Sunday, July 3, 2011

Washing Highly Flocculant Yeasts

It is not too often that I will wash a yeast cake for future batches. I figure buying liquid yeast isn't going to break the bank and I don't want to run the risk ruining a batch by pitching an infected yeast slurry. However, there are a few seasonal strains that I will occasionally wash, as I only have access to them for a few months a year. These include Pacman, Yorkshire Square, Thames Valley II, Young's, and Bedford Bitter strains. The only problem with these yeasts, Pacman aside, is that they are all highly flocculant and can be difficult to wash as the yeast tends to trap trub and hop debris as it settles.

There are a few ways to wash yeast for homebrew use. The most common and simplest method involves adding sterilized, de-oxygenated water to a yeast cake, agitating it, and decanting the suspended yeast off the layer of trub and hop particles that settles out. The decanted yeast slurry is usually put in mason jars where the yeast will further settle over a period of a few days. This method works pretty well for most yeast strains, though rarely works for highly flocculant British strains - as the yeast often bonds with trub and hop material as it settles, making it nearly impossible to get a sizable amount of pure yeast for immediate re-pitching. To get around this problem, I found a similar method that easily separates the yeast from the trub material and leaves a larger sample for repitching. Here's how it's done:

First, boil a gallon or so of water in stockpot until sterilized and de-oxygenated. Sanitize a large two quart mason jar and two smaller half quart jars with lids. Rack your beer off the yeast cake and dump a half gallon of your boiled, cooled water into the fermenter and gently swirl the water to break up the yeast cake. After a while, you will notice the yeast will settle into three layers - a creamy/whitish layer at the bottom, darker one in the middle, and a hazy one at the top. For our purposes, we want to take the yeast on the bottom. If we were washing WY1056, it would be the hazy, top liquid we would decant. However, with high flocculating yeasts, taking just the top layer would result in a thin layer of yeast that when fermented, could lead to higher than normal attenuation (slow flocculators) and cloudy beer. Once the yeast have settled a bit, decant off all but the bottom layer. Add more water to the yeast and put the slurry into the large mason jar. Let it settle in the fridge for a bit. You will notice three new layers forming. Again, decant off all but the bottom layer and add more water. Once you have a good amount of yeast mixed with just a little bit of trub, agitate the yeast and let it settle again. Continue to do so until the trub and yeast completely separate into distinct layers - a small layer of brownish trub on the bottom and a thick, hazy-white layer of yeast in suspension. I find it takes a few tries to get the yeast to fully separate from the trub. Decant the remaining liquid (should be about a quart) into the other two mason jars and refrigerate for storage.

So that is one way to wash flocculant yeast. This method works best when you have as little trub and hop material in the fermenter as possible and the fewer times you have to decant, the more yeast you will retain. As I said above, I don't wash yeast often, though this method works pretty well. However, an even better way of gathering highly flocculant yeast for re-pitching is to top crop. Most English yeasts are excellent top croppers and the process, which takes little effort, results in a lot of healthy, pure yeast that can be pitched immediately or saved for later use. I would recommend top cropping over yeast washing 99% of the time, though yeast washing is a nice alternative if you aren't around when the fermenting beer is nearing high krausen. Next time I brew with wy1318, I'll top crop some yeast and document the process. 

3 comments:

  1. I am a huge WYeast 1968 fan but it suffers from all the downsides you mention here in terms of trying to wash it. I have repitched it, trub and all, several times with good results. But I'm just about to try top-cropping using a small-scale Burton-Union system along these lines:

    http://www.byo.com/component/resource/article/117-Departments/354-build-a-burton-union-system-projects

    I love the idea of getting a decent amount of slurry, top-cropped from these British yeasts and this seems like one promising way to do it.

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    Replies
    1. Thanks for the link, looks really interesting. Though one thing I am always wary about when I see top cropping devices, is that most often you want to harvest the second rise (krausen) once the beer is just over 50% attenuated; to ensure you are getting the same fermentation/attenuation characteristics as the original yeast.

      Harvest too early or late and you can end up with yeast that will over-attenute and not flocculate and vice versa. I've dumped a few batches because of this...

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  2. Another option to rinse yeast, after you've racked your beer off the cake, use something sanitized to scoop a decent portion of the yeast into your large mason jar, add some starter wort. Once the yeast starts fermenting, it will be in suspension separated from the trub. Decant the fermenting beer to your two smaller jars and allow the yeast to ferment out, when it's finished you'll have two or three jars of clean yeast.

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