Wondering what causes that fuzzy residue to sit on your teeth when you eat spinach? The oxalic acid found in spinach binds with calcium and iron to produce this coating which actually protects your teeth in a manner similar to topical pastes used to treat tooth sensitivity. But acid is bad for your teeth, right? Not in this case. The pH of oxalic acid in cooked spinach is actually close to neutral and therefore is not harmful to your enamel.