Having thus solved the problem of spontaneous generation, a problem which was but a parenthesis forced upon his attention, Pasteur returned to fermentation. Guided by his studies on vinegar and other observations of detail, he undertook an inquiry into the diseases of wine. The explanations of the changes which wine was known to undergo rested only on hypothesis. From the time of Chaptal, who was followed by Liebig and Berzelius, all the world believed wine to be a liquid in which the various constituents react upon each other mutually and slowly. The wine was thought to be continually 'working.' When the fermentation of the grape is finished, equilibrium is not quite established between the diverse elements of the liquor. Time is needed for them to blend together. If this reciprocal action be not regular, the wine becomes bad. This was, in other words, the doctrine of spontaneity. Without support from carefully reasoned experiments, these explanations could not satisfy Pasteur, especially at a moment when he had just been proving that there was nothing spontaneous either in the phenomena of fermentation or in animal and vegetable infusions.

Pasteur tried first of all to show that wine does not 'work' as much as it was supposed to do. Wine being a mixture of different substances, among which are acids and alcohol, particular ethers are no doubt formed in it in course of time, and similar reactions perhaps take place between the other constituents of the liquid. But if the exactitude of such facts cannot be denied, based as they are upon general laws, confirmed and extended by recent inquiries, Pasteur thought that a false application was made of them when they were employed to explain the maladies of wine, the changes which occur in it through age—in a word, the alterations, whether good or bad, which wines are subject to. The 'ageing' of wine soon appeared to him to consist essentially in the phenomena of oxidation, due to the oxygen of the air which dissolves and is diffused in the wine. He gave manifest proofs of this. I will only mention one of them. New wine inclosed in a glass vessel hermetically sealed keeps its freshness; it does not 'work,' it does not 'age.' Pasteur demonstrated besides, that all the processes of wine-making are explained by the double necessity of oxygenising the wine to a suitable degree, and of preventing its deterioration. In seeking for the actual causes of injurious alterations, Pasteur, always obedient to a preconceived idea, while carefully controlling it with the utmost rigour of the experimental method, asked himself whether the diseases of wine did not proceed from organised ferments, from little microscopic vegetations? In the observed alterations, he thought, there must be some influences at work foreign to the normal composition of the wine.

This hypothesis was verified. In his hands the injurious modifications suffered by wines were shown to be correlative with the presence and the multiplication of microscopic vegetations. Such growths alter the wine, either by subtracting from it what they need for their nourishment, or, and principally, by forming new products which are the effect of the multiplication of these parasites in the mass of the wine.

Everyone knows what is meant by acid wine, sharp wine, sour wine. The former experiments of Pasteur had clearly shown that no wine can become acid, sharp—can, in a word, become vinegar—without the presence of a little microscopic fungus known by the name of mycoderma aceti. This little plant is the necessary agent in the condensation of the oxygen of the air, and its fixation on the alcohol of the wine. Chaptal, who published a volume on the art of wine-making, knew of the existence of these mycoderm flowers; but to his eyes they were only 'elementary forms of vegetation,' which had no influence whatever upon the quality of the liquid. Besides the mycoderma aceti, which is the agent of acetification, there is another mycoderm called mycoderma vini. This one deposits nothing which is hurtful to the wine, and its flowers are developed by preference in new wines, still immature, and preserving the astringency of the first period of their fabrication.

The requirements of the two sorts of flowers are such that even when the flower of vinegar is sown on the surface of a new wine, no development takes place. Conversely, the mycoderma vini sown on wines that have grown old in casks or in bottles will refuse to multiply. The mycoderma vini produces no alteration in the wine; it does not turn the wine acid. In proportion as the wine grows old the flower tends to disappear, the wine 'despoils' itself, to use a technical expression; physiologically speaking, the wine loses its aptitude to nourish the mycoderma vini, which, finding itself progressively deprived of appropriate nourishment, fades and withers. But it is then that the mycoderma aceti appears, and multiplies with a facility so much the greater that it draws its first nourishment from the cells of the mycoderma vini. The mycoderma aceti has played so large a part in the early pages of this book that it is not necessary to go back upon it here.

