How it began
In the late summer of 1883 the beer at the Carlsberg brewery in Copenhagen began to smell off and taste bitter. Emil Christian Hansen, head of the physiology department of the Carlsberg Laboratory, traced the fault to wild yeast mixed into the brewing yeast, and not only to bacteria as Louis Pasteur had assumed. He isolated a single cell of good yeast and grew it into a pure culture, which was first used at production scale in November 1883.
Before it existed
Every brewery took yeast from one batch of beer and pitched it into the next. If the yeast became infected, the infection passed from batch to batch, and the usual remedy was to fetch fresh yeast from another brewery. Brewing was unpredictable and often produced undrinkable beer.
How it works
Hansen diluted yeast in a bottle, counted the cells in a sample under a microscope on a ruled grid, and diluted further until each cubic centimeter held on average half a cell. He put such samples into bottles of sterilized wort and kept only bottles in which a single colony grew, so each culture came from one cell. With the head brewer he then built an apparatus to grow the chosen strain safely in quantity. He advised returning to the pure stock at intervals.
How it spread
Brewery owner J. C. Jacobsen gave the pure yeast away free to other brewers and promoted the method across Europe. By 1888 Hansen reported that all the major breweries in Denmark and Norway used it, that it was common in Russia, the Netherlands and Germany, and that Carlsberg had sent cultures to Asia, Australia and South America.
Who pushed back
The one place where breweries held out was the United Kingdom. Brewers there said pure cultures did not give the right taste, especially in matured beer, and Hansen himself noted that strong top-fermented beers seemed to behave differently. The United Kingdom changed its view later.
What lasted
Mixed cultures reused from batch to batch quickly disappeared from industrial brewing, and yeast became something supplied from a laboratory. Carlsberg states that its original yeast, named Saccharomyces carlsbergensis, is used in most lagers made today.
