reactions of oils and fats

7/28/2019 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)

HO - C

O

- R1

3 fatty acids

+

C

O

- R3HO -

HO - C

O

- R2

OH

OH

OHglycerol

HC

H2C

H2C

O

C

C

OC

OO

O

O

- R1

triacylglycerol

= - R2

- R3

HC

H2C

H2C

+3H20


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Acid Value

Number of mgs of KOH required to neutralize theFree Fatty Acids in 1 g of fat.

AV =ml of KOH x N x 56

Weight of Sample= mg of KOH


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Oxidation of Oils and Fats

The reaction of molecular oxygen withorganic molecules has for long been aprocess of considerable interest.

Although a wide variety of organicmolecules are susceptible to chemicalattack by oxygen, a great deal of attentionhas recently been focused on lipidsbecause of the remarkable implications oftheir 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 oxidativedeterioration 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).

CO

- O -C

H H

+ H

C

O

- O - C

H

Represent as RHR

+H

(Eq.1)


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Initiation The removal of hydrogen takes place at the

carbon atom next to the double bond.

CO

- O -C

H H

+ H

C

O

- O - C

H

Represent as RH R + H (Eq.1)


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Formation of Lipid Radical

Hydrogens on carbons next to double bonds mosteasily removed (-carbon)

H - CH2 - CH2 - CH3

H - CH = CH2

H - CH2

- CH= CH

2

CH2 = CH - CH - CH= CH2

H

Energy forH removal

(kcal/mole)

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 ofunsaturated fatty acids.

Photooxidation energy from light is captured aided bysensitizermolecules (pigments: chlorophile)

Light excites these sensitizers to the triplet state thatpromotes oxidation by type I and type II mechanisms.


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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 (1

O2)metastable, excited energy state of O2

two unpaired electrons in same orbital

triplet oxygen

ground state

2 electrons w/ same

spin in 2 orbitals

singlet oxygen

excited state

2 electrons w/ different

spin in 1 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

RH +M+2 R+ H++M+


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Initiation mechanisms-Thermal Oxidation

The energy requirements for the abstraction of H toform a lipid radikal can be supplied in the form ofthermal energy.

High temperatures (like frying) facilitate the all stagesof the chain reaction

Initiation mechanisms-Enzymatic Oxidation

Enzyme-catalysed oxidation is initiated even in theabsence of hydroperoxides. This means the enzyme aloneis 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 lipidradical (Eq.3)

R + O2 ROO (Eq.2)

ROO + R1H ROOH+ R1 (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)

R1 + O2 R1OO (Eq.4)

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, orenzyme activity.

Transion metals such as Feand 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 bytermination if the free radicals react withthemselves 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

+

+

_

.+

.-

+

CH3 (CH2)3 CH2 CH CH CH CH CH CH2 COOH

CH3 (CH2)3 CH2 CH CH CH2 CH CH CH2 COOH

.

H

.


O

O

H

O

O



O

Initiation (metal)

Propagation

Propagation

.

O2

H

OH.HydroperoxideDecomposition

CH3 (CH2)3 CH2 H C CH CH CH CH CH2 COOH

CH3

(CH2)3

CH3

O

.

H.Termination

Pentane

14 13 12 11 10 9

12 11 10 9

12 11 10 9

12 11 10 9

12 11 10 9

12 11 10 9

n

n

n

- n

n

n


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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. Enzymes5. 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

Type of Fatty Acid

18:0

18:1D9

18:2D

9,1218:3D9,12,15

Rate of Reaction

Relative to Stearic Acid

1

100

12002500


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Kinetics of Autoxidation


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ANALYSIS OF OIL OXIDATION

1. Peroxide Value

KI CH3 C OH HI CH3 C OK

O O

ROOH HI I2 H2O ROH

I2 Na2S2O3 NaI Na2S4O6

A.

B.

C.

+

+

+

+

+

+

+

2

2

2

Peroxide Value = ml of Na2S2O3 N 1000

(milliequivalent peroxide/kg of sample) Grams of Oil


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2. 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 solutioncontaining 1.0 g oil in 100 ml of a mixture of solvent andreagent.

This method determines the amount of aldehyde(principally 2-alkenals and 2,4-alkadienals ) in animalfats and vegetable oils.

3. Totox Value = 2* PV + p-AnV

Aldehyde + p-AnV Yellowish Products

(Under acidic conditions)


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Active Oxygen Method (AOM)

Determined the time required to obtain certain peroxidevalue under specific experimental conditions.

The larger the AOM value, the better the flavor stability

of the oil.

Oil Stability Index / Rancimat Methods

Oxidative Stability of Oils and Fats

OSI and Rancimat measure the change in conductivitycaused 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 ofautoxidation.

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.

reactions of oils and fats

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