Louis Pasteur was a truly gifted scientist who is best known for his contribution to the germ theory of disease, and is generally acknowledged as the founder of the science of microbiology. He also made incalculable contributions to the brewing industry - which is odd, considering the fact that he disliked beer. He had, instead, a different motivator for his work - vengeance.

By 1860, Pasteur had already accumulated an excellent reputation as a chemist. He was interested primarily in pure research, and wrote an appeal to the emperor Napoleon III for funding for a laboratory at his university, the Ecole Normale. The emperor was receptive to his wishes, and by 1868 the project was underway. Pasteur's apartment was just across the street from the building site, and he eagerly watched the process of construction. In October of 1868, however, he was felled by a massive cerebral haemorrhage. As soon as he recovered his powers of speech, he began asking about the progress of the lab, but received only vague and unsatisfactory answers. In time, he was finally able to drag himself to his window, where he made a horrifying discovery - construction had ceased almost immediately after his collapse. The emperor promised him that work would resume at once - but almost immediately, in July of 1870, construction was halted again by the onset of the Franco-Prussian War. Within three months, the emperor had been captured, and Paris was under siege. The highly patriotic scientist's nation was being humiliated; his laboratory was endangered, his son - who had enlisted in the army - was gravely ill with Typhoid, and he was too weak from his illness to do anything about any of it. In time, the war ended - but his almost maniacal hatred of Germany would never fade.

Before long, Pasteur's fertile mind had conceived a brilliant plan for the destruction of his nemesis. He knew that Germany's main export was beer - German beer tasted better and kept longer, and outsold most local brews throughout Europe. Thus, his plan was to create the mightiest beer the world had ever known, a beer that would strike terror into the hearts of Germans, and plunge their economy into flaming ruin forever. A beer possible only due the the wonders of modern science: a beer he called the beer of revenge.

Pasteur knew that the secret to beer was the yeast that fermented sugars leached from malted barley. Until the 1860s, all beers were dark porters, stouts, and ales, but in 1860 German brewers had developed yeasts that would remain active at temperatures near freezing. They sank to the bottoms of the fermentation barrels and had little effect on the taste of the beer, producing brews that were light in color, flavor, and body, and kept extremely well. These were called "lagers," as the process of bottom-fermentation was known as "lagering." Pasteur determined that spoilage was caused by contaminating organisms, and quickly determined both which strains caused the most problems and methods for excluding them from large-scale brewing. In addition, he developed strains of yeast that behaved the same way as the German lager-producing yeast, but more quickly and at higher temperatures.

The scientist's chief ally and secret weapon was Pierre-Augustin Bertin, the Head of Scientific Studies at Ecole Normale, a close and long-time friend - and a conoisseur of fine beer. Bertin methodically drank his way through Pasteur's experimental brews, providing insightful appraisals and commentary, as well as good humor to offset Pasteur's grim intensity. Before long, they had both the tools and techniques necessary for the beer of revenge - and set out to spread them across the continent.

Pasteur travelled across Europe, demonstrating his secrets to commercial brewers - save those in Germany. In addition, he wrote a book - Studies on Beer - which was an instant success and became the authoritative and indispensable guide of the brewing industry. Of course, he refused to authorize a German translation, and none was ever produced. The battle was short and the outcome decisive: to this day, very little German beer is exported, and many successful breweries - Whitbread in England, and Carlsberg in Denmark - attribute their success to the impetus provided by Pasteur's visits.

Interestingly, Pasteur's work on the beer of revenge profoundly influenced his development of the germ theory. He wrote:

"Seeing that beer and wine undergo profound alterations because these liquids have given shelter to microscopic organisms, how can one help being obsessed by the thought that phenomena of the same kind can and must sometimes occur in humans and animals?."
It also - ironically, and indirectly - contributed to Germany's preparation for World War I, as most of the breweries idled by the collapse of the German brewing industry were eventually converted to manufacture acetone for cordite production.

This account follows part of Alan Baxter's discussion of Pasteur's life in "Louis Pasteur's beer of revenge," which appeared in Nature Reviews Immunology I, 2001.

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