DATE: 06/03/2014

LIPID

OXIDATION Causes & Prevention

Sadanand Patel

Presentation Pattern

Rancidity(brief), Types

Hydrolytic rancidity(brief)

Oxidative rancidity and ways and outline

Photo-oxidation(detail)

Auto-oxidation(detail)

Feasibility of reactions: Pro-oxidants

Comparison b/w Photo and Auto-oxidation

Hydroperoxides decomposition(brief)

Factors affecting Lipoxidation(detail)

Anti-oxidant, types and mechanism(detail)

Measurement of Oxidative rancidity(brief)

Fat oxidation characteristic curve and induction period

General information

Quick history of Lipoxidation

What is Rancidity?

Rancidity is a term generally used to denote a

condition of unpleasant odours and flavours in

foods resulting from deterioration in the fat or oil

portion of a food.

RANCIDITY

HYDROLYTIC

RANCIDITY

OXIDATIVE

RANCIDITY

MICROBIAL

RANCIDITY

Active sites

Oxidation

Hydrolysis

1

2

HYDROLYTIC RANCIDITY

Partial or Complete Hydrolysis

of ester bonds

Liberation of FFA

DAGs, MAGs,, Glycerol

EX:

Development of rancid flavour in milk

Deep fat frying

Enzyme action,

Heat, Moisture

OXIDATIVE RANCIDITY Major causes of food spoilage

Extensive research have done, yet to be discovered

……extremely complex subject because

1. Sensitivity of Intermediates Isolation separation and identification

2. Effect of catalyst Pro & anti-oxidants

3. Influence of photo-oxidation and auto-oxidation simultaneously

4. Relation between oxidation rate and stability

Photo-oxidation

Enzymatic oxidation Auto-oxidation

Ways

Fats & Oils Primary Oxidation Products

Secondary Oxidation Products

Tertiary Oxidation Products

Factors Decomposition Oxidation

Hydroperoxides

Aldehydes, Ketones, Diene and triene

Acids, Epoxides, Dimers, Oxirane rings

Outline of the process

Photo-oxidation

Sunlight

Photosensitizers

Singlet oxygen

Natural Pigment Chlorophyll, Pheophytin, Flavin, Porphyrin Dye Stuff Erythrosine, Methylene blue

1 3

Triplet oxygen

Singlet oxygen

Why singlet oxygen??

Angular momentum = 2s + 1

(s is total spin)

For Triplet:

2(1/2+1/2) + 1= 3

For Singlet:

2(1/2- 1/2) + 1 = 1

sens + RH(sing) + hv → intermidiates-I(trip)

intermediats-I(trip) + O2(trip) + hv → ROOH + sens

sens + O2((trip)) + hv → intermidiates-II(sing)

intermediats-II(sing) + RH(sing) + hv → ROOH + sens

Pathways have been proposed

Pathway-I

Pathway-II

sens - photosensitizes

Mechanism

RH + O2(singlet) + hv → ROOH

Singlet oxygen is more electrophilic than triplet oxygen and can reacts 1500

times greater than triplet oxygen with high electron density moieties like, C=C

Hence it’s a type of ene type reaction,

Evidences show that it is a Free radical mechanism

Inhibition in rate by chemical species

known for interfere with Free radical

reaction.

Catalysed by light

High yield of

Hydroperoxides

Relatively long

Induction Period

Auto-Oxidation

Experimental results are explained in three step free radical scheme

RH RH + O2 R’

R’ + O2 ROO’

ROO’ + RH ROOH +

R’ + R’

R’ + ROO’

ROO’ + ROO’

K1

K2

K3

K4

K5

K6

Initiation

Propgation

Termination

R’

Nonradical product

Rate equations: Oxygen Absorption:

Eqn (4), (5) can

be neglected

Eqn (5), (6) can

be neglected

Hydroperoxides and their Decomposition Oxygen-oxygen

bond scission Alkoxy and

Hydroxyl radicals

Aldehyde and acid/ester

Or

Hydrocarbon and oxoacid /

oxoester

1

5

4

3

2

Ketone and

1ᵒ Alcohol

2ᵒ Alcohol and Free

radical

Ketone and

Hydrocarbon

A thermodynamically difficult reaction because Activation energy is 35kcal/mol

Metal Catalysts Prooxidants

This reaction is still a considerable mistery

Pro-oxidants

Transition metals, particularly those having two or more valence states with a suitable oxidation-reduction potential between them are major pro-oxidants.

