Potassium metabisulfite, Potassium pyrosulfite, K-meta, or the active ingredient in Cambden tablets. It is chemically very similar to Sodium metabisulfite, and they are both used mostly interchangeably. Sodium metabisulfite is a sterilizer and antioxidant/preservative. ...

Uses

Brewing

Potassium metabisulfite is used in the brewing industry to inhibit the growth of wild yeasts, bacterias, and fungi. This is called 'stabilizing'. It is used both by homebrewers, and commercial brewers alike.

It is sold either as a white powder, or as cambden tablets. The powder is difficult to use for small batchs of wine or beer. This is because it inhibits the action of all yeasts, wine yeasts are just more resistant than other microorganisms. If too much Potassium metabisulfite is added it will result in a 'stuck' fermentation. The typical amount is around 1/4-1/2 teaspoon per 5 gallons. So for batches smaller than this Cambden tablets are normally used.

Chemistry

It is a white odorless compound, having a monoclinic crystal structure, and a density of 2.34 g/cc. It has the chemical formula of K₂S₂O₅,and decomposes at 190 degrees Celsius, yielding (turning into) 1 K₂O (Potassium monoxide), and 2 SO₂ (Sulfur dioxide) molecules. The Potassium monoxide is a solid salt, and remains in the must. The Sulfur dioxide is a gas, which is dissolved in the must initially, but it quickly dissipates. Similar to how the Carbon dioxide, CO₂ escapes from a can of pop, when opened.

When added to must, or wort it produces Sulfur dioxide gas (SO₂). This both inhibits nearly all wild microorganisms from growing, and it acts a potent anti-oxident, protecting both the color, and delicate flavors of wine. It is not used as much for brewing beer, because the wort is almost always boiled, which kills most microorgisms anyway. It is essential to winemaking though, because the must is usually not boiled, as this rids it of most of its aroma, and flavor. Boiling also sets the pectin in fruit juice, usually resulting in a 'pectin haze' in the finished wine, that is difficult to remove. Must is the juice of freshly pressed grapes, prior to fermentation into wine. ... WORT 89. ...