making wine kit in 5 litre demijohn

[jamesbont]

Wine kit says to top up to 4.5 litres. Will this leave to much headspace in the demijohn? Can this cause Oxidation? What about when it is racked?

 

[Tim_Crowhurst]

If you're only just starting the kit, 4.5L is about right. You need plenty of head-space inside the DJ during the first week or two, to leave space for the foam generated by the fermentation, so to start with the DJ is normally filled up to the shoulder. Typically you top it up to the neck after the first racking, as it's only after that point that you need to worry about oxidation.

 

[oldbloke]

As long as you have an active yeast colony you don't really need to worry about oxidation at all.

 

[Megaross]

Co2 (produced by fermentation) is denser than air and thus sinks to the bottom to cover your brew in a blanket and protect it.

Headspace doesn't matter in primary, to be honest you want some oxygen in with it at the start, aerobic respiration of yeast colonies breeds the yeast - more yeast = less work for yeast and thus better brew.
Anaerobic respiration (where alcohol is produced) is without oxygen, so by the time the yeast has used up all the oxygen all should be well, nice healthy yeast colony processing the glucose into alcohols.

Or at least that is my understanding of it, could be wrong but that's the way it was explained to me.

After primary though you want as little air contact as is humanly possible, that's when you'll get oxidation issues.