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Extraction And Characterization Of Vegetable Oil Using Bread Fruit Seed
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1.5.3.1 Reactions
Animal
and plant fats and oils are composed of triglycerides, which are esters
containing three free fatty acids and the trihydric alcohol, glycerol.
In the transesterification process, the alcohol is de-protonated with a
base to make it a stronger nucleophile. Commonly, ethanol or methanol
are used. As can be seen, the reaction has no other inputs than the
triglyceride and the alcohol. Under normal conditions, this reaction
will proceed either exceedingly slowly or not at all, so heat, as well
as catalysts (acid and/or base) are used to speed up the reaction. It is
important to note that the acid or base are not consumed by the
transesterification reaction, thus they are not reactants, but
catalysts. Common catalysts for transesterification include sodium
hydroxide, potassium hydroxide, and sodium methoxide.
Almost all
biodiesel is produced from virgin vegetable oils using the
base-catalyzed technique as it is the most economical process for
treating virgin vegetable oils, requiring only low temperatures and
pressures and producing over 98% conversion yield (provided the starting
oil is low in moisture and free fatty acids). However, biodiesel
produced from other sources or by other methods may require acid
catalysis, which is much slower.
The transesterification reaction is
base catalyzed. Any strong base capable of de-protonating the alcohol
will do (e.g. NaOH, KOH, sodium methoxide, etc.), but the sodium and
potassium hydroxides are often chosen for their cost. The presence of
water causes undesirable base hydrolysis, so the reaction must be kept
dry. In the transesterification mechanism, the carbonyl carbon of the
starting ester (RCOOR1) undergoes nucleophilic attack by the incoming
alkoxide (R2O−) to give a tetrahedral intermediate, which either reverts
to the starting material, or proceeds to the transesterified product
(RCOOR2). The various species exist in equilibrium, and the product
distribution depends on the relative energies of the reactant and
product.
GENERAL PROPERTIES OF VEGETABLE OILS
1.6 Vegetable oils - General properties
Vegetable
oils are obtained from oil containing seeds, fruits, or nuts by
different pressing methods, solvent extraction or a combination of these
(Bennion, 1995). Crude oils obtained are subjected to a number of
refining processes, both physical and chemical. These are detailed in
various texts and articles (Bennion, 1995), (Fennema, 1985). There are
numerous vegetable oils derived from various sources. These include the
popular vegetable oils: the foremost oilseed oils - soybean, cottonseed,
peanuts and sunflower oils; and others such as palm oil, palm kernel
oil, coconut oil, castor oil, rapeseed oil and others. They also include
the less commonly known oils such as rice bran oil, tiger nut oil,
patua oil, ko_me oil, niger seed oil, piririma oil and numerous others.
Their yields, different compositions and by extension their physical and
chemical properties determine their usefulness in various applications
aside edible uses.
Cottonseed oil was developed over a century ago
as a byproduct of the cotton industry (Bennion, 1995). Its processing
includes the use of hydraulic pressing, screw pressing and solvent
extraction (Wolf, 1978). It is classified as a polyunsaturated oil, with
palmitic acid (C16H32O2) consisting 20 – 25%, stearic acid (C18H36O2) 2
– 7%, oleic acid (C18H34O2) 18 – 30%, and linoleic acid (C18H32O2)40 –
55% (Fennema, 1985). Its primary uses are food related – as salad oil,
for frying, for margarine manufacture, and for manufacturing shortenings
used in cakes and biscuits.
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