Yeast washing is another way to harvest yeast from one batch of beer to use in another batch. This is a great way to save some money on brewing costs. The benefit over simply reusing a yeast cake from a batch is that you don’t have any of the trub, or nasty proteins, hop particles and gunk that settles to the bottom during primary fermentation. With yeast washing, you get just a healthy sample of yeast without any of that extra sluff.
Ironically, the idea for this came from Wyeast Labs, an Oregon laboratory that sells brewing yeast. Instead of rewriting their recommendation on how to wash yeast, I’m simply going to link to it.
edit: link has been removed so I’m linking to Don Osborn’s tutorial instead
There’s also an interesting article on acid washing yeast to remove extra gunk and hopefully improve fermentation. I’m going to stick with simple washing for now.