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The Phantom of the Beer: yeast


We’ve gradually completed our series on the brewer’s essential raw materials. Step by step, we’ve dealt with the four most important constituents of any beer. First of all with hops and malt, then water, and finally in the end all that was missing was the phantom. Sounds spooky? Sure. But above all I think it’s exciting. And that’s why I wanted to know just exactly how this phantom, the yeast, haunts each and every beer, as it were. Herr Huber from Doemens Academy GmbH helped me to find the answers to my questions and to track down the mysterious phantom …


What exactly is your job?

I’m the Deputy Laboratory Manager and Director of the Yeast Bank and Micro-Organism Collection of Doemens Academy GmbH, as well as being a lecturer on microbiological quality control and EDP basics at Doemens e.V.

Can you describe briefly how far your work involves hops, malt, water and above all yeast as raw materials?

As Director of the Doemens Yeast Bank, I’m responsible for supplying breweries of highly disparate output categories with the pure yeast cultures they want. We supply our 160 different yeast strains in different forms: agar slant, liquid cultures, dry yeast) worldwide. As Deputy Manager of the Doemens laboratory, together with our lab team, I support the breweries and malt-houses with chemical-technical and microbiological analytics, so as to assure a high level of product quality. In addition, I assist the students of the Doemens Colleges in selecting the yeasts and micro-organisms for their examination brews, i.e. their practical finals to qualify as a master brewer.

There’s a saying that “The brewer makes the wort, the yeast makes the beer.” Would you sign up to that?

Yes, definitely. The high quality of a beer wort is an extremely important foundation for the subsequent beer. Only: without fermentation, the wort would very quickly spoil, and only a hopped sugar solution would remain. It’s only the yeast that converts the sugar dissolved in the wort into alcohol and carbon dioxide. In addition, the yeast forms what are called fermentation by-products, which lend the beer its taste-determinant components. This is what turns the wort into beer.

What does yeast have to do with the taste of the beer?

Besides alcohol and carbon dioxide, the yeast produces different aroma components during fermentation, the above-mentioned fermentation by-products. Probably the best-known of these are the banana and clove aromas in Bavarian wheat beers. The diversity is huge, particularly with top-fermenting yeasts. For example, the yeast strain can be determinant for the style of beer involved. If, for instance, you split a beer wort up into 16 separate portions, and then ferment these with 16 different yeast strains, you’ll get 16 different beers. This applies above all to the top-fermenting yeasts, but to bottom-fermenting strains as well.

What’s the difference between top-fermenting and bottom-fermenting yeasts?

From a traditional viewpoint: top-fermenting yeast rises to the top of the liquid during primary fermentation and can there be lifted off (harvested) by the brewer. Bottom-fermenting yeast sinks during primary fermentation to the bottom of the fermentation tank, and can be harvested there. In scientific terms: these are two biologically different types of yeast.

What makes the yeast sink or swim?

In purely physical terms, a yeast cell always has a higher density than the beer wort or the green beer. So both top-fermenting and bottom-fermenting yeasts ought to sink. During fermentation, carbon dioxide is produced, which rises in small bubbles. Top-fermenting yeast generally forms what are called bud clusters (see Fig. 1). These act like a fishing net, and catch the carbon dioxide. This imparts buoyancy to the entire cluster, which rises to the top. But as soon as fermentation has been completed, and there are no bubbles, this cluster will also sink to the bottom.

Fig. 1: Microscopic image of top-fermenting yeast strain 479


Since the bottom-fermenting yeast mostly occurs in the form of individual cells (see Fig. 2), it cannot “capture” the bubbles. It is kept in suspension in the liquid by the flow patterns caused by the bubbles, but during fermentation falls to the bottom more quickly.

Fig. 2: Microscopic image of bottom-fermenting yeast strain 375


Is there at present a discernible tendency towards top- or bottom-fermenting yeast?

This depends entirely on the region concerned and the philosophy of the brewery involved. For instance, many craft and industrial-scale breweries in Germany often brew with both. In the craft beer scene, the emphasis is more on top-fermented beers, while the trend is meanwhile moving towards lager-type beers again, i.e. bottom-fermented brews. 

Can you name some typical top- and bottom-fermented beers?

Examples of top-fermented beers would be wheat beer, Alt, Kölsch, ales or IPA. Bottom-fermented beers include Export, Helles, Pils, Märzen, dark beer and smoked beer.

