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Extraction And Characterization Of Vegetable Oil Using Bread Fruit Seed
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Palm oil, olive oil, cottonseed oil,
peanut oil, and sunflower oil amongst others are classed as Oleic –
Linoleic acid oils seeing that they contain a relatively high proportion
of unsaturated fatty acids, such as the monounsaturated oleic acid and
the polyunsaturated linoleic acid (Dunn, 2005; Gertz et al., 2000). They
are characterized by a high ratio of polyunsaturated fatty acids to
saturated fatty acids. As a consequence of this, they have relatively
low melting points and are liquid at room temperature. Iodine values,
saponification values, specific compositions and melting points in
addition to other physical properties have been determined and are
widely available in the literature (Williams, 1966), (Oyedeji et al.,
2006).
Other oils fall under various classes such as the erucic acid
oils which are like the oleic linoleic acid oils except that their
predominant unsaturated fatty acid is erucic acid (C22). Rapeseed and
mustard seed oil are important oils in this class. Canola oil is a type
of rapeseed oil with reduced erucic acid content (Applewhite, 1978). It
is a stable oil used in salad dressings, margarine and shortenings.
Soybean oil is an important oil with numerous increasing applications in
the modern day world. It is classed as a linolenic acid oil since it
contains the more highly unsaturated linolenic acid. Other oils include
castor oil (a hydroxy-acid oil) which contains glycerides of ricinoleic
acid (Erhan et al., 2006). Also worthy of note is that coconut oil,
which unlike most vegetable oils, is solid at room temperature due to
its high proportion of saturated fatty acids (92%) particularly lauric
acid. Due to its almost homogenous composition, coconut oil has a fairly
sharp melting point (Bennion, 1995).
1.7 Auto oxidation and oxidative stability in vegetable oils
By
definition, the oxidative stability of oil is a measure of the length
of time taken for oxidative deterioration to commence. On a general
level, “the rates of reactions in auto-oxidation schemes are dependent
on the hydrocarbon structure, heteroatom concentration, heteroatom
speciation, oxygen concentration, and temperature (Ferrari et al.,
2004).
If untreated, oils from vegetable origin oxidize during use
and polymerize to a plastic like consistency (Honary, 2004). Even when
they are not subjected to the intense conditions of industrial
applications, fats and oils are liable to rancidity (Eastman Chemical
Company, 2001; Morteza- Semnani et al., 2006). This happens more so in
fats that contain unsaturated fatty acid radicals (Charley,
1970).
Indeed the oxidisability of a vegetable oil is dependent on the level of
unsaturation of their olefinic compounds. In general terms, oxidative
rancidity in oils occurs when heat, metals or other catalysts cause
unsaturated oil molecules to convert to free radicals. These free
radicals are easily oxidized to yield hydroperoxides and organic
compounds, such as aldehydes, ketones, or acids which give rise to the
undesirable odors and flavors characteristic of rancid fats (Eastman
Chemical Company, 2001). The role of peroxides is exploited in
monitoring oxidative deterioration by measuring peroxide values (POV)
(Mochida et al., 2006).
Lipid oxidation occurs via auto oxidation or
lipoxygenase catalysis. Auto oxidation refers to a complex set of
reactions which result in the incorporation of oxygen in lipid
structures. Auto oxidation reactions are seen to progress more rapidly
in oils that contain predominantly unsaturated fat molecules; other
relevant factors include the presence of light, transition metal ions,
oxygen pressure, the presence or absence of antioxidants and pro
oxidants, temperature and moisture content. Auto oxidation reactions
occur at an increasing rate after the initial induction period. This
behavior can be explained by assuming that oxidation proceeds by a
sequential free radical chain reaction mechanism. Relatively stable
radicals that can abstract hydrogen atoms from the allylic methylene
groups in olefinic compounds are formed. Hence auto oxidation is a
radical induced chain reaction which proceeds through the traditional
stages of initiation, propagation and termination. Detailed proposed
mechanisms for these free radical chain reactions are available in
literature (Fennema, 1985).
Lipoxygenases are metal proteins with an
iron atom as the active center. They catalyze the oxidation of
unsaturated fatty acids to hydroperoxides as with auto oxidation. Enzyme
activation usually occurs in the presence of hydroperoxides, even
though enzyme catalyzed oxidation can occur even in the absence of
hydroperoxides (Fennema, 1985). As earlier stated, the more unsaturated
the fatty acid involved is, the greater its susceptibility to oxidative
rancidity. For instance, the linolenic acid esters present in soybean
oil (with twice the unsaturation as monounsaturated esters) is
particularly sensitive to even oxidation of the slightest kind, commonly
referred to as flavor reversion, resulting in beany, grassy or painty
flavors (Wolf, 1978). A highly saturated fatty acid level is confirmed
to be of benefit in terms of storage ability when compared to more
unsaturated vegetable oils (Ferrari et al., 2004). Indeed, the tendency
of an oil to combine with oxygen of the air and become gummy (known as
drying) is measured with the iodine number, which in fact is merely a
measure of the level of unsaturation of the oil in question (a higher
iodine number will indicate higher unsaturation seeing that iodine is
absorbed primarily by the mechanism of addition to the double bonds
characteristic of unsaturation) (Gunther, 1971).
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