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All about beer yeast. What is bottom fermentation, top fermentation and wild fermentation?

In the beer world we distinguish between 3 main groups under which beers are subdivided. This subheading is based on the type of fermentation that these beers have undergone.

* Bottom fermented beers

* Top-fermented beers

* beers obtained by wild or spontaneous fermentation


Bottom fermented beers:

This fermentation takes place at a temperature between 5 and 15 degrees Celsius. At these temperatures, few aroma substances, also called esters, are produced by the yeast. The result is a fairly neutral tasting and smelling end product. The beer must have a "clean" character, with the taste mainly coming from the malt and hops.

The fermentation is quite slow and takes about 10 - 15 days on average. At the end of fermentation, the yeast sinks to the bottom. 

We have only been able to brew bottom-fermented beers since the late 1800s. During that time, Carlsberg's lab developed this yeast. In the previous years we also had few resources to be able to brew this beer all year round. This beer had to be cooled and in the past you could only reach those temperatures in the winter and you could not brew in the summer.

Examples of bottom-fermented beers are: pilsner, lager, bock beer (bokbier is sometimes also brewed top-fermented in the Netherlands, but the original beer style is bottom-fermented).


Top-fermented beers:

This fermentation takes place at a temperature between 16 and 24 degrees Celsius. At these temperatures, many aroma substances or esters are produced by the yeasts. The result is a very aromatic beer with scents such as peach, banana, raisin, pear, etc etc. The smell depends on the yeast type, you have many types of top-fermented beers, each of which has its own characteristic.

Top fermentation is quite explosive and is often finished in 5 days. Very fast compared to the bottom fermentation. With top fermentation, the yeast floats on top as a thick layer.

Examples of top-fermented beers are: double, tripels (from an abbey or monastery), wheat beer, weissen beers, English Ales, porters, stouts, etc.


Wild or spontaneous fermentation:

In a wild or spontaneous fermentation, the brewer does not add yeast himself as with top fermentation or bottom fermentation. After boiling, the beer is cooled in a shallow tub and exposed to the air. Wild yeasts float in the air and often also in the buildings where these vats are located, which now end up in the beer and start fermentation.

Spontaneous fermentation is uncontrolled and gives a very special character to the beer. During exposure to the air and then also to the wooden barrels, the beer comes into contact with lactic acid and acetic acid bacteria, among other things. These bacteria acidify the beer and this is a popular process for, for example, the gueuze beers from Belgium.

With bottom-fermented beers you want to avoid all other bacteria and brew a product that is as clean as possible. With the geueze you look for those bacteria to bring certain properties into the beer.

In the past, all beers were actually slightly sour because we didn't have the knowledge and resources or the right yeasts to be able to work cleanly.


Yeasts convert sugars into alcohol and carbon dioxide.

Fermentation is a natural process to convert sugars into carbon dioxide and alcohol. We need both end products to be able to call beer beer. Fermentation is a very complex process where much more happens than just converting sugars into alcohol and carbon dioxide. So many aromatic substances are also produced that we find very pleasant in beer. Sometimes less pleasant aroma substances are produced that we want to get rid of quickly. During the bearing, we lose some of the "off" flavors again.

The choice of yeast also plays a role in how far the yeast can ferment. The degree of fermentation says something about the residual sugars that remain in the beer. These residual sugars therefore determine whether a beer is dry or quite sweet. So a brewer has to think very carefully about the end result what he wants to achieve and which yeast he needs for that. What dryness or sweetness does he want to achieve, what are the aromas he is looking for in the beer in question.

When mashing, at the beginning of the brewing process, it is determined how big the deposit will be, in other words: how many kg of malt are added in how many liters of water. With a larger deposit, more malt, more starch in the malt is broken down into sugars and more of those sugars can be fermented into alcohol. This determines how much alcohol you get in the end product.






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