Bleaching carotenoid degradation

Carotenoids are present in soybeans in a very low concentration (0.8—3.7 ppm), and the main forms are lutein and P-carotene. They are co-extracted with oil but are often removed or degraded by oil refining steps designed to remove the undesirable minor components that contribute to physical and chemical instability and undesirable color, such as degumming to remove PLs, neutralization to remove free fatty acids, bleaching to decompose lipid hydroperoxides, and deodorization to remove volatile oxidation products. 

The non-specific lipoxygenases can cooxidize carotenoids and chlorophyll and thus can degrade these pigments to colorless products. This property is utilized in flour bleaching (cf. 15.4.1.4.3). The involvement of LOX in cooxidation reactions can be explained by the possibility that the peroxy radicals are not as rapidly and fully converted to their hydroperoxides as in the case of specifically reacting enzymes. Thus, a fraction of the free peroxy radicals are released by the enzyme. It can abstract an H-atom either from the unsaturated fatty acid present (pathway 2a in Fig. 3.30) or from a polyene (pathway 2b in Fig. 3.30). 

A closer examination of the breakdown of each carotenoid in thylakoids, D-10, and D-144 (Fig.3) allows a more specific statement to be made. In thylakoids, neoxanthin violaxanthin, and 6-carotene show essentially the same bleaching kinetic with a significant lag phase whereas lutein degradation starts later and continues slower. Since both PS-II activity (Fig.2) and PS-I activity (data now shown) are inhibited before massive breakdown of pigments occurs, either the bleaching of car is not responsible for the rapid loss in activity, or only very few car and chi molecules are affected during the early phase of the experiment. From the data of Fig. 3, these specific pigments cannot be identified. 

In D-144, again lutein is the most stable carotenoid showing a short lag phase of 30 min. Neoxanthin is also retained during the first 30 min but is then the most rapidly degraded carotenoid. Violaxanthin and 6-carotene are linearly bleached with the onset of illumination. P-700 measurements in D-144 during illumination have shown that the activity of the reaction centre of PS-I is remarkably stable (data not shown) being destroyed only when less than 5% of chi and car remain present within... 

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