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What constitutes the zest of a citrus fruit?

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Gourmet chefs recognize the value of using the zest of a citrus fruit, whether a lemon, orange, lime or some other, in many of their restaurant dishes. They add color; they impart a dimension of flavor. Most of us probably automatically assume the zest of the fruit is its skin. But is it really that simple?

The anatomy of a citrus fruit

The anatomy of a citrus fruit includes a pithy white ‘center column’ surrounded by pulp – the segments or carpels containing tiny juice-filled vesicles. It is this pulpy section that people ordinarily eat when they consume a piece of citrus fruit.

Encasing the pulp is the whitish ‘albedo.’ The albedo is commonly called the ‘pith.’ Just outside the albedo lies the ‘flavedo’ or colored part of the citrus peel.

Magnifying the flavedo

The flavedo or zest of a citrus fruit, upon closer inspection, features a number of layers. The very thin outer part, visible to the eye, is called the cuticle. Pores in the cuticle control the metabolite respiration processes. The cuticle consists of a polymer matrix embedded with long-chain alcohols, alkane waxes and triterpenoids, including pale-yellow squalene.

Just below the cuticle or epidermis of a citrus fruit lie other colorful cell layers -- the hypodermal layers -- one atop another. The cuticle and hypodermal layers of a citrus fruit contain volatile oils, but relatively little acid. These oils are responsible for imparting a different flavor than citrus pulp does, and adds a bitter principle to foods.

References:

University of Florida - Indian River research and education center: Peel morphology and fruit blemishes

University of California at Los Angeles: Give me a squeeze

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