Calcium propionate is an organic salt formed by the reaction of calcium hydroxide with propionic acid (also known as propanoic acid). The Center for Science in the Public Interest indicates that calcium propionate is added to bread products to prevent mold growth. Another derivative is sodium propionate, which is often used in baked goods since the calcium may alter the effect of other materials used in making pies or cakes.
Calcium propionate has been studied extensively for potential toxicity and for any possibility that it might cause cancer. All the laboratory findings were negative, except for one study involving young, vitamin B-12 deficient rats. The study findings suggested that massive amounts of calcium propionate could result in a decrease in growth rates during the study. But, the amount used was many times more than you could ever get from a standard diet - plus, it's not known how much the vitamin B-12 deficiency had to do with those results.
In other laboratory studies, rats fed large amounts of calcium propionate (and similar compounds) over longer periods of time did not appear to have any negative outcomes. So the accumulation of research evidence indicates that calcium propionate is non-toxic and safe to use in the typical amounts currently used by food manufacturers.