Top Cropping Yeast

I thought I’d do a quick post on top-cropping yeast.  This is an easy way to get fresh, viable yeast, especially if you brew frequently.  I first tried it after watching the Brewing TV episode on open fermentation (which I also do sometimes), and I soon made it a key part of my process, deliberately selecting strains that are good top-croppers and planning a series of beers around a particular pitch of yeast.

The strains I have on slants that I know are good croppers include the following: Wyeast West Yorkshire (1469), London Ale III (1318), Ardennes (3522), Trappist High Gravity (3787).  I have some “Conan” yeast in the form of The Yeast Bay’s Vermont Ale strain.  This was originally a British yeast, cultured by Greg Noonan (watch this excellent episode of Chop and Brew for more information), and based on one fermentation I suspect it will also be a good candidate for top-cropping.  I’ve cropped versions of the Chico strain too.

I tend to plan a series of beers around a particular yeast so that I can pitch directly from brew to brew.  The yeast in these pictures is the Ardennes strain (3522), grown up from a slant last week, and then used to ferment a 1.042 blond saison/grisette. 

The night before I’m going to crop yeast, I usually sterilize some of the tools I’m going to use in my pressure cooker: a funnel and mason jar for the yeast, and some water to store it under.  Unfortunately the spoon I use to crop the yeast doesn’t fit in my cooker.  This step is probably overkill, and I don’t always use it since everything gets sprayed liberally with starsan solution just before I crop, and that ought to be sufficient.

I usually aim to crop early on the third day, after fermentation has slowed somewhat and the yeast begin to gather in thick clumps at the top of the beer (obviously this will depend on the strength of your wort and the speed of your fermentation: since I usually brew only 11.5l of sub-1.050 beer, fermentation is beginning to slow down after three days).  Here I had to do it a little early, so the yeast was more foamy than thick and I didn’t collect as much as I usually do.  Once you’ve done this a few times, you begin to get a sense of when you should crop: in Brew Like a Monk, Ron Jeffries says that “it should look like a rich lather, with a dense head on top”.  If you’re brewing a hoppy beer, the yeast that surfaces on the first few days might be coated in hop oils.  I don’t worry about this too much, but you can skim this off and then come back to collect clean yeast later after the top-layer has reformed.

I scoop the yeast up with a metal spoon and transfer it into the mason jar via a metal canning funnel.  Its best if your relaxed while you do this, but I do try to be quick to minimize the time that my bucket is open and the yeast is exposed to the environment(since this is my brew closet, there is lots of grain etc. in there, and so probably plenty of wild yeast and bacteria in the air).

Afterwards, I pour the sterilized water on top of the yeast and close the mason jar up.  At first you’ll have a milky, foamy liquid, but the yeast will eventually settle out into a creamy white mass at the bottom of the jar.  I often rouse the yeast that’s left in the bucket by stirring it up with a few gentle movements of the spoon.  If you are cropping during high krausen, the top-layer may even reform before you close things up.  In theory you need to be careful to leave enough yeast behind to finish the fermentation, but this has never been a problem for me.

As I said, I cropped early this time and didn’t collect as much as I usually do.  Still, it’s more than enough for what I have planned.  I’m going to use it in another ~1.040 wort tomorrow: Jamil’s yeast calculator, on its usual settings, says I need 30ml of yeast.  This is 40ml of thick slurry that is basically 100% yeast and very healthy. 

Sometimes I deliberately crop a little extra and grow it up in some starter wort, just to save me the hassle of cropping from the next beer.  This is especially easy if I’ve saved wort from the bottom of the kettle after a recent brew.  I may do this today, or I may just crop again for the final beer I have planned for this yeast.

So there you have it: an easy way to get multiple batches from a single pitch of yeast.  Once you get over the fear of opening up your fermenter before fermentation is over, its really very simple, and with basic sanitary precautions there shouldn’t be any significant risk of infection.

Update (24/7/14):  I’ve decided to update this post to incorporate a few things I should have mentioned, and also to describe a very small change in my process.

One thing I should have stressed is that you want to crop yeast from the second or high krausen.  If possible, I suggest fermenting in a see-through vessel with a true top-cropping yeast so that you can see what this means.  Usually the yeast will put up a thinner krausen within 12 hours of pitching, which will subside thereafter.  If you were fermenting in a bucket and just happened to peak in at this point, you might think the fermentation was over and you’d missed your chance.  However top-cropping yeasts like WY1469 will put up a second, much thicker krausen shortly after this.  This is what you want to crop from.  The time frame in the main post is about right: 3 days into fermentation is usually when I see this.

The change in my process is minor.  I’ve started to use a Bubbler Wide-Mouth carboy for my primary fermentation.  The wider mouth makes top-cropping possible, but its narrower than a bucket, giving less room for manoeuvre.  To accommodate this, I’ve taken a slotted metal spoon and bent the joint between the handle and the spoon itself to make an L-shape.  It essentially looks like a ladle.  I can now dip down into the bubbler and pull up yeast more easily.  As an added bonus, this means that the spoon also fits into my pressure cooker, so I can sterilise along with everything else.