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Yeast belongs to the fungal kingdom and are unicellular. There are about 1500 different strains of yeast, but brewer’s yeast is very different to those yeasts found in the wild. Many yeast strains can convert sugar to carbon dioxide and alcohol, but this does not necessarily mean they can make good beer. Brewing yeast has, essentially, been domesticated. In other words, we have purposefully selected for characteristics such as the ability to grow and feed off malted grains quickly, and to provide the desired beer flavours and aromas.

    During fermentation, yeast breaks down simple sugars to harvest energy for its survival, and produces carbon dioxide and alcohol, which for us, are the desired waste products. It is believed that yeast evolved this ethanol-CO2 producing ability, to prevent other microbes from also feeding on the simple sugars, as the very high acidity and alcohol levels make the environment intolerable for most organisms. This is great for us brewers.

    There are hundreds of different strains of brewing yeast, but you want to choose a strain that creates the best flavours in your beer, and not all strains will be suitable to your needs. Both literature and the web are full of helpful information regarding which yeast strain to choose for your beer.

    There are two main species of brewing yeast, namely, ale (S. cerevisiae) and lager (S. pastorianus) yeast, and within these 2 main species, there are many different strains, all producing various characteristics to a beer.

    George Fix helpfully categorised ale and lager yeasts according to their fermentation character (flavours etc) rather than by region:

    • Lager  - Dry/Crisp
                  - Full/Malty
    • Ale       - Clean/Neutral
                   - Fruity
    • Phenolic
    • Hybrid (these beers lie between the ale and lager style)
    • Eccentric (these strains produce unusual flavour compounds such as earthy, barnyard, sour or are high gravity fermenting yeasts.

    Ale strains are top fermenting (produce a large, foamy head/krausen). This is because the ale yeast surfaces are hydrophobic and the yeast cells stick to the CO2 bubbles and rise with them to the surface. They usually ferment at about 18-21⁰C.

    Lager strains are sometimes referred to as “bottom-fermenting” as during fermentation, they don’t rise (or only minimally) to the surface. They are also not usually very good flocculators, so they tend to stay in suspension for longer than the ale yeasts. This allows the yeast to reduce more of the by-products that are formed during fermentation. This results in the “clean lager” character we commonly speak of. They usually ferment between 11 and 12⁰C.

    So, how do you choose? Well, you first need to decide what the overall concept is that you want from your beer. Do you want malty and sweet, crisp and dry, clean or estery (fruity), high or low alcohol etc? Once you have decided, you can begin looking at possible yeast strains. Over the last year or so, new and exciting yeast strains have entered the South African market, so you now have a lot more choice to play with. It is daunting to begin with, but once you know what you’re dealing with, you can have a lot of fun trying the different styles. It really is amazing how much character a yeast strain can impart on a beer (see previous blog). The most important things to consider are:

    • Attenuation (do you want crisp or malty/sweet)
    • Flavour profile (clean, spicy, malty or fruity etc)
    • Flocculation (how well a yeast settles out of suspension)
    • Supply/availability
    • Temperature range (this is important if you don’t have access to temperature control)

    You can chat to your brewing buddies, look at literature, search online, or you can play with the different yeast strains yourself and pick the best one. You can do this by brewing a batch and splitting it into a number of fermenters with different yeast. Once you’ve decided on a strain, you can then manipulate temperature, oxygen levels, pitching rates and determine what has the best effect on your finished beer.

    Most yeast suppliers make our decisions easier by differentiating the yeast by ale or lager first, and then by geographic location (country, city, or region) or by its specific style name (eg: Altbier, California Common etc). Once you’ve narrowed it down, you can decide from there. But don’t limit yourself to just these. You can brew an American pale ale using a European ale yeast, for example. This will set your beer apart from all the other APA’s, and give you a lot more yeast strains to play with. Don’t be afraid to experiment.

    Good luck, and have fun!

     

    References

    White, C., Zainasheff, J. 2010. Yeast: The practical guide to beer fermentation. Brewers Elements Series, Brewers Publications. 304 pp.

    Reid, A., Ingerson-Mahar, M. 2012. If the yeast ain’t happy, ain’t nobody happy: The microbiology of beer. American Academy of Microbiology, Washington, DC. 13 pp.

    More about the Author, Megan Gemmell:

    When I am not working as a microbiologist, I am brewing and studying beer. Being the geek scientist that I am, brewing yeast has become one of my main interests. After many years of brewing on my home-made system, squeezed into the laundry, I have taken the plunge and started Clockwork Brewhouse, and I’m loving it. 

     Visit Clockwork Brewhouse http://www.clockworkbrewhouse.co.za/

    Written by Megan Gemmell — April 22, 2015

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