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Sometimes I make a large batch of wine and fill maybe two or three demijohns, sometimes four. I don't know how many demijohns I can fill until I've filled them. That means, sometimes I end up with an extra two pints or something of unfermented wine. I have avoided putting this juice/sugar/yeast mixture into a demijohn since I have been believing that a demijohn somehow needs to actually be filled.

Come to think of it, I don't where I picked up a belief like that. And I wanted to ask if it's true, or if it's perfectly fine to let my wine ferment in a half-full demijohn. ?

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  • totally fine to let a 1/2 full demijohn ferment. – Mr_road Aug 13 '17 at 11:26
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I have never had a problem with fermenting wine, beer or soda in a part filled demijohn in several years of brewing. To be fair, I have never fermented anything in a demijohn less than 1/3 full, but I have fermented many brews in demijohns that are 1/2 - 3/4 full. All turned out as expected (or nearly so...)

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Oxygen can always be a problem, and indeed, the less headspace you have on your demijohn the better.

For beer, I know that I can add 4 g of sugar per liter of headspace to get the fermentation going a little bit again, to clean the air out of the demijohn. I do not have enough experience with wine if there is also still enough yeast in suspension to do the same.

Another possibility is to fill all your demijohns almost up to the brim, and then put the rest of the wine in a PET bottle, and squeeze the air out of it.

Oh, and now I see in the last line that it is about fermenting. Like mentioned in the preceding lines, 4 g of sugar gives (appr) 1 liter of CO2. If you have 2L of juice with a Brix value of 10°, which means 10g of sugar on 100g of solution, that would mean that you would have appr. 200 g of sugar in this 2L of juice, then the fermentation would generate more than 40L of CO2, well enough to clean out most demijohns, except the biggest, I suppose.

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  • I contest that oxygen can always be a problem, in the first couple of days of fermentation sufficient oxygen is a bonus and allows the yeast to rapidly synthesis sterols which maintain flexibility of cell membranes. – Mr_road Aug 13 '17 at 11:24
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IMHO one can ferment perfectly well in a half filled demijohn. Filling a demijohn completely when fermenting can lead to airlock problems if the fermentation is vigorous - which most initially are. So some space above the ferment in demijohn is a useful thing. 7/8 and 3/4 full are common situations and I have fermented wine in a approximately half full demijohn - for exactly the reasons stated in the question. The wine in the partially filled vessel tasted just like the wine in the mostly filled vessel.I have done this several times for different wines and several beers.

IMHO Oxygen and fermenting liquids are not such bad company as one might be lead to believe....

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