"I've got a bone to pick with you!", she said.
This sounded ominous but I waited for the details. It turned out that my friend had followed my simple wine method to the letter.
"I used four 1-litre cartons of juice and the sugar syrup addition was made up to exactly a litre too. The fermentation went fine, but when I went to bottle it I only got four full litres and one about three quarters full. I thought I was making five litres of wine. Where did the rest go?"
OK. This is normal. To a first approximation, unfermented wine is just sugar dissolved in water. There's a popular misconception that when you dissolve sugar in water the volume doesn't increase. The truth is that the volume does increase, but not by as much as the volume of added sugar. The sugar molecules occupy the spaces between the water molecules. It's a neat exercise in packing but it's not perfect and the total volume does increase.
Fermentation breaks down the dissolved sugar producing CO₂ and ethanol. Some CO₂ stays in solution but most escapes as bubbles. This reduces the total liquid volume, but the effect is masked while the bubbles are present, because they take up some space, so the surface level stays artificially high. This is why volume loss appears to happen quickly, towards the end of fermentation when the bubbling stops.
Another cause of volume loss is yeast growth. The initial yeast addition is less than a teaspoonful. But the final sediment is many times greater and largely comprises dead yeast cells. Where do they come from? Well, they are organisms and so are made of carbon, hydrogen, oxygen (which come from the sugar) and also nitrogen from the fruit juice. It follows that if you remove dissolved molecules into a sediment, the volume must decrease. Again, this is masked, because the sediment also takes up space.
All of which explains that volume loss is to be expected. Now, what to do about it? Rather than top up to get that fifth litre, I prefer to capture four full litres and one 75cl bottle either for immediate drinking or, if a bit cloudy, for cooking. Waste not, want not