Reuse Yeast

Developing a Healthy House Yeast Strain

According to Chris White, (Yeast), with careful harvesting practices, an initial lab yeast culture can be reused in five or ten batches of beer. And as the yeast is reused, it becomes acclimated to the local brewing conditions. The result is faster, cleaner, more consistent fermentations. "Most brewers report the yeast "settles in" and performs best by the third generation." - White.

Top Cropping

Ale yeast strains float to the top of a carboy at the peak of fermentation, and produce a thick mat of foam called kraeusen. The foam is made up of active yeast of high viability, high vitality, and peak cleanliness. This is the yeast you want working in your next batch of beer.

With excellent sanitization practices,  you can ladle the kraeusen from a fermenting bucket, being careful to not contaminate the beer or the collected yeast. Or you can build a carboy top cropping device, using a two hole stopper or cap and a couple pieces of racking cane and flexible tubing. You can hook up very low pressure sterile CO2 from a tank to create a yeast vacuum by inserting a racking cane into the cap's other hole. You can also use the fermenter's own CO2 output to force the foam into a steril collecting flask. The device becomes a closed loop blow-off system. And the collected top cropped yeast is ready to re-pitch immediately.

Sam Scott has posted this video of his home built top cropping device for a carboy.

Bottom Cropping and Rinsing

The other option for yeast reuse is to harvest and rinse the settled yeast cake when you rack away the finished beer to a keg or bottling bucket. Siphon off the beer, leave a bit of liquid to help swirl the yeast cake back into a slurry. Wipe the mouth of the fermenter with cheap vodka to sterilize the opening. You can also quickly flame the opening for added protection against infection. Pour the slurry into a sterile container with a loose lid. The amount of slurry should only account for about 1/4 to 1/5 of the space in the container so you can top up with sterile water, leaving about 10% air space.

Harvested yeast cake, including dead cells and trub, well agitated.

Seal and shake the container(s) vigorously for three minutes to break up the flocs. Let the container(s) settle. Within ten minutes the materials will settle into layers. At the top will be a thin watery layer. In the middle will be a creamy layer of good yeast. The bottom layer will hold settled dead cells and hop bits.

After ten minutes of settling, the layer of good yeast floats on top of the trub and dry-hop particles.

Decant off the thin top layer to the drain. Decant the middle layer of good yeast into another sterile container with a loose lid. Store cold. Compost the hops and trub.

Revitalizing

You'll want to use the bottom cropped yeast as quickly as possible to avoid loss of viability and vitality. According to White, any yeast below 90% viability (living cells) should not be re-used. At the same time, you will only get good fermentation results if the viable yeast has high vitality - the cells are healthy enough to perform fermentation. If you've had the yeast in the refrigerator for a week or two and it is free of contaminating bacteria, you can do a further test of its health by revitalizing with high gravity wort.

By early on brew day, bring the yeast up to 70 ºF to 75 ºF and add sterile 1.080 SG wort at a rate of 0.50 millimeters per 10 millimeters of yeast slurry volume. After 4 to 12 hours without aeration the active yeast will turn the wort milky and dead cells will settle to the bottom of the container. Decant the top healthy portion, pitching into your fermenter.