Harvesting Wild Yeast

Generally it’s nice to brew with predictable ingredients in controlled conditions. You purchase your ingredients, follow your process, pitch your yeast, and leave your beer in a temperature stable environment. After some time the yeast consumes all the sugars and leaves behind its own flavor characters and some alcohol.

I decided that I’d carry out a little experiment to mix the process up a little. I wanted to catch some wild yeast. Yeast grows on a number of plants and also drifts through the air. In the old days people fermented things without even knowing what was causing it. Later brewer’s yeast was isolated and then carefully selected and bred to the different strains we use today. There is however no reason we cannot make use of wild yeast in our beers and see what we end up with at the end. Wild yeast tends to have a particular character and doesn’t ferment as clean as some commercial yeasts. Some beers rely on this unique character to create their particular flavor profile. In this post I describe a yeast capture I did recently. I will not describe isolating the yeast as this does require more specialized equipment, but what we’ll end up with will allow us to brew beer.

Equipment needed

You won’t need much to capture some yeast. There are two ways you can capture yeast: from the air, or from some fruit, flowers, or other piece of plant. I will cover the first one here, but generally it’ll be the same process for the latter.

So before we get started here’s what you’ll need:

  • One or two small jars with lids
  • Some muslin cloth
  • Dry malt extract
  • Small amount of hops (optional)
  • Rubber bands
  • A larger jar with an airlock in the lid

Process
First off we’re going to need to get our small jars clean and sanitized. For this I boiled them in some water for about 15 minutes with their lids and the muslin cloth.

wildyeast1

At the same time I prepared some wort with the DME aiming for a gravity of around 1.030. I used enough water to fill the two jars which was around 250ml, requiring about 20g of DME.

wildyeast2

In this case I decided to add a small amount of hops (10 IBUs) which for this batch size is around 0.75g of 8%AA hop pellets. The hops add a little bittering and also help prevent the growth of some unwanted bacteria. I figured there would be no harm in adding that much, and if it prevents bacteria growing, all the better.

wildyeast3

Once the jars were boiled I took them out (carefully) and left them to dry. Once dry I added some of the wort to each jar, placed the muslin on top and secured it with a rubber band and placed the lids back on until they completely cooled.

I decided to carry out my capture overnight in the winter, so I wasn’t too worried about the jars being a little warm, they cooled down pretty quick outside. I took the jars into my garden and placed them in two different places and removed the lids.

In the morning I replaced the lids and brought them inside. I poured both jars into a larger jar fitted with an airlock and left it at room temperature.

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Now it’s a matter of waiting to see if you caught something and if it’s any good. It’s highly advisable to leave the jar for at least two weeks. The reason for this is that bacteria growth will start shortly after inoculation and peak soon after. Then yeast will start to grow and displace the nasties. After a few days you should start seeing the tell tale signs of fermentation in your jar. Bubbles, some foam, but don’t expect anything like high krausen.

Once it’s fermented out (2-3) weeks it will be safe to taste. DO NOT test or taste it before this time however. The presence of mold doesn’t mean your wild yeast starter is ruined, it’s just a common part of the process. If you have a pH meter handy, you can check the pH and see if it’s dropped significantly. That would also indicate the yeast has been working as the lactic acid concentration will increase after around weeks 4-5.

When testing, use smell first. If it smells OK you should use a sanitized pipette to pull a sample from below the surface and give it a taste. If it tastes good then you’ve got yourself your own awesome pet yeast. Otherwise bin it and try again.

Alternative method
As mentioned earlier it is possible to harvest yeast from flowers, fruit, and other pieces of plants. For this follow the above process but instead of leaving the jars outside simply place some fruit, leaves, bark, or whatever takes your fancy, into the wort. After a few days you should see some fermentation signs and a sediment will start to settle. No need to crush the fruit for this. If you notice the fruit going off remove it with a sterilized implement. Ideally leave the fruit 2-4 days and remove. Then leave the wort for a further 30 days.

I’ve not tried this method yet, but it is a similar method to capture yeast as above. You will find that different fruit, vegetables, and such will offer you different yeast types, so it’s a little more controlled than capturing yeast from the air.

Results!
You should end up with a layer of yeast at the bottom of your jar that won’t look too dissimilar to what you’d see at the bottom of a starter. Here’s what mine looked like after 3 weeks, with a good yeasty layer at the bottom of the jar.

wildyeast6
It had a very distinct flavor. A little sour and a bit earthy, but not bad, and I didn’t suffer any ill effects from sampling it. Unfortunately I ended up ditching this batch because I left it too long to brew with. But I plan on doing another capture now when spring arrives and more plants are in bloom.

Cultivating and isolating
If you are happy with the taste of your small beer then you can proceed to propagate the yeast. You can do this with step up starters, increasing the volume of the wort by a factor of 10 each time. So start with a 200ml starter with a gravity of 1.030-1.040 and leave that to grow ideally on a stir plate. Then pitch that into a 1.5l starter wort of the same gravity. At this stage you should have enough cells for a brew with an OG < 1.060.

Unfortunately this involves some guess work as unless you have the required tools to count how many cells your capture has, we’re just estimating the cell count. But hey, it’s free yeast, so just try again.

You can isolate the yeast cells in your capture, but that is outside of the scope of the article. For this process you will require a few more items like petri dishes, wire loops, alcohol burners, agar etc. There are a few resources online about wrangling yeast like this though if you wish to take this route.

Summary
Using wild yeast can lead to some interesting and very different flavors in your beers. If you end up capturing a good strain, you can always keep a small starter batch in the fridge and propagate from this for brews in the future. The strain will evolve over time, as it adapts to your brewing conditions and intermingles with other strains in the air, but that goes for pretty much all yeast strains.

So go hunting and have fun. Hope you catch something great.

by Sven Steinbauer
I live in the English countryside with my wife, two kids, and dachshund, having moved here from Germany many years ago. I work as a software engineer and enjoy figuring out how things work and learning how to do new things. I savor learning about and exploring the processes, different techniques, and ingredients that go into creating beer. With so many variables, adjustments, and ingredients, there’s a virtually endless combination of things to try out and learn about, plus you get to drink beer at the end of it. Although I enjoy a large variety of different beer styles, I still favor German beers most of the time.

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