You may have heard of the process of moving your beer off of the yeast either to bottle or keg, or put into secondary fermentation and then using the yeast at the bottom of that fermenter to ferment another beer. I have done this 3 times and have had successful results.
Mind you, my methods might be a little unorthodox. Here are the steps that I take:
1) Plan the brew day for the second beer based on when the yeast is done.
The second time I did this, I brewed a
lager and figured in two weeks, it would be done fermenting. I began brewing the second beer, a Maerzen, exactly 2 weeks after the first. Unfortunately, while the second beer was boiling, I took a gravity sample and found that the first beer was far from done (gravity reading was 1.030). Luckily, I had enough vessels that I was able to move that beer to a secondary fermenter, use half the yeast cake for the new beer, and then return the Vienna into the original fermenter. Both beers turned out great. Vienna
2) Sanitize two vessels
One vessel will be for your fresh beer, the second vessel will either be your bottling bucket, keg, or secondary fermenter for your first beer.
3) Sanitize the path
I use a fresh rag and sanitized water and gently clean the top of one side of the first fermenter to get rid of any chunks of kreuzen or gunk left behind during the fermentation. This is probably very unorthodox, so get some other opinions. This is how I’ve done it, and it works well for me.
4) Put the yeast cake into the second beer
Go through your normal methods of infusing oxygen into the new beer (shake the fermenter, use an oxygen stone and tank or aquarium pump, etc.) and pitch the yeast.
You should see fermentation rather quickly with this method. And you just saved some money on yeast.