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Lipid Oxidation
The overall mechanism of lipid oxidation
consists of three phases:
(1) initiation, the formation of free radicals;
(2) propagation, the free-radical chain
reactions; and
(3) termination, the formation of nonradical
products.
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Lipid Oxidation
The important lipids involved in oxidation
are the unsaturated fatty acid moieties,
oleic, linoleic, and linolenic.
The rate of oxidation of these fatty acids
increases with the degree of unsaturation.
Oleic 1 times rate
Linoleic 10 times
Linolenic 100 times
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Lipid Oxidation
Initiation:RH + O2 -->R + OH
R + O2 --> + ROO
Propagation:
ROO + RH --> R + ROOH
ROOH--> RO + HO
Termination:
R + R --> RR
R + ROO--> ROOR
ROO + ROO --> ROOR + O2
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Lipid Oxidation
Where RH is any unsaturated fatty acid; R is a free radicalformed by removing a labile
hydrogen from a carbon atom adjacent to a doublebond;
ROOH is a hydroperoxide, one of the majorinitial oxidation products that decompose to formcompounds responsible for off-flavors andodors. Such secondary products include hexanal, pentanal,
and malonaldehyde.
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Lipid Oxidation of a Monoenoic Acid
oleic acid as an example, a hydrogen could be
removed from either C-8 or C-10, as these positions
are located alpha to the double bond.
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Lipid Oxidation of a Monoenoic Acid
Using oleic acid as an example, a hydrogen could be removed fromeither C-8 or C-10, as these positions are located alpha to the doublebond. Abstraction from carbon 8 results in the two radicals A and Bwhich are positional isomers of each other stabilized by resonance
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Lipid Oxidation of a Monoenoic Acid
Or abstraction from carbon 11 can occur, resulting in the two radicals
C and D:
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Lipid Oxidation of a Monoenoic Acid
Oxygen can be added to each radical to form peroxyradicals at C-8, C-9, C-10 or C-11. Addition to the 8 and10 positions yield the peroxy radicals shown above
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Lipid Oxidation of a Monoenoic Acid
These radicals may abstract hydrogens from other
molecules to yield the hydroperoxides shown
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Lipid Oxidation of a Monoenoic Acid
The addition oxygen at the 11 and 9 positions
results in the peroxy radicals
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Lipid Oxidation of a Monoenoic Acid
The subsequent addition of abstracted hydrogen
molecules results in the hydroperoxides shown
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Lipid Oxidation of dienoic acid
The situation with a dienoic acid is a little different. Whilethere are more positions a to a double bond, there is oneposition that is at two double bonds. This position is veryreactive. For linoleic acid, carbon 11 is at two doublebonds and will be removed to yield the free radical
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Lipid Oxidation of dienoic acid
There are two possible resonant structures that can result from this radical. Theradical may shift to carbon 14 with the double bond reforming between carbons11 and 12. The radical may also shift to carbon 9 with the double bond formingbetween carbons 10 and 11. Both of these cases result in conjugated structuresthat are at lower energies than are the non conjugated structures they werederived from. For this reason, the oxidation of linoleic acid yields approximatelyequal amounts of the C 13 and C 9 radical with only traces of the original C 11
radical present. The resonant structures formed are shown
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Lipid Oxidation
Once formed, hydroperoxides may break
down through a number of mechanisms. A
common breakdown scheme is called
dismutation. In this reaction ahydroperoxide reacts with another
molecule or radical to form two new
compounds.
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Lipid Oxidation
This reaction scheme is capable of generating aldehydes, ketones,alcohols and hydrocarbons. Many of the volatile compounds formedduring lipid oxidation originate through similar dismutations.
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Lipid Oxidation
Hydroperoxides are not stable compounds and given time, they willbreak down. A typical mechanism, as shown below, results in theformation of two radicals from a single hydroperoxide molecule.
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Lipid Oxidation
Both of these new radicals can initiate further oxidation.
Some metals can speed up this reaction.
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Lipid Oxidation
Note that both ions and free radicals were formed. The net reaction is
shown above
Copper was the catalyst. Copper did not initiate the reaction, but oncethe hydroperoxides were formed, it sped up their breakdown.
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Lipid Oxidation
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Lipid Oxidation
Since the reaction RH + O2 free
radicals, is thermodynamically difficult
(activation energy of about 35 kcal/mol),
the production of the first few radicalsnecessary to start the propagation reaction
normally must occur by some catalytic
means such as hydroperoxidedecomposition, light and heat exposure
and metal catalysis.
