Yeast selection is one tool you can use when designing your recipes. So what's the purpose of yeast in homebrewing award winning beer and mead? Sounds like a silly question doesn't it. "Yeast ferment sugars in the wort or must to make alcohol" seems to be the obvious answer. But when you gain some experience and begin developing your own recipes, that answer just isn't enough. You can use different strains of yeast to fine tune your beer's character or overcome some difficult brewing situations.
Basic Considerations of Yeast Selection
Pitching Rate: The pitching rate is a fundamental property of fermentation that many homebrewers get wrong. You should always know what the pitching rate should be for the beer style you want to brew. All characteristics of a yeast strain that you will find published assume you will be pitching the proper amount of healthy yeast. Under or over-pitching will yield results outside those printed specs that may or may not be obvious to most homebrewers. Use the pitching-rate calculator on MrMalty.com to determine the proper amount of yeast to pitch.
Ale or Lager Strain: Which type of yeast should I use, ale or lager? This yeast selection decision isn't as cut and dry as it seems. Many yeast strains will "cross-over" to produce characteristics that are not typical of that type of yeast. The relationship is not linear, in other words, there are grey areas in which you can manipulate the yeast to produce predictable results. You just have to be a little creative with some of the fermentation parameters such as fermentation temperature, pitching rate, oxygenation, fermentation duration, conditioning and lagering. You can use different yeast strains in a "hybrid" situation to achieve your desired goals. For example, many ale strains are tolerant of low temperatures and will produce a very clean fermentation suggesting it was performed by a lager yeast. Many lager yeast will produce the fruity esters typical of an ale fermentation if the temperature is raised to say 58°F instead of 50°F. But, under normal situations, you should be able to narrow the choice of yeast selection down to one or the other types of yeast based on the beer's style.
Yeast Viability: You can't expect your yeast to produce consistent results unless they are healthy and viable. There are a lot of factors which affect yeast viability such as age, storage conditions, how they were transported from the manufacturer and arrived in your home brewery, how many generations you have pitched and the wort density of those previous generations, etc. Again, the specs published about the particular yeast strain you are using assume you are using clean, healthy, viable yeast. When making your yeast selection decision, if possible, always use the freshest yeast available. Store your yeast cold and keep the temperature stable. Don't repitch on a yeast slurry from a high-gravity fermentation. Don't re-pitch too many times. Wash your yeast from a prior fermentation before storing. Make a suitable starter with OG of around 1.040 and step it up for higher gravity fermentations (literature states that you shouldn't step it up to a volume of more than 10 times the original). Provide nutrient and plenty of oxygen for your starters. Pitch at high krauesen if possible or allow the yeast to settle and re-pitch into a starter the day you plan to pitch to kick start the yeast. Don't store in a lot of alcohol (back to washing your yeast).
Oxygenation and Nutrients: Using olive oil instead of oxygenating aside, you must provide the yeast with plenty of oxygen and nutrients so they can get through the lag phase of their reproduction and continue with the fermentation as healthy and viable as you can make them. This means you must provide some form of nutrients for high-adjunct worts (it doesn't hurt to provide a little nutrient in every beer you ferment) and plenty of oxygen before the yeast take on the daunting task of converting all that sugar into alcohol.
Fermentation Temperature: This variable affects the outcome of your fermentation more than just about any other factor. Things like flocculation, ester and phenol production, reabsorption of intermediate compounds, higher alcohol production, attenuation, all are affected by fermentation temperature. You must do whatever you can to assure the yeast ferment at the proper temperature and keep the temperature as stable as possible. Using a controller with a + or - 1 degree of tolerance is best. Attach the temperature sensor on the side of the fermenter or in a thermowell inside the fermenting wort to control the temperature of the wort and not the ambient temperature around the wort.
Conditioning: Every fermentation produces compounds that you don't want in your finished beer. Allowing the yeast to reabsorb these compounds prior to bottling or kegging is generally called "conditioning". It can be done in a secondary fermentation where the beer is racked off the dregs of primary fermentation, it can be done by manipulating the temperature at the end of primary, or it can be done through lagering. Be patient and allow plenty of time for a proper fermentation and conditioning or lagering phase. Pulling your beer off the yeast too soon will leave the beer under attenuated with a lot of undesirable compounds. Allow the yeast to do their job and clean up these byproducts before transferring or bottling.
