Chapter three Lipid deterioration Lipid deterioration refers to the various processes that can lead to the degradation of lipids, which include fats and oils. This degradation can result in changes to the sensory, nutritional, and functional properties of the lipids, making them less suitable for consumption or industrial use. Some general consequences of lipid deterioration: Off-Flavors and Odors: Lipid deterioration can result in the formation of compounds that produce unpleasant tastes and smells. Rancidity, a common consequence of lipid oxidation, is often associated with a disagreeable odor and taste. Nutritional Loss: Essential fatty acids, such as omega-3 and omega-6 fatty acids, are susceptible to oxidation. The degradation of these nutrients can lead to a reduction in the overall nutritional quality of the affected food products. Color Changes: Some lipid deterioration processes can cause discoloration in food products. For instance, the oxidation of lipids in fats and oils may lead to changes in color, making them appear darker. Reduced Shelf Life: Lipid deterioration contributes to the degradation of the overall quality of food products, limiting their shelf life. This can result in economic losses for both producers and consumers. Health Concerns: Oxidized lipids can produce harmful compounds, including free radicals and aldehydes, which may have adverse health effects when consumed in excessive amounts. These compounds have been associated with inflammation and oxidative stress. Impacts on Food Industry: For industries involved in the production of foods containing lipids, lipid deterioration can have economic consequences. The need for quality control measures, increased testing, and potential product recalls can result in financial losses. To minimize the consequences of lipid deterioration, proper storage conditions, the use of antioxidants, and suitable packaging materials are often employed in the food industry. Several factors and mechanisms contribute to lipid deterioration: Lipolysis Lipolysis is the breakdown of lipids and involves hydrolysis of triglycerides into glycerol and FFA. FFA released in foods by lipolysis produce off-flavor. This is also termed as “hydrolytic rancidity” For example, rancidity of flavor in milk due to lipolysis of milk fat. Hydrolysis of ester (triacylglycerol) can occur in two ways: Enzymatic Moisture and heat Enzymatic lipolysis During processing and storage lipase in food hydrolyse esters bonds of triglycerides of fat and release FFA. These free FFA produces off-flavor. Lipolysis in the presence of moisture and heat. During deep frying of foods the temperature is high and the water is released.

1. Chapter three
Lipid deterioration
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2. • Lipid deterioration refers to the various
processes that can lead to the degradation of
lipids, which include fats and oils.
• This degradation can result in changes to the
sensory, nutritional, and functional properties of
the lipids, making them less suitable for
consumption or industrial use.
• Some general consequences of lipid
deterioration:
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3. • Off-Flavors and Odors:
– Lipid deterioration can result in the formation
of compounds that produce unpleasant tastes
and smells.
– Rancidity, a common consequence of lipid
oxidation, is often associated with a
disagreeable odor and taste.
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4. • Nutritional Loss:
 Essential fatty acids,
such as omega-3 and
omega-6 fatty acids,
are susceptible to
oxidation.
 The degradation of
these nutrients can
lead to a reduction in
the overall nutritional
quality of the affected
food products.
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5. • Color Changes:
– Some lipid deterioration processes can cause
discoloration in food products. For instance,
the oxidation of lipids in fats and oils may lead
to changes in color, making them appear
darker.
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6. • Reduced Shelf Life:
– Lipid deterioration contributes to the
degradation of the overall quality of food
products, limiting their shelf life. This can
result in economic losses for both producers
and consumers.
• Health Concerns:
– Oxidized lipids can produce harmful
compounds, including free radicals and
aldehydes, which may have adverse health
effects when consumed in excessive
amounts. These compounds have been
associated with inflammation and oxidative
stress. Tewodros Mebratie

7. • Impacts on Food Industry:
– For industries involved in the production of
foods containing lipids, lipid deterioration can
have economic consequences.
– The need for quality control measures,
increased testing, and potential product
recalls can result in financial losses.
• To minimize the consequences of lipid
deterioration, proper storage conditions, the
use of antioxidants, and suitable packaging
materials are often employed in the food
industry.
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8. Several factors and mechanisms
contribute to lipid deterioration:
Lipolysis
• Lipolysis is the breakdown of lipids and involves
hydrolysis of triglycerides into glycerol and FFA.
• FFA released in foods by lipolysis produce off-
flavor. This is also termed as “hydrolytic
rancidity”
• For example, rancidity of flavor in milk due to
lipolysis of milk fat.
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9. • Hydrolysis of ester (triacylglycerol) can occur in
two ways:
a. Enzymatic
b. Moisture and heat
• Enzymatic lipolysis
• During processing and storage lipase in food
hydrolyse esters bonds of triglycerides of fat and
release FFA.
