Bakers searching for traditional fruitcake and Christmas pudding recipes will quickly find that many of the best come from sources in Britain. That means some of the instructions require some translation. Many websites can help you with the necessary metric conversions, but ingredients can sometimes require more research. For example, the "mixed peel" called for in many recipes is better-known here as candied fruit peel or glaceed fruit.
As any marmalade fan knows, the rinds of citrus fruits have a concentrated flavor. That's why the zest of citrus fruit is often called for as a flavoring ingredient in its own right. In the days before citrus fruits became inexpensive and readily available, frugal cooks would often save their peels as a baking ingredient by simmering them in a heavy sugar syrup. The sugar acts as a preservative, reducing the available water in the peels until they can't support mold or bacterial growth any more.
Commercial Mixed Peel
Commercial mixed peel varies widely in quality, but most uses the same handful of basic ingredients. The peels of oranges and lemons are included in most blends, because they're inexpensive, flavorful and readily available. Many also include citron, a citrus fruit that provides no usable flesh but a great deal of fragrant peel. Many brands include pieces of other fruit, cut to the same approximate size and shape as the pieces of peel. These aren't peel, of course, but they add complementary flavors and colors. Pineapple and cherry pieces are the most common of these additions.
Using the Peel
The classic use of mixed peel is in holiday baking. Fruitcakes and Christmas puddings were traditionally made well in advance of the holiday, so any additions to the batter had to be suitable for long-term storage. Candied peel, because it was permeated with sugar syrup, was ideal for the purpose. Chop the peel into finer pieces if necessary, oiling the blade of your knife to prevent it from sticking, and add it to either the batter or the dry ingredients, as your recipe directs. Mixed peel also makes an interesting accent in white cakes, tarts, cookies and other baking.
Making Your Own
Making your own mixed peel is not complicated, though it does require some time. Cut your orange and lemon peels into long strips, approximately 1/8 inch wide, and place them in a heatproof bowl. Cover them with boiling water and soak them for 10 minutes, then drain them and repeat the process. This quick blanching removes bitter oils from the peels. Simmer the peels in a heavy sugar syrup for two to three hours, until they're completely translucent. Strain the fruit from the syrup and let it dry on parchment-lined baking sheets, then pack it into airtight containers for later use.