We’ve quoted this before but it’s worth reiterating that ‘brewers make wort, yeast makes beer’. Choosing the right strain of yeast arguably makes the biggest difference in how your end beer turns out so we thought we’d talk you through how to make the right selection.
The most obvious distinction to make in yeast strains is whether it is a lager or an ale strain. In very simple terms, there are two predominant yeasts used in brewing; Saccharomyces cerveisiae or ale yeast, and Saccharomyces pastorianus or lager yeast.
Ale yeast works best at room temperature (18-22°C) and produces the slightly ‘fruity’ characteristics that you would expect in many ales. They tend to be top fermenting and often have a higher alcohol tolerance than the lager strains. In comparison, lager strains work at much lower temperatures (8-15°C), are usually bottom fermenting and utilise the sugars found in wort more fully which gives that clean, crisp taste that is classically associated with the style.
Once you’ve decided on what kind of yeast you are going to use there are a few things to consider in terms of what flavor contribution you want from your yeast;
Esters are the flavor component which impart flower, fruit-like flavors and aromas to your beer. At the right levels these flavors can be desirable but if you don’t control your fermentation properly they can become unbalanced and overpowering.
As well as the strain of yeast that you choose there are many other factors which can have an impact on the formation of esters during fermentation. These include fermentation method, pitching rates, wort aeration and the temperature at which you ferment. One of the main reasons it’s important to properly oxygenate your wort is because an increase in oxygen available at pitching directly links to a decline in esters formed in the final beer. Which esters form and to what extent is largely strain-specific.
In some circumstances an increase in wort gravity can lead to an increase in ester formation (so higher ABV beers would have higher ester levels) which can limit the gravities that brewers can achieve whilst still remaining true to style and within the accepted ester level.
When it comes to controlling ester production, proper aeration of your wort prior to pitching is key and then controlling the fermentation temperature as much as possible will really help your beers improve. Pressure during fermentation has also been shown to limit ester production (but can also reduce yeast growth) which professional brewers achieve by brewing in large, conical fermenters.
There are over 200 carbonyl compounds that can be found in beer and contribute in some way to the flavor. Homebrewers are most likely to have heard of diacetyl and acetaldehyde which are both carbonyl compounds. Carbonyls also play a significant role in your beer’s stability.
Acetaldehyde is a common carbonyl and is what imparts the grassy/green apple flavor that brewers associate with a very young beer or a beer that has not been left on the yeast for long enough. Acetaldehyde builds up during the growth stage of the yeast cycle but typically declines when the yeast enters the stationary phase. As with ester formation, the level to which acetaldehyde will form and decline is largely dependent on the strain of yeast used. If you rack your beer off of the yeast too early the yeast is unable to remove the acetaldehyde that has formed which can lead to unacceptable levels being present in your end beer.
Diacetyl is another ‘flavor-active’ carbonyl and similar to acetaldehyde, the level to which it is present in the final beer is determined in the fermentation stage. Diacetyl imparts a “butterscotch” flavor and aroma to beer. Diacetyl in very small levels is usually acceptable by the BJCP in almost all ales and possibly even in Czech style pilsners but typically lagers will not contain diacetyl which is why brewers include a diacetyl rest in their lager fermentations, to allow the yeast time to clean up any diacetyl that has formed.
Phenols are a chemical compound which can be responsible for several off flavors in a beer such as clove and banana, spicy or medicinal. In certain quantities and certain styles (such as clove and banana in a Belgian beer or a traditional wheat beer) these flavors can be somewhat desirable though in many other instances they are considered a flaw.
All yeast strains will produce phenolics to some extent though some German, British and Belgian strains are developed to produce higher levels. Your brewing water and your method for mashing and sparging may also contribute to high concentrations of phenols.
When it comes to mashing and sparging, if you mash at too high a pH (usually above 5.5 though around 5.2 is preferred), sparge with too much water or sparge with water that is too hot, then you are likely to extract polyphenols – more commonly known as tannins. This can lead to harsh astringency or bitterness in your finished beer as well as contributing to cloudy beer.
You should also be aware of your water chemistry when mashing, as high levels of chlorine can form chlorophenols which impart band-aid flavors. Chlorophenols are always considered an off flavor in beer. If your water is particularly high in chlorine (above 200mg/l though preferably below 150mg/l) then you should either run it through a carbon filter or boil it for 20-30 minutes before brewing.
Which yeast to choose?
So you can see that there are several considerations when choosing a yeast strain which will affect the end flavor of your beer. What beer style is the strain particularly suited too? What level of esters and phenolics is it described as providing? Is the strain particularly prone to high levels of diacetyl and if so, do you have the necessary temperature control in order to do a diacetyl rest?
With an awareness of the different flavor elements that yeast can provide you’ll be able to make the appropriate choice of strain in relation to the style of beer you want to produce. To help, we’ve created a list of the styles that our yeasts are best suited for but as always, experimentation is the key! So once you know the rules, don’t be afraid to break them!