Edit: I do not advocate this method anymore. I do advocate re-using your yeast, but you should wash it first.
If you brew regularly, and especially if you like to use liquid yeast strains, you might want to consider reusing your yeast. As with everything in homebrewing, there is a long list of do’s and do-not’s, but in general, it’s pretty easy to decide if repitching on yeast will work for you.
First of all, if you don’t use liquid yeast, the only reason to re-pitch onto your yeast cake is if you’re brewing something big, like an imperial stout or barleywine. Something with a gravity over 1.080. The reason it is helpful in this case is that you have a large, recently active yeast colony, ready to go to work. Of course, you could just buy 2 packets of dry yeast, but why not use what you already have? It’s worth a shot to see how it works. After all, isn’t that why we homebrew in the first place?
Ok, so you use liquid yeast, not dry. That’s typically $8 a pop, give or take a couple bucks. That’s a big chunk of your supplies, and if you can stretch it over 3 batches, it’s suddenly no more expensive than dry yeast. In fact, many brewers will re-use yeast up to 10 batches.
So you’re in. You want to re-use your yeast. Now again, there are choices to make.
The easiest way to go is to rack off your finished beer from a previous batch, either into your bottling bucket, or into a keg. You leave behind the trub and yeast. Yum. Do this the same day, or the night before you’re going to brew. Re-cap the carboy or bucket just as if there was beer in it, and store it in a relatively cool place.
Make your wort, and after cooling down, instead of racking (or funneling) into an empty carboy/bucket, put it into your carboy with the yeast slurry left from your previous batch. Shake well (aerate), and prepare for some serious fermentation. If you have a blow off tube, you might want to keep it handy, especially if this fresh batch is a high gravity brew.
So what things are there to look out for?
First of all, some of the hop flavor from your previous batch will be in that yeast cake. Don’t do an IPA in the first batch, then a mild or wheat in the second round where you wouldn’t want that hop character (potentially) bleeding over.
Second, don’t reuse yeast if your first batch was a high gravity batch. The rule of thumb is not to reuse yeast if your batch was over 1.060. The only reasonable explanation I’ve heard about why this is, is that the alcohol in that high gravity fermentation will weaken the cell walls of the yeast and cause them to produce some undesirable flavors.
Third and last (for this article anyway), is a violent fermentation. Prepare for blowoff, and also for a hot fermentation. You may want to have an ice bath, or temperature controlled fermentation environment to avoid too hot a fermentation.
That’s it. Have fun with your yeasties!