Homebrewers have a lot of options when it comes to purchasing yeast for making beer. But even if your local homebrew shop is well stocked, you might not see the proprietary yeast strains that some commercial brewers are using, whether they're blends of several yeast strains or individual strains that have mutated over time in the brewery. These special yeasts can be the key to recreating the great flavors of your favorite beers at home...if only you could get your hands on them.
Never fear! It turns out you can acquire that unique Belgian strain used by your favorite brewery, even if there isn't a viable substitute at your local homebrew shop. In fact, the brewery is already selling it to you: it just comes in very small quantities at the bottom of the beer bottle.
Commercial breweries rely on yeast just like homebrewers do, and if they are bottle-conditioning their beers, there are likely some living yeast cells in the bottle, just waiting to ferment your next batch of beer.
Before you pour a 6-pack of your favorite beer into your fermenting homebrew, there are a few things to consider to make sure you are going to get a viable yeast supply.
What types of beers are best for harvesting yeast?
If you want to harvest yeast from a commercial beer, the only requirement is that the beer must have been bottle conditioned. That means that the beer was carbonated in the bottle via natural fermentation by the yeast. If, on the other hand, the beer was filtered and force carbonated by being pumped with carbon dioxide, there will be little to no viable yeast cells, so don't waste your time.
When trying to determine which beers are bottle conditioned and will make good prospects for harvesting, you should look to the following styles: German Hefeweizen, Belgian Wit Beerm, Belgian Trappist Ales (tripels, dubbels, etc.), and Sour Ales (if you want a little "funk" with your yeast).*
*Take extra precaution when working with sour beers, as they will often contain a blend of yeast and bacteria. Any equipment that you use for harvesting and fermenting with wild yeast and bacteria should be thoroughly cleaned before being used again with your next batch of (non-sour) beer or else you run the risk of having an accidentally infected batch.
But those aren't the only styles that are bottle conditioned. Read the label and you may find many other examples of delicious bottle conditioned stouts, IPAs, or brown ales. Sometimes the label will say "Beer on Lees" which translates to "beer on yeast," indicating that it was bottle conditioned. If you're unsure, carefully pour a bottle into a glass and then take a peek inside the bottle. If it's bottle conditioned, you should see some yeast still stuck to the bottom.
How do you harvest the yeast?
The first step is to make a yeast starter—a small batch of beer—to feed the yeast, as they're likely hungry after running out of fermentable sugars in the bottle. A normal yeast starter of about 1000 mL and an OG of 1.040 will do the trick. Here is a great tutorial on making a yeast starter. If you haven't made one before, don't worry, it's really easy.
Once the starter is made, you will want to remove as much of the yeast from the commercial beer as possible without contaminating it, so make sure the beer is chilled and has been stored standing upright. When you open the beer, give the lip a spritz of sanitizer to make sure it doesn't have any nasty bacteria living on it.
Then, carefully pour most of the beer into a drinking glass , being careful not to disturb the layer of yeast that will have formed on the bottom. Stop pouring as soon as the yeast cake at the bottom of the bottle starts to reach the lip.
Now drink your beer. While drinking the glass of beer, either spray some no-rinse sanitizer on the lip and neck of the bottle (such as Star-San) or use a grill lighter or creme brulee torch to heat sanitize the lip of the bottle.
Give the bottle a good swirl to break the yeast off the bottom of the bottle, then pour the yeasty remains into your starter.
I recommend repeating this for two or three beers (get some help if you don't want to drink them all yourself). That will insure that you have a sufficient amount of healthy yeast to start eating away at your starter.
Wrap the top of your yeast starter container (we recommend a 2-liter Erlenmeyer flask) in sanitized aluminum foil, or use a stopper with an air-lock, and let it sit. If you have a stir plate, great, if not, just give it a swirl every time you walk past it sitting on your kitchen counter. You can also easily build a simple stir plate with parts you might already have around the house.
It may be a day or two before you start to notice any activity in the starter. Remember, it's a very small amount of yeast in there. Give the starter 3 to 4 days once the activity starts, and when the liquid begins to clear, you can chill it down in your fridge to see how much yeast has built up and collected at the bottom of the flask. If needed, decant the beer off of the top and repeat the starter process to build up the amount of yeast further.
How do you use harvested yeast?
Once you have built up enough yeast, go ahead and brew your beer and pitch your harvested yeast into it, just as you would purchased yeast. At this point your yeast can be treated just like a vial of yeast you buy from a homebrew supply store.
How quickly do you need to use harvested yeast?
Once you make a yeast starter and chill it, don't leave it in the fridge for more than a couple of weeks. If you do, you may want to make a second starter to ramp the yeast back up to prepare them for the big job of fermenting your beer. Even stored in the fridge, yeast will begin to die of over time if they're not fed more sugar. If you store yeast for more than a few weeks, make a starter to feed the yeast, make sure they're still active, and to build up a higher yeast count.
What results can you expect?
You have to remember that you have harvested this yeast out of a bottle, starting from a very small amount. As long as it was a clean, single-strain culture, then it should build up just fine and work just as well as purchased yeast, but your mileage may vary.
There aren't many extra worries as long as you are careful about sanitation. Just make sure you build your yeast population up to a healthy size and pitch it in your beer within a few weeks, and you can expect the same results as purchased yeast, and maybe even a little better, because maybe, just maybe, that unique proprietary yeast strain was the missing ingredient in your perfect brew.
More Beer & Homebrewing on Serious Eats
Use A French Press to Add Flavor to Your Beer
Homebrewing Protips: Shortcuts to Make Brewing Easier
How to Host A Beer and Cheese Tasting Party
How to End Your Homebrewing Hiatus
Homebrewing: Brewing Outside
Homebrewing with Brett and Pedio: What to Expect