There is another disease very common among wines when the great heat of summer begins to make itself felt in the vintage tubs. The wine is said to turn, to rise, to spurt. The wine becomes slightly turbid and at the same time flat and piquant. When it is poured into a glass, very small bubbles of gas form like a crown upon the surface. On placing the glass between the eye and the light and slightly shaking it, one can distinguish silky waves shifting about and moving in different directions in the liquid. When the turned wine is in a cask, it is not unusual to see the bottom of the cask bulge a little and sometimes a leakage takes place at the joints of the staves. If a little opening is made, the wine spurts out, and that is the reason why the wine is said to spurt.

Authors who have written on the subject of wine attributed this malady to the rising of the lees. They believed that the deposit which always exists in greater or less quantities in the lower part of the cask rises and spreads itself into all the mass of the wine.

Nothing can be more inexact. If this phenomenon is sometimes produced—that is to say, if the deposit rises into the mass of wine—the effect is due to a sudden diminution of the atmospheric pressure, as in times of storm, for example. As the wine is always charged with carbonic acid gas, which it holds in solution from the moment of fermentation, one can conceive that a lowering of barometric pressure would cause the escape of some bubbles of carbonic acid. These bubbles, rising from the lower part of the cask, may disturb a portion of the deposit, which then mixes with the wine and renders it turbid. But the real cause of the disease is quite different. The turbidity is without exception due to the presence of little filaments of an extreme tenuity, about a thousandth part of a millimeter in diameter. Their length is very variable. It is these which, when the wine is agitated, give rise to the silky waves just referred to. Often the deposit of the casks leaves a swarm of these filaments entangled in each other, forming a glutinous mass, which under the microscope is seen to be composed entirely of these little filaments. In acting upon certain constituents of the wine particularly upon the tartar, this ferment generates carbonic acid. The phenomenon of spurting is then produced, because when the cask is closed the internal pressure of the liquid augments. The sparkling and the crown of little gas-bubbles, observed when the turned wine is poured out into a glass, is similarly explained. In a word, the disease of turned wine is nothing else than a fermentation, due to an organised ferment which, without any doubt, proceeds originally from germs existing on the surface of the grapes at the moment of gathering them, or on spoilt grapes such as are found in every vintage. It is very rare not to find this parasite of turned wine in the deposit of the wine at the bottom of the casks, but the parasite is not troublesome unless it multiplies very largely. Pasteur found the means of preventing this multiplication by a very simple remedy, equally applicable to other diseases of wines, such as that of bitterness or greasiness (maladie de la graisse).

Many wines acquire with age a more or less bitter taste, sometimes to a degree which renders them unfit for consumption. Red wines, without exception, are subject to this disease. It attacks by preference wines of the best growth, and particularly the finest wines of the CÔte-d'Or. It is once more a little filamented fungus which works the change; and not only does it cause in the wine a bitterness which little by little deprives it of all its better qualities, but it forms in the bottles a deposit which never adheres to the glass, but renders the wine muddy or turbid. It is in this deposit that the filaments of the fungus float. If white wines do not suffer from this disease of bitterness, they are exposed, particularly the white wines of Orleans and of the basin of the Loire, to the disease of greasiness. The wines lose their limpidity; they become flat and insipid and viscous, like oil when poured out. The disease declares itself in the casks or in the best-corked bottles. M. Pasteur has discovered that the greasiness of wines is likewise produced by a special ferment, which the microscope shows to be formed of filaments, like the ferments of the preceding diseases, but differing in structure from the other organisms, and in their physiological action on the wine.

In short, according to Pasteur's observations, the deterioration of wines should not in any case be attributed to a natural working of the constituents of the wine, proceeding from a sort of interior spontaneous movement, which would only be affected by variations of temperature or atmospheric pressure. They are, on the contrary, exclusively dependent on the development of microscopic organisms, the germs of which exist in the wine from the moment of the original fermentation which gave it birth. What vast multitudes of germs of every kind must there not be introduced into every vintage tub! What modifications do we not meet with in the leaves and in the fruit of each individual spoilt vine! How numerous are the varieties of organic dust to be found on the stems of the bunches, on the surface of the grapes, on the implements of the grape gatherers! What varieties of moulds and mildews! A vast proportion of these germs are evidently sterilised by the wine, whose composition, being at the same time acid, alcoholic, and devoid of air, is so little favourable to life. But is it to be wondered at that some of these exterior germs, so numerous, and possessing in a more or less marked degree the anaerobic character, should find at certain moments, in the state of the wine, the right conditions for their existence and multiplication?