Sanitization: You can make the right yeast selection decisions and get your yeast in perfect health for fermentation, but without good sanitization, it's all for nil. Get in the practice of scrubbing with the right cleaners, rinsing well and repeatedly, using the best sanitizer, sanitizing everything that will come in contact with the yeast before and after using the items, sanitizing your hands, sanitizing the mouth of your starter vessel and the scissors and yeast package before pouring them into the starter or fermenter, and never expose the yeast to the open air for any longer than necessary. It may seem like overkill, but it is an important part of brewing and will pay dividends for the remainder of your homebrewing hobby/obsession.
Yeast Selection Guide:
Once you have those basics covered, you can now look at your yeast as an important component in your recipe formulation. By yeast selection and by varying the parameters above, you can produce a myriad of characteristics in your final beer. Yeast selection can be complicated, but yeast are normally selected for a variety of parameters. These include the character of their fermentation, flocculation characteristics, the ester profile produced during fermentation, and their viability. No matter what contributions you hope to get from the yeast in your recipe, there is a strain and a set of fermentation parameters you can use to achieve those characteristics. Let's look at some factors you may want to look at when choosing a yeast strain:
Alcohol Tolerance: Each strain has a stated range of alcohol tolerance. Use the alcohol tolerance if you are planning to brew a high-gravity beer.
Attenuation: Different strains will have different expected attenuation ranges. When you want a beer that will leave some residual sweetness, choose a yeast strain that doesn't attenuate as well as the other varieties. This yeast will flocculate out and finish fermenation leaving some sugars unfermented. The end result will be the perception of sweetness in the beer. Many beer styles have some residual sweetness and one way to achieve this is by yeast selection. Other yeast strains attenuate well and leave a clean dry finish in your beer. Of course there are others that fall somewhere in between the two extremes. Use the attenuation to determine the finish of your beer. Of course controlling mash temps and grain bill has a lot to do with finish as well, but we are talking yeast here.
Fruity Ester, Phenolic, and Higher Alcohol Contributions: All yeast produce these compounds to varying degrees. Each strain produces different ester, phenol, and higher alcohol profiles and many have been identified with a particular beer style. Think of the characteristic fruity esters in an English beer, or the banana and clove profile of the Weizen yeast. These flavor and aroma characteristics have been well documented and should be available online. When in doubt, use the yeast from the beer's country of origin or one that historically was used to brew the beer. Each strain also produces a characteristic profile which you may or may not be looking for. Check out published descriptions to decide. For example, some yeast accentuate the maltiness of a beer and would be appropriate in a style that is malt forward such as many German beers, others accentuate the hop bitterness, flavors, or aromas in a beer and would be appropriate in a hoppy beer, while others have varying degrees of citrusy, woody, fruity, tart, soft, or minerally characteristics to name just a few.
Unwanted Off Flavors: Lager yeasts produce sulfur, that's just a fact of life. Some yeast strains produce prodigious amounts of diacetyl or other compounds which, depending on the concentrations, may cause off flavors in your final beer. Be aware of these contributions and how you will either remove them or live with them in your beer.
Flocculation: How a yeast strain flocculates is important for several reasons. One is beer loss. Some yeast strains flocculate well and leave a nice dense layer of yeast on the bottom of your fermenter with clean clear beer on top. Others however, don't flocculate as well and leave a loose fluffy mass of yeast which makes transfer difficult. You can expect to lose some beer unless you add a fining agent which will settle the yeast mass to the bottom and facilitate the transfer to a secondary, bottling bucket, or keg. Another consideration is the yeast left in suspension. Residual yeast in your beer can leave it with a "yeasty" character you may or may not want, depending on the style. This residual yeast floating around will affect clarity as well.
Yeast Selection Conclusion
Since we are talking about homebrewing, we won't discuss some of the other yeast selection criteria that professional brewers use, such as fermentation speed, stress tolerance, storage characteristics, and mutation stability. I'll leave these for the technical publications professional brewers read.
When designing a recipe, yeast selection should be considered with the finished beer in mind. Since there are several different parameters to consider when selecting your yeast, there will also be a large combination of these factors that you need to consider. For instance, you may want a yeast that accentuates the malt profile but leaves a some fruitness behind, with a clean and dry finish. These characteristics are all provided by the yeast selection and there should be several strains you could choose and several different fermentation profiles which would leave you with the desired character in your finished beer. The best way to decide on a yeast strain for your recipe is to read the profiles of the different strains. They are published on the manufacturer's website along with recommended styles for each strain. Other ways are to look at recipes which have been brewed by others, ask more experienced brewers in your club, or get on the forums and ask for recommendations and the opinions of others who may have some insights into your style. Just remember, all this brewing stuff is not rocket science. The best way to make award winning beer is practice, practice, practice.