I think I’ll try top cropping via the blow-off hose. I usually run my hose into a 1 gal. pitcher, filled about half-full usually with star san solution. With a good sticky yeast, like Weizen, I get a big load of top crop, but until now I’ve never thought of harvesting it. I checked Chris and Jamil’s yeast book, and searched the forum, no luck. So, if I were to give this a shot, presuming that star san would hurt the yeast, I would probably use boiled and cooled water, or beer, in the pitcher. I don’t see why it wouldn’t work. Is this one of those “duh, I missed what everybody knows” moments? or am I not seeing why it’s a bad idea?
It’s a bad idea because the pitcher will become contaminated with stuff floating in the air. You can cover it with sanitized foil to keep stuff out, but use as little boiled/chilled water as possible to make sure the pitcher doesn’t overflow.
If you want though, you can check out the “yeast recovery system” I built, it was in Zymurgy a couple of months ago. I designed it to actually return yeast to the fermenter, so it is mounted above the carboy. The blowoff goes up into the jar and liquid is able to run back in to the fermenter through another tube. Then there is an airlock mounted as well to allow CO2 to escape. This same idea could easily be modified so you run the blowoff into a sealed jar and mount an airlock on it. A couple of holes in the lid, a couple of grommets, Bob’s your uncle.
Sounds somewhat like a “Burton Union”, yes?
That’s exactly what I was going for, but I hesitate to call it a “union” because it’s just returning it to one fermenter. 
Doesn’t seem like a bad idea to me: Karlis' Hopyard: Homebrewing and Beer Blog: Top Cropping Yeast from a Carboy
Nice link. Thanks. I think I might try that rig on my next Belgian.
I’m assuming you’d want to build a starter off of the amount of yeast harvested.
Or do you get a pitchable amount?
He says in there, I think, that the yeast isn’t very thick, so I’d say you’d want a starter with it. Seems like a pretty good method to get healthy yeast. I’d like to try it as well.
Interesting link, thanks. I would still do a starter with it. I’ve never measured how much thick slurry I get, and it varies, guessing from a half cup to a cup. But, yeah, it is the best yeast of the batch. Use it.
I think I’ll try just covering the pitcher with foil. We let air in with starters, so why not let a little in with this? That will be a quick, easy setup. It’s only for a couple of days, anyway. Most of the flow is going to be outbound with all the CO2 being produced. After the major chaos is over, harvest it and replace the blowoff hose with an airlock.
Hey, thanks for linking to the site. Yes, I have so far always made a starter with the collected yeast. Though on a couple occasions I had a full 1000ml flask and probably didn’t need to do a starter. I did starter anyway since I usually am not able to pitch collected yeast straight into a new batch. Like to make sure the yeast is nice and active.
cheers!