Saturday, August 10, 2013

Isolating Wild Yeast

I think any enthusiast in the hobby of home brewing at sometime or another becomes enchanted with the multitude of yeast strains that are available. Suppliers such as Wyeast, White Labs, and the Brewing Science Institute have hundreds of options (although not all of them are offered to home brewers). Despite the options from suppliers this is just the tip of the iceberg as far as yeast are concerned. Wild yeast live all around us on trees, fruit, and even in the stomachs of bugs. Who knows what unique and wonderful yeast could be waiting to be found. I recently set out to collect some of these "Wild Yeast". I thought I would share some of what I have learned and found.

Isolating Wild Yeast

This is the procedure that I have found works best for isolating Wild Yeast. I have mostly used this method to isolate yeast from wild beers (Cantillon and Drei Fontenien) but have also used it to collect yeast from barrels.

  1. I take a small sample of the beer and add it to a container filled with sterilized low gravity wort that is made with a little bit of hops. This first step will make sure that any of the bugs (Yeast and Bacteria) are able to ferment wort and live in the increasing level of alcohol. This also works to kill or stop the growth of any bacteria that is present, which I am not really interested in (the hops should also help cut down on bacterial growth). I allow for 2 weeks of growth since some of these bugs are coming out of a bottle that has set for a while.
  2. I next take about 1 mL of the new culture from step 1 and place it on a large wort agar plate. This will give me the chance to grow up single colonies. I allow for 1 week of growth.
  3. Take any colonies that have grown on the first agar plate and transfer them to their own new wort agar plate and allow them to grow into separate single colonies. (You may notice that some of the colonies do not look similar, in which case you may have mutants).
  4. Next, take single colonies from the 2nd set of plates and grow them up in small (250 mL) wort samples. If the yeast is able to attenuate the wort in these samples they can be moved onto the final stage.
  5. Step up the cultures to about 1/2 gallon test batches. If you like the results you can save the yeast for further batches of beer.

I have used the above method to collect a dozen separate wild yeast strains. Most of the time the yeast is pretty unremarkable and has little to no aroma that it produces. Other times the yeast does not attenuate well or take weeks to finish fermenting. However, I have found several strains that I like so far and will continue to look for other keepers. It is really rewarding to spend all the time isolating a strain and then get to figure out how best to use it!