AUTOOXIDATION of LIPIDS

Activation Energy for Food Chemical Reactions

Reactions Activation Energy (Kcal/Mole)Nonenzymatic Browning Reaction 50Enzyme Catalyzed Reaction 10-15Singlet Oxygen Oxidation1-2

Flavor Quality Consumer Acceptance Nutritional Quality Essential Fatty AcidsEssential Amino Acids Vitamins Health Risks Growth Retardation Heart Diseases Carcinogens Oxidation of Foods

Oxygen History

1772 Scheele & PriestleyDiscovered Oxygen 1811 AvogadroOxygen is a diatomic molecule 1848 Faraday Oxygen is a paramagnetic molecule 1934 Herzberg Discovered singlet oxygen 1964 Foote & Wexler Singlet oxygen in photooxidation

Mechanisms of Oxidation in Foods

Triplet oxygen oxidation

Singlet oxygen oxidation

Molecular Orbital of Triplet OxygenEnergy*2S2S2Px 2Py 2Pz2Pz 2Py 2Px***MolecularAtomicAtomic

Molecular Orbital of Singlet OxygenEnergy*2S2S2Px 2Py 2Pz2Pz 2Py 2Px***MolecularAtomicAtomic*1S1S

Comparison of 3O2 and 1O2

3O2 1O2

ENERGY LEVEL 0 22.5 Kcal

NATURE DIRADICAL NO RADICAL

REACTION RADICAL ELECTRON-RICH

Molecular Orbital of Triplet OxygenMolecularAtomicAtomic2Pz2Py2Px2Px2Py2Pzs*sppp*p*2S2S1S1Ssss*s*E

General Mechanisms of Autoxidation 14 13 12 11 10 912 11 10 9+ H(CH2)4CH3CHCHCHCHCH2RCHOOPROPAGATION

12 11 10 HYDROPEROXIDE DECOMPOSION - OHCHCHCHCHCHCH2R(CH2)4CH3OOH12 11 10 9(CH2)4CH3CHCHCHCHCH2RCHO+ HTERMINATION (PENTANE)CH3(CH2)3CH2CH3(CH2)3CH3OCHCHCHCHCHCH2R+ 12 11 10 9

Major Chemical Mechanisms of Lipid OxidationA. Initiation - The formation of free alkyl radicalB. Propagation - The chain reaction of free alkyl radicals and peroxy raidcalsC. Termination - The formation of nonradical products

Factors Affecting Autoxidation 1. Energy in the form of heat and light 2. Catalysts (Metal) 3. Double bonds 4. Enzymes 5. Chemical oxidants 6. Oxygen content and types of oxygen 7. Natural antioxidants 8. Phospholipids 9. Fatty acids10. Mono- and Di- glycerides11. Polymers

Mechanisms of Oxidation++RHRHRO2ROO++ ROOR1HROOHR1+++RORROR++ROROOROORO2++222RRRRROOROOROORROORROOR

Oxidation of Mono-OlefinesDouble bond shifts toOleic acid - 4 Hydroperoxides12 11 10 9 8712 11 10 9 87 11 10 9 87981110CCCCCCOOH 11 10 9 87CCCCCCOOHCCCCCOOHCCCCCOOHCC

Hydroperoxides from Linolenate16 15 14 13 12 11 10 99121316CCCCCCOOHCCCCCCCCOOHCCCCCCCCCCOOHCCCCCCCCOOH16 15 14 13 12 11 10 916 15 14 13 12 11 10 916 15 14 13 12 11 10 9

Peroxide Decomposition

OH++orRCOHR1++R'OHROH++R'R'H++RCR1HOOHRCR1HORCR1OR'HRCR1HOHR1CH1ORR'

Effects of Metal on Peroxide Decomposition

Cu+ROOHROOH-Cu++Cu+H+ROOROOHCu++ROOHROROOH+OH-H2O+++++++++2

Formation of C C CH3(CH2)7CHCHCH3(CH2)7CHCHCH2(CH2)6COORCH3(CH2)7CHCHCH(CH2)6COOROOH

RCH3(CH2)7CHHCH++++PropagationTerminationDisproportionation+CH3(CH2)7CCHCH3(CH2)7CHHCHCH3(CH2)7CHHCCH3(CH2)7CHHCOHCH3(CH2)7CHOCHHOHRHCH3(CH2)7CHCH

CCCCHHHHRHHHHOCCCOOHCCCOHHHHHRCCCCOHHHHHRCCCH2CH2CHCHROCHCH3OCHCH2COHABHydroperoxide Decomposition