Calcium carbonate, or precipitated chalk, is used to reduce acidity in wine or must. It should be used only after adequate bench testing and with proper equipment to measure both pH and titratable acidity. When considering the use of calcium carbonate, serious consideration should be given to an easier to use product such as Acidex, the reason being that calcium carbonate binds preferentially to tartaric acid, rather than other acids in the must/juice, with the result being that only partial de-acidification will occur and acid imbalance is a real possibility.
When calcium carbonate is used, it is best to use it early in the winemaking process in order to give enough time for tartrates to re-stabilize.
2.5 grams per gallon is the recommended dose and will reduce acidity by approximately 0.1%. In volumetric terms, 5/8 teaspoon per gallon will lower acidity .15%. Use before fermentation. Do not reduce acid more than .30%.