Polyphenol oxidation is a chemical process impairing food freshness and other desirable qualities, which has become a serious problem in fruit and vegetable processing industry. It is crucial to understand the mechanisms involved in these detrimental alterations. o-Quinones are primarily generated by polyphenols with di/tri-phenolic groups through enzymatic oxidation and/or auto-oxidation. They are highly reactive species, which not only readily suffer the attack by nucleophiles but also powerfully oxidize other molecules presenting lower redox potentials via electron transfer reactions. These reactions and subsequent complicated reactions are capable of initiating quality losses in foods, such as browning, aroma loss, and nutritional decline. To attenuate these adverse influences, a variety of technologies have emerged to restrain polyphenol oxidation via governing different factors, especially polyphenol oxidases and oxygen. Despite tremendous efforts devoted, to date, the loss of food quality caused by quinones has remained a great challenge in the food processing industry. Furthermore, o-quinones are responsible for the chemopreventive effects and/or toxicity of the parent catechols on human health, the mechanisms by which are quite complex. Herein, this review focuses on the generation and reactivity of oquinones, attempting to clarify mechanisms involved in the quality deterioration of foods and health implications for humans. Potential innovative inhibitors and technologies are also presented to intervene in o-quinone formation and subsequent reactions. In future, the feasibility of these inhibitory strategies should be evaluated, and further exploration on biological targets of o-quinones is of great necessity.
Browning is a severe problem in the juice industry, which is tremendously affected by quinone (Q) reactivity. Here, the formation rate, electrophilicity, and oxidizing ability of different quinones were investigated. LC/MS was applied to monitor the loss of polyphenols and quinones in model systems during storage. Mathematical modeling of collected LC/MS data was conducted to derive the rate constants of various reactions. Polyphenols containing catechol or pyrogallol groups are more susceptible to oxidation. Periodic acid and quinone mediated oxidation were faster than the autoxidation of polyphenols. Chlorogenic acid (CQA) and 4‐methylcatechol (4MC) only containing B ring can hardly contribute to browning. They must combine with A ring to form yellow polymers. Furthermore, the electrophilicity and oxidizing ability order of quinone is: CQA‐Q ≈ 4MC‐Q > catechin quinone (CAT‐Q) ≈ epicatechin quinone (EC‐Q) > gallic acid quinone (GA‐Q). It was possible that CAT‐Q and EC‐Q would undergo nucleophilic addition with their A rings and GA‐Q's extra hydroxyl group might contribute to the electrons, resulting in lower reactivity. This work innovatively evaluated the reactivity of diverse polyphenols and quinones, attempting to illustrate their contributions to nonenzymatic browning. These findings provide a new perspective to restrain the browning in the juice processing industry.
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