Structure And Physicochemical Properties Of Carotenoids

Carotenoids are oligoterpenic lipids most commonly possessing 40 carbons and an extensive conjugated doublebond system. They are classified into two main groups: the hydrocarbon carotenes and the xanthophylls, the latter of which possess one or more oxygen-containing polar functional groups. Carotenoids may be bicyclic, monocyclic, or acyclic. Several common carotenoids naturally occurring in foods or used as color additives are illustrated in Figure 1. The conjugated double-bond system is the most influential structural feature of the carotenoids, affecting both hue and chemical stability. Carotenoids with six or more conjugated double bonds absorb visible light, and absorption maxima generally increase with increasing number of double bonds. Selected physical characteristics of common carotenoids are listed in Table 1.

The two chemical reactivities of carotenoids most relevant to food systems are oxidation and isomerization. Reaction with oxygen, either autocatalytically or enzymati-cally, is the major cause of carotenoid degradation in foods and affects both color capacity and stability. Oxidation can disrupt conjugation and result in bleaching of endogenous or added carotenoid pigments and in formation of off-flavors (5). Oxidative degradation of carotenoids can be initiated by those factors that stimulate oxidation of unsaturated fatty acids, including light, transition metals, and lipoxygenases. Environmental factors that affect the rate of oxidation include oxygen tension, temperature, water activity, and pH (6-9). Linear (zero-order) kinetics of color loss is generally observed in dehydrated foods, whereas first-order kinetics commonly apply to aqueous systems (8,10,11), indicating that water plays a protective role in oxidative decolorization of carotenoids. Carotenoids in plant foods tend to be stabilized by complexation with proteins or polysaccharides, and processes that disrupt the microstructure of foods can result in decreased chemical stability, as discussed later.

Carotenoids are synthesized, either naturally or chemically, predominantly in the all-trans configuration. However, in solution, carotenoids undergo isomerization to yield a variety of cis isomers, and thermodynamic constants have in some cases been determined (12,13). The rate of isomerization increases with temperature and oxygen exposure. Trans-to-cis isomerization generally results in small decreases in absorption maxima, and consequently slight changes in hue.

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