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Measurement of lipid oxidation
Perox ide value
Peroxides are the main initial products of autoxidation.They can be measured by techniques based on theirability to liberate iodine from potassium iodide, or to
oxidize ferrous to ferric ions. Their content is usuallyexpressed in terms of milliequivalents of oxygen perkilogram of fat. Although the peroxide value is applicablefor following peroxide formation at the early stages ofoxidation, it is, nevertheless, highly empirical. The
accuracy is questionable, the results vary with details ofthe procedure used, and the test is extremely sensitiveto temperature changes. During the course of oxidation,peroxide values reach a peak and then decline.
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Measurement of lipid oxidation
Thiobarbi tur ic acid (TBA )
TBA is the most widely used test for measuring the extent of lipidperoxidation in foods due to its simplicity and because its results arehighly correlated with sensory evaluation scores.
The basic principle of the method is the reaction of one molecule ofmalonaldehyde and two molecules of TBA to form a redmalonaldehyde-TBA complex, which can be quantitatedspectrophotometrically (530nm).
However, this method has been criticized as being nonspecific andinsensitive for the detection of low levels of malonaldehyde. Other
TBA-reactive substances (TBARS) including sugars and otheraldehydes could interfere with the malonaldehyde-TBA reaction.Abnormally low values may result if some of the malonaldehydereacts with proteins in an oxidizing system. In many cases, however,the TBA test is applicable for comparing samples of a single materialat different states of oxidation.
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Measurement of lipid oxidation
TBA - Thiobarbituric Acid
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Measurement of lipid oxidation
Iod ine Value
Iodine value is a measure of the
unsaturated linkages in fat and is
expressed in terms of percentage of iodine
absorbed. The decline in iodine value is
sometimes used to monitor the reduction
of dienoic acids during the course of theautoxidation.
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Measurement of lipid oxidation
Act ive Oxygen Method (AOM)
Iodine value or Peroxide Value is
measured over time as Oxygen is bubbled
through an oil sample
This method is also used to evaluate
antioxidants
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Antioxidants
Antioxidants function by
interfering with the chain
reaction. If the number of
free radicals can be kept
low enough, oxidation willnot occur. The following
is a model for the type of
compound that can
function effectively as anantioxidant:
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Antioxidants
In order to function well as an antioxidant a molecule must:
React with free radicals more rapidly than the free radicals react with lipid.The products of the reaction with free radicals must not be pro-oxidant.The molecule must be lipid soluble.
The free radicals formed by conjugated molecules can exist in many resonantstructures as shown below:
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Alternatives to Antioxidants
Elimination of oxygen Packaging under nitrogen;
packaging in vacuum;
packaging with an oxygen scavenger
Elimination of the sensitive substrate Replacement of polyunsaturated oils with less unsaturated oils,
such as olive oil or palm oil, that are more stable
Decreasing the rate of oxidation Storage at low temperatures;
storage in the dark; use of fats and oils that contain low levels of oxidation promoters
(eg. oxidized products and heavy metals);
use of ingredients that are naturally rich in antioxidants
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Antioxidants
BHA BHT
TBHQ
Propyl Gallate
Ascorbic Acid
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Antioxidants BHA
Butylated hydroxy anisole is a mixture of two isomers. Referred to as a 'hindered
phenol' because of the proximity of the tertiary butyl group to the hydroxyl group.This may hinder the effectiveness in vegetable oils, but increase the 'carrythrough' potency for which BHA is known.
Uses: Lard, shortenings, vegetable oils, cereals, package liners, potato products,dry soups, chewing gum, etc. Usually in combination with other primaryantioxidants.
Propyl Gallate Three hydroxyl groups make it very reactive. Lower solubility. Tend to chelate
trace minerals such as iron and form colored complexes. Are heat labile,especially under alkaline conditions.
Uses: Lard, shortening, vegetable oils, cereals, package liners, animal feeds, etc.Used alone and in combination with BHA or PG and citric acid.
BHT Butylated hydroxy toluene is also a 'sterically hindered' phenol Susceptible to
loss through volatilization in high temperature applications.
Uses: Lard, shortening, vegetable oils, cereals, animal feeds, etc. Usually usedin combination with BHA or BHT and citric acid.
