Why does spinach leave a chalky residue on teeth?
Spinach contains oxalic acid, which is released when the cell walls are ruptured, first by cooking and later by chewing. The oxalic acid combines with the calcium in saliva (and in the spinach itself) to form tiny calcium oxalate crystals that cling to the teeth, leaving behind a dry, dusty sensation. Not surprisingly, we found that the effect is exaggerated when the spinach is combined with dairy, since milk and cheese products contain far more calcium than saliva does to combine with the oxalic acid. If you are especially sensitive to this phenomenon, you could try eating your spinach raw, since without the rupturing of cells that occurs during cooking, there will be less oxalate available to form the oxalate crystals.