Fermentation is a process that converts the sugar (long-chain and short-chain molecules) obtained from starch during brewing to alcohol and CO2. The process of fermentation ends when all fermentable sugar has been converted to alcohol. A certain amount of residual sugar is needed to carbonise the beer during maturation (second fermentation). Carbon dioxide is produced during the fermentation of the sugar.
Top-fermented fermentation takes place at temperatures between 18°C and 22°C. At the end of the fermentation process, the yeast used for top-fermentation swims to the surface of the tank. Typical brands of top-fermented beers are Ale, Kölsch and wheat beer. Top-fermented beer has the advantage that it can be fermented at room temperature.
Bottom-fermented fermentation takes place at temperatures between 9°C and 12°C. As opposed to top-fermenting yeast, the yeast used for bottom-fermentation settles to the bottom of the tank. Typical brands of bottom-fermented beers are lager and Pilsner. Usually, bottom-fermented beers require a much longer fermentation.
Maturation (also called the second fermentation) takes place at temperatures between about 3 to 5°C. It is carried out under pressure in order for the beer to carbonise. If the natural carbonation is not sufficient, supplementary CO2 can be added.
The temperature of the fermentation process can be regulated at the tank via a cooling unit and a cooler. Constant temperatures are considered to improve the quality of the beverage. Varying temperatures, instead, offer the possibility to change the taste.
Yeasts are classified as members of the Kingdom Fungi. They multiply by duplication and are widely dispersed in nature with a wide variety of habitats. Brewing yeasts are special clean breed yeasts that are available in both dry and liquid form. Dry yeasts have the advantage that they have a longer lifetime. As aforementioned, there are bottom-fermenting and top-fermenting yeast strains.
Known yeast strains:
- W34/70 for lager beers like pale ale, Märzen, Pilsner (strain of Weihenstephan)
- WB06 for wheat beers
- US-05 for ales
- S23 also for lager beers (strain of VLB-Berlin)