Potentials of Afzelia Africana Vegetable Oil in Biodiesel Production
Vegetable oils are combustible materials of organic origin and have potentials for use as biodiesel. The potential of extractable vegetable oil from Afzelia africana in the production of biodiesel was investigated and its fuel properties were evaluated by the assessment of the physicochemical properties. Standard official methods were employed in extraction and characterization of oil from Afzelia africana (African oak) seed and it was found that it contained 18.50% crude lipid content on dry weight basis with a very high percentage of total fatty matter and a refractive index of 1.47 at 40°C. The acid value 4.49% was low in the oil sample while the peroxide value 6.40 meq g-1 was indicative of very low level of hydroperoxides which could initiate or propagate further oxidation of the oil, thereby improving the stability (oxidative stability) of the oils. The saponification value was 229.12 mg KOH g-1 while the iodine value indicates that the oil was non-drying and non volatile at room temperature. The value of the Cetane index was 55 and the estimated value of the heat of combustion revealed that burning the vegetable oil in a diesel engine will yield about 9209.45 cal g-1. The fatty acid distribution in the seeds of Afzelia africana expressed as the percentage area of fatty acid methyl esters composition revealed lauric acid 0.69%, myristic acid 1.54, palmitic acid 33.45%, stearic acid 6.97%, oleic acid 12.25% and linoleic acid 41.25%, showing that the seed oil was predominantly composed of palmitic acid and oleic acid families. This result showed that the oil would be stable on exposure to atmospheric oxygen during storage and use. This stability is further confirmed by the low level of free fatty acids.
Received: July 25, 2011;
Accepted: October 10, 2011;
Published: October 29, 2011
Vegetable oils come mainly from seeds. Within the seeds, oil is stored in the
endosperm (castor oil, coconut oil), the cotyledons of the embryo (peanuts,
soya beans, cotton) or in the scutellum (corn) (Simpson and
Conner-Orgazally, 1986). Some vegetable oil seeds include groundnut, cotton
seed, sunflower, palm, corn, castor linseed and rubber seed etcetera (Ikwuagwu
et al., 2000).
Afzelia africana plant is called akparata in the Igbo speaking
south-eastern part of Nigeria. The genus was named in honour of Dam Afzelius
of Uppsala, who lived in Somalia. Afzelia africana is a very attractive
medium sized tree, with bright green leaves that turn to attractive yellowish
colour during harmattan seasons in Nigeria. It has upright crown and the dropping
branches resemble an eucalyptus from a distance. Afzelia africana is
a deciduous plant that bears 6-10 hard, shiny, oblong black bean-shaped seeds
with red orange aril with extractable oil (Palgrave and Palgrave,
2002). The seeds are used as soup thickeners (Igwenyi
and Akubugwo, 2010).
Vegetable oils are now processed into other industrial products for the benefit
of mankind. These other applications of this nutrient in non-food or industrial
sectors have mounted pressure on the food use of certain food materials. For
example, carbohydrates are converted into biofuels and used as alternative biodegradable
and environmentally friendly fuels. They are converted into ethanol used as
fuel/blends for combustion engines and also as a raw material for the production
of strong drinks and other industrial products. Vegetable oils are used as biodiesel,
vanishes, raw material for paints, ink oil and so on (Igwenyi
et al., 2008). Thus, the food use and industrial applications of
plants and their products have increased over the years and this is associated
with increase in world population. This has mounted pressure on their food uses
and created a big problem in feeding the ever increasing world population (Wilcox
and Leffel, 1987).
Vegetable oils and their methyl esters have long been considered as potential
alternative fuels for diesel engines. In addition to ignition quality, the gross
heat of combustion of oil is one of more important properties in determining
its suitability for use in diesel engines (Krisnangkura,
1991). The use of renewable fuels also leads to a reduction of carbon dioxide
(CO2) in the atmosphere that contributes to the greenhouse effect
(Arapatsakos et al., 2008).
The heat of combustion of a fuel is the quantity of energy released by burning
a unit amount of the fuel and it is important with regards to fuel consumption.
The heat of combustion of oils increases with the molecular weight and varies
from about 5,900 cal g-1 for butyric acid to about 9,600 cal g-1
for stearic acid (Hall et al., 1978).
