Mainly about oxidation reaction that occurs due to activated oxygen species in lipids

1. OXIDATIVECHANGESINFOODS
BY;
K.VINITHA
2018694619
MTECH FPE

2. INTRODUCTION
Oxidative rancidity is associated with the degradation by oxygen in the air
Oxidation primarily occurs with unsaturated fats.
Oxidation reactions are one of the major sources of deterioration that occurs
during manufacturing, storage, distribution and final preparation of foods.
Oxidative reactions in foods results in destruction of valuable nutrients,
undesirable flavours and odours affecting the palatability of foods, and even in
generation of toxic compounds.

3. LIPIDPEROXIDATION
 Oxidative deterioration of lipids via free radical mechanism - lipid peroxidation.
 The reaction consists of three major steps: initiation,
propagation, and termination.
 Two principal initiation reactions -1. Homolytic scission
catalysed by metal ions and heme proteins 2. the reaction of
activated oxygen species with the lipid substrate to yield
peroxides and free radicals
Lipid hydro peroxide

4.  If preformed hydroperoxides are present, their homolytic decomposition to free radicals
can propagate new reaction chain.
 A principal function of pro-oxidant metals, such as copper and iron, or heme proteins,
such as haemoglobin - is their capacity to catalyse decomposition of preformed
hydroperoxides to initiate new oxidation chains.
 Activated singlet oxygen is formed upon decomposition of lipid hydroperoxides by iron
complexes

5.  Activated oxygen species which may be important in initiating oxidative changes in foods include;
1. singlet oxygen,1O2
2. Hydroxyl radical,
3. ozone
4. superoxide anion
5. hydrogen peroxide.
 Chemical and enzymatic reactions in biological materials generates -singlet oxygen, hydroxyl radical,
superoxide anion, and hydrogen peroxide.
 Ozone is primarily a product of photoreactions in polluted air.
 Superoxide anion and hydrogen peroxide are relatively inert toward organic molecules but can decompose
to produce the more reactive singlet oxygen and hydroxyl radical.
INITIATION OF OXIDATIVE CHANGES IN FOODS
HO•
O3
O−
2
H2O2

6. SINGLET OXYGEN 1O2
Singlet oxygen (1.O2), an electronically excited state of oxygen, reacts readily with unsaturated compounds to
yield peroxides which can initiate oxidative chain reactions
 Natural pigments such as chlorophyll, hematoporphyrins, and flavins may serve as sensitizers to
yield oxidative damaging species in the presence of O2 and visible light.
 1O2 is produced by breakdown of preformed lipid hydro-peroxides by iron complexes
 Hydrogen peroxide and O−.
2 are generated in biological systems as a result of metabolic activity
acts as a biological source of 1O2
 Oxidation of O−.
2 with oxidizing agents such as benzoyl peroxide and lauryl peroxide yields
1O2
 Enzymes produces singlet oxygen - includes bovine adrenodoxin reductase lactoperoxidase
Xanthine oxidase ,lipoxygenase , quercetin dioxygenase , prostaglandin synthase.
Generation of 1O2:

7. Reactions of 1O2
1. Singlet oxygen is an effective initiator of oxidative reactions.
2. Initiation lipid peroxidation takes place via the ene reaction to yield an allylic hydroperoxide.
3. Subsequent decomposition of the hydroperoxide yields free radicals which then initiate new
reaction chains in the propagation step of autoxidation.
4. Alkyl radicals in the propagation reactions can react with the abundant triplet oxygen to yield
peroxyl radicals.
5. Thus, once the reaction is initiated by singlet oxygen, it quickly becomes autocatalytic in the
presence of triplet oxygen.

