10. Antioxidants[1]

8/8/2019 10. Antioxidants[1]

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ANTIOXIDANT


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Antioxidants

The chemical compounds which can delay the start or slow

the rate of lipid oxidation reaction in food systems.


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14 13 12 11 10 9

- y H

CH2 CH CH CH2 CH CH CH2 R(CH2)3CH3

Initiation

13 12 11 10 9

y

+ 3O2

(C H2

)4

C H3

C H C H C H C H C H2

RC H

Mechanism of Antioxidant

Metal

Energy

Reactive oxygen species

Lipoxygenase

Substrate effect

Oxygen consumption,

Conjugated diene

Electron spin resonance

)!mv

K=109/sec


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y

(K= 10o M-1sec-1)

+ y H from RH (triglyceride)

(CH2)4CH3 CH CH CH CH CH2 RCH

O

O

Propagation

13 12 11 10 9

OH

C(CH3) 3

OCH3

y ,from

)0!1000mv

)0! 300-500mv

(K= 107 M-1sec-1)

R.

OC(CH3

)3

OCH3

.


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- y OH

CH CH CH CH CH CH2

R(CH2)

4CH

3

O

O

H

y

(C H2

)4

C H3 C H C H C H C H C H2 RC H

O

C H 3 (C H 2)4 C HO

13 12 11 10 9

13 12 11 10 9

Termination

Most reactive oxygen speciesPeroxide value

(C H2 )3C H3 C H3

)0!mv

Sensory evaluation

Volatile compounds

Transition Metal

)0!mv


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Antioxidant

O-2, 1O2, .OH, H2O2,

Cu, Fe. R, RO, ROO

R, RO,

ROO,1O2, O

-2,

-OH, H2O2,

Cu, Fe

Prooxidant Jail

Are you ready to fight the attack of prooxidants?


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Preventive Antioxidants

Superoxide dismutase

Catalase

Glutathione peroxidase

Singlet oxygen quencher

Transition metal chelators (EDTA)

Preventive antioxidants minimize the formation of

initiating radicals


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Gluthione

HC

COOH

H

CH2

O C N C C N CH2COOH

CH2

CH2

NH2

H O H

SH


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Radical Scavenging Antioxidant

Vitamin C

Tocopherol Quercetin

Anthocyanin

Radical scavenging antioxidants break free radical

chain reaction by donating hydrogen to free radicals


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Standard One-Electron Reduction PotentialCompounds Er (mV)

HO H+ / H2O 2310

RO H+ / ROH 1600

HOO. H+ / ROOH 1300

ROO H+ / ROOH 1000

R H+ / RH 600

Catechol H+ / Catechol 530

E- Tocopheroxyl H+ / E- Tocopherol 500

Ascorbate H+ / Ascorbate 282


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Resonance Stabilization of Antioxidant Radicals

OH

C(CH3)3

OCH3

C(CH3)3

OCH

3

O.

OCH

3

C(CH3)3

O

.

C(CH3)3

OCH3

O

.

OCH3

C(CH3)

O

.

R , RO , ROO RH , ROH , ROOH

E0=1000mv)0! 300-500mv (K= 107 M-1sec-1)

+


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Minimization of Lipid Oxidation

If a compound inhibits the formation of free alkyl radicals

in the initiation step, or if the chemical compound interrupts

the propagation of the free radical chain, the compound can

delay the start or slow the chemical reaction rate of lipid

oxidation.

The initiation of free radical formation can be delayed by

the use of metal chelating agents, singlet oxygen inhibitors,and peroxide stabilizers.

The propagation of free radical chain reaction can be

minimized by the donation of hydrogen from the

antioxidants and the metal chelating agents.


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Characteristics of Antioxidants

The major antioxidants currently used in foods are

monohydroxy or polyhydroxy phenol compounds with

various ring substitutions. These compounds have low

activation energy to donate hydrogen. The resulting

antioxidant free radical does not initiate another free

radical due to the stabilization of delocalization of

radical electron.

The resulting antioxidant free radical is not subject to

rapid oxidation due to its stability.

The antioxidant free radicals can also react with lipid

free radicals to form stable complex compounds


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Antioxidants

OH

C(CH3)3

OCH3

C(CH3)3(CH3)3C

CH3

OH

Butylated Hydroxy TolueneButylated Hydroxy Anisole


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OH

OHOH

COOC3H7

PropylGallate

OH

C(CH3

)3

OH

TBHQ

CH

CHO OH

OH

OH

CH3CH3

OH CHO

OH

CH

OH

CH3 CH3CH3 CH3

Gossypol

Antioxidants


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Mechanism of Antioxidants

Hydrogen donation to free radicals by antioxidants.

Formation of a complex between the lipid radical

and the antioxidant radical (free radical acceptor).


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Reaction of antioxidants with radicalsRy + AH RH + A

RO + AH ROH + A

ROO + AH ROOH + A

R + A RA

RO + A ROA

ROO + A ROOA

Antioxidant + O 2 Oxidized Antioxidant

y

y y

y

y

y y

y

yy

y


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Stable Resonance Formation of BHA

O H

C (C H3

)3

O C H3

Ry , RO y , orROO y

C (C H3

)3

O C H3

O

O C H3

C (C H3

)3

O

.