Beer yeasts are fungi, aren’t they? Precisely what fungi are they?

Exactly: in biological terms, we’re talking about fungi. The group of fungi includes firstly the familiar edible mushrooms, secondly the moulds and last but not least the yeasts themselves. In terms of form and shape, they are not comparable to the other two categories. They are microscopically small living organisms approximately 5 to 10 µm in size. In the case of beer yeasts, they are mostly round to oval in shape.

Can you explain briefly what exactly happens in the cells of the yeast?

The yeast absorbs the fermentable sugars via various transport systems, and first of all splits them into glucose or fructose. These are known as monosaccharides, and are then converted into energy for the cell, alcohol and carbon dioxide, either by breathing or fermentation. The nitrogen compounds of the beer wort are converted into cell constituents with the aid of the energy obtained, and also used for building new cells (yeast propagation).

How does the temperature affect the yeast?

This will depend on the type of yeast involved. Top-fermenting yeasts develop their activity particularly at temperatures of between 16 und 25 °C, while with bottom-fermenting yeasts primary fermentation occurs mainly at temperatures of between 8 and 12 °C. In general terms: the higher the temperature, the faster the yeast will ferment.

Is it true that you can re-use yeasts after they’ve already been used?

As I’ve already mentioned, the yeast multiplies during fermentation, and can accordingly be harvested in somewhat greater quantities. This means that the yeast, in the case of top-fermenting fermentation, is lifted off the top, or in the case of bottom-fermenting yeast harvested from the bottom of the vessel. This yeast should not be stored for too long. But it can be used again for a new beer within one week. There are wheat beer breweries in Bavaria, who for several decades have been repeatedly re-using their top-fermenting wheat beer yeasts. In the case of bottom-fermenting yeasts, use tends to be limited to up to ten times at most.

What’s the story with all the different yeast strains? How many different yeast strains are used here in Germany, for example?

Yeast is a living organism, and like all living organisms is subject to evolution. Yeasts adapt to the conditions they encounter in the environments they’re used in, and accordingly have in recent centuries also developed different strains in the different breweries. These were then, as from the 1950s, increasingly collected and isolated by different yeast banks, like the one we have here at Doemens. Each strain has different properties in terms of fermentation speed, settling behaviour, aroma formation, etc. It’s not that easy to specify a precise number of disparate yeast strains, since the differences are sometimes only very marginal. But quite definitely at least 160 yeast strains are in use, which exhibit significant differences in their gene pool.

Brewing beer without yeast – is that even possible?

No. Without yeast, there won’t be any fermentation. There is indeed something called “spontaneous fermentation”, where yeast is not added intentionally. But the wort then ferments spontaneously with the yeasts present in the air, so in the end yeast is involved after all.

Is this spontaneous fermentation also the reason, then, why yeast is not explicitly mentioned in the Bavarian Purity Law of 1516?

Yeast was unknown to Duke Wilhelm IV, which is why he didn’t include it in his Purity Law. It was not until 1683, with the development of the microscope by Antoni van Leeuwenhoek, that it became possible to detect micro-organisms. And it was only in the 1860s, then, that Louis Pasteur described alcoholic fermentation and identified yeast as the “causal agent” in this process. For example, it was already known in 1516 that during fermentation “sort of beige stuff” (yeast) is produced and when this “stuff” is re-used, fermentation can be speeded up. Right up to the present day, some breweries are still using the term “destuffification” for the yeast harvest.

What yeasts does Doemens actually offer?

We offer eight different dry yeasts from the Lallemand company (Canada), which are dried and sold all over the world in a cooperative arrangement with Doemens. In addition, we can offer starter cultures for propagation in the brewery, such as liquid yeasts for producing wheat beers, light ale, pilsner, etc.

How has yeast earned its name as the “Phantom of the Beer”?

By producing alcohol and various aromatic components, which then lend the beer its final character.


Now, unfortunately, we’ve come to the end of our journey through the world of the brewer’s raw materials. But we’re meanwhile quite a bit better informed when it comes to brewing and the ingredients required. During recent months, we’ve met up with quite a few interesting experts on each of the individual raw materials involved, and learned a lot of new facts. When you now remember once again everything about hops, malt, water and yeast, then surely the way is clear for your first delicious glass of amber nectar. So the watchword is – cheers! 🙂

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