Introduction
Yeast is a microscopic fungus, of which there are hundreds of species. It is extremely important brewing ingredient because different strains give different beer types their distinctive and characteristic flavors. When a brewery has found an ideal yeast, it will be retained for many years. However, fresh batches are produced regularly from samples kept under special laboratory conditions to prevent the built up of microbiological contamination. (Just as milk can go sour – yeast and bacteria can also contaminate beer). Individual yeast cells are invisible to the naked eye, and are carried in air current. When they grow on a suitable food source (for example fruit, such as grapes and plums) they form ‘colonies’ of cells. (These can be seen as a fine white powdery film on the skins of the fruit). Yeast can feed on a variety of sugars, converting them into energy in order to grow and multiply. When it first grows, the yeast cells need a supply of oxygen in the same way a animals do when they convert sugar into the carbon dioxide and energy. If animals run out of energy they die. But in the absence of oxygen, yeast obtains its energy from ‘anaerobic fermentation’ in which sugars are converted to alcohol and carbon dioxide. Most importantly for the brewery, yeast produces a variety of flavoring components (through side reactions), which help give the beer its characters flavors. Also use top-fermenting yeast strains that typically ferment between 65-75F and lagers use bottom fermenting yeast stains that typically ferment 50-55F. Pitching temperature should always be 80F or under because yeast will die if exposed to temperature over 110F. Too high of a fermentation temperature or higher alcohols (fusel alcohols). Yeast fermentation time will vary depending on strain, temperature, and fermentations environment, however most fermentations should be complete within 7-14 days.