• These free FFA produces off-flavor.
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11. • Lipolysis in the presence of moisture
and heat.
• During deep frying of foods the
temperature is high and the water is
released.
• Thus lipolysis is more favorable reaction
during deep fat frying of foods.
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12. • Lipid oxidation
• In all foods, the first mode of spoilage is
microbial.
• However, after microbes have been controlled
by processing, oxidation becomes the set of
chemical reactions most limiting shelf-life and
degrading the quality of foods.
• The kinetics of lipid oxidation in foods often has
a lag phase followed by an exponential increase
in oxidation rate.
• The length of the lag phase is very important to
food processors since this is the period where
rancidity is not detected and the quality of the
food is high. Tewodros Mebratie

13. • Once the exponential phase is reach, lipid
oxidation proceeds rapidly and off-aroma
development quickly follows.
• Figure: Delta-tocopherol can increase the lag phase of
the oxidation of a corn O/W emulsion
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14.  It is well known that unsaturated fatty acids and
oxygen are the components that react during the lipid
oxidation process.
 Additionally, other components can promote or
prevent oxidation reactions.
 Lipids can be oxidized by three main ways that
include complex reactions:
autoxidation,
Enzymatic - catalyzed oxidation and
photo-oxidation.
 Among the three mechanisms, autoxidation, which is
a continuous free-radical chain reaction, is the most
important process of lipid oxidation.
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15. Mechanism of autoxidation
• It is well known that unsaturated fatty acids and
oxygen are the components that react during the lipid
oxidation process.
 Normally, the autoxidation process is usually
represented as a combination of three distinct phases:
• The initiation in which free radicals occur,
• The propagation in which the number of reactive
compounds is multiplied, and
• Finally the termination in which the reactive
compounds degrade or react with each other to give
non-reactive compounds.
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16. • In fact, oxygen is in triplet electronic state while
double bonds of fatty acids are in singlet
electronic state.
• Initiation occurs as hydrogen is abstracted from
an unsaturated fatty acid.
• The resulting alkyl radical tends to be stabilized
by a double-bound rearrangement to form a
conjugated dienes or trienes.
• These alkyl radicals are the first free radicals
that initiate lipid oxidation.
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17. • Initiation
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18. Double bond rearrangement and formation
of hydro peroxides on propagation stage
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19. • Initiation step of lipid oxidation for linoleic acid.
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20. • Initiation step of lipid oxidation for oleic acid.
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21. Propagation
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22.  Termination
• The termination phase consists of the reaction
between radicals or with other non-radical
compounds (antioxidants) to give rise to non-radical
products.
• In the case of the reaction between two radicals,
radical–radical coupling and disproportionate can
occur to form a non-radical adduct.
• In fact, the reactions between peroxy, alkoxy and/or
alkyl radicals are usually represented as follows :
R• + R• → R–R
R• + ROO• → ROOR
RO• + RO• → ROOR
RO• + R• → ROR
ROO• + ROO• → ROOR + O2
2RO• + 2ROO• → 2ROOR + O2
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23. Example of a termination step of lipid oxidation under
conditions of low oxygen concentrations.
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24. • However, termination reactions are not always
efficient and may lead to new reactive
compounds.
• The mechanism that ensures termination
efficiently is the decomposition of peroxy and
alkoxy radicals to give rise to secondary
products such as alkanes, alcohols and carbonyl
compounds.
Decomposition of Hydroperoxides and Alkoxy
and Peroxy Radicals
• Lipid hydroperoxides are not considered harmful
to food quality because they are odourless and
tasteless
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25. • However, hydro peroxides are unstable
compounds, so they tend to decompose into
their alkoxy and peroxy radicals
• These radicals are further degraded into
secondary compounds that are responsible for
sensory deterioration such as odours and
flavours associated with lipid oxidation.
• The main secondary compounds released
include lipid alcohols, ketones, epoxides,
aldehydes and hydrocarbons.
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26. • The formation of these compounds is mainly
produced via alpha– or beta-scissions reactions,
and it is minimal during the initiation phase but
increases exponentially during the propagation
and termination phases.
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28. Lipid Photo-Oxidation
• Photo-oxidation is another mechanism of initiation of
lipid oxidation.
• Usually, lipid contained food products are directly
exposed to light in the supermarket to be attractive to
consumers.
• This fact promotes the photo-oxidation process, that
is much faster than autoxidation.
