Brewers tend to hawk their wares on the purity of their ingredients and the provenance of their hops. But the yeast that helps turn their efforts into beer seldom gets a mention. After decades shrouded in obscurity, the ingredient driving the whole process is finally getting more recognition.

For years, food historians have believed bakers and beer makers were engaged in a selective breeding programme, whether they intended to or not. Now there's evidence to support their claims. A paper published in the journal Cell in 2016, co-authored by Troels Prahl, a brewer and microbiologist at White Labs, a yeast distributor in San Diego and by Belgian scientists at the University of Leuven, lends credence to the idea that brewers domesticated yeast 500 years ago without even realising it.

The researchers sequenced the genome of 157 strains of Saccharomyces cerevisiae, a species of yeast used to make beer, wine and sake, to unlock their potential. In doing so, they've helped inform inquisitive brewers about how the craft has developed, and provided valuable lessons on the origins of flavour.

The DNA sequences revealed that yeast in use today has a lineage dating back to beer halls and monasteries throughout Europe. The findings could be a boon to brewers. It may help answer the question of at what point did wild and domesticated strains of yeast begin to diverge?

Why does that matter? Yeast is perhaps one of the most studied organisms in cell biology and remains one of the least understood. According to Prahl, by getting a detailed reading of the yeast genome, brewers now have a much better understanding of which yeasts are closely related to each other, how the microbes evolved over time and perhaps more importantly how various strains of yeast help flavour beer.

For much of recorded history, brewers unwittingly relied on yeast to make beer, but the conversion process remained a complete mystery to them

Mapping the yeast genome is far more than an academic exercise among science-driven brewers. “These technology-driven scientific methods have opened the Pandora’s Box. Brewing scientists, since the discovery of DNA, have been interested in how beer turns out, based on the genetics of the yeast,” Prahl says. The data collected from the research project could help researchers to design new strains of yeast to hybridise rather than resorting to genetic modification.

Whereas today’s brewers can select specific strains of yeast to elicit certain traits in beer that’s a relatively recent phenomena. For much of recorded history, brewers unwittingly relied on yeast to make beer, but the conversion process remained a complete mystery to them.

So much so that an archaic term for yeast was “goddesgood ” because it ‘cometh of the grete grace of God' (According to the British author Elizabeth David in her cookbook “British Bread and Yeast Cookery” the term dates back to the age of Chaucer.)  Beer was the handiwork of divine providence rather than microbes. Given the capriciousness of the spirit world, the outcome tended to be uneven, and bad batches of beer were not uncommon – or so the thinking of the time went.

Nonetheless, yeast has been instrumental to the production of beer for thousands of years. During the Middle Ages, European brewers relied on a technique called blackslopping. They would reserve a portions an older batch of tasty beer to start new ones. Ongoing generations of brewers sipped and tasted beer to arrive at potable suds. The thinking goes that after hundreds of years of production in man-made environments, their tinkering led to distinct regional styles of beer – lagers and ales among them – and to the domestication of yeast.

Once brewers determined how to isolate yeast to achieve a desired result, they could focus on building beer empires

“Brewers were among the first scientists and microbiologists, who were either consciously or unconsciously working toward the domestication of yeast,” said William Bostwick, author of the title The Brewers Tale.

In 1857 the history of science and beer took a decisive turn, Louis Pasteur determined that yeast was the microorganism responsible for alcoholic fermentation. In a nutshell, he determined that yeast is a single-celled fungi. A living thing, part of the mycological kingdom that thrives in moist environments provided that they have simple nutrients to consume such as sugar and amino acids.

The breakthrough subsequently led to changes in beer production. Once brewers determined how to isolate yeast to achieve a desired result, they could focus on building beer empires. The founders of the Danish Carlsberg beer company, for example, rose to prominence by purifying a strain of yeast they christened Saccharomyces carlsbergensis to establish a global brand. “Once brewers at the company determined how to use a single strain of yeast to make lager, it became the industry standard worldwide,” said Prahl.

Lager, ale, porter and stout are all names given to beer. But they are all the same concoction – it’s the details which are tweaked. “Yeast is the catalyst,” said Chris White, the CEO of the White Labs. He described the process of turning what would be a warm grainy gruel into beer. Much like the starter for sourdough bread or the bacteria used to make kombucha, beer relies on brewers yeast to give beer its spark. Yeast consumes the nutrients obtained from a mash of water, grain and hops to produce the combination of alcohol and carbon dioxide that makes beer.  

What’s frequently overlooked is that many of the qualities attributed to beer are due to the influence of yeast. “Yeasts can produce over 500 flavour and aroma compounds,” said White, affecting a beer’s alcohol level, complexion and clarity. Substances called phenols and esters produce the citrus, banana and coffee-like flavours often found in craft beers.

Yeast is a living entity and the brewer is a living entity. It’s part of a biological reaction – John Keeling, beer brewer

These qualities add dimensions to the sensory experience of drinking beer that many people attribute to the explosion of interest in the craft brewing industry and why the taste of beer continues to evolve over time. John Keeling, the brewing director at Fuller’s London facilities in west London’s Chiswick, has observed these subtle changes over the span of more than 40-year career.

“What I like about the world of beer is it’s the world of biology. Yeast is a living entity and the brewer is a living entity. It’s part of a biological reaction.” Yeast and humans are on a journey together that makes life more interesting, he says.

So the next time you raise a pint, you might want to make a toast to a microorganism and honor this phenomena and the power of small things to work wonders. Modern-day beers owe their existence to traditions stretching back hundreds of years into antiquity. Yeast has served as the engine for a beverage that has stood the test of time, turning the humdrum ingredients of agrarian societies into something magical.   

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