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The Home Brewing Blog

Brewing split test batches

Yeast is arguably one the most important ingredients in brewing. It is the yeast that turns wort into beer, producing more than 800 different flavour compounds in the process. Different yeast produce different beers, and therefore it is vital to know the performance of your yeast. It is quite possible to produce entirely different beer flavours using the same grain, hops and recipe, but fermenting each batch with a different yeast. Try it: you'll be amazed.

In comparing different yeasts, it is vital to change only one single variable between batches-- the yeast itself. The best way to do this is to brew a single batch of wort, and split that in different batches which will then be fermented with different yeasts. The wort should be nothing fancy, in fact a plain blonde wort (the type you would use for a lager or American blonde ale) is best, because it will serve to provide a platform for the yeast flavours, rather than take centre stage on the palate. You can make an all-grain wort from pale or Pilsner malt and a pinch of medium crystal, but don't make it too malty; you'll want to focus on the flavours produced by the yeast. Using malt extract is also an option. Add some neutral bittering hops, but not too much; you don't want the hops overpowering the flavours produced by the yeast. Aim for medium bitterness. Somewhere in the 23 - 27 IBU range is a good starting point.

Unless you have the capacity to produce large batches of wort, you will be fermenting small batches of beer. That means it will be convient to have smaller fermentation vessels. For a test small test batches I prefer using 2 litre PET bottles (the ones used for cool drinks). These can be fitted with an airlock either by using an adaptor bung and top hat grommet, or by drilling an 8mm hole in the cap and fitting it with at top hat grommet directly into the cap. For slightly larger batches the 5 litre PET bottles used for still prepared or spring water work well. These you have to fit with a top hat grommet nto the cap directly. (Tip: a stepper drill bit is the easiest way to drill clean holes in plastic without leaving burrs and jagged edges.)

For my latest test batch, comparing four different yeasts, I added 700 grams of dry malt extract to 6 litres of water to make about 6.35 litres of wort with a gravity of around 1.040. Six grams of Southern Star bittering hops (13.6% alpha acid) boiled for 30 minutes should result in about 25 IBU.

I used four sanitized PET bottles as fermenters, filling them with 1.5 litres of wort each (using a sanitized jug andfunnel) and closing each one in turn with a sanitized, undrilled cap. Shaking the bottle vigorously for a minute or so is enough to aerate the wort. After that all that has to be done is to pitch an amount of yeast proportional to the amount of wort, fit the airlock bubbler an wait for the fermentation to complete.


In comparing yeast performance the yeast's temperature range is one of the most important things to be considered. While all factors (wort composition, volume, temperature, aeration/oxygenation, duration of the fermentation and what not) should be kept identical in order to have the different yeast strain as the only variable between batches, not all yeasts have the same preferred temperature range. While 20°C might be a good fermentation temperature for an average ale yeast, it may be at the top of the range for a lager yeast and at the very bottom of the range for a Kveik.

One therefore has to make a choice: either ferment all batches at the same temperature, or ferment each batch at the optimum center of the yeast's temperature range. Fortunately the yeasts to be compared in this particular test all had a similar recommended fermentation temperature range (all ale yeasts with an optimum fermentation temperature around 22°C) so no specific temperature control was necessary.

 From that point on it is a good idea to keep a log and note daily observations on fermentation activity (judged by Kausen and airlock bubbling) unless all you're interested in is the final gravity and flavour. In this case I wanted details on how vigorous the fermentation proceeded and how long it took. Batch #4 was the first to complete the fermentation, followed by #3. Batches 1 and 2 took a few days longer. Incidentally, this produced a great illustration on how yeast remains in suspension during fermentation, but settles to the bottom of the fermentation vessel when the fermentation is complete. Left: batch #2, still fermenting; right: batch #4, fermentation completed.

Once the fermentation of the test batches is complete all that remains to be done is to progress as usual: take gravity readings of each batch, bottle and prime the beer, and let it condition. Alternatively, if you have a small keg and the necessary kegging equipment, you can keg the beer and force-carbonate it. Trying to use a Sodastream system to quickly carbonate a beer is not recommended, unless you want your ceiling decorated in hint-of-beerfoam. (Don't ask me how I know that.)

Using this simple method you can compare a variety of factors, ingredients and other parameters that control the character and/or quality of your beer. It's not much work, but very instructive, and if you want to become a better brewer this is a great way to educate yourself!

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