Even if the fruit were inedible, the orange, native to China, would have been grown just for its peel. Historically it was a seasoning on the order of ginger and garlic. As a medicine, the peel was something of a panacea.
There's a tradition according to a 16th century Chinese medical book of "servants, children, ragpickers and others" collecting it, drying it, and selling it to drugstores. The peel of the "coolie orange," also called the Canton orange when it hit Europe, was valued above that of the mandarin (called the tangerine after it grew in Tangiers).
Medicinally, orange peel is supposed to bring down fevers, cure acne, keep the aged in good voice, alleviate coughs, prevent convulsions, rid the body of pin worms, and act as an antidote to fish and shellfish posion. Old and dark peels -- some have been aged for a century or more -- are considered the most effective.
Orange peel is a staple in Sichuan and Fugian Provinces. It's best with meat and poultry; whole pieces added to braised duck or pork dishes with rock sugar and dark soy sauce cut the richness. For stir-fried dishes it may be minced or ground in a spice grinder; or softened in a little shaoxing wine then slivered or minced and added wine and all.
Orange peel, usually labeled "dried tangerine peel," comes in small plastic packages in Chinese groceries, on the shelf with the seasonings; but it's best to dry your own.
Use oranges or mandarins with powerful flavor and fragrant peels. The peppery tangelo or mineola, bright orange and shaped like a hand grenade, is a personal favorite as are various mandarins. Every time you eat the fruit, immediately put the peel, white-side up on a cutting board and with a sharp knife saw away the white pith down to the oil sacks. Toss the peel on a plate and let it dry in or out of the sunlight. Depending on the environment, it should dry in a week or so (it should still be flexible). Store in a jar.
Note: Orange peels does improve with age. I stay a year ahead.
BRAISED PORK WITH CINNAMON AND ORANGE PEEL (6 to 8 servings) 2 tablespoons peanut oil 3 pounds fresh pork butt, cut into1 1/2-inch cubes, fat and all 1/2 cup dry sherry or Shaoxing wine 6 cups boiling water 2-inch stick cinnamon 4 garlic cloves, smashed 6 thin slices ginger 2 to 3 pieces dried orange or tangerine peel 3 tablespoons dark soy sauce 4 approximately 1-inch cubes of Chinese rock sugar or 3 1/2 table-spoons granulated sugar 1 teaspoon salt
Heat a wok or heavy skillet over high heat until very hot. Add the oil and half the pork. Brown the meat lightly and remove with a slotted spoon to a bowl. Don't add oil, but repeat with the other half of the meat. Re-add the meat to the pan with the wine and continue to stir over the heat for a minute or so. Add the boiling water, reduce the heat; add the cinnamon stick, and simmer for 15 minutes.
Add the garlic, ginger, orange peel, soy sauce, sugar and salt; cover and cook for 2 hours or until the meat is tender, checking from time to time. (You may have to add more water). Uncover, turn the heat to high and reduce the sauce, stirring the pork, until the sauce is syrupy. (There should be little liquid left). Serve surrounded by steamed or stir-fried fresh spinach or heart of bok choy, and rice.