Carotenoids are susceptible to oxidative degradation and isomerization resulting from storage and processing conditions. These reactions result in both loss of color and biological activity and formation of often unpleasant volatile compounds. Degradation occurs upon exposure to oxygen and is accelerated by the presence of substances such as metals, enzymes, unsa-turated lipids, and prooxidants; exposure to light; and conditions that destroy cell wall and ultrastructural integrity. Heating can promote isomerization of the naturally occurring all-trans to various cis isomers. This process then affects bioavailability of the carote-noid. Processing also affects bioavailability by macerating tissues, destroying or weakening cell ultrastructure, denaturing or weakening complexes with proteins, and cleaving ester linkages, thereby releasing carotenoids from the food matrix.
Processed foods are frequently fortified with car-otenoids to increase nutritive value and/or enhance attractiveness. For example, annatto, an extract from the seeds of the Bixa orella tree, containing the carotenoids bixin and norbixin, is added to butter, margarine, and processed cheese to give a yellow-orange color to these products. Tomato oleoresin is added to processed tomato products, increasing lycopene content while enhancing their attractive red color.
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