By Harold McGee
When fresh tomatoes are cooked down to make a thick sauce, they gain some flavors—notably rose- and violet-like fragments of the carotenoid pigments—but they lose the fresh “green” notes provided by unstable fragments of fatty acids and by a particular sulfur compound (a thiazole). Because tomato leaves have a pronounced fresh-tomato aroma thanks to their leaf enzymes and prominent aromatic oil glands, some cooks add a few leaves to a tomato sauce toward the end of the cooking, to restore its fresh notes. Tomato leaves have long been considered potentially toxic because they contain a defensive alkaloid, tomatine, but recent research has found that tomatine binds tightly to cholesterol molecules in our digestive system, so that the body absorbs neither the alkaloid nor its bound partner. It thus reduces our net intake of cholesterol! (Green tomatoes also contain tomatine and have the same effect.) It’s fine, then, to freshen the flavor of tomato sauces with the leaves.