aeration vs de-gassing - differences?
- Thread starter scottv
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Aeration is adding air. Degassing is removing dissolved gases. So the difference is just the direction in which the gas is moving, be it into or out of the liquid. To get air into the liquid, you have to be kind of violent (shaking, blendering, vigorous whisking, etc). Getting gas out is a bit easier and a gentle stir will usually do it.
or of course, with de-gassing you can also use mechanical means, whether it's a hand held "mityvac" type pump or a powered vacuum pump of some sort.
Good point, you don't need to agitate to perform either operation. Aeration stones put tons of tiny bubbles into the liquid, which dissolve more easily. (You can also use them for quicker carbonation inside a keg.) Vacuum degassers will literally suck the gas out of the liquid. Thankfully, they are helpfully named so you're unlikely to mix them up. 
it sounds like it's either quite early in the ferment, when the greatest amount of CO2 is released
or
you're making the batch in a carboy and haven't left enough air space for foaming in the early stages.
Early in the ferment, I usually start slow to see how the batch foams up, if it foams quickly I slow the stirring more to see if the foam subsides some, if that doesn't seem to work I stir faster as that can also burst the bubbles and knock the foam level back down.
After 4 or 5 days, it usually doesn't foam as much, but the yeast type can have some influence on it.
Medsen suggests the use of anti-foam drops, which can be very helpful in reducing this issue.
With carboys, the shape of the glass creates a constriction, so I usually just make up the must to the target quantity, then after taking the various measurements etc, but before pitching the yeast, I remove a pint or two (presuming a 1 gallon batch) so that there's room for the foam to rise in the early stages and prevent eruptions. The reserved must is just kept in the fridge for the first 4 or 5 days, then when the main body of the must either stops foaming so much or the main body of must hits the 1/3rd sugar break, I add the reserved must back into the carboy (letting it come back to room temperature first).
Oh, and if possible you can reduce any possible mess by putting the carboy in the sink and do the stirring procedure there.
Recently I've taken to making batches, initially in a bucket, then there's usually enough space for the foam to rise as there's a greater exposed surface area for the foam to rise and bubbles to pop. At the 1/3rd break stage, I then give it a final stir/aeration (and if I'm doing SNA/staggered nutrient addition add any final nutrient then) then while all the yeast is still in suspension I siphon it into the carboy so it can be air locked off and bubble away to it's hearts content to finish.....
