A short history of yeast

A fine yeasty head on a fermentation vessel at the former Whitbread brewery in London about 1948

A fine yeasty head on a fermentation vessel at the former Whitbread brewery in Chiswell Street, London about 1948

Around 80 million years ago, when Triceratops still browsed the plains of what wasn’t yet North America, some flowering plant species developed a new strategy to spread their seeds, encasing them in a soft, fleshy cover – fruit – that became sweet and tasty as everything ripened. The fruit was then eaten by animals, which would subsequently deposit the seeds far away from the mother plant, and with the addition of some useful fertiliser as well.

It did not take long, however, for enterprising funguses to start exploiting the sugar in the ripe fruits for their own growth and development, using oxygen to break the sugar down into carbon dioxide and water, releasing energy at the same time. If there was no oxygen about they would turn the sugar, via acetaldehyde, into alcohol, and make energy that way, although they very much preferred not to: alcohol was poisonous.

These funguses mostly reproduced by budding – dividing into two, and giving each daughter cell a complete copy of her mother’s genes. However, on one occasion, at least, a daughter received two copies of her mother’s genome instead of one. The daughter’s descendants evolved this spare set of genes so that, unlike their ancestors, they could make alcohol all the time, not just in the absence of oxygen. The spare copy of the gene that created the enzyme that turned acetaldehyde into alcohol also evolved so that it could do this trick in reverse – turn alcohol into acetaldehyde.

What these changes enabled the clever little fungus – the ancestor of brewing yeast – to do when it landed on sugary fruit was to quickly flood its environment with alcohol, which was toxic to most of its microbial rivals (our double-genome yeast had, of course, also evolved greater tolerance to alcohol, to cope with the extra alcohol it now created). Once it had swamped the area with alcohol, and thus seen off rival funguses, it could then win even more energy by turning the alcohol into acetaldehyde.

The alcohol content of over-ripe fruit attacked by yeast has been tested at as high as 4.5 per cent, and there is a theory that humans like alcohol in part because our early primate ancestors learned to associate its smell with the presence of ripe, sugary fruit – a sort of ancient alcopop. Appreciating alcohol, therefore, looks to be something deep in humanity’s genes, going back to when we lived in trees and were still covered in fur.

Fermenting sugar is a rare trick: of the thousands of millions of different yeasts, only 250 can do this, and just 24 are “good” yeasts, in the sense that what they produce is palatable to humans (although even then we have to arrest the yeasts’ development at the point of maximum alcohol development, by cutting off their oxygen, before they start making acetaldehyde, which will give you a very nasty hangover.)

Yeasts also need help in tackling grain: a growing barley or wheat seed is converting its starches to sugars, and then using the sugar to make energy, and produce roots and stalks, faster than the yeast can rob the seed of its sugar. But if humans stop the seed growing by heating it until it dies, and then crush and soak the dried grain, then the yeast can get at the sugar.

You can use the wild yeasts in the air to do this, if you like, and this is the preferred method of the lambic beer brewers of Belgium, for example. But the lambic brewers rely on a natural environment filled with “good” yeasts. For many early brewers, leaving sweet wort uncovered for a few days was most likely to result in a nasty vinegary liquid, as “bad” yeasts predominated. At some point, however, brewers realised that the dregs from a “good” brew contained something that would kickstart sweet wort into producing another good brew – a point underlined etymologically by the Anglo-Saxon for yeast, doerst, coming from dros, dregs. Old Norwegian words for yeast include kveik, which comes from a word meaning “kindling”, with the idea that the yeast “restarts the fire” in the wort.

Although brewers knew that yeast was essential for fermentation, they had no idea what it did, and no clue as to the actual nature of yeast. Michael Combrune, the London brewer who was among the first to use a thermometer, wrote in 1762, for example, that yeast was “vesicles formed out of the must [or wort], and filled with elastic air”. By the early 1830s science had advanced enough to discover that yeast made alcohol from sugar, with “carbonic acid gas” – carbon dioxide – as a side-product, but how yeast did this (and even the fact that it was a living organism) was completely unknown.

It took a German, Theodore Schwann, in 1837 to show through experimentation that yeast was alive. Schwann correctly described yeast as a fungus, and called it in German Zuckerpilz, sugar fungus, which another German, Julius Meyen, translated into Latin in 1838 to give the organism its biological name, Saccharomyces. Schwann’s findings were not immediately accepted, however. Some of the leading chemists of the time, including his fellow-German Justus Liebig, dismissed Schwann’s ideas and insisted that yeast was just a residue of the decomposition of sugar. It took the great French scientist Louis Pasteur, working from the 1850s to the 1870s, to finally prove what yeast was, and have a stab at showing how it worked.

