Lipid Oxidation
Autoxidation
Polyunsaturated Fatty Acids
Free Radical Initiation
H-abstraction
O2 uptake
Diene Conjugation
Lipid Peroxides
Catalysts (Fe, Fe-O2)
Decomposition
Polymerization
(dark color, possibly toxic)
Secondary By-products
Insolublization
including rancid off-flavor
of proterins
compounds such as ketones,
alcohols, hydrocarbons, acids,
epoxides
Implications to meat (food) products
• A major cause of quality deterioration in meat and meat
products.
– Produces WOF in cooked meats
– Oxidized flavors in oils and cooked meats
– Develop rancidity in raw or fatty tissues
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•
•
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Loss of functional properties
Loss of nutritional values
Formation of toxic compounds
Forms colored products.
Factors affecting the development of lipid
oxidation in meat
• Oxygen
• Fatty acid compositions
• Prooxidants
• Antioxidants
• Processing conditions of meat
– Irradiation
– Cooking
– Grinding, cutting, mixing, restructuring etc.
• Storage: time and conditions
Process of Lipid Oxidation Autoxidation
•
•
Initiation
RH + (reactive oxygen species) ·OH -->R· + H2O
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•
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Propagation
R· + O2 ------> ROO·
ROO· + RH ------> R· + ROOH
ROOH -------> RO· + HO-
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Termination
R· + R· ------> RR
R· + ROO· ------> ROOR
ROO· + ROO· ------> ROOR + O2
Initiation
Fatty acid
.OH
.
.
or other
Free radicals
Initiation - Continued
Propagation
•
ROOH + Fe2+-complex ---Æ Fe3+-complex ---Æ RO. + OH-
•
ROOH + Fe3+-complex ---Æ ROO. + H+ + Fe2+-complex
Oxidation Products
• Hydroperoxide decomposition leads to aldehyde formation
– e.g. alkanals, hexanal
• Produces rancid flavors
• The free radicals produced damage other compounds
including vitamins and proteins
Oxidation Rates: Types of Fatty Acids
• As # of double bonds increases
– # and reactivity of radicals increases
Type of Fatty Acid
18:0
18:1Δ9
18:2Δ9,12
18:3Δ9,12,15
Rate of Reaction Relative to Stearic Acid
1
100
1200
2500
Electron States of ROS
Orbitals
• Electrons has some of the properties of a particle, and
some of the properties of a wave motion
• As a result, the position of an electron at a given time
cannot be precisely located, but only the region of space
where it is most likely to be.
• These regions are referred to as orbitals or electron shells
with a particular energy level
Quantum Numbers
• K, L, M, and N shells each can hold specific maximun number
of electrons
• Principal quantum number: n
– K-shell, n = 1; L shell, n = 2 etc
• The second *(azimuthal) quantum number: l (l = n-1).
– S, p, d, f
– Govern the shape of the orbital
•
The second quantum number: m (magnetic quantum number)
– L, L-1 …..0 ….1, …..-L
• The 4th quantum number: spin quantum number: ½, -1/2
Atoms and Bonds: Atom
Atomic weight of 1 = 1 x 10-23 g
Atomic wt ~ 0
15
Sketches of the electron density for the first
three shells
Bond Energy and Lipid Oxidation
Bond Strength (kcal/mol)
H-O-H, 119; RO-H, 104-105; ROO-H, ~90; Ar-H, 112;
ArO-H, 85; NH2-H, 107, RS-H, ~90; ArS-H, ~84
Bond energies in kcal/mol of C-H bonds in polyunsaturated
fatty acids
H H H
H
H H
—C—C—C—C=C—C—C= C—C—C—
H H H H H H H H H H
98 95 88 108 75 108 88 95
Free Radicals
• Reactive Oxygen Species
• Peroxyl radical (ROO.)
• Alkoxyl radical (RO.)
• Iron-oxygen complexes (ferryl and perferryl radicals)
• Thiyl radicals (RS.)
