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.