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Lipids in foods

Lipids occur in nearly all food raw materials with the major classes being

triglycerides (also known as triacylglycerols), which occur in fat storage cells

of plants and animals, and phospholipids, which occur in biological membranes.

PHOTO OXIDATION

TYPE II PHOTO-OXIDATION

An alternative route leading to the formation of hydroperoxides instead of

the free radical mechanism is via a photo-oxidation route. In this case, excitation

of lipids (type I photo-oxidation) or excitation of oxygen (type II

photo-oxidation) may occur in the presence of light and a sensitiser. There

is no induction period in the photo-oxidation route.

Oxygen in the environment is normally in the triplet electronic state, 3O2.

Triplet oxygen can be excited by light to singlet oxygen in the presence of a sensitizer, such as chlorophyll, according to reaction. Singlet oxygen reacts much faster than triplet oxygen with the unsaturated lipid via the ‘ene’ reaction producing an allylic hydro peroxide via a shift of a double bond.

Ketonic rancidity

Ketonic rancidity is a problem that can be encountered with some products

such as desiccated coconut which contain short-chain saturated fatty acids.

Moulds such as Eurotium amstelodami degrade triglycerides in the presence

of limited amounts of air and water.

TYPE I PHOTO-OXIDATION

In the presence of some sensitisers, such as riboflavin, type I photooxidation

occurs.Type I photo-oxidation is characterised by hydrogen atom

transfer or electron transfer between an excited triplet sensitiser and a substrate,

such as a polyunsaturated fatty acid, producing free radicals or free

radical ions.

The development of oxidative rancidity

in foods

These fats are

almost completely triglycerides, and it is these components that are of most

significance as potential sources of oxidative off-flavours in these foods.

In plant or animal tissues used as foods, the phospholipids present in

all biological membranes may be an important substrate for oxidative

deterioration.

Types and effects of rancidity

Ocampo, Juan Miguel B.

(1) initiation, the formation of free radicals;

RH + O2 -->R· + ·OH

R· + O2 --> · + ROO·

The overall mechanism of lipid oxidation consists of three phases

(2) propagation, the free-radical chain reactions;

ROO· + RH --> R· + ROOH

ROOH--> RO· + HO·

(3) termination, the formation of non-radical products.

R· + R· --> RR

R· + ROO·--> ROOR

ROO· + ROO· --> ROOR + O2

Metal-catalysed lipid oxidation

Transition metal ions in their lower valence state (Mn+) react very quickly with hydroperoxides. They act as one-electron donors to form an alkoxy radical and this can be considered as the branching of the

propagation step

Transition metals, e.g. Fe, Cu, Co, which possess two or more valence states

with a suitable oxidation–reduction potential affect both the speed of

autoxidation and the direction of hydroperoxide breakdown to volatile

compounds.

Loss of fat-soluble vitamins and pigments

The vitamins A,D, E and K, and chlorophyll and carotenoids are fat-soluble

and loss of these food components by radical-catalysed reactions may often

accompany lipid oxidation in foods containing these components.

Antioxidant effects

Other relevant reactions

Antioxidants can inhibit or retard oxidation in two ways:

END

by scavenging

free radicals, in which case the compound is described as a primary

antioxidant

by a mechanism that does not involve direct scavenging of

free radicals, in which case the compound is a secondary antioxidant.

Reaction of oxidised lipids with other food components

Lipid oxidation products may react with proteins or with nucleic acids in

food. Carbonyl compounds derived from oxidation of phospholipids may

react with proteins to lead to flavour compounds in roasted meat.

MECHANISM OF LIPOXYGENASE-CATALYSED OXIDATION

The enzyme in oilbearing

seeds, e.g. soybeans, can be an important source of hydroperoxides

formed in the oil during extraction. In vegetables, oxidative changes due to

the enzyme may lead to off-flavours during storage. The enzyme does,

however, contribute to flavour formation in some plant foods including

tomatoes and cucumbers.

The enzyme lipoxygenase (linoleate oxygen oxidoreductase, EC 1.13.11.12)

is present in a wide variety of plant and animal tissues

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