Unterhefe No. 1: the yeast that began the pure-culture standard

This is a story worth telling precisely, because it is often told imprecisely. In 1875 J. C. Jacobsen founded the Carlsberg Research Laboratory in Copenhagen — a research institution attached to a brewery, conceived not as a quality-control department but as a scientific laboratory. In 1879 Emil Christian Hansen became head of its department of physiology and led it until 1909. It was there that the first method for culturing pure yeast was developed.

To see why that was a revolution, the problem has to be looked at through the eyes of a brewer of the period. Fermentation worked, but it failed irregularly and for no visible reason. Brew after brew came out well, and then one spoiled beyond repair. The brewer had no instrument for telling cause from effect, because the working material was a suspension in which many organisms lived side by side at once.

Hansen's solution rests on a single idea that sounds obvious now and was not obvious then: if the suspension is a mixture, then start from one cell and propagate only its descendants. Everything that ferments then has one ancestor, and the result stops depending on whatever happened to be in the vessel.

The strain that came out of this Hansen named Unterhefe No. 1 — bottom yeast number one. Under that name it entered production in 1883, and under that name it still appears in the scientific literature, beside the later species name Saccharomyces carlsbergensis. Its genome was sequenced and described in 2014, and that paper calls it plainly the first established lager yeast strain, in production since 1883.

The same paper adds a detail that explains why the story continues. Lager yeasts fall into two groups: the Saaz type, to which S. carlsbergensis belongs, and the Frohberg type, exemplified by the Weihenstephan strain WS34/70. The Copenhagen strain is better adapted to growth at low temperature than the second group. That is not a technical curiosity but an explanation of why lager is the beer of cold cellars, and why the brewing industry went where it was cold.

Genomics added one more thing to the story long after Hansen. Lager yeast is not a straightforward species but a hybrid of two species of the genus Saccharomyces. Hansen therefore isolated an organism that was itself already the outcome of an earlier, accidental event — a meeting of two lineages that happened without human involvement. The pure culture was pure in a microbiological sense, but not in an evolutionary one.

The consequences reached well beyond brewing. Pure culture became the standard wherever a process depends on a microorganism: in baking, in the manufacture of organic acids, later in pharmaceutical biotechnology. The idea that an industrial process should rest on one known, described strain rather than on a local suspension of uncertain composition comes straight from here.

The form of the institution deserves some credit for this. The Carlsberg laboratory was not a quality-control department subordinate to production but a research establishment maintained beside a brewery, and that explains why a method came out of it rather than another recipe. A department of physiology under one man for thirty years could work on a problem with no deadline and no promise of a result this quarter. It would be hard to find a cleaner illustration of research output being a function of who pays, and on what terms.

It is worth noticing what was lost. Beer before pure cultures was a product of a place, its flavour depending on which microflora had settled into a particular brewery. After Hansen the same beer could be made anywhere, given the strain and a cold store. Repeatability beat locality, and only a century later did some brewers begin deliberately returning to mixed and spontaneous fermentation — not because Hansen's method stopped working, but because it works so well.

There is one further consequence, rarely discussed, that becomes visible only through genomic work. If a brewery rests on a single strain, that strain becomes property — something kept, guarded and handed on. A paper published in 2024 in Communications Biology traces the mechanism through the yeast used to brew Guinness stout in Dublin: its line can be followed back to 1903, but its origins remain unknown. The authors used Illumina and Nanopore sequencing to assemble the genomes of twenty-two Saccharomyces strains in order to establish where that particular lineage came from.

It is worth seeing what this means in practice. A brewery can know exactly which day and which brew its present culture descends from, and at the same time not know where the first one came from. Continuous record-keeping begins at the moment somebody decided a strain was worth guarding — and that moment is precisely after Hansen. Before him nobody kept a register, because there was nothing to register.

For a reader who came here for mushrooms, one sentence matters most. Yeast is a fungus. It is not a plant, not a bacterium and not a category of its own — it is a single-celled fungus among the ascomycetes. So when we say that fungi shaped the material culture of Europe, the strongest evidence is not standing in a forest. It is in the glass.

Pure culture as infrastructure, not a secret

Emil Christian Hansen’s archive dates the first production use of a pure yeast culture at Carlsberg to 12 November 1883. The achievement was not isolation alone but a system: propagating starter material, recognising contamination and distributing a reproducible culture between breweries. A letter from J.C. Jacobsen dated 20 May 1884, preserved in Royal Danish Library collection NKS 4069 4°, documents samples being sent within Denmark and abroad. The Carlsberg Foundation stresses that Hansen did not patent the method and made it freely available to the brewing industry. “Unterhefe No. 1” is therefore best understood as a technical standard and exchange network, not a magical singular organism. Modern CBS1513 enables genomic study of lager yeast, but it should not be called the unchanged contents of the original vial unless an unbroken deposit history is separately demonstrated.