Calcium propionate (along with propionic acid and sodium propionate) is used as a preservative in bread and other baked goods. It also occurs naturally in butter and some types of cheese.
It keeps bread and baked goods from spoiling by preventing mold and bacterial growth. You may be concerned about the idea of preservative use in food, but on the flip-side, you certainly don't want to eat bacteria- or mold-infested bread.
Calcium propionate has been studied extensively for toxicity and as a potential cancer-causing agent in the laboratory, and all the findings were negative. Only young and vitamin B-12 deficient rats that were fed enormous amounts of calcium propionate (many times more than is possible from a normal diet) in one study showed a decrease in growth rates.
Rats fed large amounts of calcium propionate and similar chemicals over longer periods of time have not shown any negatives effects. The accumulation of research evidence shows that calcium propionate is non-toxic and safe to use in the typical amounts currently used by food manufacturers.
There are anecdotal claims that some people who are sensitive to calcium propionate may suffer from migraine headaches triggered by exposure to foods that contain the preservative, but there doesn't appear to be any medical research that backs that claim.
Calcium propionate does not build up in any body tissues. After you eat food with calcium propionate, the body splits the calcium apart from the propionic acid, which is easily absorbed and metabolized like all other fatty acids.
Calcium Propionate. United States Food and Drug Administration. Accessed July 27, 2009. http://www.accessdata.fda.gov/scripts/fcn/fcnDetailNavigation.cfm?rpt=scogsListing&id=62
CHEMINFO: Calcium Propionate. Canadian Centre for Occupational Health and Safety. Accessed July 27, 2009. http://www.intox.org/databank/documents/chemical/calcprop/cie654.htm