The cause of these alterations having been found, a mode of preventing the development of all these parasites had still to be sought. Pasteur's first endeavour was to discover some substance which would be antagonistic to the life of these ferments of disease, while harmless to the wine itself, and devoid of any special smell or taste. But in this research success was dependent on too many conditions to be easily attainable. After some fruitless trials, the thought occurred to Pasteur of having recourse to heat. He soon ascertained that, to secure wine from all ulterior changes, it sufficed to raise it, for some instants only, to a temperature of from fifty-five to sixty degrees. His experiments were first directed upon the disease of 'bitterness.' He procured some of the best wines of Burgundy, wines of Beaune, and of Pomard, of different years—1858, 1862, and 1863. Twenty-five bottles were left standing forty-eight hours to allow all the particles suspended in the wine to settle; for, however clear wine may be, it always produces a slight deposit. Pasteur then decanted the wine with minute care, by means of a syphon of slow delivery. This last precaution was necessary to prevent the deposit from being stirred up. When there remained in each bottle only one cubic centimeter of liquid, Pasteur shook the bottle, and then examined with the microscope the residue of each bottle. He perceived in each case distinct filaments of ferment. The wines, however, were not in the least bitter to the taste, but the germs of a possible evil were there—an evil which would have been first detected by the palate when the little fungus had fully developed.

Without uncorking it, Pasteur then heated a bottle of each of these wines. The heating was carried to a temperature of sixty degrees (140° Fahr.). After the cooling of the bottles he laid them by the side of other unheated bottles of the same wines in a cellar, the temperature of which varied in summer between thirteen and seventeen degrees. Every fifteen days Pasteur inspected them. Without uncorking the bottles, he held them up against the light, so that he could see the sediment at the bottom of each bottle, and thus detect the least formation of deposit. In less than six weeks, particularly in the wine of 1863, a very perceptible floating deposit began to form in all the unheated bottles. These deposits gradually augmented, and on examining them with the microscope they were seen to be formed of organised filaments, mixed sometimes with a little colouring matter which had become insoluble. No deposit appeared in the heated bottles.

The idea of heating wines does not belong to Pasteur. Those who love to search into questions of priority will find described in the works of Latin agriculturists various methods for the preservation of wine, based on the employment of heat. To give the wine durability, they sometimes added to the vintage variable quantities of boiled must, reduced to half or two thirds, in which orris, myrrh, cinnamon, white resin, and other ingredients, were infused. But, to cite examples nearer our own time, Appert, whose preserves have become so popular, relates that he sent to St. Domingo some bottles of Beaune which had been previously heated to seventy degrees, and that he compared, on their return into France, two bottles of this wine with a bottle which had remained at Havre, and also with other bottles of the same wine which had remained in his cellar, neither of which had undergone the operation of heating. The superiority of the wine which came from St. Domingo, said Appert, was incontestable. Nothing could equal its delicacy or its perfume. But Appert did not by any means describe the wine of the two bottles which remained in France as either injured or diseased. His remark was based upon an incomplete observation. It simply stated the fact, which indeed was previously known, that a long voyage, added to the employment of heat, had an excellent effect upon the Beaune. This incident had been so completely forgotten, that it was only in 1865 that Pasteur, during the historical researches which preceded his 'Etudes sur le vin,' accidentally met with this story of the bottles of St. Domingo, and hastened to communicate it to the Academy. But in reference to this question of heating, a discussion arose as to priority, which was quite unexpected by him. A Burgundian wine grower, M. de Vergnette, having first proposed the congealing of wines as a protective influence, had afterwards spoken, without much precision, of heat as another means of preservation. On this ground he claimed for himself a great part of the invention of Pasteur's process. 'If, after having subjected some specimens of wines which are to be sent abroad to the ordeal of heating,' said M. de Vergnette, 'one sees that they have been able to resist the action of the heat, then they may safely be shipped. In the contrary case they ought not to be sent.' According to M. de Vergnette, it was to the composition of the wine, its robust condition, and good constitution, that it owed its power of supporting the heating process. Pasteur had no difficulty in demonstrating that these assertions are contradicted by experiment. Wine never changes by the moderate application of heat when air is excluded; and it is precisely when of doubtful soundness that it should be subjected to the process of heating. This operation does not alter it any more than would be the case if it were in a perfectly healthy state. All wines may undergo the action of heat without the least deterioration, and one minute's heating at the proper temperature suffices to insure the preservation of every kind of wine. Thanks to this operation, the weakest wine, the most disposed to turn sour, to become greasy, or to be threatened with bitterness, is insured against injurious change.

