Autooxidation

AUTOOXIDATION of LIPIDS
Activation Energy for Food Chemical Reactions
Reactions
Activation Energy
(Kcal/Mole)
Nonenzymatic Browning Reaction
50
Enzyme Catalyzed Reaction
10-15
Singlet Oxygen Oxidation
1-2
Oxidation of Foods
Flavor Quality
Consumer Acceptance
Nutritional Quality
Essential Fatty Acids

Essential Amino Acids
Vitamins
Health Risks
Growth Retardation
Heart Diseases
Carcinogens
Oxygen History
1772 Scheele & Priestley
Discovered Oxygen
1811 Avogadro
Oxygen 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 Oxygen

Molecular
Atomic
2Px 2Py 2Pz
*
*
Atomic
2Pz 2Py 2Px
Energy
2S
2S
*
1S
1S
Molecular Orbital of Singlet Oxygen

Molecular
Atomic
2Px 2Py 2Pz
Energy
Atomic
2Pz 2Py 2Px
*
2S
2S
1S
1S
Comparison of 3O2 and 1O2

O2
ENERGY LEVEL
O2
22.5 Kcal
NATURE
DIRADICAL
NO RADICAL
REACTION
RADICAL
ELECTRON-RICH
Molecular Orbital of Triplet Oxygen

Molecular
Atomic
2Px
2Py
Atomic
*
*
2Pz
2Pz
2Py
*
2S
2S
*
1S
1S
2Px
General Mechanisms of Autoxidation

CH3
(CH2)3
14
13 12 11
10
9
CH2 CH CH CH2 CH CH
INITIATION
(METAL)
C H3
CH2 R
- H
12
11
10
(C H 2) 4 C H C H C H
CH CH
C H2
+ O2
CH3
(CH2)4
12
11
10
9
CH CH CH CH CH
O
PROPAGATION
+ H
CH2 R
CH3
12
11
10
(CH2)4 CH CH CH CH CH
O
HYDROPEROXIDE
DECOMPOSION O
CH2
- OH
H
CH
(C H )
2 4
12
11
CH CH CH
10
9
CH CH
CH
O
C H3
(C H 2) 3
CH2
TERMINATION
C H3
(C H 2) 3
12
CH
11
CH
H
+ H
C H 3 (PENTANE)
10
CH
9
CH
C H2
Major Chemical Mechanisms of Lipid Oxidation

A. Initiation
- The formation of free alkyl radical
B. Propagation - The chain reaction of free alkyl radicals

and peroxy raidcals
C. 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 acids
10. Mono- and Di- glycerides
11. Polymers
Mechanisms of Oxidation
1. Initiation
RH
R + H
R + O2
2. Propagation
3. Termination
ROO
ROO + R1H
R + R
ROOH +
R1
RR
ROO + ROO
ROOR
RO + R
ROR
ROO + R
ROOR
2RO + 2ROO
2ROOR +
O2
O2
Oxidation of Mono-Olefines
Oleic acid - 4 Hydroperoxides
11 10
11 10 9
C C C C C C
O
O
H
12 11 10
Double bond shifts to

8
12 11 10
9 8 7
C C C C C C
O
O
H
11
C C C C C C
O
O
H
C C C C C C
O
O
H
10
Hydroperoxides from Linolenate

16
15
14
13
12
11
16
15
14
13
16
15
14
13
C
O
O
H
16
C
O
O
H
15
14
12
11
C
O
O
H
13
12
11
12
10
11
C
O
O
H
10
12
10
13
10
16
Peroxide Decomposition
H
R C R1
O
O
H
H
C
R1
OH
O
+ R'
+ R'H
O
R
+ R1
H1
or
O
R C H +
R1
H
C
R1 +
R'
OH
R' +
OH
ROH
R1 +
R'H
Effects of Metal on Peroxide Decomposition
Cu+ +
++
Cu
ROOH
RO +
ROOH
ROO + H+
2 ROOH
RO +
OH-
+
+
ROO +
Cu++
Cu+
H+ + OHH2O
Formation of C C
C H3
C H3
(C H 2 )7
(C H 2 )7
H H
C C C H2
H H
C C CH
O
O
H
C H3
(C H 2 )7
(C H 2 )6
CO O R
(CH 2) 6
C O OR
H H
C C
C H3
(C H 2 )7
+ RH
H H
C C
OH Termination
Propagation
CH3
(CH2)7
H H
C CH
CH3
(CH2)7
CH3
(CH2)7
Disproportionation
+ CH3
CH3
CH3
(CH2)7
H H
C CH
(CH2)7
+
(CH2)7 C CH
H H
C C
H
C
H
C
H
H
C OH
H
C O
Hydroperoxide Decomposition
O
C
H
A
H H H H H H
C C C C C C R
H O
O
B
H
O
O
C CH2 C
C CH 3
H
O
C
H
CH2
H
C
H H H
C C C R
CH2 CH CH R
H
O
C
H
C
H
H
C
H
C

Autooxidation

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AUTOOXIDATION of LIPIDS

Activation Energy for Food Chemical Reactions

Nonenzymatic Browning Reaction

Enzyme Catalyzed Reaction

Singlet Oxygen Oxidation

Essential Fatty Acids

Mechanisms of Oxidation in Foods

Triplet oxygen oxidation

Singlet oxygen oxidation

Molecular Orbital of Triplet Oxygen

2Px 2Py 2Pz

2Pz 2Py 2Px

Molecular Orbital of Singlet Oxygen

2Px 2Py 2Pz

2Pz 2Py 2Px

Comparison of 3O2 and 1O2

Molecular Orbital of Triplet Oxygen

General Mechanisms of Autoxidation

Major Chemical Mechanisms of Lipid Oxidation

- The formation of free alkyl radical

B. Propagation - The chain reaction of free alkyl radicals

Factors Affecting Autoxidation

Double bond shifts to

Hydroperoxides from Linolenate

Effects of Metal on Peroxide Decomposition

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