I'm looking at buying some 4.5L demijohns, I have found a recipe that I'd like to try. The recipe states to use 1L honey to 3L water and age for 8m-1y.
Considering the demijohn is 4.5L and in total there will 4L in mass I assume that when I shake to mix the honey in it will dissolve somewhat and therefore there will be a little extra room after mixing, correct? Assuming that is correct and I give or take I will have around 0.6L of room in the demijohn. I hear you don't want a lot of room inside because it can cause issues. How safe will I be with that amount of room?
Thank you.
Welcome to the forum, mead is an awesome but fickle drink. Your recipe sounds good for a drier mead, 1 L of honey should be around 1.4 kg so just over 3 lb which should ferment dry in 1 gallon. Nutrients are a must, I started using Tronozymol which has a bunch of extra nutrients alongside the DAP (base yeast nutrient) after getting a stalled fermentation, 1 tsp is probably good for your batch as I've found it leaves a flavour if you use too much. Some people like adding tannin others don't find it's needed. From my experience, don't add any acids before fermentation as it's easy to overdo them and get a stuck fermentation, they probably won't be needed in a dry mead anyway but can be added to taste afterwards.
Regarding volumes, 4L in the demijohn should be ok for fermentation as long as the yeast doesn't foam too much, I'm going to start fermenting 5L batches in a 10L bucket as then there's plenty room to de-gas the mead, then I'll rack into the demijohn for ageing. You'll want to transfer off the yeast once it's done fermenting to allow it to clear, you can top up the new demijohn with a honey water mix if you don't want to water the mead down, fermentation will start up again but shouldn't be as active allowing for less headspace.
The trick with mead seems to be de-gassing it every day for the first week or so, gentle stirring to remove the built up CO2 which hampers the yeast, folks also stagger their nutrients over the course of fermentation but with such small amounts in a 1 gal batch it's probably safe to add at the start. These steps will allow the mead to ferment smoother and become drinkable in a about 3 months rather than the 8 - 12 you've read about.
I've used Lalvin D-47 yeast, it's good but you want to keep it under 20c or it throws some odd flavours (according to the internet), Lalvin 71B is a favourite which I'm going to try next.
If you want a fast test batch, 3 x 340g jars of honey (I like Tesco orange blossom) made up to 5L (or what you can manage), 1 tsp of nutrient and fermented with either D-47 or Safale S-04 (an english ale yeast). It'll ferment dry at about 7% abv, can be primed and carbonated for a dry cider style drink (my first one) or stabilised and sweetened for a light wine (my 2nd batch).
Good luck, I'm just about to get back into mead making after finding a local-ish producer which got me to realise how tasty it can be when done right, or better than my first tries.