Brewing - Fermenting - Fermentation Methods

Fermentation Methods

There are three main fermentation methods, warm, cool and wild or spontaneous. Fermentation may take place in open or closed vessels. There may be a secondary fermentation which can take place in the brewery, in the cask or in the bottle.

Brewing yeasts may be classed as "top-cropping" (or "top-fermenting") and "bottom-cropping" (or "bottom-fermenting"). This distinction was introduced by the Dane Emil Christian Hansen. Top-cropping yeasts are so called because they form a foam at the top of the wort during fermentation. They can produce higher alcohol concentrations and in higher temperatures, typically 16 to 24 °C (61 to 75 °F), produce fruitier, sweeter beers. An example of a top-cropping yeast is Saccharomyces cerevisiae. Bottom-cropping yeasts are typically used to produce cool fermented, lager-type beers, though they can also ferment at higher temperatures if kept under 34C. These yeasts ferment more sugars, creating a dryer beer, and grow well at low temperatures. An example of bottom-cropping yeast is Saccharomyces pastorianus, formerly known as Saccharomyces carlsbergensis.

For both types, yeast is fully distributed through the beer while it is fermenting, and both equally flocculate (clump together and precipitate to the bottom of the vessel) when fermentation is finished. By no means do all top-cropping yeasts demonstrate this behaviour, but it features strongly in many English yeasts that may also exhibit chain forming (the failure of budded cells to break from the mother cell), which is in the technical sense different from true flocculation.

The most common top-cropping brewer's yeast, Saccharomyces cerevisiae, is the same species as the common baking yeast. However, baking and brewing yeasts typically belong to different strains, cultivated to favour different characteristics: Baking yeast strains are more aggressive, in order to carbonate dough in the shortest amount of time; brewing yeast strains act slower, but tend to produce fewer off-flavours and tolerate higher alcohol concentrations (with some strains, up to 22%).

To ensure purity of strain, a "clean" sample of brewing yeast is sometimes stored, either dried or refrigerated in a laboratory. After a certain number of fermentation cycles, a full scale propagation is produced from this laboratory sample. Typically, it is grown up in about three or four stages using sterile brewing wort and oxygen.