Pasteur, Louis

(1822–95), a scientific genius and gifted scholar, has left a body of work which impinges on physics, chemistry, microbiology, agronomy, and medicine. On the centenary of his birth in 1922, the Institut Pasteur in Paris published a monograph on his principal discoveries listed under the following headings:

• 1847: Molecular dissymmetry

• 1857: Fermentations

• 1862: Supposedly spontaneous generations

• 1863: Study of wines

• 1865: Silkworm diseases

• 1871: Study of beers

• 1877: Virus diseases

• 1880: Viral vaccines

• 1885: Rabies protection

Pasteur’s original work on what were supposedly spontaneous generations, or transformations, led him to interpret the process of alcoholic fermentation and to demonstrate that this, far from being spontaneous, was the result of intervention by living cells, yeast, using sugar for their own nutrition and transforming it into alcohol and carbon dioxide. ‘The chemical act of fermentation is essentially a phenomenon which correlates to a vital act…Now what for me constitutes this chemical division of sugar (into alcohol and carbonic gas), and what causes it? I admit that I have no idea’ (Œuvres de Pasteur, ii. 77). With something approaching genius, Pasteur understood the phenomenon without being able to provide a precise explanation; contemporary biochemistry was able to explain in detail the different stages of the chemical fermentation mechanism only in the first half of the 20th century.

During his career as a scientist, Pasteur must have devoted only three or four years to the study of wine. Yet in this time he achieved as much as a good specialist researcher would have been delighted to achieve in an entire lifetime. Not only did he apply his theories to fermentation and ensure the mastery of the basics of vinification and conservation of wines, he also perfected the art of adding tartaric acid, demonstrated the presence of succinic acid and glycerol, and made valuable suggestions about the role of oxygen in wine ageing.

But it was above all in the field of microbiological diseases of wine that Pasteur’s work has been most valued. One of the early problems assigned to Pasteur was to explain and prevent the vinegar spoilage of red wines shipped in barrel from Burgundy to England, as well as to try to explain some of the many faults in French wine which had become apparent at the time. He identified the following transformations in various wine constituents:

• mannitic acid: degradation of sugars

• ‘tourne’: degradation of tartaric acid

• bitterness: degradation of glycerol

• ‘graisse’: production of a polysaccharide

From his discovery of the various micro-organisms which caused different wine maladies, such as the acetobacter which turn wine into vinegar, came the whole science of bacteriology. He suggested that the application of heat (now called pasteurization) would destroy these micro-organisms and prevent microbial development, with beneficial effects on the quality of wine. The demonstration of the existence of these bacterial diseases was extremely fruitful for the science of oenology; it resulted in the progressive reduction in volatile acids in wine which was an important factor in raising quality. Pasteur’s research work on wine, and beer, also gave rise to his remarkable studies on the cause and prevention of infectious diseases in humans and animals.

Whatever the undoubted merits of Pasteur’s work, to which we owe the basis of wine microbiology, with all its practical consequences for vinification and wine conservation, it should be noted that he did not understand the positive role that lactic acid bacteria could have in degrading malic acid. Because of this it was particularly difficult to grasp the principles of malolactic conversion, which, in 1930, Jean ribéreau-gayon elucidated as a bacterial transformation which could be of great benefit to a wide range of wines. It was not until the 1970s that the rest of the wine world was convinced. Not without reason, Émile peynaud has written, ‘the evolution of oenology would certainly have been very different if Pasteur, instead of leaving us the basis of a perfect method of adding tartaric acid, had taught us to add malic acid’.