Another, measure of quality of oil used in combustion engines is the Cetane
index or number. This is a measure of a fuels ignition delay and can be
defined as the time between the start of an injection engine and the first identifiable
pressure increase during combustion of the fuel. Generally diesel engines run
well with a cetane number from 40-55 (Owen et al.,
1995). The Cetane number, therefore measures the ease with which a diesel
fuel ignites under compression and diesel engine conditions (Heywood,
Some physicochemical parameters were considered which affect the oxidative
stability of the oil under storage and usage as well as overall quality of the
oil. Some of these parameters include oil content to check the presence of extractable
oil and the quantity that can encourage industrial attention. Others include
the acid value which varies with the source of the oil and the level of refining
(Hammond, 1993) as well as the peroxide value which gives
the amount of active oxygen (mg) present in 1 kg fat (Ikwuagwu
et al., 2000). The iodine value is important as a measure of the
degree of unsaturation and a determinant in the drying and non-drying property
of the oil while the saponification value determines the amount of fatty acids
that can be liberated on complete hydrolysis of the oil etcetera.
This study was therefore aimed at the investigation of fuel properties of the vegetable oil from Afzelia africana with a view to providing baseline data for further systematic scientific research.
MATERIALS AND METHODS
Test samples: All the samples used were fresh seeds purchased from foodstuff
dealers in EKE ABA market, Abakaliki, Ebonyi State, Nigeria. They were processed
traditionally (dried and roasted) and the shells of the seeds were removed manually
and further dried in an oven at 60°C for 48 h. The study was carried out
during the dry season period of 2010 in the Department of Biochemistry, Faculty
of Biological Sciences, Ebonyi State University, Nigeria.
Oil extraction and characterization: Oil was extracted using petroleum
ether (40-60°C) in a soxhlet apparatus and the solvent was later distilled
off in a rotary evaporator. The acid value was determined as the number of milligrams
of potassium hydroxide required to neutralize 1 g of the oil (AOAC,
1990). The peroxide value was determined following procedures described
by Hamilton et al. (1992) as the amount of substance
in a sample expressed in terms of mill-equivalents of active or peroxide
oxygen per kilogram of fat or oil which oxidizes potassium iodide while
the iodine value was determined according to Wijs method as described
by Ikwuagwu et al. (2000) as a measure of unsaturation
in fats and is the number of grams of halogen added to the double bonds of 100
g fat and expressed as equivalent number of grams of iodine. The saponification
value was determined by refluxing the oil with alcoholic potassium hydroxide
(Hamilton et al., 1992) while the amount of unsaponifiable
matter was determined as the total quantity of matter present in the oil which
after saponification by potassium hydroxide and extraction using diethyl ether,
are not soluble in aqueous alkali and non-volatile at 103°C (Hoffman,
Determination of fatty acid profile: The fatty acid methyl esters of
lipids were prepared according to the AOAC method 991.39 (AOAC,
1990). The analysis of fatty acid methyl esters were carried out with a
Hewlett Packard Gas Chromatograph equipped with a hydrogen flame ionization
detector and a capillary column; CP-Sil-88 Wcott fused silica 50 mx 0.25 mM
id. (of 0.20 mM film thickness). The temperatures of injector and detector were
270°C. The initial column temperature was 170°C and then raised to 205°C
at a rate of 1°C min-1. Split ratio was 1/50. The carrier gas
was hydrogen at a flow rate of 1 mL min-1. The identification and
quantification of fatty acid methyl esters was accomplished by comparing retention
times of the peaks with those of standards.
Cetane number: Cetane number was estimated using the Klopfenstein simultaneous
equation for esters of fatty acids (Klopfenstein, 1982).
Heat of combustion: Heat of combustion was estimated using the methods
and equation that expressed the value in terms of constant volume at 15°C
according to Hall et al. (1978).
Statistical analysis: The results were presented as mean±standard deviation (x±SD).
RESULTS AND DISCUSSION
The mean value of the percentage composition of vegetable oil in Afzelia
africana was 18.50% crude lipid content on dry weight basis as shown in
|| Extraction and physicochemical characterization of oil from
Afzelia africana seeds
The comparative analysis with oil seeds showed that the seeds can be classified
as oil seeds or oil crop since it can serve as commercial source of vegetable
oils. Vegetable oils come mainly from seeds. This value for the oil content
of the seed (18.50%) was higher compared with results of oil contents of Brachystegia
eurycoma (5.87%), Tamarindus indica (7.20%) and Mucuna flagellipes
(3.77%) by Ibironke et al. (2005) but comparable
to results of nutritive properties of dehulled and undehulled samples of Brachystegia
eurycoma and Detarium microcarpum which ranged from 14.00 to 18.5%
by Uhegbu et al. (2009). The values were also
comparable to 15% reported in Detarium microcarpum by Akpata
and Miachi (2001) and 8.30% in Mangifera indica (Eddy
and Udoh, 2005), 2.80 and 3.1% fat content of Canavalia gladiata seeds
(whole and cotyledon, respectively) (Sagarika et al.,
1999). The oil composition was however lower than 54.26% reported for Turkish
sesame seeds (Unal and Yalcin, 2008) and 48.00-50% reported
for Sesame indicum L. (Mohammed and Hamza, 2008),
which are oil seeds that can provide a commercial quantity of oil for the industries.
