Yeasts that are commonly used for brewing are generally of two species, Saccharomyces Carlsbergensis and Saccharomyces Cerevisiae. These two species roughly translate into lager and ale yeast. Within each species however there are hundreds of different strains maintained and used by brewers throughout the world. I was once thought that you could catagorize an ale yeast if it top fermented and a lager if it bottom fermented (flocculated to form a cake on the bottom of the vessel) however this is not and never was true. While it works as a rule of thumb it is not an accurate way to determine yeast types.
For the homebrewer there are two viable options for yeast; The first is to use a dry yeast similar to yeast you would buy in packets for baking, or to use liquid yeast starter cultures. Yeast that is used for brewing is NOT the same as yeast used for bread, this is very important. Using a bread yeast will most likely yeild a beer you are not interested in drinking. I think there is a time in every homebrewers life they will start experimenting with silly things like using rapid-rise yeast in a batch just to "see what will happen", but be forewarned!
I will concentrate mainly on liquid yeast because most brewers move quickly to liquid yeast because of the great number of available strains from Wyeast and White Labs. Both these companies provide some form of yeast culture which can be either pitched directly into your wort or used to create a larger culture. I typically use Wyeast 50ml smack packs and then create a starter culture because they are a little less expensive than White Labs and I have found their descriptions of the yeast strains to be more accurate.
When choosing a yeast you an examine the description available for their yeast types at Wyeast's website before you purchase. Choose a yeast that matches the desired flavor profile for your beer and take note of two things, fermentation temperature and flocculation. Fermentation temperature, which is usually a range, is the temperature at which you should maintain your primary fermentation vessel while the yeast do their work. If you get outside this temperature range the yeast may produce off flavors which are considered undesireable. Usually being below the fermentation temperature will result in just a longer fermentation time with high temperatures resulting in a variety of flavors including citrus, butterscotch or others.
Once you have chosen your yeast to use and followed the directions on the package I suggest making a yeast starter culture still. Making a starter allows the yeast population to grow in a small vessel to a much larger number before you pitch it into your wort. The advantage of this is that wort is most vulnerable to bacterial infection when it is cooled but fermentation has not begun. If you pitch a yeast population that is not large enough it is possible for bacteria to grow at the same time as the yeast and to produce off flavors if not spoil your batch of beer. Thus pitching a large active colony of yeast will result in a strong quick fermentation which will kill all other bacteria if any are present even after sanitation before they have any chance at spoiling your beer.
These are the directions for making a 1/4 gallon yeast starter which is sufficient for almost any ale. If you are making a lager I would suggest doubling this recipe because at lower temperatures it takes even longer for a yeast colony to grow to sufficient size for vigerous fermentation.
Last Updated: July 12th, 2006