We're talking about different things here.
In brewing water, the sulfite reacts with the chlorine/chloramines, and the chlorine disappears as does the sulfite- that's a chemical reaction.
In adding sulfites to cider/wine/mead, they don't react and off-gas. Instead, they very very slowly dissipate. Winemakers will typically try to keep the sulfites at 50 ppm, and so adding sulfites at every other racking is a guestimate as to that level.
Sulfite works as an antioxidant and preservative in wine, although it is also beneficial in sanitizing must and killing wild yeast and bacteria. Wine yeast is very tolerant of sulfites, so it doesn't kill wine yeast. That's why winemakers use sulfites successfully.
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