METAL CATALYZED OXIDATION Alejandra Ochoa Food Chemistry April 21, 2016

What is oxidation? ■Degradation of lipids in foods ■Occurs in mono and polyunsaturated fats ■Will cause off-flavors and staling in food; may also affect appearance

Initiation: RH + O2 -->R· + ·OH R· + O2 --> · + ROO· Propagation: ROO· + RH --> R· + ROOH ROOH--> RO· + HO· Termination: R· + R· --> RR R· + ROO·--> ROOR ROO· + ROO· --> ROOR + O2 Hydroperoxides are formed; very unstable and will breakdown into two free radicals

Causes of Oxidation ■Hydroperoxide decomposition ■Light ■Heat ■Oxygen form and availability ■METALS

Sources of metals ■Myoglobin in meat ■Hemoglobin in blood ■Magnesium in chlorophyll

Metal Catalyzation ■Metals act as pro-oxidants ■Metals will act as catalysts, speeding up propagation reaction. They will not initiate oxidation ■Can also catalyze autoxidation of antioxidants ■Problem in foods containing transition metals or fortified foods –Copper –Iron (Fe 2+ and Fe 3+ ) ■Attracted to negatively charged emulsions and surfactant agents ■Function by catalyzing free radical oxidation and catalyzing decomposition of lipid hydroperoxides ■Tested by peroxide values and TBARS

Factors that will affect level of oxidation ■Level of unsaturation in fatty acids ■pH of food system/electrochemical charges in food system ■Moisture ■Oxidative state of the metal group

How to decrease metal catalyzed oxidation ■Type of metal –Antioxidant or time repressed ■Lower charge repulsion (as close to pI) = less surface area ■Positively or neutral surfactant/emulsifying agents ■Water ■Metal chelators –Oxalic acid has high affinity to chelate transition metals –Citric acid sequesters and chelates metal ions ■Antioxidants** –Usefulness dependent on metal (citric acid vs. tocopherol) –Fenton reactions ■Cu + + H 2 O 2 -> Cu 2+ + OH * + OH-

References ■Kapchie, VN., Yao, L., Hauck CC., Wang, T., Murphy, PA. Oxidative stability of soybean oil in oleosomes as affected by pH and iron. Food Chemistry, 141 (2013), pp ■Fomuso, LB., Corredig, M., Akoh, CC. Metal-Catalyzed Oxidation of a Structured Lipid Model Emulsion. Journal of Agricultural and Food Chemistry, 50 (2002), pp – ■Repetto, M., Semprine J., Boveris, A. Lipid Peroxidation: Chemical Mechanism, Biological Implications and Analytical Determination. Intech, (2012) Chap 1, pp ■Utera, M., and Estevez, M. Analysis of tryptophan oxidation by fluorescence spectroscopy: Effect of metal-catalyzed oxidation and selected phenolic compounds. Food Chemistry, 135 (2012), pp ■Akbiyik, T., Sonmezoglu, I., Guçlu, K., Tor, I., Apak, R. Protection of ascorbic acid from copper (II) – catalyzed oxidative degradation in the presence of fruit acids: citric, oxalic, tartaric, malic, malonic, and fumaric acids. International Journal of Food Properties, 15 (2012), pp ■Labuza, TP., Silver, M., Cohn, M., Heidelbaugh, ND., Karel, M. Metal-catalyzed oxidation in the presence of water in foods. Journal of the American Oil Chemists’ Society, (1971), pp

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