This article is from the WineMaking FAQ, by malak@CAM.ORG (Don Buchan) with numerous contributions by others.
Topping up your wine is the process of making your carboy as full of
wine as possible to make sure that there is as small a contact with air
as possible, therefore minimizing oxidation risks.
Some suggestions follow:
A) Make more than five gallons, particularly if you're using fresh
fruit; when racking, squeeze the pulp to get the liquid out to maximize
wine volume to begin with. Keep the extra must in the fridge until
B) Add water. This can change the sweetness and acidity of your wine.
C) Add a honey/water mixture.
D) Top off with some commercial wine of the same type as you're making.
This will keep the taste from being watered down.
E) Use an inert gas such as CO2. This can be gotten from a supplier, or
if you have access to it, use food grade dry ice. Some suppliers also
have cans of inert gas used to top up bottles of wine. CO2 can be made
by mixing baking soda and vinegar -- but -- only pour off the CO2 gas
on top, don't actually pour in the liquid!
F) Add clean and sanitized marbles or aquarium gravel to reduce the
amount of room in the carboy so the wine is closer to the neck.
When you do rack and you introduce something to your wine to top it up,
add some sulphite. Sulphite also helps reduce oxidation and will help
inhibit any bacteria introduced when racking.