Calcium citrate is the calcium salt of citric acid. It is commonly used as a food additive (E333), usually as a preservative, but sometimes for flavor. In this sense, it is similar to sodium citrate. Calcium citrate is also used as a water softener because the citrate ions can chelate unwanted metal ions. Calcium citrate is also found in some dietary calcium supplements. Calcium makes up 21% of calcium citrate by weight.
Calcium citrate is an intermediate in the isolation of citric acid from the fermentation process by which citric acid is produced industrially. The citric acid in the broth solution is neutralized by calcium hydroxide, precipitating insoluble calcium citrate. This is then filtered off from the rest of the broth and washed to give clean calcium citrate.
The calcium citrate thus produced may be sold as-is, or it may be converted to citric acid using dilute sulfuric acid.