Definition - What does Topping Up mean?
Topping up is the process of adding wine to a holding vessel; barrel, carboy, tank, etc., in order to reduce the level of oxygen that is trapped in. This reduces the probability of oxygenation and the spoilage of the wine as the headspace is reduced. Typically, wine needs to be topped off every two weeks to every month, depending on the type of vessel it is in.
WineFrog explains Topping Up
When wine is stored in hold vessels, a small amount of alcohol, or water, evaporation takes place over time. Whether it is alcohol or water, will depend on the humidity in the storage area.
If wine is stored in oak barrels, especially new barrels, the level of wine decreases much faster than if it is held in a carboy or stainless steel container. Oak is porous and thus soaks up the wine itself and/or allows for a greater evaporation rate. For new barrels, wine will need topping up about every 2 weeks. This keeps from the wine getting too much exposure to oxygen which can oxygenate the wine or invite in bacteria which can spoil it.