3.2 OXIDATION OF OIL/FAT Lipid oxidation is one of the major causes of food spoilage. In edible oils and fat-containing foods, it leads to the development of various off flavors and off odors, generally known as oxidative rancidity, which renders the foods less acceptable. It may also be able to decrease the nutritional value of food and in some cases may produce potentially toxic products. It may be sometimes desirable as in the case of cheese.

Oxidation is caused by a biochemical reaction between fats and oxygen called as autoxidation and it is the main reaction involved. The lipids of foods can be oxidized by (i) non enzymic and (ii) enzymic mechanisms. Lipid oxidation generally occurs after a long induction period. Once started it is generally a very rapid reaction.

(i) Non enzymic oxidation Non enzymatic lipid oxidation (Autoxidation) proceeds by a free radical mechanism. It is catalysed by light and free radical-producing substances and yields hydroperoxide (ROOH). This primary product is relatively unstable. It enters into numerous reactions involving substrate degradation and interaction, resulting in numerous compounds of various molecular weigh, flavour generating and biological significance.

Free radical mechanism A free radical is a compound with an odd number of unpaired electrons. H H H — C —C — C — ↓ — C —C• — C — + H• H H When initiated two free radicals are formed. These radicals are very reactive and generally do not have long life time.

Lipid oxidation There are three main steps i. Initiation ii. Propagation iii. Termination

i. Initiation The initiation occurs by direct attack of oxygen in its most stable form on double bonds of fatty acids (RH). The presence of a double bond in the fatty acid (RH) weakens the C-H bonds on the carbon atom adjacent to the double bond and so makes H removal easier. Oxygen attack at the end carbon of the double bond and forms hydrogen peroxide. Hydroperoxides breakdown in several steps to form free radicals. Initiator RH +O2 → ROOH → Free radicals (R• , ROO •)

The carbon radical tends to be stabilized by a molecular rearrangement to form a conjugated diene. Oxygen attack the end carbon of the double bond and forms hydrogen peroxide. Hydroperoxide breakdown, in several steps, to form free radicals.

ii. Propagation Once the initial radicals have formed, the formation of other radicals proceeds rapidly. The new radicals will not be at the double bond. To remove a hydrogen from a double bond requires 80 Kcal/mole. To remove a hydrogen alpha to a double bond only requires 15 Kcal/mole. As a peroxyl radical is able to abstract H from another lipid molecule (adjacent fatty acid), especially in the presence of metals such as copper or iron, thus causing an autocatalytic chain reaction. The peroxyl radical combines with H to give a lipid hydroperoxide (or peroxide). This reaction characterizes the propagation stage.

The peroxyl radical combines with H to give a lipid hydroperoxide (or peroxide). This reaction characterizes the propagation stage. R •. + O2 → ROO •. ROO•. + RH → ROOH + R •.

The carbon radical tends to be stabilized by a molecular rearrangement to form a conjugated diene. * Under aerobic conditions conjugated dienes are able to combine with O2 to give a peroxyl (or peroxy) radical, ROO. H H H H — C = C — + O2 → — C — C — * O O*

ii. Propagation Once the initial radicals have formed, the formation of other radicals proceeds rapidly. The new radicals will not be formed at the double bond. To remove a hydrogen from a double bond requires 80 Kcal/mole. To remove a hydrogen alpha to a double bond only requires 15 Kcal/mole. As a peroxyl radical is able to abstract H from another lipid molecule (adjacent fatty acid), especially in the presence of metals such as copper or iron, thus causing an autocatalytic chain reaction.

The peroxyl radical combines with H to give a lipid hydroperoxide (or peroxide). This reaction characterizes the propagation stage. R. + O2 → ROO. ROO. + RH → ROOH + R.

iii) Termination R • + ROO• → ROOR 2ROO• → ROO-OOR Any kind of alkyl radicals (lipid free radicals) R• can react with a lipid peroxide ROO• to give non-initiating and Non-propagating species such as the relatively stable dimers ROOR or two peroxide molecules combining to form hydroxylated derivatives (ROH). R• + R• → R-R R • + ROO• → ROOR 2ROO• → ROO-OOR

Effects of Lipid Oxidation in Foods When lipids in food are oxidised, some of the product formed impart odour and flavours, usually undesirable, to the food. The free radicals generated during the oxidation reaction and some of the molecules formed when oxidized compound decombos (aldehydes, acids, alcohols, ketones etc.) can interact with and alter other constituents including pigments, vitamins, proteins and amino acids. These interactions can result in colour, texyure and nutritive value. If foods containing oxidized lipids are consumed, the oxidation products could be involved in reactions leading to pathological changes. For e.g. malonaldehyde and oxidation product of certain polyunsaturated fatty acids found in many foods, is a potential carcinogen.

(ii) Enzyme catalysed lipid oxidation Enzyme reaction starts with the action of lipolysis. Released polyunsaturated fatty acids are then oxidized by either lipoxygenase or cyclooxygenase to form hydroperoxides or endoperoxides, respectively. Then these compounds are hydrolysed to yield a variety of breakdown products, which are responsible for the characteristic flavours of natural products.

Pro-Oxidants Transition metals, particularly those possessing two or more valency states and a suitable oxidation reduction potential between them are effective pro-oxidants. e.g.copper, iron, manganese, cobalt and nickel If present even at very low concentrations(0.1ppm) can decrease the induction period and increase the rate of oxidation. Trace metals are naturally occurring in all food tissues and all fluids of biological origin (eggs, milk and fruit juices) and they are present in both free and bound forms. Heme compounds are also important pro-oxidants.

Antioxidant An antioxidant is a molecule that can delay onset, or slow the rate of oxidation of oxidisable material. Oxidation reactions can produce free radical. In turn, these radicals can start chain reactions. Antioxidants terminate these chain reactions by removing free radical intermediates, and inhibit other oxidation reactions by acting as hydrogen donors or free radical acceptors. ROO. + AH → ROOH + A. Antioxidants are found in varying amounts in foods such as vegetables, fruits, grain cereals, eggs, meat, legumes and nuts. Antioxidants are widely used as preservatives in food and as ingredients in dietary supplements.

. Natural antioxidants: Ascorbic acid and tocopherols, Synthetic antioxidants: Propyl gallate, isoamyl gallate, tertiary butylhydroquinone (TBHQ),butylated hydroxy anisole (BHA) and butylated hydroxy toluene (BHT). Used as help guard against food deterioration Antioxidants can be directly added to vegetable oils or to melted animal fats after they are rendered. Food products can also be dipped in or sprayed with solutions of antioxidants