Seasoned brewers know that behind every successful beer, there is a very precious yeast at work. When it comes to beer – from ale, to stout, to lager – yeast is a crucial ingredient used to ferment sugars into alcohol.
A new study by a team of researchers from Trinity and UCD has uncovered new links between the yeasts used to brew Ireland’s iconic stout and those used to brew lager and ale.
The team studied the genomes of yeasts sourced from distilleries, bioethanol plants, bakeries, laboratories, as well as natural environment sources such as coconut water, in an effort to obtain a clearer picture of yeast evolution.
It took nearly two years to complete the necessary lab-based experiments and computer analyses of DNA, but now Chandre Monerawela, PhD student in the School of Genetics and Microbiology at Trinity and first author of this study, can finally breathe a sigh of relief, as he has just seen his results published in the FEMS Yeast Research journal.
He looked at the genomes of 76 groupings of yeasts with similar characteristics- known as strains- and compared them, in an attempt to uncover the origins of lager yeasts. “I looked at specific DNA regions to see if they were present or not,” says Monerawela.
Lager yeasts are classified as either Group I or Group II. The team of researchers was surprised to discover that Group II lager yeasts share DNA with stout yeasts and, interestingly, with strains that are used to brew a South Indian brew called ‘Toddy’.
Not only does the yeast used in brewing stout, which attains its dark colour thanks to the use of roasted barley or malt, turn out to be one of the parents of lager yeast, but it is possible that colonial connections contributed to the exchange of yeast strains between India and England.
Lager yeasts are known to be hybrids born out of a fusion of two different yeasts and one of the aims of the study was to find out where these two different yeasts came from.
The researchers were able to show that Group I and II yeasts arose by independent fusion events with a yeast species that was discovered in 2011. This species, Saccharomyces eubayanus, was identified in Patagonia, South America, and, just last year, it was found in China and Tibet, but never in the wild in Europe. So why is it present in some of our beers?
Around 500 years ago, trading ships carrying precious goods from the New World and China into Europe may have transported something less obvious, but no less precious in their cargoes: the ancestors of lager yeasts.
“It is possible that this yeast strain was brought back into Europe through trade routes along the Silk Road or possibly through exploration of the New World,” says Dr Ursula Bond, associate professor in Microbiology in Trinity and one of the researchers behind the study.
This would have coincided with the introduction of laws in Bavaria in 1553 that restricted the fermentation of beer to the winter months. “These laws led to natural selection of yeasts that would grow and ferment at low temperatures, and it turns out that this new species, this hybrid, is very good at fermenting at low temperatures.”
Indeed, while ales are fermented at temperatures between 20 and 30°C, lagers require colder temperatures, between 8and 15°C. Another difference is that, in ales, yeasts rise to the top after fermentation, while in lagers they sink to the bottom.
“So this study suggests that when this new yeast arrived in Europe it didn’t just fuse once, but there were two fusions that happened around 500 years ago, one with an ale-like yeast, which is similar to the ales you find in central Europe, and the other with a stout yeast. But since stout is only really brewed in the British Isles, what that means is that when this other yeast came into Europe, it must have met this British Isle yeast or a yeast that is now used in the British Isles,” explains Bond.
Another unexpected connection to arise from the study was that of stout yeast and Indian ‘Toddy’ yeast. “The South Indian drink is brewed from coconut water,” explains Bond. “If you get a tender coconut it has sugary sap inside and yeast can get at that sap and then convert it into an alcoholic beverage.”
Once analysed, those yeasts fell into the same category as the stout ales. But why would yeast in India be similar to stout yeast? Perhaps it is linked to colonial connections between Britain and India, but further research is needed. “That’s something we’re looking into now. We’re going to sequence the ‘Toddy yeasts’ and compare them to the stout yeasts and see if there is any connection there,” says Bond.
Evolution in action
These yeast species are probably only about 500 years old- a blink of an eye in evolutionary terms. Despite this, changes are happening in their genomes and this study was able to highlight some of these very early-stage evolutionary processes.
“These genomes are really dynamic. They are changing all the time. One of the things that happened is that the two genomes coming in from the two parents have actually recombined to create a new set of chromosomes. Another process we see is that you nibble away at the ends of chromosomes and genes get lost as evolution comes about. It’s really interesting that you can study something that’s very early on in the evolutionary process,” says Bond.
Future work on yeast genetics will involve looking for yeasts from different parts of the world and comparing their DNA. Already, projects such as the “1001 Genomes Project”, whereby groups are getting together to sequence 1001 yeast strains, are expanding our collective knowledge of yeast evolution.
“Eventually we hope to have thousand of strains for analysis,” says Bond. “This involves not just looking at what we have in our collections but going out into the wild and starting to look at other yeasts that are in the environment.”