8. Contd..
 Photo-oxidation of various unsaturated vegetable oils takes place at wavelengths > 340 nm.
 Photo-oxidation can be retarded by compounds that quenches 1O2 . α- tocopherol quenches 1O2 (it did not
prevent effectively build up of peroxides in the photo oxidizing oils).
 The quenching effect of α -tocopherol is offset by its oxidation to unstable hydroperoxides.
( results in Three important implications)
1. The natural phenol cannot prevent the build-up of peroxide species,
2. Its ability to act as a free radical scavenger will be impaired
3. The vitamin E content of the foodstuff would be reduced.
 The most efficient singlet oxygen quenchers can prevent singlet oxygen addition to allylic double bonds and
provide protection at a concentration of .01% by weight - quenching criteria is given by β-carotene, but it
may impart an objectionable orange color to the food.

9. Protection against 1O2 :
 Deterioration of vegetable oils by singlet oxygen can be minimized most effectively by removal of natural
pigments and peroxides during the refining process.
 Inclusion of a nontoxic effective a oxygen quencher in foods and beverages improves their shelf life
substantially.
 Terpenoid compounds such as limonene and α-terpinene readily react with 1O2 to yield peroxides.
 oxidation of cholesterol to form an allylic hydroperoxide oxidized cholesterol remains to be determined;
however, cholesterol α-oxide is carcinogenic.
 Food additives can react with or quench singlet oxygen.
 Peptide and disulphide bonds apparently are not susceptible to cleavage by singlet oxygen .
 Among amino acids, methionine, histidine, tryptophan, tyrosine, and cysteine are affected principally,
whether as free amino acids or in peptides.
 Conformation of a protein or polypeptide may make some residues less susceptible to photo-oxidation than
others.
Contd..

10. HYDROXYL RADICAL
,
Generation of HO• * Radiolysis by high-energy radiation such as ɣ rays and X-rays is a source of
hydroxyl radicals.
* Single electron reductants such as Fe+2, Cr +2, Co +2, Cu +1 and Ti ~3 react
H2O2 with to generate hydroxyl radicals.
* Biologically catalyzed reactions may serve as sources of hydroxyl radical.
HO•

11. Reactions of HO•
 With unsaturated fatty acids, hydroxyl radical adds to the double bond - yields the hydroxylated free
radical. resultant radical reacts with oxygen - yields peroxyl radicals which propagate the chain
oxidation of lipids.
 Allylic abstraction of hydrogen atoms from unsaturated fatty acids serves to generate alkyl radicals.
 With aromatic compounds, hydroxyl radical generally adds to the aromatic system to yield a
hydroxylated product.
 With saturated compounds, hydroxyl radical generally abstracts a hydrogen atom from the weakest C-H
bond to yield a free radical.

12. Protection against HO•
 Defence against hydroxyl radical in biological systems - preventing the
formation of H2O2 and O−
2 which may serve as precursors for HO•
The enzymes superoxide dismutase and catalase would destroy these
precursors.
Probably synthetic or natural antioxidants such as BHA and tocopherols are
principal sources of protection from oxidative reactions.
 Compounds which readily donate electrons or hydrogen atoms are
good scavengers of hydroxyl radicals.

13. Ozone
Generation of
o3
Ozone is formed in the atmosphere by photochemical reactions which are
facilitated as reactive species and released into the air over cities and industrial
areas
By terpenes released into the air by plant life
Polluted air can be a factor in affecting the oxidative stability of products

14. Reactions of o3
 Ozone reacts readily with polyunsaturated fatty acids (PUFA) decomposes rapidly to the Criegee
zwitterion .
 Criegee zwitterion reacts with active hydrogen compounds such as alcohols or acids to yield the
corresponding hydro peroxides.
 These peroxides might decompose by homolytic scission to initiate free radical peroxidation of
lipids.
+ + ++ +
ROH
Zwitter ion
Zwitter ion
+

15. Protection against o3
Phenolic antioxidants such as BHA , BHT and the tocopherols would react to
inhibit peroxidation initiated by free radical generation from ozone.
 The zwitterion forms N-oxides of amines
 Ozone readily oxidizes thiols in amino acids, peptides, and proteins . Cysteine is oxidized to cystine and
cysteic acid.
 Glutathione and the thiol groups of proteins are oxidized to the corresponding disulphide and other
products.
+ RCHO +
CONTD...

