The role of brewer's yeast in the beer production process is to convert some types of sugars, such as maltose, sucrose and raffinose, which are present in the wort into ethanol and carbon dioxide. During
fermentation, by-products such as esters and higher alcohols, which are desirable in some types of beer (Weizen, Ale), appear in smaller amounts and in some not at all. Thus, alcohol is produced thanks to yeast.
Two strains of yeast “Sacharomyces” are used in beer production, the top-fermenting strain being called “Sacharomyces cervisiae” and the bottom-fermenting strain “Sacharomyces carlsbergensis”. The main difference between these two related yeast strains lies mainly in the temperature at which they work.
The top-fermenting yeasts work optimally at a temperature of 15 - 25 ° C, at which higher the above-mentioned aromatic substances (esters) are also produced. This is the reason for the fruity aroma of top-fermented beers. For yeasts used for bottom fermentation, the optimum temperature is 8 - 12 ⁰C. This method of fermentation does not produce aromatic esters and the resulting beer has primarily a hop and malt flavor and aroma.
Secondary fermentation can then continue to a lesser degree even at temperatures approaching the freezing point and may result in deep fermentation when the beer “lies”. The secondary fermentation process already produces a minimum of alcohol, but the beer is naturally saturated with carbon dioxide and gains more flavor. This process lasts for weeks, and the yeast is slowly working until it consumes all the sugar. Sometimes a combination of both types of fermentation is also used, where the main fermentation takes place at higher temperatures and then yeast is added for the bottom fermentation and the beer is fermented at lower temperatures.
Another type of fermentation is the so-called spontaneous fermentation, used mainly in Belgian Lambic beers. In this type of fermentation, wild yeast and bacteria are used, that is, naturally occurring in the air. Some breweries closely guard their bred yeast variants from the competition.