In Part 1, we went over some yeast basics and characteristics of Ale and Lager yeast as a very simple overview of the fermentation process. In Part 2, we will go over how understanding the right conditions for fermentation will maximize the good flavor potential of the yeast and, just as important, minimize the bad flavor potential. The primary factors are temperature, oxygen, the amount of fermentable sugars, the amount of yeast pitched, and viability of the yeast.
It is important to stay within the temperature range of the variety used during the fermentation period. Too far above the temperature range and the yeast can produce a variety of exciting flavors, almost all of which taste bad. Too far below and the yeast may just sit there and do nothing. Especially with Ales, too high of a temperature can set the right conditions for bacteria to beat the yeast to the sugars, which can contaminate the batch.
Oxygen is a factor that a lot of homebrewers forget about. Prior to pitching the yeast, it is important to agitate the wort or use a beer stone (air stone) to dissolve oxygen in the wort. Gasses, like oxygen, dissolve better at lower temperatures and the heat of the brewing process depletes much of the oxygen dissolved in the wort. Therefore, it is best to let the wort cool to the required temperature before stirring, shaking, or otherwise agitating the wort to dissolve the necessary oxygen.
The more fermentable sugars in the wort, the higher the potential alcohol content of the finished beer. The desired alcohol content is an important consideration in choosing which yeast variety to use. Ale and Lager yeasts do not tolerate high levels of alcohol well. They will usually keep fermenting until the alcohol level reaches about 8.5%. Around this point the yeast can become stressed and produce undesirable flavors and aromas as they stagger away drunk. For beer styles that have a higher than 8.5% Alcohol by Volume percentage (% ABV), it is best to use a yeast variety that has a higher alcohol tolerance, such as special strains for brewing yeast or even champagne yeast.
When determining how much yeast to pitch into the wort, it is better to pitch too much than too little. Too little can cause a prolonged lag time, giving other beasts, like bacteria, the edge to beat the yeast for dominance of the sweet sugars. Ideally the lag time between pitching the yeast and the yeast being thoroughly engaged in the fermentation process should not be more than a day. Worrying about the amount of yeast to pitch only really comes into play with homebrewers who culture their own yeast or for recipes with high specific gravity wort (more fermentable sugars) where the risk of contamination is slightly higher. Most commercially purchased yeast varieties, whether dry or liquid, are more than enough for a typical five-gallon batch. Be sure to follow the directions for preparing the yeast carefully to ensure the yeast get a good start. For high specific gravity beers, seriously consider making a yeast starter or doubling the amount of yeast pitched. Carl will be talking about yeast starters in a future post, hopefully soon.
Yeast viability is not much of an issue with commercially purchased varieties, as long as it is stored properly and used before the expiration date. Similar to determining the amount of yeast to pitch, viability only really becomes a factor for those who culture their own variety of yeast or harvest it from their favorite commercial beer. Old, tired, and/or mutated yeast can inhibit a clean fermentation process and produce a lot of uncharacteristic bad flavors.
Regardless of the variety of yeast you choose to craft your beer, it is important to give the little guys the right environment to maximize their career potential. Though the flavor impact of yeast on beer is often less subtle than its hops or malt counterparts, it can give a beer the flavor edge it needs to be great. Happy brewing!