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Thurs April 9
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12
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11
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10
4
3
Tues April 14
1
5
2
Tues April 14
2
6
1
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7
13
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8
12
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8
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Tues April 21
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10
Lipids
Saturated Fatty Acids
8
7
CH3 CH2
O
5
3
4
6
2
1
CH2 CH2 CH2 CH2 CH2 C OH
Octanoic Acid
Unsaturated Fatty Acids
8
CH3
7
CH2
5
6
CH2 CH2
4
CH2
O
3
2
1
CH2 CH2 C
OH
3 - Octenoic Acid
8
7
CH3 CH2
O
5
3
4
6
2
1
CH2 CH2 CH2 CH2 CH2 C OH
3, 6 - Octadienoic Acid
Fatty Acids
Melting Points and Solubility in Water
Melting Point
z
Solubility in H2O 2
Fatty acid chain length
Characteristics of Fatty Acids
Fatty Acids
M.P.(C)
mg/100 ml
in H2O
C4
-8
C6
-4
970
C8
16
75
C10
31
6
C12
44
0.55
C14
54
0.18
C16
63
0.08
C18
70
0.04
Where Do We Get Fats and Oils?





Derived from plant and animal sources
Several commercial processes exist to extract food grade oils
Most are refined prior to use
During oil refining, water, carbohydrates, proteins, pigments,
phospholipids, and free fatty acids are removed.
In general, fat and oil undergo four processing steps:





Extraction
Neutralization
Bleaching
Deodorization
Oilseeds, nuts, olives, beef tallow, fish skins, etc.

Rendering, mechanical pressing, and solvent extraction.
Lipid Oxidation
Effects of Lipid Oxidation

Flavor and Quality Loss





Nutritional Quality Loss



Rancid flavor
Alteration of color and texture
Decreased consumer acceptance
Financial loss
Oxidation of essential fatty acids
Loss of fat-soluble vitamins
Health Risks


Development of potentially toxic compounds
Development of coronary heart disease
LIPID OXIDATION and Antioxidants




Fats are susceptible to hydrolyis (heat, acid, or lipase enzymes)
as well as oxidation. In each case, the end result can be
RANCIDITY.
For oxidative rancidity to occur, molecular oxygen from the
environment must interact with UNSATURATED fatty acids in
a food.
The product is called a peroxide radical, which can combine with
H to produce a hydroperoxide radical.
The chemical process of oxidative rancidity involves a series of
steps, typically referred to as:



Initiation
Propagation
Termination
Simplified scheme of lipoxidation
H H H H
H H H H
H H H H
R C C C C R
R C C C C R
R C C C C R
H
H
+ Catalyst
H
*
+ Oxygen
H
O
O
Initiation of Lipid Oxidation

There must be a catalytic event that causes the initiation of
the oxidative process


Enzyme catalyzed
“Auto-oxidation”

Excited oxygen states (i.e singlet oxygen): 1O2









Triplet oxygen (ground state) has 2 unpaired electrons in the same spin in
different orbitals.
Singlet oxygen (excited state) has 2 unpaired electrons of opposite spin in the
same orbital.
Metal ion induced (iron, copper, etc)
Light
Heat
Free radicals
Pro-oxidants
Chlorophyll
Water activity
Considerations for Lipid Oxidation
 Which
hydrogen will be lost from an unsaturated
fatty acid?
 The longer the chain and the more double
bonds….the lower the energy needed.
Oleic acid
Radical Damage,
Hydrogen
Abstraction
Formation of a
Peroxyl Radical
Propagation Reactions
Initiation
Ground state oxygen
Hydroperoxide
decomposition
Peroxyl radical
Start all over again…
Hydroperoxide
New
Radical
Hydroxyl radical!!
Secondary Products: Aldehydes
Mechanism of Photooxidation
Chlorophyll
3O
1
or
2
Singlet Oxygen Oxidation
1
O2
HOOC
OOH
HOOC
OOH
HOOC
OOH
HOOC
OOH
HOOC
Autoxidation
9
HOOC
10
12
13
11
8
14
H
H
9
10
12
+
13
11
8
9
10
12
14
13
10
9
11
+
8
12
13
11
14
8
14
O2
OO
10
9
8
12
13
11
14
Singlet Oxygen Oxidation
1
O2
HOOC
HOOC
OOH
OH
O
Nonanal
Chemical Tests for Oxidation
Lipid Oxidation
Hydrolysis
Peroxide Value
Oxidation Tests
LIPID OXIDATION
Reactants and Products
35
Lipid System Under
Oxidizing Conditions
30
25
Oxygen Uptake
20
Peroxides
15
Secondary Products
10
5
0
1
2
3
4
5
Time
6
7
8
9
Free Fatty Acids (FFA’s)


