Sorrel

Sorrel is the startlingly sour leaf of several European relatives of rhubarb and buckwheat that are rich in oxalic acid: Rumex acetosa, scutatus, and acetosella. Cooks use them mainly as a source of acidity, and they also provide a more generic green aroma. Sorrel readily disintegrates with a little cooking into a sauce-like puree that complements fish, but whose chlorophyll turns drab olive from the acidity. The color can be brightened by pureeing some raw sorrel and adding that to the sauce just before serving.