There are four main components of beer: water, malt, hops and yeast. This article is the last of a four part series that will be covered in that order.
When combined, water, malt and hops create most of what we know as beer. However, without yeast, all you have is a batch of flavored water or wort– there's no alcohol. Yeast is what converts sugars in the wort into alcohol. As such, you can imagine just how important this living organisim is to the brewing process. Without it, you wouldn't have much of anything worth drinking. However, just how do these hungry guys work? What are the best types to use in making beer?
Yeast works through metabolism. In essence, it consumes sugar and exudes alcohol and carbon dioxide. It's a pretty simple system. As long as they have a sufficient supply of sugar (and the alcohol content doesn't exceed the yeast's resistance level), it will continue to produce alcohol and carbon dioxide. Of course, brewing is more than just dumping a load of these slurry creatures into your wort. You need to know the characteristics of the yeasts typically used in brewing beer. That word there, "typically" is important. This is because, technically, a wide range of different yeasts can be used, including bread yeast. However, different types produce different flavors, scents and other characteristics, so not all are ideal for brewing beer.
Beer yeasts are generally slotted into two primary categories. These are top-fermenting and bottom-fermenting yeasts. Top-fermenting forms a frothy head at the top of the fermentation vessel. Most ales (and ale derivatives like stout and porter) are brewed using top-fermenting yeast. Bottom-fermenting yeasts are often used to make lagers, though they can also be used for ales. Bottom-fermenting yeasts usually have a higher tolerance for alcohol and also create a beer with dryer characteristics (they're a bit more akin to the type used in winemaking).
Wild yeast can also be used in brewing beer, as it is, literally, everywhere. However, using these wild beasts is not particularly advisable (even in fruit beer brewing), as it can lead to problems with control of taste and smell. Wild yeast is not very predictable, though lambic beers do make use of a general type (there are several varieties of common wild yeast where these beers are traditionally brewed).
Yeast is possibly the most important component of brewing, simply because there is no beer without it. While water, hops and malt give the beer flavors, color and other characteristics, the type of yeast you use will also have an immense impact on the finished product, and will dictate everything from its alcohol content to is dryness and more.