There are two basic categories of color compounds: conjugated polyenes and metalloporphyrins. The former includes carotenoids, annatto, anthocyanins, betanain, dyes, and lakes. The effect of conjugation is lowering the n-n* transition energy from the highest occupied molecular orbital to the lowest unoccupied molecular orbital. Increased conjugation in the molecule shifts the absorption maximum to a higher wavelength. Substituent groups with lone pairs of electrons tend to increase n conjugation by resonance.

Carotenoids consist of a basic structure of eight repeating isoprene units that are highly conjugated. In the conjugated system, the terminal double bond has the highest electron density and is most susceptible to oxidative attack. Degradation proceeds from the end to the center of the molecule, resulting in a progressive shortening of the polyene chain (53).

Anthocyanins are flavonoid compounds characterized by the flavylium nucleus. The flavylium form is stabilized by resonance with the positive charge delocalized throughout the entire structure, making the anthocyanin molecule intensely colored. The structural transformation of anthocyanins in aqueous medium was thoroughly investigated in the early 1980s. Flavylium salts exist in equilibrium in different forms: flavylium cation (AH +), quinoidal base (A), carbinol pseudobase (B) and chalcone (C). The equilibrium between AH+ and A involves the transfer of proton from the C5, C7, or C4' hydroxyl groups to a water molecule. In the hydration reaction, the water molecule is preferentially added to the C2 of the pyrylium ring of AH+, resulting in colorless B. The conversion of B to C is a base-catalyzed tautomerization (Fig. 8). Because most isolated natural anthocyanins when placed in slightly acidic medium (pH 3-6) exist largely in the colorless forms (B and C), these fundamental studies should have practical implications. Evidence indicates that substitution pattern at various positions of the anthocyanin molecule influences the equilibrium constants of these reactions and hence the distribution of the colored (AH+, A) and colorless (B, C) species (54).

The two best known examples of metalloporphyrins found in food are the myoglobin and chlorophylls. A porphyrin metal complex possesses 19 re-electrons in an 18-atom ring. The main effect of the metal on the transitions is the conjugation of the metal ¡m orbital with the porphyrin n orbital (55). The splitting of the S orbitals of the metal ion (due to the porphyrin) exhibits additional loss of degeneracy from the theoretically predicted octahedral symmetry. In oxymyoglobin heme complex the iron coordination positions are directed to the four porphyrin nitrogens, and in the fifth and the sixth positions, to histidine F8 and 02 (or H20), respectively. The role of the protein globin is to stabilize the steric and electronic configuration of the iron heme, and to facilitate the back-bonding of electrons from the iron to the n* orbital of the oxygen (Fig. 9). The hydrophobicity of the heme pocket excludes the binding of ionic ligands such as CN~, OH~, and the closely packed amino acid side chains restrict the size and orientation of the ligand (56).

Chlorophylls, unlike myoglobin which contains a transition metal ion, are porphyrins complexed with the alkali earth metal Mg++. The compounds are hydrophobic because of the a long chain C20 phytol esterified to the propionic acid side chain at C7. The magnesium atom of chlorophyll can be readily replaced by weak acids or other metals such as copper, zinc, and iron. The free base obtained after removal of the metal is pheophytin, with the transition intensity shifted to a lower wavelength. Hydrolysis of the phytyl chain by alkali or enzyme yields the chlorophyllides, which are water-soluble and green-colored, with spectral characteristics similar to those of the chlorophylls. Removal of magnesium from the chlorophyllides yields pheophorbides, which have the same spectral properties as those of the pheophytins. These types of conversion have been frequently implicated as causes of color loss in processed green vegetables (57,58). Copper complexes of pheophytin and clorophyllide are stable to acid and are used as food colors in some European countries (59).

There is a considerable interest in colorants from natural resources. Cape jasmine (Gardenia jasminoides) have been investigated for the production of carotenoid and cro-cin. The flower is also rich in flavonoids. Another example is the mold Monascus purpureus, which has been used in Asian countries for centuries. The pigments produced are a mixture of red, yellow, and purple polyketides (60). Pigments extracted from algae, yeasts, and insects have also

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