Harvesting Yeast from Bottle Conditioned Beers

Want to get a little bit closer to that clone you’re making? Want to save money building up your yeast bank? Consider harvesting yeast from the bottle.

Harvesting yeast from a commercial beer has been around a long time, and why shouldn’t it? So much flavor of a beer comes from the yeast used, so when you’re attempting a clone, or building your stock of yeast strains, this skill should definitely be in your bag of tricks. Most commercial breweries use strains easily available to homebrewers, but sometimes you’ll hear a rumor of a “house” strain or “variant” used that you just have to get your hands on.

This brings us to two very important caveats that I must put out there:

  1. Some breweries mention their “special” yeast purely for marketing effects and may just be a run-of-the-mill strain, and;
  2. Not all breweries bottle condition their beer using the fermentation yeast strain (many Bavarian Hefes, as well as Belgians, use lager yeast for conditioning). I’m not going into which do what, mainly because I do not know, nor do I want to mislead anyone. I’m just putting this out there to be aware, there is a lot of info out there that may or may not be true. Do some research, or better yet, start by harvesting yeast and making a similar brew, then if successful you can at least talk yourself into the fact you harvested the coveted fermentation strain!

Waking the Yeast Up

prepping bottle for harvesting yeast

Now onto the process. The first and most enjoyable step in harvesting from the bottle is drinking the beer. Done. But the important point here is that as soon as you crack open the brew, you need to treat it as sanitary as possible, just as you would with a chilled wort. You’re harvesting the yeast, but anything else that goes along for the ride will get cultured as well, likely at a higher replication rate since the yeast are in pretty bad shape from storage. So pour the beer into a glass carefully leaving the sediment and a half inch or so of beer, flame the rim of the bottle, and cover it with some StarSan-ed or pressure-cooked foil until you’re ready to harvest.

There are two main ways to get the yeast back into growing phase. The first is the easiest, and also the least scientific. Simply boil and cool some fresh wort, and add a small amount to the bottle. Let it set for a few days covered and add more wort when you see fermentation start. Step the culture up by adding around 5x the amount of wort currently in the culture until you have enough for a batch. I’m not going into any more detail because I don’t recommend this method. Since you are growing everything that is in the bottle, you can’t control if it’s yeast, bacteria, or mold. Additionally, some breweries reportedly use multiple strains in conditioning and this defeats the purpose of harvesting particular strains.

I suggest taking the yeast and growing them on wort agar plates or slants and picking individual colonies that look healthy and are completely devoid of any contaminants. Yeah, it takes a while. And yeah, it’s a lot of extra steps. If you’re in this for simplicity, I guess you can just stop reading now. If  you’re trying to get a good population of healthy yeast of the strain you are targeting, let’s get started.

Harvesting Single Colonies

harvesting yeats prepping agar plates

The first step in harvesting single colonies is to prepare wort agar plates in advance. There is a small list of equipment needed for agar plating/slanting: agar powder, pressure cooker, petri dishes or small vials, stainless wire or inoculating loop, and mason jars. The general steps are fairly straightforward:

  1. Prepare a 2% (2g in 100ml fresh 1.035 SG wort) agar solution and pressure cook in a mason jar for 10 minutes with the jar lid loose.
  2. Allow the agar to cool to around 70°C (158°F), and pour 10ml into each 100mm petri dish or fill the vial halfway and keep at a slant while the agar gels.
  3. Allow the dishes or vials to cool.
  4. Seal the plates or screw the lid of the vials and keep at 4°C (39°F)

Next you want to transfer the yeast in the bottle to the plate, and try to grow individual colonies from a single cell. Swirl the bottle around to re-suspend the yeast and take a sterilized stainless wire or inoculating loop and dip into the bottle and transfer enough times to get a little drop on the agar plate. Spread the drop around the plate with the loop and seal the plate with tape. Flip it upside down and keep in the dark. After a couple days, you’ll see individual colonies start to grow on the plate. Mold will be obvious, and bacteria will usually be brown-reddish and shiny. Yeast will be creamy and not shiny.

After a few days, take the sterilized loop and transfer as much of a single colony to a new plate (you can put 4 or more on one new plate in sections) and smear it around. This will give you a nice pile of healthy yeast for a small starter when this plate grows up for another few days.

harvesting yeast colonies in agar plates

At this point, if you’re so inclined and have the materials, you can test for different strains by looking under a microscope (strains can differ in size and general look), or better yet, making a small starter and tasting the beer without hops and see if you can detect any differences. If so, label them accordingly so you know which are which.

Inoculating the Initial Starter

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Make some fresh 1.035 wort and chill. The amount of yeast you add to the wort is a bit subjective, but I would target a 1 square inch of yeast smear per 200 ml of wort. Take a spoon, dip it in alcohol and light it on fire (away from the alcohol) and allow it to burn out. Take the sterilized spoon and scrape as much yeast as you can and add it to a jar with the wort. Allow this to ferment completely, and add 5x the wort volume in the jar until you have enough for your intended test batch. I would suggest a test batch for all yeast strains which you are unsure of, since you don’t want to waste a whole 5 gallon batch on an unknown strain.

Storing the Yeast Strains

After you're doing harvesting yeast, you’ll want to safely store them for future batches. This can either be done short term (1-6 months) by keeping them on the agar plate in the fridge, or making a small starter from the yeast on agar and freezing the cells for storage of up to a few years. The procedure for freezing is as follows:

  1. Make a 25% v/v stock solution of USP glycerin (most drug stores carry this) in water (12.5ml glycerin, 37.5ml water) and pressure cook for 10 minutes.
  2. Make a 50ml 1.035 SG starter from the agar plate as above and allow to settle.
  3. Add the cooled 25% glycerin solution to the yeast and swirl, making a slurry with final glycerin concentration of 12.5%
  4. Place the jar in a container filled with alcohol to the liquid level in the jar to reduce the freezing rate in the freezer (this can be skipped, but cell viability will be reduced if placed directly into the freezer. Cells prefer to freeze at a rate of 1°C per minute).
  5. After 24 hours the frozen slurry can be removed from the alcohol and stored for several years.

To inoculate a new starter for a brew, quickly unfreeze the slurry to the temperature of a new starter wort and dump it in. You want to target no more than a 10x dilution from the frozen slurry. So if you made a 100ml frozen slurry as written above, add the whole contents to a 1000ml starter. You can also divide the 100ml slurry into smaller tubes or jars before freezing, but remember to keep the 10x dilution rule in play. So if you made individual 10ml frozen slurry tubes, you’ll want to inoculate 100ml starters and step up to your desired pitching volume.

As you can see it’s pretty easy to begin harvesting yeast cultures from commercial brews, but just keep in mind most beer is filtered before bottling and therefore contain no yeast, and not all breweries use the fermenting yeast strain for bottle conditioning. However, this mystery is part of the fun so have at it!

written by Pete Geisen

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