Louis Pasteur: His Life and Labours

by René Vallery-Radot

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?