Don’t take your yeast for granted. Pouring it out with the trub when your beer has finished fermenting wastes a valuable resource. If you brew on a relatively regular basis, you won’t need to buy a new package every time. Reusing yeast will save you a few bucks, but the greater advantage is that you can jumpstart the next batch with a much larger pitching volume.
There are two main methods for collecting yeast for reuse, depending on whether you pull it from the secondary fermentor or the primary. Each has its advantages, offering a trade-off between simplicity and predictability, with neither choice being wrong. Convenience can also factor into your decision, depending on when you’ll be ready to brew and how soon you want to use this same yeast.
Take it Easy: Secondary Collecting
Reusing yeast from the secondary is easier than popping a new pack of Wyeast. Less trub is present than in primary, so you can rack your fresh wort on top of the yeast cake and have an active fermentation almost immediately. If you’re like me and you prefer to start the new batch in a clean carboy, you can add the step of pouring the slurry into a fresh carboy (don’t forget to sanitize the neck and lip of your secondary with a spritz of sanitizer). Note that you shouldn’t need to oxygenate your wort at all. Yeast cells need oxygen during their reproductive phase, but at this concentration, they won’t need to do much reproducing.