That depends on the size and shape of the container, and whether the wine is
fermenting or still. The amount of surface area exposed is more important
the depth of the air-space. Thus a (narrow-mouth) carboy with 4" of airspace
is much better than a (wide-mouth) primary with 1".
While the wine is still fermenting vigorously, you can have (and often need)
30-40% headroom, and the exposed surface area is not a great concern.
When the ferment slows down, it will still produce enough CO2 to protect a
red-wine in a primary, but you should probably move a white wine to a
(narrow-mouth) carboy, and top up to the shoulder.
When the ferment has stopped completely, top up to ~1.5" of airspace for a
1-gallon or 4-litre jug, ~2" for a 2.8 or 3 gallon carboy, and ~3" for a 5
or 6 gallon carboy. These figures are based on my personal experience, and
are the minimum amount necessary to allow for the fluid expansion with a
10ºF temperature increase (so the stopper isn't forced out and/or the wine
doesn't end up in your airlock). If you can be sure that there will not be
such a temperature increase (even if there is a power failure and your
refrigeration system goes south for the winter), you could use 1/2-3/4" on
all vessels, but the same surface area would be exposed, and there would be
little difference in oxidation. If you anticipate a greater temperature
increase during aging, increase the gap, or (preferably) consider installing
a better refrigeration system.
Yes, but in the narrow "bottle-neck" of a carboy, there isn't much surface
area exposed. So, I think, the rate-of-transfer is slow enough, and the
volume-of-air : volume-of-wine ratio small enough, that there will not be
much noticeable difference in oxidation between a 1/2-3/4" and a 2-2.5" gap,
as long as the wine level is in the narrow "bottle-neck". The difference
would be more significant in a wine bottle, because the air : liquid ratio
is much greater. This is only a "seems logical" conclusion on my part. I
have no empirical evidence to prove the theory, and I may well be incorrect.
Regardless, there must be some compromise. Ideally, there would be no
air-space, and thus no worry of oxidation or acetefication. But liquid
expands with significant hydraulic force upon temperature increase, and some
allowance for this must be made.
I've had 3 refrigeration-system malfunctions in 2 years (and of course they
occurred on the hottest days of the year). In each case, carboys that were
topped up more than the levels I mentioned in my previous post either pushed
the stopper out or burst. Those that were not, did not. Since I also use a
layer of cling-wrap held in place with two rubber bands _over_ the stopper,
all my wine was saved (except in the carboys which burst because the stopper
However, if I were ill or out of town, or didn't notice the stopper had
popped, the cling-wrap might have come off too. Had I used airlocks during
aging, rather than solid stoppers (as some people do), the wine would have
entered the airlock, mixed with the sterilizer, which (when the temperature
dropped) would have contaminated the wine.
Thus I have determined it best to leave such a large airspace in the
carboy --- if I rack when the wine is at cellar temperature. If I rack when
it is at room temperature, I would reduce the airspace by about 30%, since
some of the thermal expansion will have already taken place. What other
choice is there?
If I had a well-insulated underground cellar, where the temperature would be
stable even if the refrigeration system failed (assuming one was even
needed), then I would top up to 1/2-3/4", and eliminate the (hopefully)
small difference that the larger gap causes.
How large an air-space do you use in your large tanks and barrels?
The other thing that occurred to me is that, if one were to top up the
carboys to 1/2" when the temperature is as warm as expected, there would be
absolutely NO need to top them up when the airspace subsequently increased
due to temperature drop. There would be no more air in the larger space. It
will simply have "de-compressed", expanded to fill the space available, just
like Boyle predicted.
I wish I had a proper cellar deep in the ground. :(