Meanwhile the Bavarian brewer Gabriel Sedlmayr had been working at his family’s Spaten brewery in Munich, perfecting a style of brewing using a yeast that thrived at cold temperatures and then sank to the bottom of the fermenting vessel when it had finished, and which, after long storage, produced a stable, long-lasting, sparkling beer. Because the yeast Sedlmayr used to make his “store beer”, “lager bier” in German, dropped to the bottom, it became known as “bottom-fermenting yeast”, while “warm fermentation” yeasts were called in contrast “top-fermenting yeasts”.

This is, as it happens, a misnomer, because while all cold fermentation yeasts fall to the bottom of the vessel when their work is done, some warm fermentation yeasts rise to the top and some, as cold-loving yeasts do, fall to the bottom. Whatever, because Sedlmayr’s methods produced a more stable beer than other brewers could make, his yeast and his cold fermentation brewing methods became widely used, in Europe and around the globe. One brewer who took Sedlmayr’s yeast home with him was JC Jacobsen of the Carlsberg brewery in Copenhagen. There, from 1880, Emil Christian Hansen worked in the brewery laboratory, discovering that normal “pitching” yeasts used in brewing were a mixture of several different strains. Hansen set to isolating single cells of yeast to get a pure strain, multiplying them and testing them for their beer-making properties.

Eventually, in 1908, Hansen isolated one strain that he felt performed best of all, and which he named after his employer, Saccharomyces carlsbergiensis. What Hansen could not know, but which genetic studies have now shown, is that S carlsbergiensis is a hybrid, of standard brewing yeast, Saccharomyces cerevisiae (which gets the last part of its name from the Latin for beer), and a related variety, S bayanus, which is more commonly found on grapes.

However, using pure strains seemed to solve the problems brewers had always had that were caused by wild yeasts infecting the brew, and Hansen’s methods were copied around the world – except in Britain, where brewers found brewing with single strains did not give them the complexity of flavours they were looking for. It took someone else working in the Carlsberg Laboratories in Denmark, Niels Hjelte Claussen, in 1903, who found one major reason why, when he isolated from a sample of English stock, or aged beer an entirely different type of yeast, which he named “British fungus”, Brettanomyces in Latin. Brettanomyces typically arrives on the scene after Saccharomyces has given up, and, in beers that are stored for a long time, it attacks and turns into alcohol the sugars normal brewing yeast won’t touch. (It also gives a ripe, not-quite-mouldy flavour to aged beers that is appreciated by many, though winemakers hate “Brett” flavours)

As geneticists began to get to grips with yeasts used in brewing in the 20th century, Saccharomyces‘ habit of swapping genes among species the way children in school playgrounds swap Top Trumps cards caused much confusion. S carlsbergiensis was relegated by yeast biologists to a sub-variety of a group of cold-loving yeasts called Saccharomyces pastorianus, after Louis Pasteur. Another type of yeast, S uvarum, that forms hybrids with S cerevisiae, was defined, then lumped in with S bayanus, and in the past couple of years has been pulled out and made a separate species again.

If you’re getting your information about yeasts from a book more than five or ten years old, it’s probably wrong, yeast genetics has advanced that much. In particular it’s wrong to say, as some writers still do, that lager is brewed with Saccharomyces carlsbergiensis, or S uvarum. Modern lager yeasts seem to be hybrids of three different yeasts, in different proportions, S cerevisiae, S uvarum and S bayanus.

Yeast has a marvellous habit of evolving to suit its environment, helped by the Saccharomyces‘ gene-swapping. For this reason, different strains rapidly develop in different breweries, and under different fermenting styles, which is why, for example, a yeast strain used in the Burton Union method of fermentation would not work well if suddenly used in Yorkshire stone squares, where a different strain of yeast has evolved.

Some brewers say their yeast even evolves to suit a particular barley strain, and problems arise if a new type of barley is suddenly used. This kind of adaptability can be observed at home if you make “sourdough” bread: the first batch, made by letting wild ambient yeasts settle on and raise the dough, will not be as good as subsequent batches, which are made with a “starter” of uncooked raised dough from the previous batch, because the yeasts will be doing a better job as the ones that thrive best, and produce the most carbon dioxide to raise the dough, come to dominate.

Update

For an update on part of this story, see this post

About these ads

5 thoughts on “A short history of yeast

  1. Pingback: Slashfood Ate (8): Beer blog round-up | FuzzyClam.com

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s