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Lesson on yeast

Yeast, nature’s wonderAbout bottom-cropping, top-cropping and spontaneous yeasts

How does it produce alcohol and carbonic acid? For centuries brewers had no idea. Something happened with the wort. Something invisible, something incomprehensible turned the sugar from the wort into alcohol and carbonic acid. What was it? No idea! Now we know that it is yeast that is responsible for the transformation. As ever, yeast remains the most mysterious ingredient in beer. No wonder they call it ‘the soul’ of beer.

Anthoni van Leeuwenhoek was the one to solve the first piece of the puzzle in the 17th century. With the help of his microscope he discovered the existence of yeast cells. However, he had no idea of what these organisms were capable of.

Louis Pasteur

It wasn’t until two centuries later that Louis Pasteur solved the largest part of the puzzle. He discovered that yeast cells were responsible for the production of alcohol and carbonic acid. He also saw that bacteria in beer could add unwanted tastes, which meant that brewers often had to dispose of their brews because they were unpalatable.

The scientist clued into the fact that he could eliminate those harmful elements through heating. This process is what we now know as ‘pasteurisation’. Pasteurisation gives dairy products and beers a longer shelf life.

Emil Hansen

After Louis Pasteur’s amazing work, things moved forward very fast. At the Danish Carlsberg, a scientist called Emil Hansen succeeded in isolating a yeast culture. This meant that the brewer could control the process better, thereby drastically minimising the chances of sour brews.

Bottom fermented beer

The yeast cell Hansen worked with is now known as Saccharomyces Pastorianus. This yeast works best under low temperatures, between 5 and 12 degrees celsius. This yeast is a bottom-cropping yeast because it is mostly active in the bottom of the kettle. Bottom fermented beers have a longer conditioning time and are aromatically more subdued. Examples of bottom fermented beers include Pilsner, Lager, and Bock beers, however, the latter is now often being brewed with top-cropping yeast.

Examples of beers are the Brooklyn Lager, Heineken H41 and the Lager from the Brouwerij Noordt.

Top fermentation

Top-cropping yeast, also known as Saccharomyces Cerevisiae, provides the counterpart to bottom fermented beers. Top-cropping yeast prefers higher temperatures, between 15 and 25 degrees celsius and is mostly active in the top of the kettle. Top fermented beers require less conditioning time and often have fuller, fruitier and floral aroma’s. Examples of top fermented beers include IPA’s, Weizen beers and Porters.

Examples of beers are Lagunitas IPA, Jopen Mooie Nel, Erdinger Hefeweizen and Kompaan Bloedbroeder.

Spontaneous fermentation

The use of these yeast strains means the brewer has less control during the process. And yet, more and more beers are being brewed the old-fashioned way. These beers are called spontaneous fermentation beers. These Lambic beers mostly derive from the Zenne Valley, the area South West of Brussels. To produce this beer, the wort is pumped into what is known as “cooling vessels”. In the cooling vessels - a type of large bath tub - wild yeast that might be drifting in the air is given the opportunity to settle into the sugar-rich wort. Shortly afterwards, the wort is put into wooden barrels where the yeast is given plenty of time to work it’s magic.

These Lambic beers form the basis for Geuzes. This is the result of a blend of Lambic beers of varying ages. It’s a true art to create a great Geuze. The blenders are known as Geuze-stekers and are held in high esteem in the beer world. Geuzes are sour beers that you can sometimes keep for decades and which age beautifully like a good wine.

Examples of beers are Mort Subite Gueuze, Boon Geuze Mariage Parfait and the Oud Beersel Oude Geuze.

Mixed fermentation

The fourth variety of beer is mixed fermentation beers. These are warm fermented beers that have either been mixed with spontaneous fermentation beers or to which lactic acid bacteria have been added. Examples are the Vlaams Rood and the Vlaams Bruin. A world renowned example of a mixed fermentation beer is Liefmans Goudenband.

Another well known beer is Orval, a Trappist beer brewed with Brettanomyces yeast. The yeast gives the beer its typical character and ensures that Orval has a clear flavour evolution.

For many brewers, yeast is the trick-of-the-trade, the key ingredient in any recipe. Both the top-cropping and bottom-cropping yeasts have many strains, each with their own unique characteristic flavour. Remember this the next time you are savouring the aroma of bananas and cloves when drinking a nice glass of Saison or knocking back a Weizen!

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