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Beer Fermentation
Ale Fermentations
Fermentation Stages
The fermentation stages in traditional ale fermentation systems (e.g., ale top skimming systems, Burton Unions, and Yorkshire squares) can be described as follows. After the wort is run into the fermenter and the yeast added, the yeast begins building cell walls and reserves, which is referred to as the "lag phase." At the end of the lag phase, the yeast begins to divide. The first visible signs of fermentation is the appearance of fine bubbles on the wort surface at the sides of the fermenter. These bubbles gradually spread, until the surface of the wort is completely covered. After about 18 hours, the bubbles thicken and their color changes to light brown as the cold break rises to the surface of the wort. At the same time the specific gravity starts to fall as does the pH, with a subsequent rise in temperature and yeast count.
Eventually, maximum fermentation is reached (usually after 36-48 hrs) and the yeast surface is covered with a white foam of constantly moving pinnacles and crevasses as carbon dioxide rises to the surface. Gradually, the yeast activity slows, and the color of the head changes from white to pale cream as yeast start to rise to the surface to replace the foam. The yeast will rise for some time, eventually forming a thick covering.
Fermentation Vessels
A wide variety of fermentation systems are in use today, including cylindroconical fermenters and the more traditional fermentation systems such as the Yorkshire square, the Burton Union, and the ale top skimming system. While vertical cylindroconical fermenters are increasingly common, there are many brewers who prefer to produce their ales using traditional fermentation systems. The most commonly found traditional ale fermentation system in the U.K. is the ale top-skimming system. The Burton Union system is now confined to a single brewery in Burton-on-Trent, and the Yorkshire square system is found in Yorkshire and Midland breweries.
Click on the following topics for more information on beer fermentation.