Tonight I ran into this headspace problem when racking my Bunratty Pyment. (See
"Meade" Into Mead:
http://www.gotmead.com/smf/index.php/topic,1664.0.html).
I decided this was a good time to experiment, so I took a rubber stopper I use for syphoning purposes which has two holes in it, placed it on the newly racked mead jug, and ran a hose to a small plastic bottle. In the bottle I had 1/2 tsp. of generic acid blend (I believe it's 50% citric, 25% malic, and 25% tartaric) dissolved in approx. 4.5 oz. of water. I then hastily scooped 1 tsp. of baking soda (bicarbonate of soda) into the water bottle. The latter end of the hose was stuck into a single hole stopper, and I quickly affixed this to the top, holding it on for about two minutes. The CO2 thus produced had the time to run through the hose, fill up the jug, and push out all the air inside through the second hole on the fermenting jug. After this, I quickly airlocked the mead vessel again.
The acid bled is a perfect choice for this process, as it won't infect the mead with dangerous microflora, nor lend any off flavors. The CO2 is invisible, so I can't say for
certain how well this worked, but I'm pretty confident it filled up the headspace in the jug with clean, neutral smelling, food-grade CO2 -- and it was very simple and inexpensive to do. There was no mess, and the acid/water/soda combo did not foam up much at all, since there's nothing else in that mix to make the bubbles hold their form.
I believe this is a viable and safe alternative for people who live far from sources of food-grade CO2, or for anyone on a tight budget (that's me and me).
Great idea, Mu!
-David