Transcript Reactions of Oils and Fats

Reactions of Oils and Fats
Reactions of Oils and Fats
• Hydrolysis
• Oxidation
• Hydrogenation
• Esterification
Hydrolysis
• Chemical (Autocatalytic)
• Enzymatical (Lipase)
H2C
OH
3 fatty acids
O
O
O C - R1
O
O HO - C - R1
HC OH +
= HC O C- R2 +3H20
HO - C - R2
O
H2C OH
O
glycerol
H2C O C - R3
HO - C - R3
triacylglycerol
H2C
Acid Value
Number of mgs of KOH required to neutralize the
Free Fatty Acids in 1 g of fat.
AV =
ml of KOH x N x 56
Weight of Sample
= mg of KOH
Oxidation of Oils and Fats
• The reaction of molecular oxygen with
organic molecules has for long been a
process of considerable interest.
• Although a wide variety of organic
molecules are susceptible to chemical
attack by oxygen, a great deal of attention
has recently been focused on lipids
because of the remarkable implications of
their oxidative damage.
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)
Oxidation of Lipids
• Autoxidation of Lipids is the oxidative
deterioration of unsaturated fatty acids via
an autocatalytic process consisting of a
free radical chain mechanism.
• The chain of reaction includes
– Initiation
– Propagation
– Termination
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.
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).
H
O
H
C
-O- C
H
O
•
C
+ H•
-O- C
Represent as
RH
R• + H• (Eq.1)
Initiation
• The removal of hydrogen takes place at the
carbon atom next to the double bond.
H
O
H
C
-O- C
H
O
•
C
+ H•
-O- C
Represent as
RH
R• + H•
(Eq.1)
Formation of Lipid Radical
• Hydrogens on carbons next to double bonds most
easily removed (-carbon)
Energy for H removal
(kcal/mole)
H - CH2 - CH2 - CH3
100
H - CH = CH2
103
H - CH2 - CH = CH2
85
CH2 = CH - CH - CH = CH2
65
H
 H on carbon next to double bond easier to remove
Initiation mechanisms
•Photosynthesized Oxidation (Photooxdation)
•Metal Catalysis
•Thermal Oxidation
•Enzymatic Oxidation
Initiation mechanisms-PO
Light, in the presence of oxygen, promotes oxidation of
unsaturated fatty acids.
Photooxidation energy from light is captured aided by
sensitizer molecules (pigments: chlorophile)
•Light excites these sensitizers to the triplet state that
promotes oxidation by type I and type II mechanisms.
Initiation mechanisms-PO
• In type I photosensitized oxidation, the triplet state
sensitizer abstracts a hydrogen or electron from the
unsaturated oil, producing radicals that initiate chain
propagation
sens light sens*
sens* + RH
R• + H•
• In type II photooxidation, the energy of the triplet sensitizer
is transferred to molecular oxygen, converting it to its
excited singlet state.
sens* +
3O
light
2
sens + 1O2
Initiation mechanisms-PO
•Singlet oxygen more reactive than triplet oxygen
RH + 1O2
RO• + •OH
ROOH
RO• provides free radical to start propagation
•Initiated by singlet oxygen (1O2)
–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
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+
Initiation mechanisms-Thermal Oxidation
• The energy requirements for the abstraction of H to
form a lipid radikal can be supplied in the form of
thermal energy.
• High temperatures (like frying) facilitate the all stages
of the chain reaction
Initiation mechanisms-Enzymatic Oxidation
• Enzyme-catalysed oxidation is initiated even in the
absence of hydroperoxides. This means the enzyme alone
is able to overcome the energy barrier of this reaction
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)
R• + O2
ROO•
(Eq.2)
• During propagation, peroxy radicals can react
with lipids (others R1H or same RH) to form
Hydoperoxide (ROOH) and a new unstable lipid
radical (Eq.3)
ROO• + R1H
ROOH+ R1•
(Eq.3)
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)
R1• + O2
R1OO•
(Eq.4)
Hydroperoxides (Eq.3) are unstable and can
degrade to produce radicals that further accelerate
propagation reactions (Eq.5) and (Eq.6)
ROOH
2ROOH
RO• + OH•
ROO• + RO• + H2O
(Eq.5)
(Eq.6)
Propagation
• Hydroperoxides are readily decomposed by
– high-energy radiation,
– thermal energy,
– metal catalysis, or enzyme activity.
