What even IS Yeast?

Posted by Barley Beaver on

You’ve probably heard the saying “Brewer makes the wort, yeast makes the beer”, and it’s actually very hard to find a more accurate statement that describes the role yeast plays in beer production. Of course, the work of the brewer is substantial and without them there would be no beer as we know it today. However, it’s really hard to overestimate the importance of brewer’s yeast in the process of beer making, since it’s the very agent that converts the maltose in the wort into alcohol we all appreciate beer for. And any professional brewer knows really well that without understanding the yeast, how it works, and how to use it correctly, brewing good beer is impossible. But fret not, as it’s not that complicated – all you need to know are some basic rules and yeast attributes that will help you brew the best beer out there. But first, a little overview of what is yeast and how it is employed in the production process of beer.


Yeast is a single-cell fungal living organism that can be found virtually everywhere in nature. There are thousands of different species of yeast, each having hundreds upon hundreds of strains with their own properties, energy source preferences and products of metabolism. What we know as brewer’s yeast is actually a single genus called Saccharomyces, with two distinct species used for fermenting beer – Saccharomyces cerevesiae (ale yeast) and Saccharomyces pastorianus (lager yeast). Each of these species has numerous strains with slightly different traits, which we’ll get into a little down the text. Yet, the main feature these organisms share is that they ensure the process of fermentation that is paramount to beer production.


If you’ve ever brewed beer or at least read about the principle, you probably know that yeast converts the sugar (maltose in the case of beer) contained in the wort into alcohol and CO2. Of course, this is an extremely simplified view of the process, since there are hundreds of chemical compounds produced, some reprocessed, some evacuated during the fermentation process completely, with many stages in-between. But despite its flaws, this model will certainly do. The most important thing here is understanding the simple fact that yeast is a living organism with certain properties varying from strain to strain, which has to be ensured with optimal conditions in order to deliver the results desired by the brewer. And it’s the brewer’s job to make sure that all the conditions are met and the yeast has all that is needed to turn wort into beer.


Doing all this isn’t rocket science, though it requires a certain degree of precision and thoroughness. Luckily for brewers, the yeast labs that produce commercial brewer’s yeast make things a lot easier by providing precise recommendations and variables that describe the properties of each strain. No matter whether using liquid yeast, dry yeast, or yeast slurry harvested from the previous batch of beer, every yeast strain can be characterized by the following attributes: Attenuation, Flocculation, Alcohol Tolerance, and Optimal Fermentation Temperature. Knowing these attributes will help you pick the right strain for any recipe and understand the optimal working conditions for the given strain. Let’s go in more detail over each of them and see how these attributes relate to yeast.



Simply put, it’s the yeast’s ability to convert maltose into alcohol, expressed in percent. The absolute value of 100% is when a strain processes all the sugars contained in the wort into alcohol and CO2 without leaving any residual sugars in the beer. For brewer’s yeast the values usually vary between 60% to 90% depending on the strain, and it depends on the style you want to brew which strain should be used.

Attenuation is fairly simple to understand. For example, a wort with Original Gravity of 1.052 (13 Plato), which is fermented with an yeast strain that has attenuation value of 75% should deliver a beer with Final Gravity of 1.013 (3.3 Plato) and ABV 5.13%. Now the “should” part refers to the fact that in reality things can go differently and you can end up with a higher or lower apparent attenuation, which should still be within a certain range around the theoretical value.

The higher the attenuation of the yeast strain the drier is the beer and the more alcohol you are able to obtain from the wort of the same Original Gravity. Respectively, the lower the attenuation, the less alcohol the yeast produces and the more residual sugars remain in the beer, rendering a fuller body and sweeter taste. And it’s easy to choose an yeast strain adequate for a particular style when knowing its attenuation. For instance, a very dry Saison will require a strain with high attenuation, whereas a full-bodied sweet porter will need lower attenuation values.



This attribute describes the yeast’s ability to form clumps after metabolizing the sugar in the wort, which subsequently settle down to the bottom of the fermentation vessel to form the yeast sediment. High flocculation values mean that the yeast strain will form these clumps quite soon into the fermentation process and form a solid and dense sediment, resulting in a very clear body. Low flocculation values mean that the yeast will not form these clumps and stay in suspension even after the fermentation is complete.