Initiate / Accelerate the reaction

Sources 1. Traces metals are naturally occurring components of all food tissues.

2. Originate from the soil where bearing material grow.

3. Metallic equipments used in handling, extraction and processing

4. Residual quantity of Spent bleaching earth, Nickel catalyst etc.

1

3

2

Factors Influencing the rate of Lipid Oxidation

Fatty Acid Composition

Free Fatty Acid

Temperature

At lower content no effect on oxidation

but in higher, it promotes

Number, position and

geometry of double bonds

As temperature increase rate of

oxidation increase but simultaneously

availability of oxygen decrease

Pro-oxidants

Surface Area

Oxygen Concentration

Rate of oxidation is proportional to

the surface area exposed to air or

water(in case of emulsion)

Supply of oxygen is unlimited so rate is

independent of oxygen pressure, but at

very low pressure rate is proportional to

oxygen pressure

Effective promoter of lipid oxidation The Ea’s

for C-H and C-C scission reactions are

slightly higher than bond energies 98.4

kCal/mol and 83.1 kCal/mol, respectively,

and this is equal to photon energy available

only at wavelengths <254 nm (UV Region)

Radiant Energy

Synergists

Moisture

Metal chelating agent deactivate the metals

hence no further initiation takes place

Citric acid, Ascorbic acid, EDTA etc.

Water activity measurement is a new

concept, for dried food aw < 0.1 (No such

effect on rate of Oxidation is detected)

As increasing the aw and keeping

between 0.65 - 0.85 increase the rate

and beyond the range oxidation rate is

enormously high and constant to aw

Anti-oxidant

Anti-oxidants

Monohydric and Polyhydric phenols derivatives are excellent hydrogen or electron donators and their radical are highly stable due to resonance delocalization

Natural Anti-oxidants Synthetic Anti-oxidants

Sesamol Gossypol Tocopherol Tocotrienol Ginistein Flavonoids

BHT BHA TBHQ PG TBHP

Antioxidants do not prevent oxidation but they slow it down, thereby extending

the induction period and hence the shelf-life of fat-containing foods.

Prevention

Mechanism

Photo-oxidation is not inhibited by the antioxidants used for autoxidation but by singlet oxygen quenchers of which the best known is -carotene.

Synthetic

Antioxidants

Natural

Antioxidants

Gossypol

Natural

Antioxidants

1. Peroxide Value

2. p-Anisidine Value

3. Ultra Violet Spectroscopy

4. Total Polar Component

5. Iodine Value

6. Active Oxygen Method

Measurement of Oxidative Rancidity

1. Peroxide Value

Indicator of initial oxidation suffered by sample.

Measured on the basic of ability of peroxides to liberates iodine from

potassium iodide

Expressed in terms of milliequlivents of oxygen per kilogram of fats.

2. Anisidine Value

In the presence of acetic acid, p-anisidine react with aldehyde

producing a yellowish colour.

The molar absorbance at 350nm increases if the aldehyde

contains a double bond conjugated to the carbonyl double bond.

3. Thiobarbituric Acid Test

Oxidation products of unsaturated system produce a colour reacton

with TBA.

Many alkanals and alkenals produces yellow colour compounds at

450nm

Dienal produce a red colour pigment at 530nm

General characteristics curve of fat oxidation

Induction Period

Without antioxidant

Induction period:

After a certain critical amount of oxidation has occurred, the reaction enters to its second phase characterized by a rapid accelerating rate of oxidation, this period is called induction period

With antioxidant

Induction Period

General Information

Level of Anti-oxidants

BHA NMT 100 ppm

PG NMT 200 ppm

BHT NMT 200 ppm

TBHQ NMT 200 ppm

Natural Anti-Oxidant GMP

Effect of Free radicals on cell

Quick History of Lipid . Oxidation

Henri Braconnot, 1815 Acids were formed during rancidification

J.L. Bolland,1947

Detailed kinetic study of antioxidant (ethyl linoleat and hydroquinone)

F.D. Gunstone, 1943-47

Auto-oxidation and effect of temperature

T.P. Hilditch,1943-47

M. Karel, 1988

Combined study of lipid oxidation and water activity(aw)

A. Tappel, 1950

Vitamin E as a biological antioxidant

Gilmont and Levenson, 1946

Photooxidation

H.S. Rawals, 1970

Fat oxidation characteristics and induction period

BIBLIOGRAPHY