Clear and white. The nickname of this dish with onion and tofu is clear and white. Onion mixed with tofu is an ordinary home-cooked dish, which many people like to eat. But this way of eating is not scientific. Because tofu is rich in nutrients such as protein and calcium. Onions contain a lot of oxalic acid. When tofu is mixed with onion, the calcium in tofu combines with oxalic acid in onion to form white precipitated calcium oxalate, which destroys the calcium in tofu.