There are two uses for potassium metabislfute:
BEFORE FERMENTATION: Works as a sanitizer to control spoilage bacteria and indigenous wild yeast that may already be present on the fruit and in the winery.
AFTER FERMENTATION: Works as a stabilizer, preservative, and anti-oxidant. Helps preserve color and clarity.
Typical usage prior to fermentation: Use 1/16 teaspoon per gallon to sanitizate grapes or fruit in preparation to make grape wine, fruit wine, or cider.
Typical usage after fermentation: Use 1/8 teaspoon per gallon to stabilize and preserve grape wine, fruit wine, or cider.