DATEDATE : 06/03/2014: 06/03/2014

LIPIDOXIDATION

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

HYDROLYTICRANCIDITY

OXIDATIVE RANCIDITY

MICROBIAL RANCIDITY

Active sites

Oxidation

Hydrolysis

1

2

HYDROLYTIC RANCIDITY

Partial or Complete Hydrolysis of ester bonds

Liberation of FFADAGs, MAGs,, Glycerol

EX:Development of rancid flavour in milkDeep fat frying

Enzyme action,Heat, Moisture

OXIDATIVE RANCIDITYMajor causes of food spoilage

Extensive research have done, yet to be discovered

……extremely complex subject because

1. Sensitivity of IntermediatesIsolation separation and identification

2. Effect of catalystPro & anti-oxidants

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

4. Relation between oxidation rate and stability

Photo-oxidation

Enzymatic oxidationAuto-oxidation

Ways

Fats & OilsPrimary Oxidation Products

Secondary Oxidation Products

Tertiary Oxidation Products

Factors DecompositionOxidation

HydroperoxidesAldehydes, Ketones, Diene and triene

Acids, Epoxides,Dimers, Oxirane rings

Outline of the process

Photo-oxidation

Sunlight

Photosensitizers

Singlet oxygen

Natural PigmentChlorophyll, 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=CHence 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 DecompositionOxygen-oxygen bond scission

Alkoxy and Hydroxyl radicals

Aldehyde and acid/esterOrHydrocarbon and oxoacid / oxoester

1

5

43

2

Ketone and 1ᵒ Alcohol

2ᵒ Alcohol and Free radical

Ketone and Hydrocarbon

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

Metal CatalystsProoxidants

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

Sources1. 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 placeCitric 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

SesamolGossypolTocopherolTocotrienolGinisteinFlavonoids

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