• Transion metals such as Fe and Cu
ROOH + M+
RO• + OH• + M+
(Eq.7)
ROOH + M2+
ROO• + H+ + M+
(Eq.8)
ROO• + RO• + H2O
(Eq.6)
2ROOH
Termination
• The propagation can be followed by
termination if the free radicals react with
themselves to yield non-reactive (stable)
products, as shown here:
R• + R•
RO• + R•
ROO• + R•
RR
ROR
ROOR
ROO• + ROO•
ROO•R + O2
• Carbonyl compounds (aldehydes and
ketones)and hydrocarbons
Pentane Formation from Linolenic Acid
CH3
(CH2 )3
14
13
CH2
CH CH
12
11
Initiation (metal)
(CH2 )3
CH2
CH CH
O
COOH
CH2 n COOH
CH2
11
CH CH
O
O
H
CH2
10
9
CH
CH
CH2 nCOOH
+ H.
.
10
CH
_
12
(CH2 )3
CH
CH
O
Hydroperoxide
Decomposition
CH3
9
CH
11
12
(CH2 )3
n
+ O2
Propagation
CH3
10
CH
CH CH
12
(CH2 )3
CH2
11
.
CH2
Propagation
CH3
CH CH
- H.
12
CH3
9
10
CH2
CH
9
CH-
CH2 n COOH
.OH
11
10
CH
CH
CH CH
9
CH
CH2 n COOH
.
O
CH3
(CH2 )3
CH.
2
O
+ H C
12
11
CH CH
.
Termination
+ H
CH3
(CH2 )3
Pentane
CH3
10
9
CH
CH
CH
2 n COOH
Oxidation Product
• Primary Oxidation Products
– Hydroperoxides
• Secondary Oxidation Products
– Aldehydes and ketones
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. Free Fatty acids
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
18:2D9,12
18:3D9,12,15
Rate of Reaction
Relative to Stearic Acid
1
100
1200
2500
Kinetics of Autoxidation
ANALYSIS OF OIL OXIDATION
1.
Peroxide Value
O
O
A.
KI + CH 3
B.
ROOH + 2 HI
I2 +
C.
I2 + 2 Na2 S 2 O3
2 NaI +
Peroxide Value =
C
OH
HI
+ CH 3
H2 O +
Na2 S4 O6
ml of Na2S2O3  N  1000
(milliequivalent peroxide/kg of sample)
Grams of Oil
C
OK
ROH
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 solution
containing 1.0 g oil in 100 ml of a mixture of solvent and
reagent.
This method determines the amount of aldehyde
(principally 2-alkenals and 2,4-alkadienals ) in animal
fats and vegetable oils.
Aldehyde + p-AnV
Yellowish Products
(Under acidic conditions)
3. Totox Value = 2* PV + p-AnV
K232 and K270
• Oxidation of PUF is accompanied by an increase
in the UV absorption of the products.
• Lipids containing methylene-interrupted dienes
and trienes show a shift in their double-bond
position during oxidation due to isomerization
and conjugate formation.
• The resulting conjugated dienes exhibite an
intense absorption at 232 nm; similarly
conjugated trienes absorb 268 nm.
• K232 and P.V correlate well in the early stages
of oxidation.
Oxidative Stability of Oils and Fats
Active Oxygen Method (AOM)
Determined the time required to obtain certain peroxide
value under specific experimental conditions.
The larger the AOM value, the better the flavor stability
of the oil.
Oil Stability Index / Rancimat Methods
OSI and Rancimat measure the change in conductivity
caused by ionic volatile organic acids, mainly formic acid,
automatically and continuously.
Antioxidants
• Primary Antioxidants
– Chain-breaking antioxidants are free radical
acceptors that delay or inhibite the initiation
step or interrupt the propagation step of
autoxidation.
• Secondary Antioxidants
– Act through numerous possible mechanisms,
but they do not convert free radicals to more
stable products.
Primary Antioxidants
• R• + AH
• RO• + A•
• ROO• + AH
RH + A•
ROA
ROOH + RH
Natural and Synthetic Antioxidants
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 oxygen’s excess energy and
dissipate the the energy in the form of heat.