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Yeast not only produces alcohol, but also creates most of the flavour in beer. However, many old strains of yeasts have been forgotten about today. Two scientists want to hunt them down and bring them back to life.
A steep staircase disappears down into bottomless darkness. Mathias Hutzler descends the 153 slippery steps. Following closely on his heels is Steven Wagner. In their hands, both men are balancing large polystyrene boxes containing sterile petri dishes, tweezers and small plastic test tubes. When they reach the bottom of the stairs, they place the boxes on the damp floor and look around curiously. Pale neon light illuminates the unrendered walls. Water drips from the ceiling. It is so cold that every breath is visible in the air. This is where their hunt for yeast begins today.
Mathias Hutzler manages the yeast centre at the University of Weihenstephan. Steve Wagner teaches genetics at Central Washington State University and is carrying out a sabbatical at Weihenstephan. They met at the World Brewing Congress in Portland and immediately discovered that they shared a passion in the single-celled micro-organisms that turn grain into beer: yeast. Together, the two men are on a mission.
Yeast produces 80% of the flavour in beer
They want to find new strains of yeast, or rather, very old strains of yeast. The yeasts that were used in beer brewing decades or centuries ago, which have eventually been forgotten. Brewers replaced them with high-performance strains that ferment faster and are more robust. Unique combinations of flavours were lost in this selection process. Partially because of the different yeasts, beer used to show much more diversity in flavour.
This is because yeast produces around 80% of the flavour in beer. To a large extent, it’s the yeast that determines whether a beer has notes of banana, freshly cut grass or something more like grapefruit. If you use the same wort but add different yeast strains, it creates completely different beers. “The differences are so enormous that any layman would immediately notice them,” says Mathias Hutzler. This hunt for yeasts is also helpful for the production of non-alcoholic beer: a constantly growing sector within the industry.
The first place the two scientists are looking is Vulkan Brauerei’s cellar, located in Mendig in Germany’s Eifel region. According to the brewery, it is the deepest beer cellar in the world. Hutzler and Wagner put on white coats and plastic hair and shoe covers. Then, they trudge through the vaulted cellars, which resemble limestone caves.
The temperature in this old mine is a constant six-to-eight degrees Celsius. During its heyday, 28 different breweries had their vats of beer fermenting down here. Then, when refrigeration units arrived, that was the end of beer cellars like this. In the 1980s, the last remaining brewery moved out of the cellar.
Hunting for yeast amongst old junk
Huge fermentation tanks were left underground here. Today, with rust having ardently nibbled through these steel giants, you can only vaguely make out their once-white colouring. Hutzler grabs a ladder, leans it against the tank and climbs up. Then, he scratches around the inside of the tank with a spatula. Anything that comes loose, he catches in a small plastic test tube. Next, the two yeast hunters start on the old pipes. They cut off hoses, scrape around crumbly rubber seals and poke cotton swabs into dark pipes. Matthias Hutzler is pleased that there is so much stuff still lurking around in the cellars. His motto on his yeast hunt: “the grimier, the better”. His hope is that the brewers didn’t clean the tanks properly back then, that beer dripped out from pipes, or that foam overflowed from the tank. If that’s the case, there is a good chance that yeasts or their spores can still be found in the cellar today.
Because yeasts are microscopically small, brewers didn’t even know about their existence for many thousands of years. They only put water, malt and hops into a tank. The yeast made its way into the tank all by itself. It floated in from the air, clung to hops or malt, or fell directly into the tank from the brewers’ beards.
Even today, some brewers are relying on this kind of spontaneous fermentation once again. Instead of deliberately adding a pure yeast culture to their brew, they leave the tank open and wait for uncultivated yeasts from the environment to fall in and start to do their work. If you like things to be a bit more controlled, you can look to winemakers who rely on a mixture of different varieties; in addition to the uncultivated yeasts present in their must, they add large quantities of pure yeast to the mixture. The latter types of yeast are in the majority and take charge. This makes it easier to control fermentation and the development of flavours.
A momentous hybridisation
For thousands of years, top-fermenting yeasts (Saccharomyces cerevisiae) were mainly responsible for converting malt sugar into alcohol. It was only about 500 years ago that a momentous hybridisation between top-fermenting yeasts and an uncultivated yeast called Saccharomyces eubayanus occurred. This led to the formation of the bottom-fermenting lager yeast (Saccharomyces pastorianus).
Since then, there have been increasing references in the scientific yeast community to indicate that this hybridisation took place in Germany: Bavaria, the Upper Palatinate Forest and the area around Munich are all noted particularly often. The Czech region of Bohemia is also widely considered to be a likely origin.
