Each homebrew fermentation results in a massive increase in yeast cells. If your beer tastes good and is under 6% ABV, that leftover yeast is most likely unstressed and eager to do more. So why place an order to buy more yeast when you have recently dumped more living yeast cells than you need to start the next fermentation?
Yeast Rinsing to the Rescue!
You can harvest viable yeast with the yeast rinsing process. In essence, you dilute the leftover yeast slurry with sterile water and let it settle, then harvest the healthy middle portion. Yeast is usually harvested from the primary fermentation vessel. (When you harvest from secondary, you favor cells that don't flocculate or clear a beer as easily.)
- A day before transferring the beer, prepare the jars and sterile water. You will need a large pot, three half-pint mason jars and lids (designed for food canning), a one quart (or one liter) glass container with a wide top, tongs, a hot pot-holder or silicone gloves, a small spoon, sanitizer in a spray bottle, and a funnel.
- Place containers and lids into the pot with clean tap water.
- Bring to a boil and continue for 15 minutes. Cut the heat source and remove each container with the tongs. Refill the containers with the just-boiled water and close. After cooling for an hour, place containers in a refrigerator.
- The day you're ready to transfer (or bottle) your beer, pull the sealed, pre-boiled water samples out of the fridge first so they can get closer to the temperature of the beer.
- Rack your beer off the yeast sediment. Sanitize the rim of the carboy or fermenter. Pour in the one-quart of cool preboiled water. Swirl the liquid. When the swirling subsides, pour some of the yeasty liquid back into the quart container, leaving the re-settling bottom layer behind
- Let the contents settle. Discard the clear layer from the top of your container, replacing from a jar of pre-boiled water. Agitate, then allow to fully settle again.
- Sanitize a spoon. Discard the clear liquid layer. Toss the top layer of yeast with the spoon. Pour the creamy middle yeast layer into one or more sanitized mason jars, topping with your cool pre-boiled water. Discard the bottom layer.
- You may be able to yield several jars of quality, creamy yeast to keep in your refrigerator. You can hold them for a few months if they do not turn brown or begin to smell "off." When ready to brew, use the yeast from one of the jars to make a starter for a new batch.
What About Washing?
Yeast rinsing removes alcohol from the yeast cells while discarding the top and bottom layers. Yeast washing, on the other hand, inhibits bacteria by adding phosphoric acid to the yeast. While washing can work on the homebrew scale, rinsing is more popular.
When you tour professional operations, you'll find most breweries reuse yeast for three to seven generations or even longer. Be sure to smell and taste your yeast to be sure it's in good shape each time. Be prepared: Keep a few sachets of dry yeast on hand in case the latest generation of your rinsed-off population isn't top quality.