Potassium metabisulfite

Potassium metabisulfite, K₂S₂O₅, is a white crystalline powder with a pungent sulfur odour. The main use for the chemical is as an antioxidant or chemical sterilant. It is a sulfite and is chemically very similar to sodium metabisulfite, with which it is sometimes used interchangeably. Potassium metabisulfite is generally preferred out of the two as it does not contribute sodium to the diet.

Potassium metabisulfite has a monoclinic crystal structure which decomposes at 190°C, yielding potassium oxide and sulfur dioxide:

K₂S₂O₅(s) → K₂O(s) + 2SO₂(g)

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Uses

Wine

Potassium metabisulfite is a common wine or must additive, where it forms sulfur dioxide gas (SO₂). This both prevents most wild microorganisms from growing, and it acts as potent antioxidant, protecting both the color, and delicate flavors of wine.

Typical dosage is 1/4 tsp potassium metabisulfite per 6 gallon bucket of must (yielding roughly 75 ppm of SO₂) prior to fermentation, and 1/2 tsp per 6 gallon bucket (150 ppm of SO₂) at bottling.

Winemaking equipment is sanitized by spraying with a 1% SO₂ (2 tsp potassium metabisulfite per L) solution.

Beer

Potassium metabisulfite is sometimes used in the brewing industry to inhibit the growth of wild yeasts, bacteria, and fungi. This is called 'stabilizing'. It is used both by homebrewers and commercial brewers alike. It is not used as much for brewing beer, because the wort is almost always boiled, which kills most microorganisms anyway. It can also be added to strike water (the water used to mash the barley) in order to remove chloramines which can cause phenolic off flavors in beer. (A.J. DeLange)

See also

• Campden tablets
• Sulfite
• Sodium metabisulfite