Gypsum is a natural form of calcium sulfate, CaSO4, with varying degrees of water of crystallization (usually 2 H20). It is relatively insoluble in water and is the main constituent of permanent hardness in water. When in the crystalline or dry powder form, it is also known as “Alabaster” or “Plaster of Paris.”
In brewing it is perhaps best known as the main mineral in the well water of Burton Upon Trent, England, and is widely attributed to be the essential component of the water for pale ale and India pale ale brewing; as such it is the main ingredient in the Burtonizing salts for brewing water treatment.
Gypsum’s positive effects are to reduce wort pH, improve malt extraction efficiency through enhanced amylolytic activity, give a buffering capacity to the wort, balance the hop flavor for highly hopped beers, improve wort clarity, and remove phosphates and proteins in the wort trub. However, this latter effect of removing phosphate ions (as insoluble calcium phosphate) can be overdone and adversely affect the fermentation if the wort is too depleted of phosphate ions. Similarly a high sulfate content in brewing water and wort can affect the beer flavor, producing the famous Burton stench (sometimes also known as the “Burton snatch” or “sulfur-bite”—a distinctive smell of hydrogen sulfide, which, when concentrated, becomes reminiscent of rotten eggs).
The amount of calcium sulfate required or added depends on the base water calcium sulfate content. Brewing water with 150 to 1200 mg/l of calcium sulfate is typical but will be varied according to the wort strength and beer type to be brewed.