Common inhibitors of calcium absorption are phytates. Phytic acid is a substance commonly found in plant-based foods, and the salts formed between calcium and phytic acid cannot be absorbed. Similar to phytic acid, oxalic acid forms insoluble calcium oxalate with calcium, which cannot be absorbed by the intestines. To reduce the loss of calcium from food, vegetables high in oxalic acid, such as spinach and amaranth, should be blanched in boiling water before cooking to reduce their oxalic acid content. Foods high in calcium should generally not be cooked with vegetables; therefore, using tofu to cook spinach for calcium supplementation is a misconception.
Cereals contain phytase, an enzyme that can break down phytates to release calcium, thereby increasing calcium's bioavailability. Phytase is most active at 55°C. Therefore, to increase phytase activity, rice can be first soaked with an appropriate amount of water before washing.
In flour and cornmeal, adding a leavening agent and extending the fermentation time can both promote the hydrolysis of phytic acid, leading to an increase in free calcium.