TBHQ Tertiary-butylatedhydroquinone is an extremely potent antioxidant. Had been
used extensively in non food applications prior to gaining approval in food.
Uses: Lard, cottonseed oil, potato chips, corn flakes
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Antioxidants
Combinations Antioxidants are usually combined to take advantage of their
differing properties.
For example BHA may be combined with PG and citric acid. Thecitrate chelates metals, the propyl gallate provides a high level of
initial protection while the BHA has good carry throughproperties.
Reasons for Combinations Take advantage of different properties
Allow for better control and accuracy
May provide synergistic effects
Combinations may provide more complete distribution in somefoods
More convenient to handle
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Antioxidants
Treatment Pastry Cracker
Control 2 3
.005 TBHQ 2 7
.001 TBHQ 3 10
.020 TBHQ 4 5.005 BHA 8 12
.010 BHA 21 22
.020 BHA 27 33
.005 BHT 5 10
.010 BHT 10 14
.020 BHT 19 21
.005 PG 2 3
.010 PG 5 6
.020 PG 3 11
Stability of
Bakery Products
(AOM
Days of stability)
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Antioxidants
Uses of Antioxidants
Fats and oils (less effective in higher polyunsaturates)
Foods made with fats (potato chips, nuts, candies, pre-mixes, frozen pies)
Foods with fatty constituents (peppers, other spices,
cereals, dehydrated vegetables, citrus oils, chewing gum)
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Antioxidants
Natural Antioxidants
Should not cause off flavors or colors
Must be lipid soluble Must be non toxic
Should have carry through properties
Must be cost-effective
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Natural Antioxidants
Rosmariquinone
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Natural Antioxidants
SesameContains sesamol.
Reported to be more
effective in lard than
BHA or BHT.
OatsOats have been recognized
to have antioxidant
properties. Over 25 phenoliccompounds have been
identified in oats. Many
derived from caffeic and
ferulic acid.
Sesamol
H d ti
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Hydrogenation
Treatment of an oil with hydrogen and a
suitable catalyst to decrease the number
of double bonds and increase the degree
of saturation
H d ti
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Hydrogenation
Rate is determined by:
Nature of substrate
Type and concentration of catalyst
Pressure (Concentration of hydrogen)
Temperature
Agitation
H d ti
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Hydrogenation
Stages in Hydrogenation
Transfer and/or diffusion
Adsorption
Hydrogenation/Isomerization
Desorption
Transfer
H d ti
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Hydrogenation
Transfer and adsorption are critical steps
in controlling the degree of isomerization
and selectivity of the reaction.
Transfer of reactants and products to and
from the bulk liquid oil phase and the
surface of the catalyst.
H d ti
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Hydrogenation
Diffusion
Diffusion of reactants into pores on the
catalyst surface. Diffusion of products out ofthe catalyst surface pores.
H d ti
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Hydrogenation
Selectivity
Define selectivity as the ratio of the rate of
hydrogenation of linoleic acid to that of oleicacid.
Commonly observed selectivities range for 4to 50.
This would mean linoleic acid is hydrogenated4 to 50 times faster than oleic acid
Desire highly selective catalysts. Why?
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Characteristics of some food lipids
Lipid Iodine Value % Saturated % Oleic % Linoleic
Olio Oil 46.8 47.6 50.1 2.3
Butter Oil 39.5 57.8 38.3 3.9
Chicken Fat 86.5 23.4 52.9 23.7
Cocoa Butter 36.6 60.1 37.0 2.0
Corn Oil 127.0 8.8 35.5 55.7
Cotton Seed 106.0 26.7 25.7 47.5
Lard 66.5 37.7 49.4 12.3
Olive Oil 89.7 2.9 89.5 7.6Palm Oil 53.6 47.3 42.9 9.8
Peanut oil 93.0 17.7 65.5 25.8
Safflower Oil 144.0 5.7 21.7 72.6
Soybean Oil 136.0 14.0 22.9 55.2
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Hydrogenation
Rate of oxidation of fatty acids, their esters and triglycerides.
