Thursday, December 17, 2009

Top-Crop Till You Drop

Nowadays, the price of yeast (particularly liquid yeast) can make up a fairly sizable chunk of the cost of a batch of homebrew. At up to $9 a vial or pouch (depending on where you shop), this can be a substantial portion of a batch's cost, especially for brewers who buy hops and grain in bulk. Good thing is, there's a simple method for dealing with this. Reusing yeast isn't just a way for frugal homebrewers to save a buck (something we tend to be fans of) but it's also great for building up large, healthy quantities of yeast to ensure great fermentations down the line.

Probably the most popular method of harvesting yeast is to do so after fermentation, when the beer is racked out of primary leaving all that yeast behind. This is effective for gathering up a big quantity of yeast (and depending on your brewing schedule, pitching new wort directly onto a yeast cake can work), but the technique is not without its disadvantages.

Perhaps the main fallback is this: the muck left behind after racking is not purely yeast. There will also be trub from the kettle, consisting mainly of proteins and hop material (subject to that batch's hopping rate and whether pellets, bags, etc. were employed). Harvesting yeast slurry after primary means picking up some non-yeast material, in all likelihood.

Some brewers, as an alternative, choose to collect yeast via top-cropping, whereby the yeast is skimmed off the surface of the wort during the height of active fermentation. This ensures that the goods you are getting are clean, active, healthy and lively yeast cells.

And it's easy to do. I start with a small canning jar that gets a quick soak in some sanitizer. Then I add a small amount (a couple ounces only) of filtered water to the jar, which then goes into the microwave to just to make sure nothing's alive in there. I set the lid on top while things cool off; the steam helps to ensure everything's sanitary even though, yes, I'd already given the jars a sanitizer bath. Extra precautions can't hurt when you're dealing with something as important as your yeast.

The ideal time to top-crop is shortly after active fermentation has kicked into high gear. Give the yeast enough time to move any hop material to the side of the fermenter, but don't wait so long that the yeast mat has fallen too much back into the beer, or you might not find yourself with enough skimmable yeast to fill your jar. A visit to Jamil Zainasheff's yeast pitching rate tool will give you an idea of how much slurry you'll want for your next batch.

I scoop the yeast off with a sanitized spoon and stir it into the water in the jar, which helps knock the yeast off the spoon and will also form a thin, protective layer on top once things settle out in the fridge. With a label affixed identifying the yeast strain, its generation number and date of collection, this little jar of wonder is ready to live in the back of my fridge until it's time to unleash its magic on another bucket of sugar water.

Yeast slurry can be saved in the fridge for several months. Inside a couple weeks, you can usually simply repitch the slurry right into the next batch; longer and a small starter might be helpful to wake things up. Jamil's calculator will help to figure out the yeast viability based on its age and give an idea of how much extra slurry should be pitched accordingly.

Congratulations! You just got to play with nature's coolest fungus*, improved the quality of your future batches and saved some cash at the same time. Now go blow that money on beer.

* Up for debate


nate said...

Great article, Brad. When I discovered this I was so relieved. the cost of yeast is killer. I can easily keep a moderated ale at $.50 to $.60 by storing my own yeast. It's good practice, since that's how the big boys do it. I've even got a strain of wild Toledo, OH yeast in my fridge that I try to keep going.

Brad said...

"Wild Toledo, OH yeast"? Please elaborate! An old jug of apple cider perhaps?

Saving and reusing yeast is definitely the way to go. I will often collect yeast from the cake after I rack out of primary (there's lots of yeast down there), but I want to top-crop more often as you get cleaner (and some say mightier) yeast and it's fairly easy to do.

Karlis said...

Hey really liked the article. You are really able to keep your yeast for a few months with no spoilage? That's great, I haven't tried to store mine for that long. I have a post here on top-cropping from a carboy in case anyone is interested:

Brad said...

Karlis, I know I've stored yeast for a couple months in the past. As it gets older the viability drops so you'll have to pitch more slurry and/or wake it back up with a starter (not bad policy anyway when the yeast has a little age on it).

I can't quite recall how long I've gone in between reuses. As a general rule I try to reuse the slurry as quickly as I can, so I plan my brews accordingly.

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