Further decomposition of hydroperoxides C-C-C=C-CHO C-C-C=C-CHO O O H R-CHO + OHC-CH2-CHO (malonaldehyde) Formation of malonaldehyde is one of the major products of lipid oxidation. Malonaldehyde can cross-link with proteins, enzymes and DNA and cause health problems.
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Further decomposition of hydroperoxides
C-C-C=C-CHO
C-C-C=C-CHO O O H
R-CHO + OHC-CH2-CHO (malonaldehyde)
Formation of malonaldehyde is one of the major
products of lipid oxidation.
Malonaldehyde can cross-link with proteins,
enzymes and DNA and cause health problems.
Thiobarbituric acid (TBA) test
Measuring TBARS (Thiobarbituric acid reactive
substances) is a general test used to evaluate the
extent of lipid oxidation.
Oxidation products of unsaturated systems produce
a colour reaction with TBA.
Colour results from condensation of two molecules
of TBA and one molecule of malonaldehyde.
2 mol TBA + malonaldehyde red colour
The product can be measured quantitatively at 530
nm using a spectrophotometer.
Role of metal ions in lipid oxidation
Metal ions can catalyze the oxidation of lipids.
Metals possessing two or more valency states and a
suitable oxidation-reduction potential between them
are effective pro-oxidants. e.g. Fe, Cu, Mn, Co
Even at concentrations as low as 0.1 ppm, they can
decrease the induction period and thereby increase
the rate of oxidation.
Trace amounts of heavy metals are found in edible
oils originating from:
• the soil in which the plant was grown
• the animal
• metallic equipment used in processing or storage
Role of metal ions in lipid oxidation
They are also naturally occurring components of all
foods and are present in both free and bound forms.
Mechanisms for metal catalysis of oxidation are as
follows.
1. Activation of molecular oxygen to give singlet oxygen
and peroxy radical.
-e- 1O2
Mn+ + O2 M(n+1) + 3O2
+H+
HOO0
2. Direct reaction with the unoxidized substrate.
Mn+ + RH M(n-1)+ + H+ + Ro
Role of metal ions in lipid oxidation
3. Acceleration of hydroperoxide decomposition
Mn+ + ROOH M(n+1)+ + OH- + ROo
Mn+ + ROOH M(n-1)+ + H+ + ROOo
Lipoxydase (Lipoxygenase) catalysed oxidation:
This is an enzymatic reaction which needs O2.
1,4 pentadiene structure is also needed.
RH + Lipoxydase + O2 RH + O2
Lipoxydase
ROOH ROOH R + OOH
+ Lipoxydase Lipoxydase
Lipoxydase
Lipoxygenase catalysed oxidation ……
• Lipoxygenase is present in plants and animals.
• Activation energy for the above reaction (3-4 kcal/
mol) is low compared with autooxidation hence it can
take place at low temperature, even at refrigeration
temperature.
• Under frozen conditions aw is low and the mobility of
reactants is low. Hence the rate of reaction reduced.
• Enzymatic cleavage of ROOH yields a variety of
breakdown products which are responsible for the
characteristic flavor of natural products.
• This reaction can be inhibited by phenolic anti-
oxidants (tocopherol, hydroquinones etc.)
Hematin catalysed oxidation ……
• Hematin compounds present in many food tissues
are also important pro-oxidants. E.g. myoglobin,
haemoglobin, cytochrome
• Even in well bled tissues very low amount of hematin
is present. Fe3+ is involved in this reaction.
• This reaction is different to other 2 forms due to the
requirement of pre-formed hydroperoxides.
ROOH + hematin
RH
Ro + carboxyl compounds + hematin
The activation energy for reaction is 3.3 kcal /molecule.
ROOH + hematin
Antioxidants
• Substances that can delay the onset or slow down
the rate of oxidation
• Main lipid soluble antioxidants used in foods are
monohydric or polyhydric phenols
E.g. Tocopherol, BHT, BHA, PG, TBHQ
• For maximum efficiency, they are used in combination
with metal sequestering agents
Mechanism of action:
A substance delays autooxidation reaction, if;
• it inhibits formation of free radicals
• it interrupts propagation of free radicals
Antioxidants
• Antioxidants inhibit free radical formation by:
• Quenching singlet oxygen
• Chelating metal ions
• Decomposing hydroperoxides
• An antioxidant inhibits the chain reaction by acting as
hydrogen donor (free radical acceptor) for Ro and
ROOo radicals.
Ro + AH RH + Ao
ROOo + AH ROOH + Ao
• The resulting antioxidant radical will not initiate new
free radicals and they may undergo a variety of
reactions forming stable products.
Antioxidants…..
Ao + Ao AA
Ao+ ROOo ROOA
Dihydric phenols dismute to yield quinones with the
formation of original antioxidant.
Ao + Ao AH + quinone
Effectiveness of an antioxidant is influenced by its,
• Chemical potency
• Solubility in oil (accessibility to free radical)
• Volatility (stability during heating, storage)
Synergism of Antioxidants
• Synergism occurs when a mixture of antioxidants
produces a greater activity than the sum of the
activity of each antioxidant in the mixture, when
tested individually.
• Two types of synergism are recognized:
1. Action of mixed free radical acceptors:
ROOo + AH ROOH + Ao
Ao + BH AH + Bo
The presence of the second antioxidant (BH) will
have a sparing effect since it regenerates the primary
antioxidant (AH).
E.g. Phenolic antioxidant and ascorbic acid
Synergism of Antioxidants
• Phenolic antioxidant is the primary antioxidant (more
effective one) while ascorbic acid is the synergist.
• It is possible for two phenolic antioxidants to exhibit
synergism in a similar way.