Acid Methyl Ester Triglyceride
Oleic 1 1 1
Linoleic 27 30 27
Linolenic 77 87 97
Arachidonic 114
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Hydrogenation
Before After
Unsaturated Saturated
Liquid Solid
Cis Cis/Trans
Effects of Hydrogenation
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Hydrogenation
* is point of catalyst link
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Hydrogenation
* is point of catalyst link
Obtain both cis and trans isomers
H
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Hydrogenation
Parameter SelectivityFormation of
Trans bonds
Reaction
Rate
Correlation Direction
Temperature Positive Positive Positive
Pressure Negative Negative Positive
Concentration Positive Positive Positive
Agitation Negative Negative Positive
The effects of processing conditions on hydrogenation
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Hydrogenation
The effects of hydrogenation include:
IsomerizationTemperature
D 9 cis 13.4 C
D 9 trans 44 C
D 12 cis 9.8 C
D 12 Trans 40 C
Hydrogenation
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Hydrogenation
Method
Oil is heated with catalyst (Ni), heated to
the desired temperature (140-225C), thenexposed to hydrogen at pressures of up to
60 psig and agitated.
An example of heterogeneous catalysis.
Hydrogenation Conditions
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Hydrogenation - Conditions
Starting oil must be: Refined
Bleached
Low in soap
Dry
The catalysts must be: Dry
Free of CO2 and NH4
Hydrogenation
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Hydrogenation
Heterogeneous Catalysts
Most commonly utilized
Catalysts and reactants exists in different
physical states
Hydrogenation reaction takes place on
surface of catalyst
Nickel containing catalysts are mostfrequently utilized
Hydrogenation
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Hydrogenation
Nickel Catalysts Typical Ni catalyst is usually reduced Ni
dispersed in the absence of air into hardened fatto stabilize it. In such systems, the support plays
an essential role in determining the specificreactivity of the catalyst.
Advantages of Nickel Availability
Low Cost
Inert nature of metal to the oil
Hydrogenation
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Hydrogenation
Hydrogenation Limitations
Selectivity is never absolute
Little preference for C18:3 over C18:2
Important amounts of trans acids are formed
Selectivity and isomerization are linked
Hydrogenation
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Hydrogenation
Isomerization
An equilibrium will be established betweenpositional and geometric isomers in the
mixture. Double bonds that are reformed tend to
have a trans/cis ration of 2:1. All transwould be expected if there were no stericconsiderations.
Hydrogenation
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Hydrogenation
Isomerization
Purposes
Convert liquid fats to plastic fats
Improve oxidative stability
Covert soft fats to firmer fats
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Frying Mass Transfer
Water in a frying food migrates from the center to the surface. Aswater is removed at the surface due to heating, water is 'pumped' tothe surface. The rate of water loss and its ease of migration throughthe product are important to the final characteristics of the food.
Heat Transfer
Water evaporation from the surface of a frying food also removesheat from the surface and inhibits charring or burning at the surface.The heat of vaporization of water to steam removes much of theheat at the food/oil surface.
Heat Removal
As long as water is being removed at a sufficient rate, the surface ofthe food will not char. Subsurface water in the food will also conductheat away from the surface and towards the center of the product.
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Frying Interior Cooking
The transfer of heat to the interior of the product by water will resultin cooking of the interior of the food. Want enough heat to 'cook' theproduct, but not enough to cause damage - example -French fry
Oil - Food Interactions
Ideally the food products should have similar dimensions and thus,similar surface to volume ratios. Once an equilibrium is establishedall processes should be the same unless there are changes inequipment function or in oil composition.
Oil
The properties of oil change with frying. New oil has a high heatcapacity that diminishes with use. Other factors such as viscositymay change dramatically with use
F i St f il
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Frying - Stages of oil
Break in oil.White product, raw, ungelatinatized starch at center of fry; no
cooked odors, no crisping of the surface, little oil pickup by the food.
Fresh OilSlight browning at edges of fry; partially cooked (gelatinization)centers; crisping of the surface; slightly more oil absorption.
Optimum OilGolden brown color; crisp, rigid surface; delicious potato and oilodors; fully cooked centers (rigid, ringing gel); optimal oil absorption.
Degrading OilDarkened and/or spotty surfaces; excess oil pickup; product moving
towards limpness; case hardened surfaces.
Runaway OilDark, case hardened surfaces; excessively oily product; surfacescollapsing inward; centers not fully cooked; off-odor and flavors(burned).
F i Q lit f il
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Frying - Quality of oil
Indicators of frying oil quality:
Total polar compounds
Conjugated dienes
FFA
Dielectric constant
Color
pH