Lycopene is a bright red carotenoid pigment found in tomatoes and other red fruits, and is the most common carotenoid in the human body. Its name is derived from the tomato's species classification, Lycopersicon esculentum. Ironically, the highest natural concentrations of lycopene in food are found not in tomatoes, but in watermelon.
Due to its ubiquity, lycopene has been licensed for use as a food coloring. Research has shown it is a powerful antioxidant, with associated health benefits. The potency of lycopene as an antioxidant is increased by food processing. For example cooking tomatoes (as in the canning process) increases the bioavailability of lycopene and promotes isomerisation to antioxidant forms of the chemical.
Lycopene is not water-soluble and instantly stains any sufficiently porous material, including most plastics. While a tomato stain can be fairly easily removed from fabric (provided the stain is fresh), discolored plastic defies all efforts to drive out the lycopene with hot water, soap, or detergent. (Bleach will destroy lycopene, however.) Plastics are especially susceptible to staining if heated, scratched, oiled, or pitted by acids (such as those found in tomatoes).
The color of lycopene is due to its many conjugated double carbon bonds (known as chromophores ). Each double bond reduces the energy required for electrons to transition to higher energy states, allowing the molecule to absorb visible light of progressively longer wavelengths. Lycopene absorbs most of the visible spectrum, so it appears red.
If lycopene is oxidized (for example, by reacting with bleaches or acids), the double bonds between carbon atoms will be broken, cleaving the molecule into smaller molecules each double-bonded to an oxygen atom. Although C=O bonds are also chromophoric, the much shorter molecules are unable to absorb enough light to appear colorful. A similar effect occurs if lycopene is reduced; reduction may saturate and join lycopene molecules, converting the double bonds to single bonds as a result.
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