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 ValueIndicator 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 TestOxidation 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
Top Related