Brew for Africa - 011-949-1009 / 076-173-9196

The Home Brewing Blog

Cultivating yeast from a bottle of beer

Traditionally all breweries used to bottle condition their beers like home brewers still do: by adding priming sugar to the beer at bottling time, and letting the residual yeast take care of carbonating and maturing the beer. This was fine when beers were sold more or less locally, but in this day and age of globalization many breweries prefer to filter their beers to remove the yeast and then either force-carbonate the beer (by forcing carbon dioxide gas into the beer under pressure) or, if the beer is bottle conditioned, add a bottling yeast which provides carbonation.

The main advantage to the brewer of using a bottling yeast is twofold. Firstly, it allows the brewery to keep their own house yeast in-house rather than releasing it to all and sundry. But secondly, and this is the main argument for bottling yeasts, is that a bottling yeast is usually a variety of wine yeast or another yeast strain that can't ferment maltotriose.

Maltotriose is a malt sugar that is fermented out by yeasts that have a high attenuation (and produce a dry beer) but not, or only partially, fermented by yeasts that have a lower attenuation (and produce a sweeter, more full-bodied beer). One aspect of bottle conditioning is that many yeasts used in traditional, bottle-conditioned beers ferment maltotriose partially and rather slowly, which means that the beer becomes dryer and more highly carbonated over time as more maltotriose is fermented in the bottle. This in turn means that bottles of different ages and/or stored under different conditions will produce different versions of the beer. Some will be dry and highly carbonated, while others will be far less dry and have less carbonation. Because bottling yeasts do not ferment maltotriose at all but only ferment the priming sugar, the final product will be more consistent when poured.

However, there are still traditional English and Belgian breweries that do not filter their beers and leave it up to the residual (house) yeast to condition the beer. This means that you, too, can brew with their yeast! All you have to do is to cultivate the yeast still left in the bottle to the point where it will ferment a batch of beer for you.

My preferred method of cultivating yeast from a bottle of beer is as follows:

  1. Get a bottle of your favourite traditionally brewed beer. Try to find out if the brewery in question uses a bottling yeast. They will usually tell you. Also try to get a bottle of beer that is as fresh as possible, since the viability of the yeast diminishes over time. Three months is usually the limit, but you may get lucky with older bottles as well.
  2. Drink the beer. (Yes, this is the hard part!) When pouring the beer, leave the yeast in the bottle along with about 2-3 cm. of beer. Let the residue stand, covered (a sanitized piece of aluminium foil will do nicely) until it has lost all its carbonation.
  3. Make about half a litre of starter wort with an OG of 1.040 or so, using 50 grams of dry malt extract into 500ml of water, boiled for 10 minutes along with 2-3 high alpha acid hop pellets for bacterial control and some good quality yeast nutrient. Let it cool to room temperature. Then add some of the starter wort to the bottle. Add about the same amount as the volume of beer stil in the bottle. Do not add too much wort! The number of viable yeast cells in the bottle will be low, and these few cells can't handle too much wort at this point. The beer still in the bottle will dilute the wort to a gravity of about 1.020 or so, which is about ideal. Cover the rest of the starter wort and store it in the fridge.
  4. Cap the bottle with a sanitized crown seal. Then shake the bottle like it owes you money and you're a Mafia enforcer. Two or three minutes of this will usually aerate the wort sufficiently.
  5. Remove the cap and fit an adaptor bung, a top hat grommet and an airlock bubbler. Let the bottle stand at fermentation temperature for a few days until you can see some carbon dioxide gas production. This may start very slowly since the amount of viable yeast is small, and it may not last long. If there is no visible activity at all your residual yeast may be past the point of viability.
  6. When the fermentation activity begins to die down (depending on the amount and viability of the yeast in the bottle this can take anything from one to five days) remove the airlock and pour the contents into the remainder of the starter wort. Place the starter wort on a stir plate for aeration and agitation. Let the starter wort ferment.
  7. Step up the starter to double the volume (i.e. 1 litre of starter wort). This should give you enough yeast to brew a 19-25 litre batch of beer.

 As always when working with yeast cultures, work as cleanly as possible! Spraying bottle and flask necks with sterilant, soaking everything in a good sanitizing agent and using a gas flame on all bottle and flask necks prior to transferring liquid in and out of them are all vitally important.


Find us on Facebook ▲ Back to top