Pasteur’s Parasites


In the midst of the seemingly relentless medical assault on the consumption of wine, the authoritative figure of Louis Pasteur offers wine drinkers some muchneeded support. It was, after all, this giant of 19th-century science who pronounced wine to be “the most healthful and most hygienic of beverages”, and who also believed that “a bottle of wine contains more philosophy than all the books in the world”. And Pasteur’s own most extensive study of wine, the Études sur le vin of 1866, is much more than merely a scientific treatise or a record of experiments.

The extraordinary expansion of the French economy under Louis-Napoléon in the mid-19th century had exposed underlying problems in French viticulture which had been disguised while France’s economy was more agricultural than industrial, and while most French wine was drunk — and was made to be drunk — very young and in the locality where it was grown. But the commercial treaties which had opened up vast and tempting markets for French wine had also exposed those wines to the rigours of transportation and to extreme variations of climate. The result had been an unacceptable degree of spoilage. As an English wine merchant had written to Pasteur in October 1863: “The French are astonished that the trade in French wine has not expanded since the treaty of commerce. The reason for this is simple enough. To begin with we were eager to trade in these wines, but it wasn’t long before we learned to our cost that to do so led to great losses and infinite embarrassment because of the maladies to which they are subject.”

The scale of the problem was immense. In mid-19th century France two million hectares were given over to the cultivation of the vine. This produced 50 million hectolitres of wine, with an annual total value of 500 million francs. But if the problems associated with the transportation of French wine could be overcome and a global market for this luxury product be secured, that value would increase dramatically. The stakes were high.

The problem was severe enough — and the potential rewards great enough — for the Emperor himself to take an interest. In July 1863 he had commanded Pasteur to investigate the problem of wine spoilage and to propose remedies. A little over two years later, in December 1865, Pasteur presented the fruits of his research to the Emperor at the palace of Compiègne. It is not an exaggeration to say that our empirical understanding of the chemistry of wine and wine-making begins at the moment Pasteur bowed before Louis-Napoléon. Pasteur paid generous tribute to his predecessors in the study of wine and fermentation — men such as Chantal, Lavoisier, and Fabroni. Nevertheless, his own contribution was transformative.

Before 1866 men’s understanding of how to make wine, and of the changes wine undergoes as it matures, was to a large extent a question of customary practices and hunches, hampered however by prejudices which actively concealed from their eyes what was happening in front of them. (It should also be remembered that Pasteur enjoyed the immense technical advantage of powerful microscopes.) For those who went before Pasteur it was an article of belief that wine was always in chemical motion, and that defects arose when the interactions between the various chemical elements within wine — sugars, acids, alcohol — were somehow out of balance. When that imbalance occurred the various maladies to which wine was subject would arise. These defects the French had carefully discriminated. Principal among them were l’acétification, la pousse, la graisse, and l’amertume. But, although the French had an extensive and flexible vocabulary for the various kinds of spoilage, in practical terms this helped them not at all, because their beliefs also committed them to the counsel of despair that the various changes wine went through happened spontaneously and in a manner that eluded investigation. Condemned, as they believed themselves to be, to ignorance about the causes of the various maladies of wine, how could they set about discovering remedies in a scientific manner? Pasteur’s findings completely overturned this picture. On the basis of microscopic observations he showed that all the maladies of wine were the result, not of some mysterious and occult process, but rather of the proliferation within wine of fungal parasites. All organic infusions, Pasteur insisted, contain such microbial parasites. Wine, itself an organic infusion, was no exception to this rule. At a stroke the problem of wine spoilage was rescued from the domain of mystery and put within reach of a practical solution.

The key to preventing spoilage was to inhibit the activity of these parasites. Pasteur demonstrated how to do so by heating the wine for a short period up to around 60°C. This “chauffage” was enough to kill off all the fungal parasites, but — according to Pasteur — did not harm in any way a wine’s qualities of taste or bouquet. He himself had arrived at this conclusion on the basis of extensive comparative tastings. But he was dismayed to find that the wine experts of the day did not agree with him. Men’s imaginations, he regretfully concluded, exerted an influence over their taste. There was, he discovered, “une certaine prévention” in these experts against the practice of “chauffage” he was proposing. That view still holds the field today. Pasteurisation is now used only for bulk wines of low quality.

Revolutionary though his ideas were, Pasteur was nevertheless very attentive to — and respectful of — the time-honoured practices used by the vigneron. Although he naturally understood that the manner in which these beliefs were held was frequently superstitious, he could also see that they might embody a germ of practical wisdom based on generations of observation. They were “le fruit d’une expérience raisonnée”. Some of the most interesting pages in Études sur le vin describe the fieldwork Pasteur did among the vignerons of the Jura, and record in some detail the vineyard and cellar practices of that region. Indeed, it was these apparently anomalous practices, particularly those followed by the makers of vin jaune, which impelled Pasteur to undertake microscopic investigation of the “fleurs” which form on the surface of vats of fermenting wine, and so put him on the track of the role played by microbial contamination in the spoilage of wine.

It is this interleaving of history, anthropology and science which gives Études sur le vin its peculiar charm, and which makes it a classic in the literature of wine.

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