For thousands of years, brewers knew that something turned sweet wort into beer. They skimmed the foam from one batch to start the next, trusted “godisgood” (a medieval English name for the yeasty froth), and blamed bad luck when a batch went sour. In the nineteenth century, science finally caught up with that knowledge, with dramatic consequences.
Seeing the invisible
In the 1680s Antonie van Leeuwenhoek looked at fermenting beer through his microscopes and saw tiny globules, but he did not know they were alive. In the 1830s three researchers, Charles Cagniard de la Tour, Theodor Schwann and Friedrich Kützing, each proposed that yeast was a living organism that multiplied during fermentation. Powerful chemists, especially Justus von Liebig, pushed back, arguing that fermentation was a purely chemical breakdown. The debate raged for decades.
Pasteur’s studies on beer
Louis Pasteur settled much of the argument. Through work in the 1850s and 1860s, he showed that fermentation is carried out by living yeast, and that spoilage, sourness and off-flavours are often caused by other microorganisms that sneak in. His technique of gentle heating to kill unwanted microbes, later called pasteurisation, was first applied to wine.
In 1876 he published Études sur la bière (Studies on Beer). He was partly driven by patriotism. After France’s defeat in the Franco-Prussian War of 1870–71, he hoped French brewers could rival the Germans. Pasteur urged brewers to keep their fermentations clean and free from contamination. But he still assumed that a brewery’s yeast was basically one organism.
Hansen and the pure culture
That assumption was overturned at the Carlsberg Laboratory in Copenhagen, founded in 1875 by the brewer J. C. Jacobsen. There, the mycologist Emil Christian Hansen showed that a brewery’s yeast was often a mix of strains, some of which caused off-flavours and “sick” beer. In 1883 he developed a method for growing a culture from a single yeast cell. The first pure lager strain, known as Carlsberg bottom yeast No. 1, went into production that year. It was later named Saccharomyces carlsbergensis.
Jacobsen chose not to patent the method and shared the yeast and technique with other brewers. Within a few decades, pure-culture yeast was standard in lager breweries around the world. Around the same time, refrigeration machines such as Carl von Linde’s, tested with Munich’s Spaten brewery in the 1870s, let brewers ferment cold lagers year-round.
Lager yeast held one more secret. Geneticists established that it is a hybrid of ordinary ale yeast, S. cerevisiae, and a cold-tolerant wild species. In 2011 researchers identified that missing parent, Saccharomyces eubayanus, in the forests of Patagonia. Related strains have since turned up in other parts of the world. Exactly how and when the hybrid arose in European breweries is still being studied. Today lager yeast is usually classified as Saccharomyces pastorianus, a name that honours Pasteur.
Why it matters
Understanding yeast turned brewing from an art based on tradition into one supported by microbiology. Clean, pure cultures made beer more consistent and less likely to spoil, and made the global spread of pale lager possible. Every modern yeast bank, from industrial breweries to homebrew shops, owes something to the patient lab work of Pasteur, Hansen and their predecessors.