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Extraction And Characterization Of Vegetable Oil Using Bread Fruit Seed
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Based on studies by
Toshiyuki. (1999), the oxidative stability of refined vegetable oils is
found to be determined considerably by the fatty acid composition, the
tocopherols content and the carbonyl value (Toshiyuki, 1999). When
observed at frying temperatures, it is seen that in general, non-refined
oils prove to have a better stability than refined oils (Gertz et al.,
2000). This could be attributed to the fact that refining steps, in
particular deodorization, remove a percentage of the tocopherols, which
act as natural anti-oxidants in vegetable oils (Applewhite, 1978). Corn
oil has a better stability than soybean oil, while rapeseed oil is seen
to give a better performance than olive oil. This can be explained in
terms of their compositions (Isbell et al., 1999). When investigated at a
temperature of 110oC, vegetable oils still show the trend of increased
stability in the unrefined state than when refined. Meadow foam oil is
reported as the most stable oil in the study conducted by Isbell et al.
(1999). High oleic sunflower oil and crude jojoba oil also had good
values of oxidative stability (Isbell et al., 1999). Other studies
indicate that the presence of free fatty acids has a pro-oxidant effect
on vegetable oils (Frega et al., 1999). Hence refining practices are
important, seeing Aluyor and Ori-Jesu 4839 that improper handling and
raw material abuse can result in the stimulation of enzymatic activity
which could produce free fatty acids (Applewhite, 1978). Further
investigations on manufacturing practices also reveal research which
indicates the importance of the solvent used in the extraction of
vegetable oils. Traditional solvents utilized such as hexane or
petroleum ether have the characteristic of extracting only non-polar
species. Isopropanol however, as documented by Oyedeji et al. (2006)
would extract some polar and high molecular weight compounds. Among
these compounds are the natural antioxidants and pigments in oilseeds
which presence lead to extended shelf life and hence better oxidative
stability (Oyedeji et al., 2006).
1.8 Antioxidants and stability of vegetable oils
Numerous
experimental works have established the positive effect of
anti-oxidants on the oxidative stability of vegetable oils for both
edible uses and industrial uses. An important class of anti-oxidants
consists of the phenolic compounds butylhydroxyanisole (BHA),
butylhydroxytoluene (BHT), propyl gallate, and tert-butyl
hydroquinone (TBHQ). Their use in vegetable oils meant for domestic and industrial processes is widespread.
Vegetable
oils in their natural form possess constituents that function as
natural antioxidants. Amongst them are ascorbic acids, _-tocopherole,
_-carotene, chlorogenic
acids and flavanols (Ullah et al., 2003).
Tests conducted to investigate the effectiveness of natural
anti-oxidants contained in red pepper oil added to soybean and sunflower
oils indicate that they provide variable protection against light
induced auto-oxidation.
In the above mentioned study on the
inhibitive effect of natural antioxidants contained in red pepper oil,
it was additionally observed that the phenolic anti-oxidant
butylated
hydroxytoluene (BHT) shows more effectiveness generally than natural
anti-oxidants (Ullah et al., 2003). In the work done by Robert (2005),
the common phenolic anti-oxidants were tested for their effectivenessin
improving the oxidative stability of biodiesel obtained from soybean
oil. Dunn monitored the oxidative stability by means of pressurized
differential scanning calorimetry (P-DSC). For both static and dynamic
conditions, improvements in oxidative stability are observed with the
application of anti-oxidants, which included BHA, BHT, TBHQ, propyl
gallate (PrG) and α-tocopherol. The work of (Dunn, 2005) further showed
that the relative effectiveness of the different anti-oxidants differed
for static and dynamic conditions, although all showed superior
performance when compared with α-tocopherol.
A recent area of
interest in antioxidant research is concerned with finding effective
replacements for the conventional synthetic antioxidants from among
various natural extracts from plant species which are seen to possess
antioxidant properties. Such research is in the main prompted by the
reported possibility of synthetic antioxidants having adverse health
effects on humans exposed to them. Specifically, they are known to
contribute to liver enlargement and an increase in microsomal activity
(Khanahmadi et al., 2006; Morteza- Semnani et al., 2006). Maduka et al.
(2003) investigated the effectiveness of a Nigerian alcoholic beverage
additive, Sacoglottis gabonensis stem bark extract as an antioxidant for
common stored vegetable oils. Inhibition of lipid peroxi-dation was
found to be comparable to inhibitions obtained with treatment with
vitamins C and E (Maduka et al., 2003).
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