Degree of hydrolysis (hydrolytic rancidity)
High level of FFA means a poorly refined fat
or fat breakdown after storage or use.
Peroxide Value
l
Measures peroxides and hydroperoxides in an
oil which are the primary oxidation products
(usually the first things formed).
l
The peroxide value measures the “present
status of the oil”. Since peroxides are
destroyed by heat and other oxidative
reactions, a seriously degraded oil could have
a low PV.
Peroxide Value
KI + peroxyl radical yields free Iodine (I2)
l The iodine released from the reaction is measured in the
same way as an iodine value.
l I2 in the presence of amylose is blue.
l I2 is reduced to KI and the endpoint determined by loss of
blue color.
4I + O2 + 4H
2I2 + 2H2O
Thiobarbituric acid (reactive substances) TBA OR TBARS
Tests for end products of oxidation – aldehydes, Malonaldehyde
(primary compound), alkenals, and 2,4-dienals
lA pink pigment is formed and measured at ~530 nm.
TBARS is firmly entrenched in meat oxidation research and is a
method of choice.
TBARS measure compounds that are volatile and may react further
with proteins or related compounds.
High TBA = High Oxidative Rancidity
HEXANAL Determination
l
Good indictor of the end products of oxidation
(if there are any).
l
l
Standard method in many industries.
Aldehyde formation from lipid oxidation.
l
Nonenal is also a common end-product
Conjugated Fatty Acids
During oxidation, double bond migration
occurs and conjugated fatty acids are
formed.
They absorb light efficiently and can be
monitored in a spectrophotometer.
R C C C C C C C C R
TECHNIQUES OF MEASURING OXIDATIVE
STABILITY
Induction Period: is defined as the length of time
before detectable rancidity or time before rapid
acceleration of lipid oxidation
Iodine Value
 Measure
of the degree of unsaturation in an oil
or the number of double bonds in relation to
the amount of lipid present
 Defined as the grams of iodine absorbed per
100-g of sample.
 The
higher the amount of unsaturation, the
more iodine is absorbed.
 Therefore the higher the iodine value, the
greater the degree of unsaturation.
Iodine Value
A
known solution of KI is used to reduce excess ICl
(or IBr) to free iodine
R-C-C = C-C-R + ICl  R-C-CI - CCl-C-R + ICl
[Excess]
(remaining)
scheme: ICl + 2KI  KCl + KI + I2
 The liberated iodine is then titrated with a
standardized solution of sodium thiosulfate using a
starch indicator
 I2 + Starch + thiosulfate = colorless endpoint
 Reaction
(Blue colored)
Iodine Value
Used to characterize oils:

