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7/28/2019 Reactions of oils and fats
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Reactions of Oils and Fats
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Reactions of Oils and Fats
Hydrolysis
Oxidation
Hydrogenation
Esterification
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Hydrolysis
Chemical (Autocatalytic)
Enzymatical (Lipase)
HO - C
O
- R1
3 fatty acids
+
C
O
- R3HO -
HO - C
O
- R2
OH
OH
OHglycerol
HC
H2C
H2C
O
C
C
OC
OO
O
O
- R1
triacylglycerol
= - R2
- R3
HC
H2C
H2C
+3H20
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Acid Value
Number of mgs of KOH required to neutralize theFree Fatty Acids in 1 g of fat.
AV =ml of KOH x N x 56
Weight of Sample= mg of KOH
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Oxidation of Oils and Fats
The reaction of molecular oxygen withorganic molecules has for long been aprocess of considerable interest.
Although a wide variety of organicmolecules are susceptible to chemicalattack by oxygen, a great deal of attentionhas recently been focused on lipidsbecause of the remarkable implications oftheir oxidative damage.
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Oxidation of Oils and Fats
The results of the oxidation of fats and oils is the
development of objectionable flavors and odors
characteristic of the condition known as
oxidative rancidity.
Loss of shelf-life, functionality and nutritional
value.
Adverse health effects (carcinogenic)
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Oxidation of Lipids
Autoxidation of Lipids is the oxidativedeterioration of unsaturated fatty acids via
an autocatalytic process consisting of a
free radical chain mechanism.
The chain of reaction includes
Initiation Propagation
Termination
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What is Free radical?
A free radical is a group with an odd
number of unpaired electrons.
They are extremely unstable and
immediately react with another molecule to
form stable substances.
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Initiation The initiation of lipid oxidation starts with the
removal of an hydrogen atom from unsaturated
TGs or FFAs (RH) to form a free radical (R)
(Eq.1).
CO
- O -C
H H
+ H
C
O
- O - C
H
Represent as RHR
+H
(Eq.1)
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Initiation The removal of hydrogen takes place at the
carbon atom next to the double bond.
CO
- O -C
H H
+ H
C
O
- O - C
H
Represent as RH R + H (Eq.1)
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Formation of Lipid Radical
Hydrogens on carbons next to double bonds mosteasily removed (-carbon)
H - CH2 - CH2 - CH3
H - CH = CH2
H - CH2
- CH= CH
2
CH2 = CH - CH - CH= CH2
H
Energy forH removal
(kcal/mole)
100
103
85
65
H on carbon next to double bond easier to remove
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Initiation mechanisms
Photosynthesized Oxidation (Photooxdation)
Metal Catalysis
Thermal Oxidation
Enzymatic Oxidation
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Initiation mechanisms-PO
Light, in the presence of oxygen, promotes oxidation ofunsaturated fatty acids.
Photooxidation energy from light is captured aided bysensitizermolecules (pigments: chlorophile)
Light excites these sensitizers to the triplet state thatpromotes oxidation by type I and type II mechanisms.
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Initiation mechanisms-PO
Singlet oxygen more reactive than triplet oxygen
RH + 1O2 ROOH RO + OH
RO provides free radical to start propagation
Initiated by singlet oxygen (1
O2)metastable, excited energy state of O2
two unpaired electrons in same orbital
triplet oxygen
ground state
2 electrons w/ same
spin in 2 orbitals
singlet oxygen
excited state
2 electrons w/ different
spin in 1 orbital
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Initiation mechanisms-Metal Catalysis
Metal ions (e.g. Fe, Co, Cu) can also initiate reaction
found naturally in foods, from metal equipment
RH +M+2 R+ H++M+
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Initiation mechanisms-Thermal Oxidation
The energy requirements for the abstraction of H toform a lipid radikal can be supplied in the form ofthermal energy.
High temperatures (like frying) facilitate the all stagesof the chain reaction
Initiation mechanisms-Enzymatic Oxidation
Enzyme-catalysed oxidation is initiated even in theabsence of hydroperoxides. This means the enzyme aloneis able to overcome the energy barrier of this reaction
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Propagation
This highly reactive lipid (alkyl) radical (R) can
then react with oxygen to form a peroxy radical(ROO) in a propagation reaction (Eq.2)
During propagation, peroxy radicals can react
with lipids (others R1H or same RH) to form
Hydoperoxide (ROOH) and a new unstable lipidradical (Eq.3)
R + O2 ROO (Eq.2)
ROO + R1H ROOH+ R1 (Eq.3)
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Propagation
This lipid radical (R1) will then react with oxygen
to produce another peroxy radical (R1OO)resulting in a cyclical, self-catalyzing oxidative
mechanism (Eq.4)
Hydroperoxides (Eq.3)are unstable and can
degrade to produce radicals that further accelerate
propagation reactions (Eq.5) and (Eq.6)
R1 + O2 R1OO (Eq.4)
ROOH RO+ OH (Eq.5)
2ROOH ROO+ RO+ H2O (Eq.6)
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Propagation Hydroperoxides are readily decomposed by
high-energy radiation, thermal energy,
metal catalysis, orenzyme activity.
