An Interesting Observation With Breathable Bungs

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Mildly Amused
Supporting Member
Jan 19, 2022
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Central Alberta
Today I racked some wines and bottled my Malbec from last november. I have 8 6 gallon carboys with breathable bungs. I topped them all up today.

I am pretty anal about headspace. I am typically a half inch under the bung and ALL of these carboys were to the bottom of the neck. SOOO.. Who thinks that the one way action of the breathable bungs would allow for evaporation? None have any signs of oxidation so I have no worries about them not functioning well. Just an interesting consideration that I WILL be paying attention to in the future.
That's really interesting. I don't have breathable bungs (yet) but your experience is worth further research.

I have the opposite issue - wines I put into bulk earlier in the year have ZERO headspace, touching the stopper. I assume it all has to do with temperature and barometric pressure.
I assume it all has to do with temperature and barometric pressure.
Absolutely. I have popped solid bungs and noticed volume increases from summer to winter, but I wonder if there is an avenue for micro oxidation, which I question, but concentration with the angels share is definitely an interesting concept.
In a barrel out in the wine shed - sure, in a plastic or glass carboy in a controlled temp environment - I wouldn't think so, unless your humidity is way low. I also use the breathable bungs and I see the level fluctuate with the pressure, but that's all.
To resolve the question of evaporation, can't you compare current weight with the weight when you filled the carboy? You do weigh your carboys, don't you?🤓

I had to check my Wine Exotic Info Notebook (yes, seriously, WEIN pronounced "wine" 😅 ). The thermal expansion of wine is .166ml per degree F so, depending on the temperature (and pressure) when you filled the carboy, maybe it's not evaporation.
* Silicone as a polymer will not transmit water, but it will transmit a low level of oxygen. In the scheme of things a 1.25 inch round opening is small enough that I would ignore the oxygen transmission.
* as the name implies they can burp to release pressure and I would assume that slow ingresses also could happen to balance pressure, ,,, but it isn’t significant mass transfer as long as the rubber is clean/ properly seated.
* your original post suggests evaporation. My guess would be that you filled the carboys mid summer when the temperature could be 5 to 10C warmer. Decreasing the wine temperature by 5C will shrink the volume enough that a 3cm neck would show a change in height.

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