Zur Chemie der Oxidation phenolischer Verbindungen

Phenolic compounds, a group of secondary plant substances, have gained much attention because of their proposed positive effects on human health such as the antioxidant activity and cancer prevention. During food processing, phenolic compounds are modified by chemical, enzymatic, and thermal processes in which oxidations are of significance. In this regard, the conversion of phenol into quinone is an important step that has only been postulated in many studies. Therefore, in this work, two different trapping reactions for the determination of reactive quinoid intermediates are developed and the influence of the chemical structure on the oxidation is investigated. Quinoid intermediates can either be detected indirectly via redox reaction with benzolsulfinic acid (BSS) or quinones can be trapped and determined directly by using o-(2,3,4,5,6-pentafluorobenzyl)-hydroxylamine. With both trapping reactions a great number of quinoid intermediates from a variety of phenolic compounds is verifiable. Furthermore, both methods are applicable to food matrices, too, here tested with potatoes and onions (inner and outer layers). The oxidation is initiated by different oxidants which show an important influence on the degradability of phenols and the formation of oxidation products. Thus, the oxidation via laccase leads to the highest loss of phenols and thermal forced oxidation as well as base catalysis show the lowest oxidation potential. The highest amounts of quinones (trapping adducts) are detected with FeCl3-mediated and Fenton oxidation, whereas enzymatic oxidation results in a great number of various oxidation products. Beside organic acids, which are evidenced by Fenton reaction, a lot of polymer compounds are formed (up to degree 6 of polymerisation), especially in enzymatic oxidation, which are investigated via colour formation and LC-MS. Some possible reaction mechanisms of base catalysis are given because of the results from LC-MS-analysis, too. Through the use of variously substituted phenolic compounds, differences in the oxidation behaviour can be noticed. Thus, e. g. methoxylation or a nearby carboxyl group leads to a stabilisation of phenols. If the formation of a quinoic structure is impeded, due to factors like a glucose molecule in B-ring of flavonoids, the degree of oxidation and the formation of quinones are minimised.