16. SUPEROXIDE ANION
 Reduction of oxygen to water involves acceptance of four electrons with formation of several
intermediates
 The first of these intermediates, perhydroxyl radical (HO.
2), or its ionized form superoxide
anion ( O−.
2 )
 In the presence of suitable substrates, peroxidase enzymes is an important source of O−.
2 in
food materials.
 Oxidation of several compounds sulphite, thiol, haemoglobin , flavins, quinones etc. yields
(O−.
2 ).
 Enzymatic sources of O−.
2 are the flavoprotein dehydrogenases ,xanthine oxidase, dihydroorotate
oxidase, aldehyde oxidase, etc. Flavoprotein dehydrogenases generates free O−.
2 whereas oxidases
reduce oxygen to H2O2 and hydroxylases form O−.
2 bound to the enzyme.
Generation of O−.
2

17. Reactions of O−.
2
 The HO.
2 and O−.
2 are relatively unstable radicals and readily undergo a dismutation reaction forming
hydrogen peroxide and oxygen. Of these two radicals, O−.
2 is more stable than HO.
2.
 In a food system nucleophilic addition to an ester or triglyceride would generate an acid
peroxide radical which might initiate new oxidation chains.
 O−.
2 is not capable of initiating peroxidation by abstracting an allylic hydrogen atom .The
reaction of O−.
2 with diene hydroperoxides (LOOH), impurities in polyunsaturated fatty acids
Perhydroxyl radical HO.
2 behaves as a weak acid having a pKa of 4.88

18. Protection against o−.
2
 Scavenging of O−.
2 by superoxide dismutase or by compounds which easily react with
O−.
2 such as vitamin C, thiols, hydroquinones, and catechols would not only destroy
radical O−.
2 but presumably would prevent formation of the more damaging a singlet
oxygen and hydroxyl radical from O−.
2 .
 In this way, stability of oxidizable compounds in food increased.
 A product of these reactions is hydrogen peroxide which may participate further in
oxidative reactions. In this regard, catalase might be an excellent additive to any system
designed to prevent oxidative reactions.

19. HYDROGEN PEROXIDE H2O2
 Hydrogen peroxide is formed in these systems by transfer of two electrons to 3O2 or by one electron
reduction of 3O2 to O−
2 followed by dismutation reactions.
.
Generation of
H2O2
* In living organisms-microsomes, mitochondria, tissue homogenates, liver slices,
whole perfused liver, aerobic microorganisms,and phagocytizing granulocytes.
* By oxidases - includes D-amino acid oxidase, uricase, a-hydroxy acid oxidase,
and glucose oxidase.
* By autoxidation of ascorbic acid in a reaction catalysed by copper or other
transition metals.

20. Reactions of H2O2
 Hydrogen peroxide is the least reactive molecule among all the activated oxygen species.
 During base catalysed dismutation of H2O2 ,
1O2 of is formed:
 The oxidative properties of H2O2 are increased greatly by other reagents which result in formation of HO˚
radical or 1O2 by reaction with H2O2 .
 Oxidative ability of H2O2 increases in the following situations:
1. Addition of ferrous or cuprous salts to H2 O2 generates HO˚ radical
2.The disproportionation of H2O2 may give rise to 1O2
3.Singlet oxygen is formed by halide-peroxidase catalysed degradation of hydrogen peroxide
H2O2 +HO¯ HO2¯ +H2O
H2O2 + HO2¯ H2O+ HO¯ + 1O2

21. Protection against H2O2
*Cytotoxic
*The toxicity of H2O2 might also be explained by HO˚ and 1O2 arising from it.
*Reactivity of HO˚ radical and 1O2 toward lipo-protein material they might cause
damage to essential cellular components
In living organisms, defence against H2O2 toxicity is accomplished by keeping its
steady state concentration low.
A low steady-state concentration of H2O2 is maintained by the enzymatic actions of
catalase and peroxidase. Both types of enzymes contain
hematin as a prosthetic group and catalyse similar reactions.
2H2O + 3O2
catalase
H2O2+ H2O2