I don't do this but maybe I should, I know a lot of people do. Sometimes referred to as over-building. Make a bigger starter than you need and save some. Then you know that the yeast is ok and you aren't risking the brew going wrong, alcohol stress, hop debris etc before you harvest the yeast. Here is a description of the method:
Make your starter at least 24-36 hours before you plan to pitch, I’ve learned that older strains tend to take a bit more time than more recently harvested yeast, so plan accordingly. Using a good yeast calculator, overbuild your starter by 100 billion cells. BrewUnited’s Overbuild Harvest function makes this so incredibly easy, you can even download a spreadsheet version. Usually, the extra amount will be less than or right about 1 liter.
After at least 24 hours on the stir plate (longer if shaken or using older yeast), you can harvest the yeast. Since I like to crash and decant, I always harvest my yeast the night before I brew. To do this, sanitize an appropriately sized Mason/Ball jar using whatever sanitizer you prefer, remove your flask of starter from the stir plate, secure the stir bar using a magnet, and fill the sanitized jar with the proper amount of slurry.
Place the jar in the fridge and pull it out a few hours before you make your next starter. That’s basically it!
Previously, I encouraged people to replace the beer on top of the yeast with pre-boiled and chilled water if they planned to store it for longer than a couple weeks. However, I’ve recently been made aware of some fairly convincing evidence suggesting this is both unnecessary and potentially harmful to the yeast. I started leaving the starter beer on top of the harvested cake and I’ve experienced no noticeably negative impact. For these reasons, I no longer plan to replace the beer with boiled water and recommend others follow suit.
How many generations of yeast can be harvested before it goes bad?
I’ve only anecdotal evidence, so keep that in mind. At the writing of this article, I have two strains of yeast, WLP090 San Diego Super Yeast and WLP002 English Ale Yeast, both have been harvested 13 times. The 12th beer each fermented tasted great with no noticeable differences compared to the original vial. I’ve used this method successfully with ale, hybrid, and lager strains.
What makes this method any better than rinsing (“washing”) yeast?
To start with, I personally think it is much easier and far less time consuming. Additionally, the yeast fermented a wort of ~1.040 OG that had no hops in it, so it is arguably less stressed and ultimately cleaner. I also like that I’m only harvesting a single jar of yeast as opposed to multiple jars that would eventually end up in the trash.
How much yeast is actually being harvested using this method?
I don’t know. I wish I knew, but I’ve yet to find anyone with the know-how to do an official count. As I mentioned before, I’ve been assuming 100 billion cells, though I fully accept this number is likely way off. Still, it has worked great for me.