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
(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:
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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