A few days ago, under a thread with the banner "Wine Aging" I made some
comments about the effect of temperature on the ability of airlocks to
protect wine. I promised to post more later.

Air locks are designed to be a constant pressure valve to protect the wine.
Of course during fermentation CO2 is given off, the pressure in the vessel
tries to go up and the excess volume bubbles out through the airlock.
Perfect. If the temperature of bulk aging wine increases, the wine expands,
pressure tries to goes up, and what ever is in the head space bubbles out
through the airlock. Fine. If the wine expands too much and all the head
space is expelled, then wine goes out through the airlock. Not so fine.
You can watch your wine you can prevent this.

But if the temperature drops too much, a different scenario takes place.
Outside air will bubble through the airlock into the carboy. If this
happens it is not a very good thing. So how much protection does an airlock
provide and how important is this?

I checked two types of airlocks. The S shaped airlock with three bubble
chambers on each side will hold 11 cc's of liquid when properly filled. The
3 piece cylindrical airlock will hold 9 cc's from bottom of the floating
valve to where liquid spills through the tube so it provides 9 cc's of
protection. (I did not have one of the S locks with one chamber on each
side free to test.) For this analysis I will use the 9 cc smaller airlock

The question now is how much of a temperature change will these volumes
protect against before air bubbles through the airlock. This is effected by
the volume of the wine and the starting temperature. I assume 25 deg C as
the starting temperature and determined the temperature drop the airlock
will protect against for different size carboys. I found the following:

Carboy Temp Drop
(US gal) (deg C)
1 9.7
3 2.2
5 2.0
6 1.2
10 0.8
12 0.7
13 0.6

This shows some pretty small numbers, but what are the restrictions of the
calculation.
1) I used the expansion of water. I found the expansion table for alcohol
but not for 13% alcohol, so I considered water to be a better estimate. I
suspect that wine will have more expansion and therefore less protection but
I can not say for certain.
2) I do not make a correction for expansion of the glass vessel but this
should be very small.
3) This is based on temperature changes of the wine, not the room. Over a
one day/night temperature cycle the room may change more than the wine.
(Thanks to Jack Keller for pointing this out.)

Now what does this mean? Standard airlocks will protect carboys of 5-6 gal
or smaller from temperature changes in a temperature controlled home and may
well protect the larger carboys. But if you are keeping wine in an
unairconditioned room or in a cellar that sees temperature changes of
several degrees C. You may want to take all this into account.

You should be aware of day to night temperature changes, changes associated
with cold fronts, and seasonal changes. Seasonal changes, though they may
cause the largest change will probably not be too important as there will
only occur once a year. Day night changes may not be important as the wine
may not change temperature fast enough to matter. But if it does it will be
catastrophic as it will happen 365 times a year. Changes associated with
cold fronts and heat waves may be the ones you really have to be concerned
about.

Ray Calvert