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. ?