By Harold McGee
The intensely flavored citrus peel has long been used to flavor dishes (for example, dried orange peel in Sichuan cooking), and as a preparation in itself in the form of candied rind. The outer epidermis contains the aromatic oil glands, while the underlying white, spongy, pectinrich albedo usually contains protective bitter phenolic substances. Both the oil with its terpenes and the antioxidant phenolics are valuable phytochemicals. The bitters are water-soluble, while the oils are not. Cooks can therefore leach the peel repeatedly with hot (rapid) or cold (slow) water to remove the bitter compounds, then gently cook the peel if still necessary to soften the albedo, and finally infuse it with a concentrated sugar syrup. Through all this processing the water-insoluble oils stay largely in the rind. Marmalade, a sugar preserve that includes citrus peel, was originally a Portuguese fruit paste made with quince, but by the 18th century the highpectin, readily gelled sour orange had begun to replace the quince. Marmalade made with sweet oranges doesn’t gel as readily and lacks the characteristic flavor, including a bitterness that helps balance the sugar.