A good example of a low flocculating yeast is the traditional German Heffeweizen strain, which delivers very hazy beer. That haze you see in a typical glass of Weizen is the yeast that didn’t settle down to the bottom of the fermenter. A high flocculating strain is the traditional British ale yeast used for Bitters, ESBs, Porters and other traditional British styles. Forming clumps that drop like a stone to the bottom of the fermenter, these beers are usually quite transparent without undergoing additional filtration.

Take note that highly flocculating yeast strains usually have lower attenuation values due to the fact that the yeast cannot manage to metabolize all the sugars before dropping out into sediment. Medium flocculating strains have good attenuation and produce fairly clear beers. High flocculating strains produce the haziest beers, while the attenuation values may vary from strain to strain.


Alcohol Tolerance

Alcohol is toxic for yeast when it reaches a certain concentration in the beer, and it’s the concentration value that defines the strain’s alcohol tolerance. The higher the alcohol tolerance value the more alcohol the yeast can produce before it becomes toxic to the organism. Most commercial yeast strains have alcohol tolerance equal to about 7-8% ABV, which allows fermenting the vast majority of beer styles. However, if you have a high gravity wort and try to ferment it with an yeast strain that has moderate alcohol tolerance, not only will you fail to get the desired final gravity, but will also get many off-flavours in your beer due to the rapid yeast die-out in a highly toxic (alcoholic) environment. That’s why knowing the alcohol tolerance of a strain is very important, especially when brewing stronger styles that require specialized yeast with higher alcohol tolerance values.


Optimal Fermentation Temperature

You probably know that the working temperature range is different for ale and lager yeast. For top fermenting ale yeast it’s typically between 15 and 22C (60 to 72F), whereas for lager yeast it’s between 10 and 15C (50 to 60F). However, within this range, each strain has a value, at which it works the best and produces optimal results.

For example, the common Chico ale yeast strain that is used for fermenting clean American IPAs has an optimal fermentation temperature of about 18C (65F), where it produces the cleanest and most neutral beers. Ramp up the temperature to 24C (75F) and the yeast will start producing undesired ethers, phenols and higher alcohols that will affect the beer’s flavour. Bring the temperature down to 15C (60F) and the fermentation will become sluggish, with low apparent attenuation and signs of yeast stress.

That’s why knowing the optimal fermentation temperature for the chosen strain is important, as the brewer will have to control the conditions for fermentation if they want to produce the best results with a given strain. Some yeast strains have a rather broad range of optimal temperatures, while others may start showing signs of stress even within a couple degrees of temperature fluctuation.


Besides these attributes, an important aspect that defines every yeast strain is its flavour profile. Some strains produce very little ethers and phenols, which result in very clean beers, the flavour of which is primarily defined by the grain bill and the hops used. Other strains have a very pronounced ether and phenol profile, which affects the beer’s taste and aroma to a great extent. An excellent example of such a strain is traditional Hefeweizen yeast, which is responsible for the banana and clove flavour German wheat beers are famous for. For each strain the flavour profile is individual and should be taken into account when selecting yeast for a particular recipe. However, the yeast producer will usually choose a commercial name that reflects the most appropriate style the strain is suited for. So if the yeast is called Belle Saison, it’s pretty obvious that this strain won’t be very useful for brewing a Stout with. Though many exciting brewing experiments involve using yeast that is seemingly inappropriate for the style. But that’s a topic for a totally different discussion.


To sum it all up, knowing your yeast and how to work with it is a very important aspect of brewing, regardless of scale. No matter how good your malt, hops or water may be, if you don’t know hot to use your yeast, don’t provide the right working conditions for this organism, or simply choose a wrong yeast strain, your beers will never be as good as they could be. But getting all these things right isn’t as hard as it seems. And hopefully, this article will help you gain full confidence when working with yeast, and inspire you to brew the best beers for you and your friends to enjoy.

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