HOW MUCH SULPHITE IS NEEDED?
Neither SO2 nor sorbate kills yeasts; they inhibit them, and can prevent microbial activity, but only if cell counts are low. If you have a mounting problem, they won’t do a good job in controlling it. The amount of sulphite needed depends on the pH of the wine the lower the pH the less you need (at pH 3.2, you need 21ppm (21mg/L) free SO2; at pH 3.5, you need 50ppm (50mg/L) free SO2.) This has to do with A) the fact that the active form that inhibits bacteria forms better at lower pH’s and B) the lower the pH, the better the acidity in the wine is in itself able to protect the wine. The following is the pH dependant equilibrium. The forms depicted in the left are favoured by higher pH’s; the right by lower pH’s.