Nothing is more simple than to realise the condition of heating in bottles. After having firmly tied down the corks, the bottles are placed in a water-bath. An iron basket is here useful. The water ought to rise up to the wire of the cork. Among these bottles is placed a bottle of water, into which the bulb of a thermometer is plunged. The bath being heated, as soon as the thermometer marks fifty or sixty degrees Centigrade, the basket is withdrawn. The subsequent soundness of the wine is thus insured.

But if Pasteur had overlooked nothing in his efforts to prevent or arrest the evil changes of wine, he still saw that full confidence was not felt in the efficacy of a process which must, it was thought, damage the taste, or the colour, or the limpidity of the wine. After having invited the judgment of people in society, whose preference, if they felt any, was generally for the heated wines, Pasteur wished to have a more decisive opinion. He addressed himself first to wine merchants and others practised in detecting the smallest peculiarities of wines; and afterwards he organised a grand experiment in tasting. On November 16, 1865, a sub-commission, nominated by the representative commission of the wholesale wine-sellers of Paris, repaired to the École Normale and examined a considerable number of specimens. After a series of tastings, which recognised, if not a superiority over the heated wines, at least a shade of imperceptible flavour, which, however, it was admitted, would escape nine-tenths of the consumers, Pasteur, fearing that there remained still in the mind of the majority of the commission a slight prejudice against the operation of heating, and that imagination, moreover, had some share in determining shades of flavour, proposed that at the next sitting there should be no indication which of the samples of wine had been heated and which had not. The commission, having no other desire than to arrive at the truth, at once accepted this proposition.

The resulting uncertainty as to whether the heated or the unheated wines were to be preferred was so absolute as to be comical. It is unnecessary to say that the heated wines had not experienced the least alteration. At a certain point Pasteur, who was astonished at the extraordinary delicacy of the palate of these tasters, employed a little trickery. He offered them two specimens absolutely identical, taken out of the same bottle. There were preferences, very slight it is true, but preferences gravely expressed for one or the other glass. The commission, making allusion in its report to this special tasting experiment, was the first to allow with a good grace that the differences between the heated and non-heated wines were insignificant, imperceptible if they existed, and that the imagination—added the report—was not without considerable influence in the tasting; since the members of the commission had themselves fallen into a little experimental snare.

Thus Pasteur, after having revealed the causes which determine the alterations of wines, had found the means of practically neutralising them. By the application of heat, and without producing any change in the colour or flavour of the wines, he had been able to insure their limpidity, and to render them capable of being indefinitely preserved in well-closed vessels. If these wines, being afterwards exposed too long to the air, were again threatened with alteration, it was because the air brought to them new living germs of those ferments which had been destroyed by the heat. But germs from this source are so trifling compared with those contained in the wine itself, that one may almost say the heating process renders the wine unalterable even after it has been rebottled in contact with the air. Thus, by a series of experiments which left nothing to chance, one of the greatest economic questions of the day was solved. Wines could be kept or transported into all countries without losing their flavour or their perfume. These experiments of the laboratory were destined to have an extensive application; for very soon arrangements were made for heating wine in barrels, the inquiry thereby assuming the proportions of a public benefit.