• Nitric oxide (.NO)
Free Radical Half-Life at 37ºC
Radical
Symbol
Half-Life Time
Hydroxyl
.OH
one nanosecond
Singlet Oxygen
1O
one microsecond
Superoxide
.O2−
one microsecond
Alkoxyl
.OL
one microsecond
Peroxyl
LOO.
ten milliseconds
Nitric Oxide
NO.
few seconds
2
Radical Reaction Potentials
Radical
mV
.OH
(hydroxyl)
+2300
.LO
(alkoxyl)
+1600
LOO. (peroxyl)
+1000
.GS
+920
(glutathione)
.HU-
(urate)
+590
.Toc
(tocopherol)
+480
.Asc-
(ascorbate)
+282
Fe3+-EDTA
+120
Catalysts
• Transition metals: Fe, Cu, Mg, Ni etc.
• Iron in lipid peroxidation
–
–
–
–
–
–
Loosely bound iron
Tightly-bound iron
Stored iron
Heme iron
Iron-complexes (Ferryl and perferryl)
Hematin
Measurement of lipid oxidation
•
Direct measurement of free radicals
– Electron spin resonance
– Spin trapping methods
•
Indirect approach: Measures markers of free radicals
– Thiobarbituric acid reacting substances (TBARS)
– Lipid chromatography: Fluorometric compounds
– Gas chromatography: Gaseous compounds
– Conjugated dienes (CD)
– Peroxide value
– Iodine Value
Effect of ROS on Degenerative Diseases
Gastro intestinal
Eye
Skin
Heart
Hepatitis
Liver injury
Cataractogenesis
Retinal damage
Dermatitis
Age pigment
Heart attack
Teeth
Periodontis
Vessels
Atherosclerosis
Vasopasms
Reactive Oxygen Species
Multiorgan failure
Cancer
Brain
Trauma
Stroke
Joints
Arthritis
Lung
Asthma
Hyperoxiia
Antioxidant Defenses in Biological
Systems
• Fat-soluble cellular membrane consists
– Vitamin E
– beta-carotene
– Coenzyme Q (10)
• Water soluble antioxidant scavengers
– Vitamin C
– Glutathione peroxidase,
– Superoxide dismutase
– Catalase
Antioxidant Enzymes and Mechanisms
2O2.- + 2H- -------------------------Æ H2O2 + O2
(superoxide dismutase)
2H2O2 -------------------------Æ 2H2O + O2
(catalase)
2GSH + H2O2 ------------------------Æ GSSG + H2O
(glutathione peroxidase)
GSSG + NADPH -----------------------Æ 2GSH + NADP(Glutathione reductase)
GSH: reduced glutathione, GSSG: oxidized glutathione
Main mechanisms for inhibition of
oxidative reactions
1. Interrupt the free-radical chain mechanism
2. Function as being preferentially oxidized - poor
protection
3. Reducing agents
4. Chelating agents for free iron
Chain-Breaking and Free Radical Scavengers
Synthetic Phenolic antioxidants
•
BHA
•
BHT
•
PG
•
TBHQ
Natural antioxidants
•
Flavonoids
•
Polyphenols
•
Tocopherol
OH scavengers: mannitol, formate, thiourea, dimethylthiourea, methanol,
ethanol, 1-butanol, glucose, tris-buffer, or sorbitol
Synthetic antioxidants
BHT
BHA
TBH
Q
PG
Natural antioxidants
Flavone
Isoflavone
Epicatechin
Quercetin
Tocopherol
Sesamol
Resonance Stabilization of Antioxidant Radicals
Iron Chelating Agents
• Phosphate
• EDTA
• Citric acid
• DTPA
• Desferrioxamine
Ideal Antioxidants
• No harmful physiological effects
• Not contribute an objectionable flavor, odor, or
color to the product
• Effective in low concentration
• Fat soluble
• Carry-through effect – no destruction during
processing
• Readily available
• Economical
• Non-absorbable by the body
Antioxidants in Meat Processing
1. Dietary supplementation of antioxidants
– Vitamin E
– Ascorbate
– Selenium
2. Modification of fatty acid composition
3. Addition of antioxidants during processing
4. Inhibition of oxygen contact
5. Smoking