 During this process, hydroperoxides are formed in the
presence of sensitizers (as chlorophyll, riboflavin,
myoglobin, and heavy metals) and light.
 Therefore, photo-oxidation is an alternative route for
the formation of hydroperoxides instead of the free
radical mechanism explained in the autoxidation
process.
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29. • The first step of photo-oxidation is the excitation of
singlet sensitizer by absorbing light energy, giving rise to
the excited triplet sensitizers.
• Then the photo-oxidation reactions could be divided into
three main pathways:
• In the first pathway, excited triplet sensitizers (3S*)
react with molecular oxygen (3O2) and produce singlet
oxygen (1O2) via a triplet-triplet annihilation mechanism .
• This is the most common mechanism for the production
of singlet oxygen.
• Then, the singlet oxygen can react directly with moieties
of high electron density of double bonds of unsaturated
fatty acids producing a hydroperoxide without the
formation of the alkyl radical.
S + (light)  3S*
3S* 3O2  1O2
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30.  Secondly , excited sensitizer can react with triplet
oxygen and produce superoxide radical anion (O2•−)
by electron transfer.
• This reactive oxygen species could abstract hydrogen
from unsaturated fatty acids and initiate the lipid
oxidation.
 3S* + 3O2  O2•−
 Thirdly, superoxide radical anion reacts with
hydrogen peroxide and produces both, hydroxyl
radical and singlet oxygen, which can react directly
with fatty acids and initiate lipid oxidation. This
reaction is catalysed by the presence of metals.
(H2O2 + O2•− → HO• + OH− + 1O2)
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31. • Finally, the excited triplet sensitizer can also abstract
hydrogen from an unsaturated fatty acid, resulting in
the production of alkyl radical.
• Then, this alkyl radical reacts with molecular oxygen
giving rise to a peroxy radical that can abstract
hydrogen from an adjacent fatty acid initiating the
free radical chain reactions mechanism, as described
above in the propagation phase .
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33. Enzymatic Lipid Oxidation
• Enzymatic oxidation refers to the oxidation
reaction involving enzymes, and there are two
kinds of enzymes involved in lipid oxidation,
namely lipoxygenase (LOX) and
hydroperoxidase.
• The oxidation process is divided into three steps:
• LOX protein has no oxidation effect on saturated
fatty acids (such as stearic acid),
monounsaturated fatty acids (such as oleic acid)
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34.  1. The dehydrogenation transfer of linoleic acid
by LOX to generates radicals, and the Fe3+ of
LOX is reduced to inactive Fe2+ state;
 2. Oxygen and radical oxidize to produce
peroxy-radical, which is accompanied by the
generation of O2
·–;
 3. The peroxy-radical is reduced by Fe2+ of LOX
to form hydroperoxide, and LOX is converted to
the active state Fe3+
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36. • Factors Affecting Lipid Oxidation
 Type of fatty acid, saturated or unsaturated.
 Storage condition
• Time
• Temperature
• Packaging and O2 concentration
• Metal ions
 Physical state of the material, emulsion, porosity,
surface.
 Water activity aw = 3 is a minimum
 Presence of other compounds, antioxidants
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38. What is an Antioxidant?
“Substance that prevents or delays oxidation.”
Anti-oxidants prevent or limit the actions of free
radicals usually by removing their unpaired
electron and thus converting them into something
far less reactive.
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39. • What is a “Free” Radical?
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Any atom (or atom within a
molecule) with at least one
unpaired electron in its
outermost shell/ orbital
Why do we want to limit their
actions?
•Highly reactive
•Free radicals damage
membranes (lipids), proteins,
& DNA
Paired
Electrons
Stable Molecule
Unpaired
Electron
Free Radical

40. How Antioxidants Reduce Free Radicals
ANTIOXIDANT FREE
RADICAL
Unpaired Electron
Electron Donation

41. • Antioxidant mechanisms of compounds that are
used to increase the oxidative stability of foods
include the control of free radicals, prooxidants,
and oxidation intermediates.
• Control of Free Radicals
• Many antioxidants slow lipid oxidation by
inactivating or scavenging free radicals, thereby
inhibiting initiation, propagation, and β-scission
reactions.
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42. • Free-radical scavengers (FRSs) or chain
breaking antioxidants can interact with peroxyl
(LOO∙) and alkoxyl (LO∙) radicals by the
following reactions.
• LOO∙ or LO∙ + FRS → LOOH or LOH + FRS∙
• Antioxidant efficiency is dependent on:
 the ability of the FRSs to donate hydrogen to a
free radical.
i.e. Any compound that has a reduction potential
lower than the reduction potential of a free radical
(or oxidized species) is capable of donating its
hydrogen to that free radical unless the reaction is
kinetically unfeasible.
