Spinach contains oxalic acid, which is released when you chew it. “When the calcium in your saliva combines with the oxalic acid, calcium oxalate crystals are created,” says Jennifer Moltoni, administrative coordinator at the Department of Oral Medicine, Infection and Immunity at the Harvard School of Dental Medicine.
Because calcium oxalate is not soluble, explains Moltoni, “it deposits on your teeth, which results in the gritty, spinach-mouth feeling.” An interesting note: The sensation worsens if you drink milk while eating spinach, since this introduces even more calcium into the mix.
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