Oxygenation -Carboy not full

Oxygen has the potential to damage meads. It alters the structure of molecules that it interacts with including phenolic elements that produce aroma and flavor. With enough oxidation, the aroma and flavor go flat and lose their fruity character. The mead may take on the aroma of sherry or Madeira wine (generally considered a fault in table wines), and if it gets bad enough the mead can end up tasting like wet cardboard. Having had a couple of batches do this, I know for a fact that it can occur.

The presence of oxygen also allows acetic acid bacteria, Brettanomyces, and other spoilage organisms to grow, so if any are present, and the oxygen is there to support them, they will ruin the mead. This is not a common occurrence, but I have seen it happen.

Notwithstanding the above, most meads (especially traditional meads) are much less sensitive to the effects of oxidation than are white wines or beers which really suffer quickly (in many cases) if they are exposed to oxygen. In the case of beer, generally you want to avoid oxidation even when the yeast are actively fermenting, which is different from mead. Usually with meads, the big concern with protecting it from oxygen comes after fermentation is complete.

This means it is okay to ferment a 1 or 2 gallon batch in a 5 gallon primary. During fermentation, the tremendous volume of CO2 released scrubs the air/oxygen out of the primary so the space is filled with CO2 at the end of fermentation. After fermentation is over, each time you open the container (to add sulfites, or oak, or to take a sample for tasting) or each time you rack, there is some exposure to oxygen. If you open the contain, and have 2 gallons of space in there, it will rapidly fill with air, and you'll have the potential for harm. This is why we recommend keeping mead in containers that are full or topped up (or otherwise protected from oxygen).

Are we overprotective? Probably.
But having lost a couple of batches to oxidation along the way, and having successfully caused some oxidation intentionally in some batches of Meadeira, I choose to exercise caution. In a thread called "bucket aging?" you'll find some discussion about one mead maker who finds no problem with oxidation in his meads leaving them alone in a bucket which is sort of at the other end of the spectrum. There is room for more than one approach when it comes to managing oxygen exposure, so you have to pick you path, and be ready to make adjustments if you find you run into problems.

Medsen