It is the outer protective layer of a fruit. The peel of citrus fruits is bitter and generally not eaten raw, but is used in cooking. The outermost, coloured part of the peel is called the zest, which can be scraped off and used for its tangy flavour. The fleshy white part of the peel is bitter when raw, but becomes sweet when candied, a process involving boiling with sugar. This recipie is extremely adaptable to many citrus fruits: oranges, lemons, limes, and grapefruits all work well.