Until a few months ago selecting yeast for my beers was based on style, but after that somewhat arbitrary. If I were making an Irish stout, for example, I would just pick up a Irish ale yeast and call it a day. Recently I started venturing outside the normal style guides and there was no “go to” yeast for my beer as it did not really fit any particular style. These adventures lead to a fair amount of research on different strains. I also had to understand the differences in the yeasts and how this would effect the beer in the end. After many hours on the internet I came to think of yeast drastically differently. I must admit that before I viewed different yeast strains as fairly irrelevant and all (within reason) would make the beer I wanted. Now I realize that yeasts can provide up to 500 individual flavors into your beer. There is perhaps no bigger decision to make in you beer than the selection of the right yeast. Here is some of what I learned that I hope will help others.
This refers to the percentage of sugars that the yeast will consume and turn into alcohol before Flocculation. A typical range for beer yeast is between 60% and 80%. A yeast with a 60% attenuation will produce a sweeter beer with a lower ABV, as it will leave more sugar behind. Likewise a yeast with an 80% attenuation will produce a drier beer with a higher ABV.
I have to admit I had really no idea what this was until I did my research. This represents how well the yeast will stick together and fall out of solution, clearing your beer. Most yeast will Flocculate given enough time but yeasts with higher Flocculation will clear better and faster. If you want a nice clear beer you will want a medium to high Flocculating yeast. It must be noted however that very high Flocculating yeast can stick together and fall out of suspension too soon causing incomplete fermentation. Beers that are intended to be cloudy will usually use a low Flocculation yeast while beers that are meant to be clear will generally use higher Flocculation yeasts.
Optimum Fermentation Temperature:
This is pretty strait forward. This is the temperature range where the yeast performs best. If you can temperature control your fermentation than this is not much of a factor. I do not at the moment so this is a factor for me. My basement, where I ferment, is 66°F year round so a yeast with a 68-72°F Optimum range might not be the best choice if I had an alternative. Granted 2°F most likely would make no real difference but I like to stay in the optimum range if I can. I also try and stay on the lower end of the optimum range. Beer can ferment at 5-10 degrees higher than ambient temperature. By staying on the lower end of the range, even if it is a few degrees warmer at the center of the fermenter, I am still with in the recommended temperatures. Again this is just my process that I follow most of the time. Feel free to do what you want here, this is just an illustration of how I use the temperature ranges in my selection process. This is by no means the only or necessary the best way it is just an example.
This is the limit in which the yeast will become stressed and eventually die due to the level of alcohol in your beer. This is pretty self-explanatory but you would not want to use a yeast with a low alcohol tolerance in a very high gravity beer unless you want a an incomplete fermentation.
Above we covered some of the main statistics you can gather from most yeast manufactures websites for particular strains. Most of the time they will also include some sort of text description as well here are some things to look for in the descriptions that may influence your choice.
Some yeast strains can produce sulfur odors and tastes when stressed. Others will routinely produce sulfur smells even if nothing is wrong (Hef, and many wine yeasts). Sulfur smells will usually go away with time but it is good to know if a particular yeast strain is prone to sulfur production. That way if you smell sulfur you will know if it is normal or not.
With a good bit of research, I have still not found a good description of what these actually are but they are usually the product of higher than recommended fermentation temperatures and yeast stress. These produce fruity, nutty, or vanilla flavors in your beer. Some styles this is good for example Belgians and Hef’s (notice these yeasts normally have a higher optimum fermentation temperature at least partially for this reason). In other styles this is not desirable such as some English and American styles. Whether Esters are desirable or not really depends on the style and what you want in your beer, this is just something to note when selecting your yeast.
The Information gathered here is accurate as I know it. I do not claim to be a microbiologist so there may be some misunderstandings included here. Please comment if I have made any errors and I will correct them and learn something new.