Unbuffered pure ascorbic acid (vitamin C). Used in wine and beer to retard oxidation, or browning. A strong anti-oxidant, ascorbic acid can be used when racking to prevent oxidation, and the resulting browning. It will provide protection for a few days, after which point any unused ascorbic acid will become inactive. Often used in conjunction with metabisulfite. It will raise the acidity.
Usage: 1 tsp per 25 gallons, or just under 1/4 tsp for 5 gals.