Yeast Flavours 1
Anyone new to brewing will quickly find themselves overwhelmed by the multitude of yeast varieties available to them and understanding what the differences can be can be overwhelming. Yeast, like other fungus, are extremely adaptable and very easily domesticated and because of that, various strains have evolved to produce various flavour characteristics called excretion products. These products and flavours are based on the metabolic balance of the yeast which, in turn, is greatly influenced by many factors. In future articles, I'll be discussing in more detail how to influence this metabolic balance, but today, we'll focus on just one: strain.
As mentioned previously, beer yeast strains have evolved over time based on the particular beer brewed in a region. At first, yeast become domesticated - that is, although they undergo no genetic changes, they quickly "behave" when they have a good thing going - that good thing being wort, in this instance, though other fermented products using other yeast species will also become domesticated similarly. However, because life is a competition, genetic changes will soon occur that allows them to better take advantage of the situtation in which they find themselves.
It should be noted that strains and species are different. Kind of. When we talk about yeast, there are a few species, but many strains. Species include Saccharomycese cerevisae for common "ale" strains, Saccharomyces pastorianus, formerly carlsbergensis for lagers, descended from Saccharomyces uvarum. Generally, the accepted difference between these two species is that one ferments at higher temperatures and the other at lower, but hybrids exist because of the aforementioned domestication and genetic mutations. Also, pastorianus can utilise melibiose, which is a type of sugar.
In addition, there are "wild" species, generally in the Brettanomyces category, though mutliple new ones are being identified, including Kreik strains, which are extremely genetically diverse and not particularly well studied, though that is sure to change in the near future.
But this page is for new brewers, so let's not get too carried away.
In general, when thinking of how you want your beer to taste, yeast strain should be the first thing you look at and they will fall into a couple of large categories - but first some vocabulary:
Flocculation is the ability for the yeast to clump together and fall out of suspension. Good flocculators will fall out quickly and leave a relatively clear beer. These strains are usually chosen for cask ales which can't rely on filtration.
Attenuation is the ability of a yeast to turn all of the available fermentable sugars in a wort into excretion products (usually ethanol). 76% is a low attenuator while 81% would be a very high attenuator. Most fall in between. This tends to be inversely proportional to the yeast's flocculation
These strains tend to ferment producing few excretion products other than ethanol. They also tend not to flocculate that well and will take a little bit longer to finish, but will produce a higher attenuation.
These strains tend to be good flocculators with lower attentuation than their American cousins. They also tend to produce fruitier excretion products, known as "esters". Esters can range from strawberry to banana and often add complexity to very simple ales.
These strains are thought to be descended from English strains and have been adapted to tolerate a higher amount of glucose, which normally acts as an inhibitor for certain functions above 1%. They produce a lot of esters and attenuate on the higher end. They also tend to tolerate much higher ethanol levels than many other yeast strains. Banana and bubblegum are characteristic flavours.
These strains are similar to Belgian strains in that they will also produce high levels of esters. They will also ferment extremely dry under the right conditions and will produce spicey clove phenolics. It belongs to the PoF+ yeast strain grouping, which stands for Phenolic off-Flavour positive. Normally when brewers encounter clove phenolics, it's because of a wild yeast contamination (or dirty tap lines). These strains produce it without being contaminated and it's an expected part of the flavour profile.
Wheat strains are another PoF+ strain and produce the characteristic banana-and-clove profile found in hefeweizens. There can be a hint of sulfur, but it shouldn't be more than that. They are moderate attenuators and fairly good flocculators which is why hefeweizen breweries store their kegs upside down.
Very little differentiates one lager strain from another as the goal is a clean flavour profile with no esters and especially no phenolic off flavours. Attenuation will give you a drier or maltier palate on the finish, but they generally are poor flocculators and require lower temperatures to function properly. They will excrete sulfur, like hefeweizens if poorly maintained.
Hybrid strains are not actually hybrids, but cerevisae strains that have adapted certain pastorianus behaviours and vice versa. Two well-known styles exist that commonly use these "hybrid" strains - Kölsch and Steam Beer. In the case of Kölsch, the bishop of Cologne had forbidden brewers from using the newly popularised lager strains, so they simply lagered with ale strains instead. The result was a slightly fruity, but otherwise clean, light beer. As for Steam Beer, they were using lager yeasts, but had no effective means of refrigeration to keep it cool, and so it was pumped onto the roof of the factory into coolships where the steam would be visible on the roof. The lager strain could utilise melibiose and therefore, have a higher attenuation rate, though the higher fermentation temperatures tended to produce strawberry esters.
In Yeast Flavours 2, I'll go over two other factors that greatly affect the metabolic balance of yeast - trub and oxygen.