ANTIOXIDANT

Antioxidants

The chemical compounds which can delay the start or slow the rate of lipid oxidation reaction in food systems.

14 13 12 11 10 9

- H

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

Initiation

13 12 11 10 9

+ 3O2

(C H2)4

C H3 C H C H C H C H C H

2RC H

Mechanism of Antioxidant

Metal

Energy

Reactive oxygen species

Lipoxygenase

Substrate effect

Oxygen consumption,

Conjugated diene

Electron spin resonance

mv

K=109/sec

(K= 10o M-1sec-1) + H from RH (triglyceride)

(CH2)4CH3 CH CH CH CH CH2 RCH

O

O

Propagation

13 12 11 10 9

OHC(CH3)

3

OCH3

from

01000mv

0 300-500mv

(K= 107 M-1sec-1)

R.

OC(CH3)

3

OCH3

.

- OH

CH CH CH CH CH CH2 R(CH2)4CH3

O

O

H

(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

)3

C H3 C H3

0mv

Sensory evaluation

Volatile compounds

Transition Metal

0mv

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?

Preventive Antioxidants

Superoxide dismutase Catalase Glutathione peroxidase Singlet oxygen quencher Transition metal chelators (EDTA)

Preventive antioxidants minimize the formation of initiating radicals

2O2·- H2O2 O2 + H2O

2GSH GSSG + 2H2O

NADP+ NADPH + H+

Superoxide dismutase

2H+

Catalase

Glutathione Oxidase

Glutathione Reductase

NADP+ Reductase

Superoxide dismutase

Gluthione

HC

COOH

H

CH2

O C N C C N CH2COOH

CH2

CH2

NH2

H O H

SH

Singlet Oxygen Quenching Mechanism of Carotenes

1O2 + 1-CAROTENE 3O2 + 3-CAROTENE

3 -CAROTENE 1 -CAROTENERADIATIONLESS

Prooxidant Activities of Transition Metals

Hydroperoxide decomposition to form peroxy or alkoxy radical.

Fe3+ + ROOH Fe2+ + ROO + H+

Fe2+ + ROOH Fe3+ + RO + OH-

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

Fe3+ + RH Fe2+ + R + H+

Activation of molecular oxygen for singlet oxygen formation.

Fe2+ + O2 Fe3+ + O-2 1O2

Radical Scavenging Antioxidant

• Vitamin C • Tocopherol• Quercetin• Anthocyanin

Radical scavenging antioxidants break free radical chain reaction by donating hydrogen to free radicals

Standard One-Electron Reduction Potential

Compounds E (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

- Tocopheroxyl· H+ / - Tocopherol 500

Ascorbate· H+ / Ascorbate 282

Resonance Stabilization of Antioxidant RadicalsOH

C(CH3)3

OCH 3

C(CH3)3

OCH3

O.

OCH3

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)

+

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.

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

Antioxidants

OH

C(CH3)3

OCH3

C(CH3)3(CH3)3C

CH3

OH

Butylated Hydroxy TolueneButylated Hydroxy Anisole

OH

OHOH

COOC3H7

Propyl Gallate

OH

C(CH3)3

OH

TBHQ

CH

CHO OH

OH

OH

CH3CH3

OH CHO

OH

CH

OH

CH3 CH3CH3 CH3

Gossypol

Antioxidants

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).

Reaction of antioxidants with radicalsR + AH RH + A

RO + AH ROH + A

ROO + AH ROOH + A

R + A RA

RO + A ROA

ROO + A ROOA

Antioxidant + O2 Oxidized Antioxidant

Stable Resonance Formation of BHA

O H

C (C H 3)3

O C H 3

R , RO , or ROO

C (C H 3)3

O C H 3

O

O C H 3

C (C H 3)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

.

..

O

C H 3 H 2

O H

C H 3

C H 3

H 2

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

C H 3 C H 3

-tocopherol

O 2

C H 3

C H 3

C H 3

O C H 2

O

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

C H 3 C H 3 C H 3

O H

- tocoquinone

Tocopherol and Oxygen Reaction

Mechanisms of Metals in Accelerating Lipid Oxidation

Hydroperoxide decomposition to form peroxy or alkoxy radical.

Fe3+ + ROOH Fe2+ + ROO + H+

Fe2+ + ROOH Fe3+ + RO + OH-

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

Fe3+ + RH Fe2+ + R + H+

Activation of molecular oxygen for singlet oxygen formation.

Fe2+ + O2 Fe3+ + O-2 1O2

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)

O

M

CO

CH2

O CO

CH2

NCH2

CH2

O

O NC

OCH2

COCH2

Metal Ions – EDTA Complex Formation

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) other phenolic antioxidants, or with (2) various metal chelating agents.

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 .

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.

Possible Future Antioxidants

1. Polymeric antioxidant.

2. Antioxidant attached to the packaging materials.

3. Development of new, non-absorbable polymeric

antioxidants for use in foods.

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.

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

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-available

Economical

Not absorbable by the body

Biochemical Control of Lipid Oxidation

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%

Glucose oxidase/catalase Reaction Mechanism.

Glucose oxidase/catalase reaction:

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

2H2O2 2H2 O + O2

The net chemical reaction is:

2 Glucose + O2 2 Gluconic acid

Glucose Oxidase

Catalase

Glucose Oxidase

Catalase

Kinds of Antioxidants

Natural antioxidants:1.Tocopherols (delta>gamma>beta>alpha)2.Nordihydroguaretic Acid (NDGA)3.Sesamol4.Gossypol

Synthetic antioxidants:1.Butylated Hydroxy Anisole (BHA)2.Butylated Hydroxy Toluene (BHT)3.Propyl Gallate (PG)4.Tertiary Butyl Hydroquinone (TBHQ)

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:SafetyAntioxidant effectivenessOff-odorOff-colorConvenience of antioxidant incorporation to foodsCarry-through effectStability to pH and food processingAvailabilityCost

Non-adsorbable, if possible

Antioxidants for Different Food Systems

A small surface-to-volume ratio – PG and TBHQ

A large surface-to-volume ratio – BHA and BHT

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.