Following hydrogenation
 During oil refining (edible oils)
 Degree of oxidation (unsaturation decreases
during oxidation)
 Comparison of oils
 Quality control
Iodine value: g absorbed I2/ 100 g fat
Iodine Value of some oils (Table 14-2)
Source
Beef Tallow
Olive, Palm,
Peanut
Corn, Cottonseed
I2 Value
<50
< 100
100 - 130
Highly saturated
High in 18:1
High in 18:1 and 18:2)
Linseed, Soybean,
safflower, conola
> 130
18:1, 18:2, 18:3
Fish
>150
18:1, 18:2, 18:3
(longer chains)
Chemical Tests
Saponification Value
Saponification Value
Saponification is the process of breaking down or
degrading a neutral fat into glycerol and fatty acids
by treating the sample with alkali.
Heat
Triacylglyceride ---> Fatty acids + Glycerol
KOH
Definition: mg KOH required to titrate 1g fat
(amount of alkali needed to saponify a given amount of fat)
Typical values: Peanut = 190, Butterfat = 220
Lipid Oxidation
Primary Drivers
 Temperature-basic
 Water
rxn kinetics
Activity
 Both
high and low Aw
 At low Aw, peroxides decompose faster and metal
ions are better catalysts in a dry environment
 Metal
Ions-catalysts
 Light-energy source
 Singlet Oxygen- ROS, highly electrophilic
 Reacts
1,500 times faster at C=C than ground state O2
 Enzymes
ie. Lipoxygenase (LOX)
Implications to food products
A



major cause of quality deterioration
Develop rancidity in raw or fatty tissues
Produces WOF in cooked meats
Oxidized flavors in oils
 Loss
of functional properties
 Loss of nutritional values
 Formation of toxic compounds
 Forms colored products
Production of toxic compounds
 Many
secondary by-products of lipid oxidation
are potential carcinogens
 Hydroperoxides
are known to damage DNA
 Carbonyl
compounds may affect cellular signal
transduction
 Aldehydes:
 Epoxides
4-OH-nonenal and malondialdehyde
and hydrogen peroxide by-products are
known carcinogens
Lipid Oxidation
Factors affecting the development of
lipid oxidation in foods
 Fatty
acid compositions
 Oxygen, free radicals
 Pro-oxidants
 Antioxidants and additives
 Processing conditions of food




Irradiation
Cooking
Grinding, cutting, mixing, restructuring etc.
Packaging
 Storage:
time and conditions
Bond Energy and Lipid Oxidation
Types of Fatty Acids and Oxidation Rates

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
Lipid Modifications
Hydrogenation
 Method
Oil is heated with catalyst (Ni), heated to
the desired temperature (140-225°C), then
exposed to hydrogen at pressures of up to
60 psig and agitated.
Hydrogenation - Conditions

Starting oil must be:





Refined
Bleached
Low in soap
Dry
The catalysts must be:


Dry
Free of CO2 and NH4
Hydrogenation
 Hydrogenation Limitations
 Selectivity
is never absolute
 Little preference for C18:3 over C18:2
 trans-fatty acids acids may be formed
Altering Fats for Oxidative Stability
 Blending
solid and liquid fats/oils
 Hydrogenation (full or partial)
 Random inter-esterification to change sn
positions
 Natural
re-arrangement
 Addition of desired fatty acids
 Targeted
 1,3
inter-esterifications
lipases for a 1, 3 inter-esterification
Interesterification
 Exchanging
positions from one glyceride to
another to alter chemical composition and
physical properties
sn-1
O
sn-2
P
sn-3
S
P
O
S
S
O
P
P
S
O
Cocoa butter
 Palmitic,
 95%
stearic, and oleic acids
of the fat
 Predominant sn
positions:
 sn-2
oleic acid
 sn-1 or 3 palmitic or stearic acid
 POS:
40%
 POP: 15%
 SOS: 27.5%
sn-1
P
sn2
O
sn-3
S
Enzymatic Interesterification
 Lipase
catalyzed
 Lipozyme (immobilized)
 Selective FA-interchange on sn-1, 3
positions
The Polar Paradox Theory
 In
bulk oils, with water and
phospholipids…
 Polar antioxidants are more effective in
non-polar or less polar systems
 Non-polar antioxidants are more effective in
polar systems
 Due to an “interfacial phenomenon”?
Polar Antioxidants

Most effective in
nonpolar or less polar
environment


Bulk oils
Located at the oil-air
interface or in reverse
micelles

High amount of
oxidants present here
Yellow Oil
Blue water
Phospholipids
Non-polar Antioxidants

Most effective in polar
environment


Oil-in-water emulsions
Located at the water-oil
interface


Dissolved in oil droplets
of the emulsion
Allows access to
oxidizing agents located
in the water phase


Peroxides
Oxidizing metals