Tartaric acid crystals, polarized light micrograph. Tartaric acid is an organic acid that occurs naturally in grapes, bananas, and tamarinds. It is used in the manufacture of fizzy drinks, and as a component of baking powder. Other uses of tartaric acid include medicine, dye-making and the silvering of mirrors. Dark field illumination. Magnification: x140 when printed at 10 centimetres wide.