A novel role for non-heme iron in myoglobin oxidation: An examination of the antioxidant effects of iron chelating compounds in meat and myoglobin model systems.

摘要

Myoglobin (Mb) oxidation, and the subsequent browning, is the primary basis for consumer rejection of fresh retail beef. Considerable effort has been directed by the industry towards the development of techniques that can enhance color stability. However, the underlying mechanism of Mb oxidation has been studied extensively, but is still not entirely understood. Current models focus on the role of lipid oxidation by-products, pH, and anionic ligands as promoters of Mb oxidation. It is known that chelation of iron and copper delays Mb oxidation and browning, but a clear role for these metals has not been established in any current Mb oxidation mechanism, other than indirectly as promoters of oxygen radical formation and lipid oxidation.;The objective of the current study was to examine the possibility that iron plays a more direct role in Mb oxidation, and that metal chelators such as milk mineral (MM) and sodium tripolyphosphate can inhibit this action. In Study 1 (Chapter 3), MM, a colloidal calcium phosphate of large molecular weight and undetermined structure, was demonstrated to be a high-affinity iron chelator. This supports previous works studying the antioxidant effect of MM in raw and cooked meats. In Study 2 (Chapter 4), non-heme iron was found to stimulate Mb oxidation even in the absence of lipid, showing for the first time that the role of ferrous (Fe2+) iron was not limited to promoting lipid oxidation, but instead has a yet-to-be determined role as a pro-oxidant factor in Mb oxidation. In Study 3 (Chapter 5), iron was found to promote Mb oxidation under standard atmospheric conditions, while in high oxygen systems this effect was not seen. Previous observations in meats found high oxygen atmospheres have a protective effect. Addition of catalase did not affect Mb oxidation. However, in iron-containing systems, catalase significantly slowed Mb oxidation, while MM addition completely reversed the stimulatory effect of added iron. In Study 4 (Chapter 6), Type I radical-quenching antioxidants were found to rapidly reduce ferric iron to the ferrous form. This strong reducing ability accounted for the pro-oxidant effects of rosmarinic acid and eugenol shown for the first time in a lipid-free Mb model system. Finally, the effect of Type I antioxidants was compared to Type II antioxidants in raw ground beef. It was found that Type I antioxidants were highly effective at preventing Mb oxidation in the presence of lipid. Of the Type II chelators examined, only MM was able to delay Mb oxidation as well as the Type I antioxidants, possibly because it is not as susceptible to enzymatic hydrolysis.