Reactions of Oils and Fats

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Reactions of Oils and Fats
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Reactions of Oils and Fats

• Hydrolysis
• Oxidation
• Hydrogenation
• Esterification

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Hydrolysis

• Chemical (Autocatalytic)
• Enzymatical (Lipase)

3H20
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Acid Value
Number of mgs of KOH required to neutralize the
Free Fatty Acids in 1 g of fat.
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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.

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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)

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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

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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.

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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
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Initiation

• The removal of hydrogen takes place at the carbon
  atom next to the double bond.

H
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Formation of Lipid Radical

• Hydrogens on carbons next to double bonds most
  easily removed (?-carbon)

100 103 85 65

• H on carbon next to double bond easier to remove

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Initiation mechanisms

• Photosynthesized Oxidation (Photooxdation)
• Metal Catalysis
• Thermal Oxidation
• Enzymatic Oxidation

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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.

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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
• In type II photooxidation, the energy of the
  triplet sensitizer is transferred to molecular
  oxygen, converting it to its excited singlet
  state.

sens sens sens RH R H
light
sens 3O2 sens 1O2
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Initiation mechanisms-PO

• Singlet oxygen more reactive than triplet oxygen
• RH 1O2 ROOH RO OH
• RO provides free radical to start propagation
• Initiated by singlet oxygen (1O2)
• metastable, excited energy state of O2
• two unpaired electrons in same orbital

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Initiation mechanisms-Metal Catalysis

• Metal ions (e.g. Fe, Co, Cu) can also initiate
  reaction
• found naturally in foods, from metal equipment

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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

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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)
• 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)

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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)
• Hydroperoxides (Eq.3) are unstable and can
  degrade to produce radicals that further
  accelerate propagation reactions (Eq.5) and
  (Eq.6)

ROOH RO OH (Eq.5)
2ROOH ROO RO H2O (Eq.6)
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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)
2ROOH ROO RO H2O (Eq.6)
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Termination

• The propagation can be followed by termination if
  the free radicals react with themselves to yield
  non-reactive (stable) products, as shown here
• Carbonyl compounds (aldehydes and ketones)and
  hydrocarbons

R R RR RO R ROR ROO R
ROOR ROO ROO ROOR O2
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Pentane Formation from Linolenic Acid
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Oxidation Product

• Primary Oxidation Products
• Hydroperoxides
• Secondary Oxidation Products
• Aldehydes and ketones

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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

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Oxidation Rates Types of Fatty Acids

• As of double bonds increases
• and stability of radicals increases
• Rate increases

Rate of Reaction Relative to Stearic
Acid 1 100 1200 2500
Type of Fatty Acid 180 181D9
182D9,12 183D9,12,15
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Kinetics of Autoxidation
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ANALYSIS OF OIL OXIDATION
1. Peroxide Value
Peroxide Value ml of Na2S2O3 ? N ?
1000 (milliequivalent peroxide/kg of sample)
Grams of Oil
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• 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
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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.

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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.
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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.

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Primary Antioxidants

• R AH RH A
• RO A ROA
• ROO AH ROOH RH

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Natural and Synthetic Antioxidants
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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 oxygens excess energy and
  dissipate the the energy in the form of heat.