Mute Dog Fermenting

How to harvest wild yeast

A number of people have inquired about my process of capturing wild yeast for use in my beers so I figured I should write up a detailed post about my process, which is actually fairly low tech and easy for anyone to do. I don’t capture yeast from the air, I’ve never tried so dont ask me anything about how to do that. I’ve captured almost all of my yeast from fruit and that’s what I’m going to show you how to do here.

Step 1: Find some fruit.

Almost all sugary fruit has got yeast hanging out on it waiting to get access to that sugar to have a feast. I recommend using fruit you grew yourself or wild foraged that way you know for certain that it hasn’t been treated with any chemicals or irradiated to kill those wonderful yeasties that you’re after. That said, I’ve captured yeast from dates imported from Oman and Mexico, and even from some cherries I got at walmart so if you don’t have access to a fruit filled garden don’t let that stop you. If you are going to get fruit from the store, I recommend organic, though it’s not required.

Step 2: Make a (hopped) starter.

Make yourself a low gravity starter (1.020-1.030), 300mL is plenty if you get something good you can step it up later. Add a touch of yeast nutrient to the starter. If you want to decrease your chances of getting lactobacillus and pediococcus (and other bacteria) drop a hop pellet or two into your starter when you’re boiling it. You don’t need to do a long boil for the hops, they are anti-bacterial even when not isomerized. Don’t bother trying to decant off the hop gunk or filter them out, it doesn’t matter and you’ll just contaminate your wort which is reserved for the next step.

Juniper berries and branches in a starter
Juniper berries and branches in a starter

Step 3: Add the fruit to the starter.

Take the fruit you’ve selected and add it to your starter. You have now pitched the yeast into your starter. Swirl it up really well to add some oxygen and then put an airlock on the starter. I know this is against the logic of starters where you want lots of oxygen for yeast to be able to reproduce, but with a wild inoculation there are other things that need a good supply of oxygen that you don’t want to grow, like acetic acid bacteria and mold. Put the airlock on, you can use foil when you’re stepping up something good later on. It may take a week or even longer before you see activity, don’t despair. While there is yeast on fruit, there’s generally not a whole lot of it so be patient.

Step 4: Determine if you got something worth pursuing.

Once you start seeing activity, you should get some sort of a krausen on your starter. Now it’s time to determine if you got something good. Take the airlock off and give it a good sniff. If it doesn’t smell like vomit you’re probably good to go. If it smells good leave it alone to let it finish fermenting and flocculate out. Once the foam has died down and the yeast has settled stick it in the fridge to crash. Make a step up starter at this point if you think you’ve got something good. Once it’s crashed use a thief/baster to decant the ‘beer’ off the top of the yeast layer. I stick a racking cane cap on the tip of my baster so I don’t have to worry as much about sucking up yeast from the bottom.

Smell the beer and if it doesn’t smell off, give it a taste to see what you’ve got. Look for esters and phenols, as well as off flavors. If you do have some butyric acid (pukey or parmesan flavors) in small quantities don’t worry, you may have had some enteric bacteria initially but they have a very low alcohol tolerance so they should have died off after the yeast got going. If you step this up that flavor will likely be gone. Additionally brettanomyces can eat the butyric acid and make wonderful pear flavors out of it. If your beer is sour you probably got lacto as well as yeast, if this is what you wanted, great! if not, you can dump it and start over or try washing the yeast which I’ll have to write a different post about.

If it smells and tastes good, step it up with a bigger starter to build up a pitchable population for a batch of beer. Congrats you’ve got some wild yeast to make delicious beer with!

Yeast from an Omani Date
Yeast from an Omani Date

Some additional notes:

If you want to increase your chances of getting something good that’s not contaminated with something awful you could add the same fruit to a few starters, I’ve never actually done this though. My success rate has been better than 50% for getting something good that makes good beer vs getting things that were just weird/off or moldy that I dumped.

Some fruits are likely more prone to have nasty on them: Strawberries seem to rot if you look at them wrong, I’ve never used one to harvest yeast but I feel like that would be a waste of time and effort. Apples are often used to start sourdough cultures so if your looking for sour bugs that’s a good place to start, otherwise I’d avoid it. Obviously don’t use overripe or rotting fruit. I’ve had good luck getting just yeast cultures off of grapes and dates in particular, raw honey is another good one to try.

If you were going for just yeast, you probably got a mix of saccharomyces and brettanomyces in your starter, keep this in mind particularly if you are bottling the beer, the brett can keep on working on higher chain sugars for a long time and you may end up with overcarbed beer or even exploding bottles, this can be minimized if you store the bottles in the fridge after conditioning. Personally I like to save off a gallon of my first beer with a new wild yeast and just let it sit for 6+ months to see if it does anything interesting. I have one strain that starts out fruity like english strains and after a few months those give way to spicy saison-like phenols. This is likely the action of both sacc and brett, but I don’t have a microscope so I don’t know for sure.

I generally harvest the dregs from my first batch in a jar that I keep in the fridge. Then when I want to make an new batch with that yeast, I take a small amount from the jar and step that up in a starter. You can however keep harvesting the dregs and reusing them each time, this will eventually select for yeast that is more flocculant and yeast that gets going quicker, in other words any brett and other yeast that takes its time will be weeded out.

Filed under wild yeast.