Brewers have always reused yeast, though they did not always know it. The German Beer Purity Law, "Reinheitsgebot," from 1516 said that beer could only be made with three ingredients: barley, hops, and water. But German brewers following this law carried over a small portion of each batch to the next, not knowing that this practice transferred the all-important yeast from the old batch to the new. Families in Scandinavia once passed down prized beer-brewing sticks that were used to stir the wort and magically induce fermentation...by introducing yeast to each new batch of beer.
Modern commercial brewers reuse yeast for several fermentations, often up to 40 or 50 batches, usually by pumping yeast directly from the bottom of one cylindroconical fermenter into the next. They do it because they can get vigorous fermentations over and over again, and because it would be cost-prohibitive to buy enough yeast to ferment each commercial-sized batch.
So reusing yeast is not new. And it is common practice in commercial breweries. But it is rarely done by homebrewers. And for good reasons. First, homebrewers brew on a small scale, so the cost of yeast is usually not prohibitive. Second, homebrewers usually do not brew as frequently as commercial operations, so reusing yeast requires storing it, which can be difficult and damaging to yeast health. Third, homebrewers often make very different beers from batch to batch that require different yeast strains, as opposed to commercial breweries, which brew the same style over and over again.
Nevertheless, there are times when homebrewers should reuse yeast in order to make better beer:
- You are making the same beer twice or three times in a row: reusing yeast will typically deliver a better fermentation on the second batch.
- You are making a beer that requires the same kind of yeast as the previous beer, for example, an American pale ale followed by an American IPA. Reusing yeast will typically deliver a better fermentation on the second batch.
- You are making a very high-gravity beer: brew a low-gravity beer first to build up enough yeast cells to ferment the high-gravity beer. This is when reusing yeast really shines. The first beer is simply a huge yeast starter, with the added bonus that you can drink it. I did this recently with a Schwartzbier (O.G. 1.056) followed by a Baltic Porter (O.G. 1.093).
Here is how to rinse and reuse yeast.
Sanitize. Sanitize. Sanitize. Any time you transfer yeast from one container to another and propagate it, you run the risk of introducing wild yeasts and bacteria. So do your utmost to keep it clean. Sanitize.
Sanitize. When primary fermentation has finished and the yeast have dropped to the bottom of the first beer, rack the beer to a keg, to a bottling bucket, or to a secondary, leaving behind about a quart of beer above the yeast cake.
Sanitize. Cover and shake the fermentation bucket or carboy to break up the sediment and suspend the yeast in the beer.
Sanitize. Wipe the rim of the fermentation vessel with a 70% ethanol solution (I use Everclear and water) to sanitize it, then pour the slurry into a clear sanitized (with Star San) container that is roughly four times the volume of the slurry. Cover loosely with (sanitized) foil.
Sanitize. You get the picture. Boil a volume of water, about three times the volume of yeast, to sanitize it. Cool to room temperature and add it to the yeast. Shake the container vigorously for a minute or two to fully suspend the yeast and break up all of the chunks.
Allow the slurry to settle for at least 10 minutes. You should see a layer of crud sink to the bottom. This is hops and proteins and dead yeast cells and poorly-behaved yeast cells. Above this layer you should see a buffy cloudy layer. This layer contains the healthy yeast cells you want to ferment your next beer. Above this is a clear layer of mostly water.
Pour off and discard the top clear layer of water. Then decant the buffy layer of healthy yeast cells into another sanitized container and cover with foil. Discard the remainder.
Yeast can be stored, covered loosely with sanitized foil to allow carbon dioxide to escape, for up to seven days at 33 to 36°F. However, it is best to use it immediately or within a day or two. You should not reuse yeast more than three times because the risk of infection is too great.
Here are some suggestions for beers to brew with reused yeast: first brew an English mild, then a barley wine. Or try a Munich helles followed by imperial pilsner. Like darker beers? Make a robust porter then and imperial stout. Hopheads, go with an IPA and follow it up with a double IPA.
For further reading, consult Yeast by White and Zainasheff.