fermentation transforms the juice of the grape into wine, and, as already shown, is caused by the yeast or ferment, which finds its way into the must; and by this fermentation the sugar of the grape is changed principally into alcohol, and carbon dioxide, or carbonic acid gas. And in order to show the relations between the sugar and the alcohol produced, it is necessary to say something about the chemical constituents of each.

Sugar.—In general terms, cane sugar may be expressed by the chemical formula, C₁₂H₂₂O₁₁, or, in other words, one molecule contains 12 atoms of carbon, 22 of hydrogen, and 11 of oxygen.

And the general term glucose, or grape sugar, may be expressed by the formula C₆H₁₂O₁₆, or one molecule contains 6 atoms of carbon, 12 of hydrogen, and 6 of oxygen.

If, instead of using the word atoms, we use the word pounds, the chemical formula may be made clear to the unscientific. Taking the formula for cane sugar, already given, it simply means that 342 pounds contain the following ingredients, in the following proportions:

And the formula for glucose means that 180 pounds contain:

And the formula for water means that 18 pounds contain:

In fermentation, it is glucose which is immediately transformed, although cane sugar ferments also; but, before doing so, it becomes changed or inverted into glucose, and one molecule takes up a molecule of water, and produces two of glucose, thus:

Or, in the production of alcohol, 100 lbs. of pure cane sugar are equal to 105.26 lbs. of pure grape sugar.

The general formula for alcohol is C₂H₆O, and for carbonic acid CO₂.

Alcohol by Weight and by Volume.—The quantity of alcohol contained in a given mixture of alcohol and water may be expressed as per cent. by weight, or per cent. by volume. The first method is usually used by chemists, and the second in commerce. If we have 100 lbs. of a mixture of alcohol and water of which 10 lbs. are alcohol and 90 lbs. water, it contains 10 per cent. of alcohol by weight. If, however, we have 100 gallons of a mixture in which there are 10 gallons of alcohol and 90 gallons of water, we say that it contains 10 per cent. by volume of alcohol. This will serve to illustrate the meaning of the terms per cent. by volume and by weight, although it is well known that, owing to shrinkage, 10 gallons of alcohol and 90 gallons of water do not produce quite 100 gallons of mixture.

Whenever merchants and wine makers use the term per cent. of alcohol, they mean per cent. by volume or measure; and whenever the expression is used in this work, it is used in that sense, unless otherwise expressed.

Fermentation—Its Products.—Per cent. Sugar to per cent. Alcohol.—In theory, glucose, during the process of fermentation, is entirely changed into alcohol and carbonic acid; the two substances produced containing the same elements as glucose, and no others. If there was no loss of sugar, or degeneration, as it is called, the reaction would be exactly expressed as follows:

And the old authorities said, if 180 parts of glucose produce 92 of alcohol, 100 will produce 51.1111, thus:

180 : 92 :: 100 : x = 51.1111,

And again, if it takes 100 parts of glucose to produce 51.1111 alcohol, how much does it take to produce 1 per cent. by weight?

51.1111 : 1 :: 100 : x = 1.9565.

These figures are now true only of that part of the sugar which is transformed into alcohol and carbon dioxide.

Different Authors.—Pasteur has shown that a portion of the glucose was changed into succinic acid and glycerine, and as the result of one of the experiments which he gives, out of a large number, it appears that 100 parts of glucose produce about 48.46 of alcohol, and it would require 2.063 to produce 1 per cent. of alcohol by weight, and 1.65 to produce 1 per cent. by volume.

But this eminent chemist’s experiments were conducted in the laboratory, and under the most favorable circumstances, so that no loss by evaporation could occur—conditions under which fermentation on a large scale is never carried on.

Dr. Guyot states that it takes about 1.5 per cent. of grape sugar to produce 1 per cent. of alcohol, which is even less than is required according to Pasteur, and is manifestly too little. And the statement has been made, that a must containing 20 per cent. of sugar will produce 13 per cent. of alcohol, which is impossible.

J. J. Griffin quotes Pasteur, and estimating the average loss to be 4½ per cent. of the sugar, deduces the figures .4881 as the per cent. by weight of alcohol produced by 1 per cent. of grape sugar. Dubief says that it takes 1.7 per cent. of cane sugar to produce 1 per cent. of alcohol by volume. Mr. Joseph Boussingault gives his experiments on musts fermented in small vessels under conditions similar to those under which fermentation is carried on in wine making on a large scale; and the result of his researches is that the product in alcohol is about 90 per cent. of what the chemical theory calls for: say, .46 by weight for 1 of sugar, or 1.7 + glucose for 1 per cent. of alcohol by volume. Mr. M. Boussingault gives it as the result of his experiment, that it takes 1.8 per cent. of sugar to produce 1 per cent. of alcohol.

So that it is pretty safe to say that it takes on an average about 1.8 of sugar to make 1 of alcohol, making some allowance for loss by evaporation, etc.

As has already been stated in the chapter on Musts, 1 per cent. for every 12 should be deducted from the percentage of sugar shown by the hydrometer for other matters than sugar.

If, therefore, we have a must which shows 24° by the saccharometer, we will deduct two, and call the remainder 22, sugar. Although it is not strictly correct to say that 22 divided by 1.8 will give the per cent. of alcohol which may be expected after fermentation, owing to the well known variation between per cent. by weight and by volume, as the figures increase, yet it is sufficient for all practical purposes.

