Yeast are a very small, single cell organism which are present everywhere. For many years, Beer makers have cultivated one particular type of yeast, Saccharomyces cerevisiae, which has been refined into hundreds of strains particularily suited to creating great beers.
Yeast available to home brewers generally comes in two types, liquid and dried. Each having pro's and cons although its generally accepted that better beers can be produced using liquid strains which is backed up by home brewing competition results.
Yeast's characteristics are usually described in terms of taste profiles (fruity, dry or clean) and also by the level of Flocculation and attenuation.
For flocculation strains are rated either high, medium or low depending on how quickly they drop out of solution following fermentation. Highly flocculant strains make beer that falls clear very soon after fermentation is over. With a less flocculant strain, you would have to wait longer for the beer to clear (or cool it down to coax the yeast drop out of solution).
Attenuation is a measure of what percentage of the wort’s malt sugars the yeast consume. It is calculated by subtracting the final gravity from the starting gravity and dividing this result by the starting gravity. For example, if you made an OG 1.048 ale and it finished at a gravity of 1.012, the apparent attenuation would be 75% [(48-12)/48 = 0.75]. The higher the percentage then the higher final volume that is likely to be achieved from a given starting wort.
Using these measures and depending on the style of beer the brewer is looking to create, yeast choices can be made and these decisions are critical in the final outcome. The brewers choice of yeast can effect the final flavour as much as any other choice of ingredient.
Dried yeasts are easy to use, cheap and they have a much longer shelf life than liquid yeasts but chioce of dried yeast stains is very limited in comparision to liquid and it generally accepted that they do not have such a great depth of flavour.
Liquid yeasts require refridgeration, have a limited shelf life and are more expensive. They available in a huge range which gives the home brewer a great scope for experimenting.
Liquid yeasts come in various packages and one of the most popular brand is Wyeast labs. Wyeast liquid yeasts come as smack pack foil bags. They contain a pouch of yeast nutrient, which when smacked, breaks and activates the yeast. The pack then swells over a few hours (longer if the yeast is older) and is ready to pitch or to make up into a starter. They come in 2 pack sizes Propagator and Activator. The Propagators contain 25 Billion yeast cells and the Activitators contain 100 Billion yeast cells. Most beers (5 Gallons up to 1.060) will require 100 Billion so the Activators can be pitched direct whereas the Propagators need to be made up in a starter over a few days so they can multiply up to the correct pitching rate. Usually a 1 litre starter will be sufficient which is made by boiling 100g of spraymalt in 1 litre of water then allowing to cool. The yeast is then added and allowed to grow over a few days prior to pitching.
Although liquid yeast are more expensive, they can easily be reused by storing some of the slurry from primary fermentation in a sterilsed jar in the fridge for a couple of weeks before repitching into a new wort. They can be reused several times making them as economical as dried yeasts and the taste gets better which each repitch.
If you've yet to try liquid yeast then we highly recommend that you give them a go and taste for yourself the effect they have on flavours.
Check out our great brewing forum for tips, advice and brewing banter.
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