C (C H 3 )3

O C H 3

O

O C H 3

C ( C H 3 ) 3

O

RH, ROH +

or ROOH

.

.

.


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O

C H3

H2

O H

C H3

C H3

H2

( C H2

)3

C H ( C H2

)3

C H ( C H2

)3

C H ( C H3

)2

C H3

C H3

E-tocopherol

O 2

C H3

C H 3

C H3

OC H 2

O

C H 2C ( C H 2 ) 3C H ( C H 2) 3C H ( C H 3) 2

C H3

C H3

C H3

O H

E- tocoquinone

Tocopherol and Oxygen Reaction


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Mechanisms of Metals in Accelerating Lipid Oxidation

Hydroperoxide decomposition to form peroxy or alkoxy radical.

Fe3+

+ ROOH Fe2+

+ ROO y + H+

Fe2+

+ ROOH Fe3+

+ ROy

+ OH-

Formations of alkyl free radical by direct reaction with fats and oils.

Fe3+

+ RH Fe2+

+ Ry + H+

Activation of molecular oxygen for singlet oxygen formation.

Fe2+

+ O2 Fe3+

+ O-

21O2


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Kinds of Metal Chelators

Metal chelators deactivate trace metals that are free or salts of fatty

acids by the formation of complex ion or coordination compounds.

1. Phosphoric acid

2. Citric acid

3. Ascorbic acid

4. Ethylene-Diamine-Tetra-Acetate (EDTA)


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O

M

C

O

CH2

O C

O

CH2

NCH2

CH2

O

O N

CO

CH2

C OCH2

Metal Ions EDTA Complex Formation


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Synergism in Lipid Oxidation

Synergism occurs when mixtures of antioxidants

produce a more pronounced activity than the sum of

the activities of the individual antioxidants when

used separately.

To have maximum efficiency, primary antioxidants

are often used in combination with (1) otherphenolic antioxidants, or with (2) various metal

chelating agents.


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Factors Affecting the Efficiency of Antioxidant

1. Activation energy of antioxidants to

donate hydrogen should be low

2. Oxidation potential should be high

3. Reduction potential should be low

4. Stability to pH and processing.

5. Solubility in oil should be .


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Antioxidant Safety

Food Additive, Meat Inspection, and Poultry Inspection Acts.

Total concentration of authorized antioxidants added singly or in

combination, must not exceed 200 parts per million by weight

on the basis of fat content of the food.


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Long-Term Safety of Monomeric Antioxidants

Pathological effect.

Carcinogenic potential

Interactions with enzymes

Effects of reproduction

The exact nature of the metabolism rate in man.


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Isolation and Identification of Oxidation Product of

2,6-Di-(Tert-Butyl)-4-Methylphenol

H O C H 2 C H 2 O H

3,3' ,5,5'-Tetra-Bis-( Tert-Butyl)-4,4'-Dihydoxyl-1,2-Diphenylethane

C H C H

3,3',5,5'-Tetra-Bis-( Tert-Butyl)- Stillbenequinone

OO


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Ideal Antioxidants

No harmful physiological effects

Not contribute an objectionable flavor, odor, or color to the fat

Effective in low concentration

Fat-soluble

Carry-through effect No destruction during processing

Readily-availableEconomical

Not absorbable by the body


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Biochemical Control of Lipid Oxidation


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Biochemical Control of Lipid Oxidation in Mayonnaise

Composition of Mayonnaise Composition (%)

Soybean oil 77.0Whole egg 7.0Water 7.0Vinegar 3.0Egg yolk 2.0Glucose 1.0Fructose 1.0Salt 0.9Natural Flavor 0.1

100%


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Glucose oxidase/catalase Reaction Mechanism.

Glucose oxidase/catalase reaction:

2 Glucose + 2O2 + 2H2O 2 Gluconic acid + 2H 2O2

2H2O2 2H2O + O2

The net chemical reaction is:

2 Glucose + O2 2 Gluconic acid

Glucose Oxidase

Catalase

Glucose Oxidase

Catalase


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Kinds of Antioxidants

Natural antioxidants:

1.Tocopherols (delta>gamma>beta>alpha)2.Nordihydroguaretic Acid (NDGA)

3.Sesamol

4.Gossypol

Synthetic antioxidants:

1.Butylated Hydroxy Anisole (BHA)

2.Butylated Hydroxy Toluene (BHT)

3.Propyl Gallate (PG)

4.Tertiary Butyl Hydroquinone (TBHQ)


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Choices of Antioxidants

Different antioxidants show substantially different antioxidant effectiveness in

different fats and oils and food systems due to different molecular structures.

We should consider the following:

Safety

Antioxidant effectiveness

Off-odor

Off-color

Convenience of antioxidant incorporation to foodsCarry-through effect

Stability to pH and food processing

Availability

Cost

Non-adsorbable, if possible


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Antioxidants for Different Food Systems

A small surface-to-volume ratio PG and TBHQ

A large surface-to-volume ratio BHA and BHT


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Application of Antioxidants to Foods

Direct addition of antioxidants to oil or melted fat.

Addition of antioxidants to the food after they are

diluted in oil.

Spraying antioxidant solution in oil on the food or

dipping food into antioxidant solution.

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10. Antioxidants[1]

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