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43. • For example, FRSs including α-tocopherol (E°ʹ =
500 mV), catechol (E°ʹ = 530 mV), and
ascorbate (E°ʹ = 282 mV) all have reduction
potentials below that of peroxyl radicals (E°ʹ =
1000 mV),
• Therefore capable of donating their hydrogen to
the peroxyl radical to form a hydroperoxide.
• i.e. Reduction potential is a measure of the
tendency of a chemical species to gain electrons
and undergo reduction.
mV - millivolts
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44. • The efficiency of the FRS is also dependent on
the energy of the resulting free radical
scavenger radical (FRS∙).
• Effective FRSs also produce radicals that do not
react rapidly with oxygen to form
hydroperoxides.
• Phenolic compounds possess many of the
properties of an efficient FRS.
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45. Most common FRSs in foods
 Tocopherols
 Synthetic Phenolics
 Plant Phenolics
 Ascorbic Acid and Thiols
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46. • Control of Prooxidants
• The rate at which lipids oxidize in foods is very
much dependent on prooxidant concentrations
and activity (e.g., transition metals, singlet
oxygen, and enzymes).
• Control of prooxidants is therefore a very
effective strategy to increase the oxidative
stability of foods.
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47. • Control of Prooxidant Metals
• Iron and copper are examples of important
prooxidant transition metals that accelerate lipid
oxidation by promoting hydroperoxide
decomposition.
• Chelators inhibit the activity of prooxidant metals
by one or more of the following properties:
 prevention of metal redox cycling;
 occupation of all metal coordination sites;
 formation of insoluble metal complexes;
 and/or steric hinderance of interactions between
metals and lipids or oxidation intermediates
(e.g., hydroperoxides)
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48. • The main metal chelators found in foods contain
multiple carboxylic acid (e.g., EDTA/ acid and
citric acid) or phosphate groups (e.g.,
polyphosphates and phytate).
 Ethylenediaminetetraacetic/EDTA
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49. • Control of Singlet Oxygen
• singlet oxygen is an excited state of oxygen that
can promote the formation of lipid
hydroperoxides.
• Carotenoids are a diverse group (>600 different
compounds) of yellow to red colored polyenes.
• The activity of singlet oxygen can be controlled
by carotenoids by both chemical and physical
quenching mechanisms.
• Singlet oxygen vs triplet oxygen? ?
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50. • Control of Lipoxygenases
• Lipoxygenases are active lipid oxidation
catalysts found in plants and some animal
tissues.
• Lipoxygenase activity can be controlled by heat
inactivation and plant-breeding programs that
decrease the concentration of these enzymes in
edible tissues.
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51. • Control of Oxidation Intermediates
• Compounds are found in foods that indirectly
influence lipid oxidation rates by interacting with
prooxidant metals or oxygen to form reactive
species.
• Examples of such compounds include
superoxide anion and hydroperoxides.
• Superoxide Anion
• Superoxide participates in oxidative reactions by
reducing transition metals to a more active state
or by promoting the release of iron bound to
protein.
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52. • Peroxides
• Peroxides are important intermediates of
oxidative reactions since they decompose via
transition metals, irradiation, and elevated
temperatures to form free radicals.
• Hydrogen peroxide exists in foods as a result of
direct addition (e.g., aseptic processing
operations) and formation in biological tissues
by mechanisms including the dismutation of
superoxide by SOD/superoxide dismutase and
the activity of peroxisomes and leukocytes.
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53. Quiz
1. What are three main ways of lipid oxidation ? 2
points
2. What are factors affecting oxidation ? 2 points
3. What is the role of isomerization in the initiation
stage of autoxidation ? 1 points
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54. Group Assignment
1. Polymerization of fat and oils
 Polymers are formed in fats and oils by processes as either thermal
polymerization or oxidative polymerization.
 Reaction ?
 Products ?
 Effect of products on food quality ?
2.Thermal oxidation reaction
 Mechanism ?
 Difference with autooxidation ?
 Examples of food processing thermal oxidation can occur ?
3. Artificial/synthetic antioxidant
 Why we need them ?
 What is their limitation ?
 Application on food products ? d
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55. Paper work submission: 06/06/16
Presentation by: 07/06/16
Group 1. eshetu, abdi and misgana
Group 2. yonas, jaber and mhiretu
Group 3. zerihun, bahir, tiringo and khali
Rearrangement of the groups is not allowed!
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