The calcium content is higher in parboiled rice, even without intentional enrichment, because calcium carbonate is typically used as a milling aid with parboiled rice. Bran removal is more difficult in parboiled rice so an abrasive material, like calcium carbonate (ground limestone), is added to the brown rice as it enters the milling process. Most of the calcium carbonate exits with the bran stream but naturally, some stays on the surface of the milled rice.