Transion metals such as Feand Cu
ROOH +M+
RO+ OH
+M
+
(Eq.7)ROOH +M2+ ROO+ H++M+ (Eq.8)
2ROOH ROO+ RO+ H2O (Eq.6)
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Termination
The propagation can be followed bytermination if the free radicals react withthemselves to yield non-reactive (stable)products, as shown here:
Carbonyl compounds (aldehydes and
ketones)and hydrocarbons
R+ R RR
RO+R ROR
ROO+R ROOR
ROO + ROO ROOR + O2
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Pentane Formation from Linolenic Acid
+
+
_
.+
.-
+
CH3 (CH2)3 CH2 CH CH CH CH CH CH2 COOH
CH3 (CH2)3 CH2 CH CH CH2 CH CH CH2 COOH
.
H
.
CH3 (CH2)3 CH2 CH CH CH CH CH CH2 COOH
O
O
H
O
O
CH3 (CH2)3 CH2 CH CH CH CH CH CH2 COOH
CH3 (CH2)3 CH2 CH CH CH CH CH CH2 COOH
O
Initiation (metal)
Propagation
Propagation
.
O2
H
OH.HydroperoxideDecomposition
CH3 (CH2)3 CH2 H C CH CH CH CH CH2 COOH
CH3
(CH2)3
CH3
O
.
H.Termination
Pentane
14 13 12 11 10 9
12 11 10 9
12 11 10 9
12 11 10 9
12 11 10 9
12 11 10 9
n
n
n
- n
n
n
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Oxidation Product
Primary Oxidation Products
Hydroperoxides
Secondary Oxidation Products
Aldehydes and ketones
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Factors Affecting Autoxidation
1. Energy in the form of heat and light
2. Catalysts (Metal)
3. Double bonds
4. Enzymes5. Chemical oxidants
6. Oxygen content and types of oxygen
7. Natural antioxidants
8. Phospholipids
9. Free Fatty acids
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Oxidation Rates: Types of Fatty Acids
As # of double bonds increases # and stability of radicals increases
Rate increases
Type of Fatty Acid
18:0
18:1D9
18:2D
9,1218:3D9,12,15
Rate of Reaction
Relative to Stearic Acid
1
100
12002500
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Kinetics of Autoxidation
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ANALYSIS OF OIL OXIDATION
1. Peroxide Value
KI CH3 C OH HI CH3 C OK
O O
ROOH HI I2 H2O ROH
I2 Na2S2O3 NaI Na2S4O6
A.
B.
C.
+
+
+
+
+
+
+
2
2
2
Peroxide Value = ml of Na2S2O3 N 1000
(milliequivalent peroxide/kg of sample) Grams of Oil
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2. p-Anisidine Value.
p-AnV is defined as 100 times the optical density
measured at 350 nm in a 1.0 cm cell of a solutioncontaining 1.0 g oil in 100 ml of a mixture of solvent andreagent.
This method determines the amount of aldehyde(principally 2-alkenals and 2,4-alkadienals ) in animalfats and vegetable oils.
3. Totox Value = 2* PV + p-AnV
Aldehyde + p-AnV Yellowish Products
(Under acidic conditions)
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Active Oxygen Method (AOM)
Determined the time required to obtain certain peroxidevalue under specific experimental conditions.
The larger the AOM value, the better the flavor stability
of the oil.
Oil Stability Index / Rancimat Methods
Oxidative Stability of Oils and Fats
OSI and Rancimat measure the change in conductivitycaused by ionic volatile organic acids, mainly formic acid,automatically and continuously.
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Antioxidants
Primary Antioxidants Chain-breaking antioxidants are free radical
acceptors that delay or inhibite the initiation
step or interrupt the propagation step ofautoxidation.
Secondary Antioxidants
Act through numerous possible mechanisms,
but they do not convert free radicals to more
stable products.
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Primary Antioxidants
R+ AH RH + A
RO+ A ROA
ROO+ AH ROOH + RH
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Natural and Synthetic Antioxidants
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Secondary Antioxidants
Chelators: citric acid, EDTA
Oxygen Scavengers and Reducing
Agents: Ascorbic acid, ascorbyl palmitate,
Singlet Oxygen Quenchers: Caretenoids
(beta-carotene, lycopene, lutein)
Deplete singlet oxygens excess energy and
dissipate the the energy in the form of heat.