AUTOOXIDATION of LIPIDS

1
AUTOOXIDATION of LIPIDS
2
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
3
Oxidation of Foods

• Flavor Quality Consumer
  Acceptance
  
• Nutritional Quality Essential Fatty
  Acids
• Essential Amino Acids
• 
  Vitamins
  
• Health Risks Growth Retardation
• Heart
  Diseases
• 
  Carcinogens

4
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
•  

5
Mechanisms of Oxidation in Foods

• Triplet oxygen oxidation
• Singlet oxygen oxidation

6
Molecular Orbital of Triplet Oxygen
Molecular
?
Atomic
Atomic
?
?
?
?
2Px 2Py 2Pz
2Pz 2Py 2Px
?
?
Energy
2S
2S
?
7
Molecular Orbital of Singlet Oxygen
Molecular
?
Atomic
Atomic
?
?
?
?
2Px 2Py 2Pz
2Pz 2Py 2Px
?
Energy
?
2S
2S
?
?
1S
1S
?
8
Comparison of 3O2 and 1O2

3O2
1O2 ENERGY LEVEL 0
22.5 Kcal NATURE
DIRADICAL NO RADICAL REACTION
RADICAL ELECTRON-RICH


9
Molecular Orbital of Triplet Oxygen
Molecular
Atomic
Atomic
s

p

p

p
p
2Pz
2Py
2Px
2Px
2Py
2Pz
s
E
s

2S
2S
s
s

1S
1S
s
10
General Mechanisms of Autoxidation
14 13 12 11 10 9
?
12 11 10 9
(
C
H
)
C
H
C
H
C
H
C
H
C
H
C
H
R
C
H
2
4
3
2
O
O
PROPAGATION
H
?
?
11
12 11 10
C
H
C
H
C
H
C
H
C
H
C
H
R
(
C
H
)
C
H
2
2
4
3
O
HYDROPEROXIDE DECOMPOSION
- OH
?
O
H
12 11 10 9
(
C
H
)
C
H
C
H
C
H
C
H
C
H
C
H
R
C
H
2
4
3
2
O
?
12 11 10 9
O
?
C
C
H
C
H
C
H
C
H
C
H
R

C
H
(
C
H
)
C
H
2
3
2
3
2
H
TERMINATION
H
?
(PENTANE)
C
H
(
C
H
)
C
H
3
2
3
3
12
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
13
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
14
Mechanisms of Oxidation
R
H
R
H



R
O
R
O
O



2
R
O
O
R
H
R
O
O
H
R





1
1
R
R

R
R



R
O
O
R
O
O
R
O
O
R



R
O
R

R
O
R


R
O
O
R

R
O
O
R


R
O
R
O
O
2
O

2
R
O
O
R

2

2

15
Oxidation of Mono-Olefines
Oleic acid - 4 Hydroperoxides
Double bond shifts to

11 10 9 8
7

11 10 9 8
7
C
C
C
C
C
C
C
C
C
C
C
C
9
8
O
O
O
O
H
H
12 11 10 9 8
7
12 11 10 9 8
7
C
C
C
C
C
C
C
C
C
C
C
C
11
10
O
O
O
O
H
H
16
Hydroperoxides from Linolenate
16 15 14 13 12 11
10 9
C
C
C
C
C
C
C
C
9
O
O
H
16 15 14 13 12 11
10 9
12
C
C
C
C
C
C
C
C
O
O
H
16 15 14 13 12 11
10 9
C
C
C
C
C
C
C
C
13
O
O
H
16 15 14 13 12 11
10 9
16
C
C
C
C
C
C
C
C
O
O
H
17
Peroxide Decomposition
H
H
R
C
R
1
R
C
R
O
H
1


O
O
O

H

R
'

R
C
R
R
'
H

1
R
'
H

O
O

R
C
H
R

1
1
H
or

R
C
R
'
R
O

1
R

R
C
H

1
O
H
R
O
H
O
H

R
'


18
Effects of Metal on Peroxide Decomposition


-

C
u

R
O
O
H
C
u


R
O
O
H



C
u




H
R
O
O
C
u
R
O
O
H



-
H
O
H


R
O
R
O
O

R
O
O
H
2


H
O
2
19
Formation of C C
H
H
C
H
(
C
H
)
C
C
C
H
(
C
H
)
C
O
O
R
3
2
7
2
2
6
H
H
C
H
(
C
H
)
C
C
C
H
(
C
H
)
C
O
O
R
3
2
7
2
6
O
O
H
H
H
C
H
(
C
H
)
C
C

3
2
7
20
H
H
C
H
(
C
H
)
C
C

3
2
7
O
H
Termination


R
H

H
H
Propagation
C
H
(
C
H
)
C
C
O
H
3
2
7
H
H
H
H
C
H
(
C
H
)
C
C
H
3
2
7
C
H
O
(
C
H
)
C
C
3
2
7
H

Disproportionation
R

H
H
C
H
(
C
H
)
C
C


3
2
7
H
H
C
H
(
C
H
)
C
C
H
3
2
7

C
H
(
C
H
)
C
C
H
3
2
7
21
Hydroperoxide Decomposition
O
H
H
H
H
H
H
C
C
C
C
R
C
C
C
H
H
O
O
B
A
H
O
O
C
C
C
H
2
O
H
H
C
C
H
3
C
H
C
H
C
H
C
H
R
H
2
2
H
O
H
H
O
H
H
H
H
R
C
C
C
C
C
C
C
R
C
C
H
H
H