Capturing Yeast

Do you want to capture your very own wild yeast culture or strain? With the right tools, and these Three Easy Methods, you can capture yeast right in your own backyard.

Before proceeding, checkout Bootleg Biology’s Backyard Yeast Wrangling Tool Kit. Everything the experimental homebrewer needs to capture wild bugs, create agar plates, and isolate wild yeast.

*** OTHER/OPTIONAL TOOLS *** Star San (or other trusted sanitizer)

Wide-Mouth Mason Jars

Cheese Cloth

Growler/Jug with Air Lock (optional) Parafilm (stretchable plastic wrap will also work if you already have some in your kitchen) Centrifuge Tubes (sanitized; optional) Plastic Pipettes (sanitized/sterile; optional) Sterile Cotton Swabs (optional) pH Meter (not cheap, but quickly becomes your best friend; optional) Good Notes (not optional)

Method #1: Wild Yeast Starter Jars

The following is how S. arlingtonesis was sourced from the ambient air in Arlington, VA. This is a modification of The Mad Fermentationist’s Ambient-Spontaneous Yeast Starters technique. The eventual goal is to isolate a strain, so changes have been made.

Create A Yeast Bath: Day One

Create a starter as you typically would for a homebrew batch of beer. For ease, use a 1/10 ratio of filtered water to malt extract (1.040 gravity). A weaker gravity will work just as well, use whatever you’re comfortable with. Create enough volume to fill up three or more jars half way (the more jars, the more likelihood of success). Boil the liquid mixture with a small amount of hops for at least 20 mins. You can boil longer with more hops, but you’re just trying to sterilize the wort and pickup the antibacterial properties from the hops (Hops will primarily slow the growth of Lactobacillus. Of course, you may in the future want to cultivate Lacto to have your own sour strain. But let’s save that for another day). Once the liquid has cooled enough so that you won’t burn yourself if it spills, pour into sanitized mason jars. If you have a pH meter, now’s a good time to check the pH level; this will be helpful later when verifying if the right kind of fermentation occurred. Cover with cheese cloth (to prevent insects from crashing your yeast party) and keep in place by screwing on the jar lids or tightening with a rubber band. Place the jars overnight outside, in a herb or vegetable garden, under a fruit tree or even in your beer cave. Any place with good air flow, vibrant vegetation and free of curious woodland creatures will work fine. Please don’t put near a septic tank, an alley or other potentially contaminated place (it’s disgusting, and probably not safe).

Bring ’em Back Home: Day Two

The next day, bring in any cooled jars left outside. Remove the cheese cloth. You can leave the liquid in the jars, but if you do so make sure to cover the surface of the liquid to prevent contact with the air. This will help reduce the chances of surface mold growth. You’ll probably still get mold, but this’ll probably make you feel more pro-active. Another option is to pour the liquid into a growler or jug and cover with an air lock. Leave in a dark, room temperature place.

Monitor For Signs of Fermentation: Day One + Two Weeks

Within a few days there should be the tell-tale signs of fermentation, with a small amount of CO2 bubbles coming to the surface of your jar or minor air lock activity. Don’t worry if you don’t see signs of activity quickly. Compared to a normal starter that has had billions of yeast cells accustomed to barley sugar, a comparatively small amount of wild microbes have started colonizing your wild yeast starters. ABSOLUTELY DO NOT TASTE until fully fermented out. Never taste any starter unless active fermentation has been verified for a sustained period of days. Roughly two weeks (assuming each jar or growler has fermented out) it should be safe to taste. Make sure you scrape off any mold, unless that’s your thing (please just do it). 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, compare the current pH to the reading you took before putting the jars outside. If the pH dropped significantly you’re probably safe. If the pH increased for any of your jars, discard the liquid and move on. Use the smell test first. If it smells good (honey, citrus, etc), you’ve probably got something fun. Creamed corn is a common aroma from certain wild yeast, and these strains often do not flocculate well. If it smells good, use a sanitized pipette to pull a sample from underneath the surface. If it tastes remotely palatable, and has dropped clear…you likely have something INCREDIBLY AWESOME. You’re now ready to create agar plates so you can isolate your own yeast. You know you want to.

EDITOR’S NOTE: Always practice common sense. If it smells horrible, and looks horrible just don’t taste it. Try one of your other test jars. If you look at the picture above, the jar in the middle has a reddish/orangeish hue. A good sign that this jar should not be tasted. Good thing, because it eventually grew a friggin MUSHROOM. That is all.

EDITOR’S NOTE #2: After reading this page, Ingo, mentioned that he adds lactic acid to lower the pH when making sourdough starters. If you’re worried about uninvited guests in your Wild Yeast Starter, this could be a good route to take. Acidulated malt could be used as well.

Method #2: Collecting Samples From Fruit & Vegetables

The microbes that live on fruits are primarily yeasts and fungi, whereas vegetables typically have lactic-acid-producing “sour” bacteria like Lactobacillus and Pediococcus in residence. Keep this in mind when attempting to capture microbes from your local vegetation.

Collecting wild yeast samples from fruit and vegetables is significantly easier than creating a Wild Yeast Starter (Method #1). Look for fruits and vegetables that appear to have a fine dusting on their skin or have started to rot (but don’t use anything that’s already fallen on the ground!). Even if dust isn’t present, you’re pretty much guaranteed to collect some amount of wild yeast.

As pictured to the right, it’s as simple as putting several fruit in sanitized centrifuge tubes, or glass jam/mason jars. Pour low gravity (1.010-1.020) unhopped starter wort into the container with fruit. Only pour enough to cover the fruit or vegetables, but leave enough room for fermentation activity at the top. It’s not necessary for fruit to be crushed for fermentation to take place. Wrap the seal between the lid and container with plastic wrap. Shake vigorously to aerate, and then leave in a dark, room temperature place. Within a few days you should see signs of fermentation taking place. A yeast sediment will eventually form at the bottom. It’s time to create agar plates. You can do it!

Method #3: Sterile Cotton Swabs

Sterile cotton swabs are small and come individually wrapped, making them easier to travel with than a jar of starter wort.

Going on vacation to a fun locale? Bring a cotton swab and a tube. Flying internationally and afraid you can’t bring fruit back because of curious customs officials? Cotton swab, tube. Visiting a sour brewery willing to give you a tour of their fermentation vessels? You guessed it. (But don’t be a jerk, either ask them permission to swab in their brewery, or do it when no one’s looking).