Calcium lactate and calcium lactate gluconate are white crystalline salts made from the reaction of lactic acid and calcium carbonate. It is often times found in hard cheeses as aging and ripening occur and is sometimes added to foods such as cantaloupe to increase firmness and promote shelf stability. In medicine it is used as an antacid and calcium supplement.
In Modernist cooking it is most often associated with the spherification process where its reaction with sodium alginate forms a gel like membrane. In basic spherification, the calcium lactate is added to the liquid that will form the spheres whereas in reverse spherification it is added to the water bath the spheres will ‘cook’ in.
When used in Basic Spherification it is typically it is typically added at a 0.5% ratio whereas in Reverse Spherification it is added at a 3.0% ratio.
You can typically find it here