Originally Posted by T_Baggins
Hmm.. interesting, I hadn't ever heard of adding at intervals... I just use before pitching yeast. As long as all the equipment is sanitized properly during racking I don't see the use for it.
It's used as an antioxidant.
If you use it at every other racking (which guestimates 50 ppm throughout), then the bound S02 prohibits oxygen uptake in the wine.
The most common defect in wine is oxidation, so anything you can do to prevent it is a good thing. It's probably ok in wines that are drunk very young, but it's pretty routine for winemakers to try to keep the sulfite level at 50 ppm.
It dissipates quickly, so if it's added at the beginning, it's long gone before bottling time.