Kettle gravity post-boil stratification

[BugHunter]

Hey every, just a quick question here. The system I work on is a 20 bbl steam jacketed kettle with floor-level tangental inlet. During the sparge I observe, as expected, a wort gravity gradient throughout the kettle fill. My question though is this: shouldn't a full vigorous boil force the wort solution into equilibrium?

I ask because quickly after shutting the steam jackets off I can begin to observe stratification in my gravity measurements, and I can't for the life of me figure out why. Is this a solution particulate issue? Temperature gradient? Something else.

I'll admit right out the gate: my strength is in microbiology, so there's probably something I'm missing. Any assistance in figuring out the cause of this observation would be appreciated, and any technical papers you could point me to even more so.

[Bainbridge]

I've noticed the same thing. Not a chemist, but thinking maybe it's something to do with sublimation and the boundary layer on the surface? Like there's some evaporative cooling that happens even as its boiling that 'rains' some, now essentially distilled, water back on the top. But I have no idea. Really interested if anyone has the answer.

[brewfun]

My question though is this: shouldn't a full vigorous boil force the wort solution into equilibrium?

Yes. And it should stay that way.

I ask because quickly after shutting the steam jackets off I can begin to observe stratification in my gravity measurements, and I can't for the life of me figure out why. Is this a solution particulate issue? Temperature gradient? Something else.

If your boiler has a hard time staying above 10psi while ramping up to a boil, I'll guess that you have too much condensate hanging around in the jacket(s) before the boiler calls for return water. That area of wort isn't going to move as much as the wort above it in the upper regions of the jacket. You want the steam to finish condensing just as it leaves the jacket, not before.

The convection flow should be from the vessel floor, up the walls and to the center, where it falls back to the bottom. A gas fired system usually runs the other direction. I've seen kettle schematics with an offset bottom jacket that is supposed to amplify convection.

I'll bet that you don't "see stratification" post whirlpool.

It raises a question for me: why not measure gravity just before knockout?

I get some steam loss during both and with extended whirlpool times, I can see as much as 0.5 Plato rise after the boil ends.

[SeattleBrewer]

Post boil evap

Between the end of boil and the end of runoff for many people is at least one hour, more if you have a longer whirlpool rest and/or cast out. You aren't boiling but there is a not-insignificant amount of evaporation that occurs during that time. So that would look like stratification over the hour, when in fact you are still slightly concentrating your wort. Try taking readings right after boil and off of the fermenter immediately after cast out is finished and record those differences. Try and find a common percentage and you should be able to dial it in every time.

I remember hearing Dan Gordon of Gordon Biersch explain this concept, and they take post cast out readings as their OG. He mentioned that many craft brewers don't actually get the real OG of their beer because of post boil evaporation.

I'd also love if you were able to publish that information on this forum and let us know what you find in regards to post boil evap vs. boil evap percentages, etc.! Could be a big help to all us trying to get the real numbers.

Cheers,
Greg