Let us then divide 22, the supposed sugar in the must, by 1.8, the amount required to produce 1 per cent. of alcohol, and we obtain 12 and a fraction. Now the total indication by the saccharometer was 24 per cent.; if we divide this by two we get the same result in round numbers.

Hence the rule: one-half of the figure indicating the total per cent. by the saccharometer (hydrometer) is approximately the per cent. of alcohol to be expected in the wine.

Owing to the fact that the loss by evaporation and degeneration may vary greatly in different cases, this will be only a rough estimate, but it will prove as satisfactory as any method that can be adopted, and it corresponds very closely with the statement made by N. Basset, that in actual practice, a must of 20 per cent. gives only 7.88 per cent. of alcohol by weight, which corresponds with 10 per cent. by volume, nearly; and it is the rule given by Petiot and Dr. Gall for a natural must.

It seems, however, from what follows below, that this is only true of a normal must, but that a different rule applies to one of a very high degree of sugar.

Limits of Sugar and Spirit.—It is said that when a solution or a must contains over 35 per cent. of sugar, it will not ferment; nor will a wine or other alcoholic mixture which contains 20 per cent. of spirit ferment. Boireau says that the maximum of alcohol which a wine can attain by the fermentation of the richest must is between 15 and 16 per cent., and those wines which show a higher degree have been fortified. He says that the highest degree of spirit ever observed by him in a natural red wine was 15.4 per cent., when it was a year old; from that time the strength diminished, but the wine always remained sweet.

There is, however, a remarkable case given, and which seems to be well authenticated, of an Australian wine which contained naturally, by fermentation, 32.4° of British proof spirit, which is equal to about 18.21 per cent. And Vizitelli states that Mr. Ellis, of the firm of Graham & Co., asserts that perfectly fermented Alto Douro wine will develop 32° proof spirit, or 18 per cent. of alcohol, and when made exclusively from the Bastardo grape, as much as 34°, or about 19 per cent. of spirit. And Mr. Vizitelli adds that he is satisfied from what he saw at Jerez, that sherry wines which have had merely 1 or 2 per cent. of spirit added to them will in the course of time indicate 34°. To produce these results would seem to require more than 35 per cent. of sugar, according to our rule; but while it is approximately correct to say that 2 per cent. of sugar produces 1 per cent. of alcohol as long as we are dealing with a must of 24 or 25 per cent. and under, it may not be true of a must of 30 to 35 per cent., for the other solid matters probably do not increase in proportion to the sugar. Therefore, to reconcile this high degree of alcohol with the statement that a must containing over 35 per cent. of sugar will not ferment, we must use Pasteur’s figures, and then we will find that by them 35 per cent. of sugar is capable of producing over 20 per cent. of alcohol.

Temperature.—The temperature most favorable to fermentation—that is, at which it commences most promptly, and goes on the most rapidly—is between 77° and 95° F., and it does not cease until the temperature descends below 49°, or rises above 140°. If the temperature is favorable, fermentation ought to commence in ten or twelve hours from the time the pomace is put into the vat, or the juice into the barrel. In countries where the weather is cold at the wine making season, it is necessary that the grapes should be gathered in the heat of the day, or fermentation will be long in commencing; and if the weather continues unfavorable, so that the grapes do not become warmed by the sun, it is even necessary to heat a portion of the must artificially, and pour it into the vats or casks, or to raise the temperature of the fermenting house.

Mr. Maumené also recommends that the vats be surrounded with mats of loose straw, four or five inches thick, to be kept in place by a covering of linen cloth; and in this way the temperature produced by the fermentation may be maintained in cool weather, without resorting to fires in the fermenting house.

It is not necessary, however, that the temperature of the surrounding atmosphere should be as high as that indicated as most favorable to fermentation; for it commences readily in a temperature of about 70°, and the liquid will soon rise to 85° or 95°, by the heat developed during the process; and unless the surrounding temperature descends below 65°, this heat will be maintained, and the fermentation will not be checked. Dr. Guyot says, however, that, to make fine wines, it should be maintained at 68°, at least; and that, in other cases, it should not be allowed to fall below 60°.

Fermenting Houses.—It is important not only that fermentation should commence promptly, but that it should be maintained regularly; and although a great amount of heat is developed by fermentation, yet the must is liable to cool during the night and cold days, unless the vats and casks are protected from the change of temperature, whereby the fermentation may be checked, to the injury of the wine. The natural conclusion is that the must ought to be fermented in closed places. In California, however, it is not necessary to construct the fermenting house with the same care required in colder climates, where it is deemed desirable to furnish them with double windows and doors. It cannot be denied that good wine is made in this State, in places where the vats remain out of doors, shaded only by trees; but the practice is not to be encouraged, for the fermentation will be checked if the temperature of the surrounding atmosphere goes to 60° and below. In constructing a fermenting house, it ought to be so arranged, when practicable, as to be on a lower level than that of the stemmer and crusher, and higher than the cellar; for then the pomace and must can be run immediately into the vats and casks, and, after the first fermentation, the wine can be drawn off through a hose into the casks in the cellar, thereby saving time and labor.