2. Combined action of a free radical acceptor and a
metal chelating agent:
• Metal chelating agents are compounds which can
partly deactivate trace metals present.
• When antioxidant property of a free radical acceptor
is enhanced by the presence of a metal chelating
agent synergism occurs. E.g. citric acid, phosphoric
acid, polyphosphates.
Choice of the Antioxidant
• Antioxidants exhibit substantial differences in their
effectiveness when used with different types of fatty foods
and under different processing and handling conditions.
• Factors to be considered in selecting an anti-oxidant are;
• Chemical potency of the antioxidant
• Ease of incorporation into the food
• Carry-through characteristics
• Sensitivity to pH
• Hydrophilic-lipophilic properties
• Tendency to produce off-flavour or off-colour
• Availability
• Cost
Choice of the Antioxidant
• In bulk oils – TBHQ and PG are more effective.
• In oil-water emulsions, polar lipid membranes,
intracellular micelles of neutral lipids - more lipophilic
antioxidants, such as BHA, BHT and tocopherols are
the most effective.
Thermal Decomposition
• Heating of food produces various chemical changes,
some of which are important to flavor, appearance,
nutritive value and toxicity.
• Different nutrients in food undergo decomposition
reactions and also interact among themselves in
extremely complex ways to form a large number of
new compounds.
• Lipid oxidation at high temperatures is complicated:
thermolytic and oxidative reactions taking place
simultaneously.
• Both saturated and unsaturated fatty acids undergo
decomposition when exposed to heat.
Thermal decomposition
I. Thermal, non-oxidative reactions of SFA:
Heating of saturated triglycerides to >200 oC yields
detectable amounts of hydrocarbons, acids and
ketones due to thermolysis.
II. Thermal, oxidative reactions of SFA:
Even though SFA are more stable to heat than their
unsaturated analogs, above 150 oC they can also
undergo oxidation, giving rise to a complex decomposition
pattern.
Major oxidative products are, series of carboxylic acids
and hydrocarbons: 2-alkanones, n-alkanals, n-alkanes,
1-alkenes and lactones.
Thermal decomposition
III. Thermal non-oxidative reactions of USFA:
Unsatutared fatty acids form dimeric compounds and low
molecular weight compounds during high heat in the
absence of oxygen.
IV. Thermal oxidative reactions of USFA:
At elevated temperatures oxidative decomposition of
USFA takes place very rapidly.
Major compounds formed at high temperatures are
qualitatively the same as that of room temperature
autooxidation. But at elevated temperatures
hydroperoxide decomposition and secondary oxidation
are extremely rapid.
Fatty acids, esters
and triglycerides
Saturated Unsaturated
Thermolytic Oxidative Thermolytic Oxidative
reactions reactions reactions reactions
(α,β,γ attack)
acids alkanes acyclic and volatile and
hydrocarbons alkenes cyclic dimeric
propenediol alkanals dimers products of
acrolein lactones autooxidation
ketones carboxylic acids
Generalized scheme for thermal decomposition of lipids
Chemistry of Frying
• Foods fried in oil, contribute significantly to the
energy in the diet because 5-40% of the oil can be
absorbed to food.
• During deep-fat frying, foods contact oil at high
temperatures (around 180 oC) and is exposed to air
for a variable period of time.
• Thus frying has the greatest potential for causing
chemical changes in food.
Chemistry of Frying
Behaviour of the frying oil:
• The physical and chemical changes that can be
observed in the oil during frying include;
• Increase in viscosity
• Increase in free fatty acid content
• Development of a dark colour
• Decrease in iodine value
• Decrease in surface tension
• Changes in refractive index
• Increased tendency to foam
Above changes are due to following classes of
compounds produced from the oil, during frying.
Chemistry of Frying……
1. Volatiles:
• Oxidative reactions involving the formation and
decomposition of hydroperoxides, lead to the
formation of saturated and unsaturated aldehydes,
ketones, hydrocarbons, lactones, alcohols, acids
and esters.
• Volatiles produced vary widely depending on the type
of oil, type of food and the heat treatment.
• They reach a plateau value with time probably
because a balance is achieved between the formation
and loss of volatiles.
Chemistry of Frying
2. Non-polymeric polar compounds of moderate
volatility (E.g. hydroxyl and epoxy acids):
These compounds are produced due to various oxidative
pathways involving the alkoxy radical.
3. Dimeric- and polymeric acids, and dimeric- and
polymeric glycerides:
These compounds occur from thermal and oxidative
reaction combinations of free radicals. Polymerization
results in an increase of viscosity of the frying oil.
4. Free fatty acids:
These compounds arise from the hydrolysis of
triglycerides in the presence of heat and water.
Chemistry of Frying
Behaviour of the food during frying:
• Water is continuously released from the food into the
hot oil. This produces a steam distillation effect,
sweeping volatile oxidative products from the oil.
• Released moisture also agitates the oil and hastens
the hydrolysis making more FFA available.
• Blanket of steam produced above the surface of oil
tends to reduce the amount of oxygen available for
oxidation.
• Volatiles may develop in the food itself or from the
interaction between food and oil.
Chemistry of Frying
• Food absorbs varying amount of oil during deep fat
frying. E.g. in potato chips the final fat content is
about 35 %.
• Food itself may release some of its endogenous lipids
(e.g. fat from chicken) into the frying medium and
consequently the oxidative stability of the new mixture
may be different from that of the original oil.
• The oil may get darken at an accelerated rate due to
the presence of food.
• Extensive decomposition due to uncontrolled frying
operation can be a potential source of damage to
sensory properties, nutritive value and safety of food.