Many homebrewers have asked the question if they could reuse the yeast that settles out from one batch of beer in another batch. The quick answer to that is yes. There are two cases of reusing your yeast: first is making a similar batch of beer immediately after the first batch; and secondly, using the yeast several weeks or months after the first batch. I will address these two cases separately. But in both cases sanitation is extremely important.
- Immediate Reuse or within one week
When you are racking your beer from the primary fermentor to the secondary, is the time that you should be collecting the yeast. It’s easy to do it. Just sanitize a quart jar and lid, sanitize the neck of the primary fermentor, and pour from the fermentor into the jar. [Some people wipe the fermentor lid with alcohol and flame it to sanitize.] Loosely cap the jar so that if you have some continued fermentation, the glass jar will not explode! Clean out the primary fermentor and pour the quart of yeast solution back in. You can then pour a fresh batch of wort directly into the fermentor. This is the easiest method of reusing yeast.
If your next batch of wort won’t be ready for up to a week, you can store the glass jar of yeast in your refrigerator and reuse as is.
Pitching onto a yeast cake:
Some brewers will pitch a fresh batch of wort directly on the yeast cake and trub from a previous batch of beer. This is not a recommended procedure because of the excess amount of yeast and trub present in your fermentor. You can obtain some off-flavors from this procedure because of the excess quantities. One quart of yeast/trub is sufficient for a rapid fermentation and will minimize any off-flavors.
- Reusing yeast up to about 3 months.
Collect the yeast following the above method and store the lightly capped jar in your refrigerator. About 5-7 days prior to brewing, prepare a starter using the yeast. [See FAQ on preparing a starter] Use the starter in your fresh batch of wort.
I do not recommend using this procedure for storing yeasts longer than 3 months although I have stored yeasts successfully for 6 months.
- Washing yeast
Some people like to wash their yeast to purify the yeast away from the trub. I don’t think this is necessary but I’ll post the Wyeast recommended instructions for your information:
If you wash yeast, you’ll have to collect more than a quart because you’ll lose a lot when you wash.
- Yeast from Primary or Secondary?
You will notice that this procedure recommends collecting the yeast from the primary fermentor. This is because this yeast has the most desirable characteristics of the yeast strain. This is the yeast that has a rapid fermentation start, produces the most alcohol, and has the flocculation (settling out) characteristics that you desire. You can also collect yeast from your secondary but that yeast will be a more recent offspring of the original yeast and may have somewhat different characteristics than the primary yeast. Realistically, you can probably collect from either source for one or two reuses of the yeast, but collecting from the primary will give the best chance of maintaining the yeast strains’ properties.
- Reuse for how many generations?
The general consensus seems to be 3-5 generations before you start having contamination issues.