Evidence for the true origin is still missing though. The uncultivated Eubayanus yeast has never been found in Europe before. Hutzler thinks it’s simply too good at hiding itself. So, what would be the best possible outcome of this yeast hunt? “Finding Pastorianus and Eubayanus yeast in the same place,” says Hutzler, and pulls out a small silver device reminiscent of a portable speaker. In fact, it’s an instrument that filters micro-organisms from the air. It’s used in case the yeast spores don’t stick to walls and fermentation tanks at all, but instead float through the air and watch the absurd scene below from a bird’s eye view.
From a small glass flask, Hutzler pours a caramel-coloured liquid into a Petri dish and places it into the analyser. Then he ceremoniously takes ten steps backwards and listens to the muffled humming that accompanies the air filtration. After five minutes, the device goes silent and Hutzler pours the contents of the Petri dish into various glass flasks containing beer wort or a nutrient solution tailored to yeast.
The test brews actually ferment
After five hours, the two researchers return to the surface. Chilled to the bone, but happy. Back in the laboratory, they heat the flasks and store them at different temperatures. This should breathe life into the yeast spores. In many of the flasks, nothing grows. But in some, yeast cells actually start dividing and multiplying.
The two scientists have searched for old yeasts in 15 different locations. They have found 300 different yeast strains and reanimated them in the laboratory. Among them are many uncultivated yeasts that have not been domesticated by humans and have probably never been used in a brewing tank before. However, 14 of the yeast strains found probably do have a past as brewing yeast. Using three of these, they even brewed some beers and served them at BrauBeviale 2018. “Personally, I enjoyed it very much,” reports Hutzler. Other scientists and beer drinkers have also given him positive feedback. Commercial success is building up slowly. Various breweries have bought some of these three yeast varieties and are now developing new products with them.
Special yeasts for making alcohol-free beers
Because most of the sugar is left over in the wort, these beers taste sweeter. To develop rounded beer flavours still, the yeasts used need to produce sufficient acidity. This results in a rounded flavour profile.
This production process is much cheaper than removing the alcohol from the beer by vacuum rectification or special filtration methods. However, the production process needs to be absolutely sterile, or the beer needs to be pasteurised. Otherwise you run the risk of unintentionally letting other yeasts fall into the tank and the malt sugar will end up fermenting after all.
It’s not only old cellars that are a good source of new yeast strains: foreign drinks are as well. In his doctoral thesis, Konstantin Bellut, a former student at Weihenstephan, analysed yeasts from the fermented tea drink Kombucha to see their suitability for brewing. He concluded that the yeast genera Candida, Hanseniaspora, Torulaspora and Zygasaccharomyces are suitable for brewing and produce nice flavours. Mainstream yeasts used for making red wine or sparkling wine can also help to make beer develop completely new flavours.
In the meantime, some breweries have summoned Mathias Hutzler, the yeast hunter to their premises. He conducts a search for yeast strains in and around a client’s brewery, and then makes any yeasts that he finds exclusively available to that client. The yeast not only allows breweries to extend the flavour profiles of their beers, but also use the hyperlocal yeast as an additional selling point when trading, catering to those who consciously focus on regional products.
The price for a professional yeast hunt like this varies depending on the amount of work involved. Searching a small cellar starts at about €300. If large areas in and around the brewery are included in the search, it can cost €2,000 or more. On top of that, there are laboratory costs for analysing the samples collected.
Those who want to make it cheaper can also play the role of yeast hunter themselves by collecting environmental samples in sterile containers and sending them straight to Weihenstephan for analysis. In this case, the cost of identifying the yeast strains is only around €130 per sample.
Frisinga, Tropicus and Nebulosa
This is a first step polishing up the image of yeast. To Hutzler’s regret, brewers are openly writing the names of the hops and malts that they use directly on their beer bottles, but customers are hardly ever told which yeasts are used. Presumably, this is also connected to them having some rather cryptic names up until now. One of the most frequently used yeasts is called TUM34/70, but that’s neither sexy nor memorable.
That’s why Hutzler sat down with his colleagues and came up with new names for the yeasts. Names that sound nice and tell a story, or that describe the yeast’s most important property. Frisinga, for example, was discovered in Freising. Tropicus delivers fruity flavours and Nebulosa produces a cloudy beer. Mathais Hutzler would be delighted if these names soon started appearing on beer labels.
In 2019, Hutzler went hunting for yeast again. This time in forests and nature reserves in the Upper Palatinate, Lower Bavaria and Franconia. Over the past four years, we have learned which trees and berries Saccharomyces yeasts grow on,” he explains. So, this time, the focus was on oak, ash, elm and certain berries. He is still hunting for Saccharomyces eubayanus. “Either we have been looking in the wrong places or we haven’t taken enough samples”, Hutzler says assuredly. Of course, there is a third possibility too: Saccharomyces eubayanus may have become extinct in Germany, the home of bottom-fermented beers. However, Hutzler will only accept this possibility after he has looked absolutely everywhere.
