Yeast is technically an ingredient, but yet it really isn’t. Yeast can be thought of as the machine by which wort is turned into beer. Before the discovery of yeast, this process was considered somewhat magical, and beer was often fermented by wild strains of yeast, and sometimes by a strain of yeast found on a family’s stir stick or in previously used fermentation barrels. In fact, the Reinheitsgebot of Germany stated that the only permissible ingredients in beer were barley, hops, and water. Often, fermenting beer would get contaminated with bacteria and/or undesirable strains of yeast, and barrels would be burned – not because of the contamination but because the barrel was “cursed.” A “stir stick” that was used in successful batches of beer would be considered blessed and would be treasured by its owners, not realizing that it was just infected with a desirable strain of yeast.
There are two main types of beer yeast; Ale and Lager. Ale yeast is top-fermenting and prefers temperatures from 60–70°F (15 – 21°C). Ales are typically more complex than lagers, with more flavour and aroma components. Lager yeast is bottom-fermenting and prefers cooler temperatures. It typically results in a cleaner tasting beer, without the estery character common to ales.
Yeast comes in two main forms, dried and liquid. Dried yeast can be of very good quality (avoid that which comes with tins of malt extract, though), and is often more quickly active in a beer. It can produce a limited range of ales—beware that no dried yeast is truly a lager yeast, nor will it provide the distinctive lager character. Dried yeasts are generally more effective if rehydrated prior to use. Liquid yeasts are harder to store as they must be kept chilled prior to use. There are many more strains of liquid yeast, and thus one can much more exactly reproduce a particular beer or style. Dried yeast is typically used by beginners and liquid yeast by more advanced brewers, although there are exceptions on either end of the scale.