Fermentation Vacuum

About a month ago, I posted a question on the Mead forum wondering about fermentation vacuum. It was just as i put the Mead wort in the primary, and pitched the yeast, i noticed that i was pulling a vacuum in my bubbler. There were some good responces, but everyone thought that i did not cool the wort enough (even though it was around 75°F). Today is the second day that i racked it, (now its in for the long haul for a few months) and i filled the bubbler, and within 5 minutes, it started to pull a vacuum. There is no dispute that the temperature is correct, since i cool my entire apt to fermenting temps and keep it within 2° that the mead is now not pulling a vacuum du to cooling. Why is this?? Any answers? I know this is not the mead forum, but i think this phenomenon would occur with beer as well. Thanks for any input.

(I would post a picture of my bubbler with the vacuum, but i dont know how.)

The only ways to create a partial vacuum within a fermenter are:
1. to shrink the volume of wort in the fermenter, or
2. to increase the volume of the fermenter that is sealed from the outside air.

Cooling the wort could cause 1, and if the carboy was colder than the wort, the wort would cool.

Having the bung gradually rise in the neck of the carboy could cause 2. This could happen if the bung was sanitized in a wetting agent such as star-san, and it was pushed in hard.

Either way it won't last very long, and the possibility of getting infections in the wort because of it are very remote.

-a.