YEASTories and BACTheories 2: Key physiological and metabolic parameters when choosing your brewing yeast

Take a few minutes to search in yeast commercial suppliers catalogs and you will quickly find out the large number of different strains that are available for brewing beer and other beverages. Brewing yeast species were initially isolated from nature and later also domesticated in different parts of the world under different environments, which resulted in a variety of yeast strains with great physiological differences. The invention of the microscope had a key role on the initial identification and characterization of the different strains. Nowadays, the continuous development of microbiological and genetic tools along with new analytical techniques has contributed to a deeper understanding of the specific capabilities and limitations of each strain, as well as for the identification of novel yeast types.

Every year the number of fully characterized yeast strains increases and there are a few companies with culture banks that have a great variety of yeast strains: White Labs, WYEAST, Fermentis, Lallemand, Mangrove Jack’s, Imperial Yeast (organic) or CooLAB (organic), among others. In each website, you can find descriptions of each strain that will help you choosing the right strain for the intended beer type.

Start simple

If you are in doubt, start simple and brew with a yeast type which is a “work horse”, meaning that it will efficiently work for a great variety of beer styles. Still, there are some factors important to consider when choosing the yeast for alcoholic fermentation:

Attenuation – how much sugar can the yeast convert into alcohol. Usually, commercial suppliers divide the yeast strains in low, medium and high attenuation, varying from approximately 65 to 85%. The specific attenuation will impact not only the alcohol % but also the mouthfeel and flavor;

Flocculation level – how easy does the yeast cells settle after fermentation. This is an important feature when you wish to re-use the yeast to another fermentation. Besides that, a low flocculation yeast can lead to a lower attenuation, resulting in a worty flavor. On the other hand, if your yeast of choice has a high flocculation, the final beer will tend to be cloudier and you will be able to taste the yeast, like in weißbier or witbier;

Alcohol tolerance – alcohol level that inhibits and potentially kills your fermenting yeast. Choosing a strain that can stand the alcohol percentage you are planning to reach is extremely important, especially in those styles that require a high alcohol % such as Imperial Stout or Belgium Ale;

Temperature – each strain has a range of temperatures where it can grow, and it is important to know both the optimal and the extreme temperatures that the yeast can stand;

Metabolite production and sensoric properties – what kind of flavors and aromas are produced by the yeast strain. There are several metabolites (intermediates or final products of yeast metabolism) that can contribute to the sensoric properties of the finished beer: esters, carbonyl compounds, phenolics, higher “fusel” alcohols and fatty acids:

• Esters are the resulting compounds from a reaction between an acid and an alcohol, and they are often associated with fruity notes in beer (e.g. ethyl acetate or isoamyl acetate). The specific types of esters formed, as well as their concentration, are strain-specific but the fermentation conditions also influence the ability of the yeast strain to produce them.  For instance, there are reports that high gravity brewing and high fermentation temperatures (20-25^oC) result in higher levels of esters (as in some ale beer types).
• More than 200 compounds with a carbonyl functional group have been found in beer, contributing for both its flavor and stability. Diacetyl and acetaldehyde are examples of carbonyl metabolites and probably the most “unwanted” compounds by brewers (except in some very specific beers), since they are considered off-flavors. Both the formation and conversion rates of those metabolites is strain-dependent, so the time that you will need to get a matured beer will depend on your yeast of choice. This is particularly important in large-scale production where time is a key control parameter.
• Phenols are commonly associated with a medicinal or spicy aroma, and some specific types add astringency and/or bitterness in the finished beer. For instance, the earthy aroma present in Brett beers (fermented with Brettanomyces yeast) is directly linked to the formation of phenolic compounds.
• When present in abundant levels, higher fusel alcohols, such as propanol and butanol, can result in fruity, floral and/or wine-like notes. Their formation can have a positive impact in ale beers but normally are not desired in ale types.
• Fatty acids are essential elements in the yeast central metabolism, but they can also be broken down into staling compounds such as (E)-2-nonenal, which will give a “cardboard” character in the finished beer.

In addition to the points mentioned above, when brewing at large-scale breweries there a few other parameters to consider when choosing the right yeast: stress tolerance, fermentation yield and productivity, mutation stability, among others. These are especially important for the re-usage of yeast in several fermentation cycles, which is a must in large-scale breweries to sustain the economical viability of the production process.

The number of identified and characterized yeast strains will increase more and more over the next years. I personally believe that some unique flavor profiles are yet to be found, and that will consequently expand the range of beer styles. If you are already brewing, what are your favorite yeast strains and how did you choose them? Tell us your yeastperiences in the comments below.

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Sources

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jib.49 

https://www.esa.org/esablog/research/spontaneous-fermentation-the-role-of-microorganisms-in-beer/http://www.wyeastlab.com/fermentation

https://www.nature.com/scitable/topicpage/yeast-fermentation-and-the-making-of-beer-14372813

https://www.whitelabs.com/

https://www.grainfather.com/blog/week-60-choosing-a-yeast-strain-for-your-beer/

http://www.equippedbrewer.com/equipment-and-supplies/how-to-choose-the-right-yeast-for-your-craft-beverage

https://beerandbrewing.com/how-to-choose-a-yeast-strain/

http://scottjanish.com/esters-and-fusel-alcohols/