The oil content was also found to be lower than castor seed 50%, cotton seed
30%, linseed 40% and palm kernel 50% (Carr, 1989).
The very high percentage (98.47%) of total fatty matter in the oil in Table
1 indicates that the oil was rich and predominantly fatty materials. The
seeds of Afzelia africana had refractive index of 1.47 at 40°C. Refractive
index of a fat is the ratio of speed of light at a defined wavelength to its
speed in the fat itself. This value varies with wavelength and temperature and
also varies with the degree and type of unsaturation as well as the type of
substitutions of component fatty acids and with accompanying substances. Refractive
index is widely used in quality control to check for the purity and adulteration
of fatty materials and to follow hydrogenation and isomerization (Hoffman,
The acid value 4.49% was low in the oil sample. This value was comparable to
the values of 3.0, 12.2, 0.8, 2.1 and 2.5 reported for sunflower, cotton seed,
groundnut, olive oil and coconut oils respectively used as edible and industrial
oils (Engler et al., 1983). It was also observed
that there were no significant differences with the values reported by Anhwange
et al. (2004) for Detarium microcarpum and Moringa oleifera
(Lam). The value was comparable to 1.68-5.05 reported by Igwenyi
et al. (2008) for Raphia vinifera seed pulp oil and 9.0-21.6
reported by Ikwuagwu et al., (2000) for rubber
seed oil used in the preparation of biodiesel. This result showed a potential
for the industrial application of the oil in biodiesel production. The values
however accounted for the low level/presence of free fatty acids in the oil
as an indicator of the little presence and low extent of hydrolysis by lipolytic
enzymes and oxidation (Ranken and Kill, 1993). The value
is comparable to the free fatty acid value of <10 reported in the survey
of plant oil engines and fuel specifications for running on rapeseed oil (Krause,
1998). The low value showed that the oil if exposed without addition or
treatment with antioxidants will be stable over a long period of time and protected
against rancidity and peroxidation. This low level of the free fatty acids is
an indication that the components were predominantly composed of triacylglycerols
and presence of natural antioxidants (Ikwuagwu et al.,
2000; Igwenyi et al., 2008).
The peroxide value in meq g-1 for Afzelia africana was 6.40.
There was no significant difference when compared to the value of 7.45-8 meq
KOH g-1 in Sesame indicum L. reported by Mohammed
and Hamza (2008). The value was indicative of very low level of hydroperoxides
which could initiate or propagate further oxidation of the oil thereby improving
the stability (oxidative stability) of the oil and corroborates the result of
the free fatty acid values. The value was comparable to the peroxide value of
less than 3 meq g-1 reported in the survey of plant oil engines and
fuel specifications for running on rapeseed oil (Krause,
1998). The peroxide value is also comparable to 2.00-6.00 reported by Igwenyi
et al. (2008) for Raphia vinifera seed pulp (mesocarp) oil
and the reports of Ikwuagwu et al. (2000) for
rubber seed oil and Engler et al. (1983) for
sunflower oil. This showed that the oil would be stable and this stability is
further confirmed by the low level of free fatty acids. This is attributable
to the presence of natural antioxidants in the oil which are effective in slowing
down the rate of oxygen absorption by reacting with the fatty acid peroxy free
radicals (Hoffman, 1986). This property of the oils which
are comparable with those of rapeseed, sesame, sunflower and groundnut seeds
suggests their suitability and use as edible oils (Ibironke
et al., 2005).
The saponification value in mg KOH g-1 was 229.12. The saponification
value was comparable to 189-191 mg KOH g-1 reported by Mohammed
and Hamza (2008) for Sesame indicum L. seeds grown in Jigawa State
of Nigeria and 179-220 mg KOH g-1 reported by Anhwange
et al. (2004) for Detarium microcarpum and Moringa oleifera.
An increase in saponification value in oil increases the volatility of the oil.
It enhances the quality of the oil because it shows the presence of lower molecular
weight components in 1 g of the oil. The saponification value is the number
of milligrams of potassium hydroxide required to neutralize the fatty acids
liberated on complete hydrolysis or saponification of 1 g of the oil. It is
inversely proportional to the molecular weight of the oil since 1 g of oil or
fat containing low molecular weight fatty acids will have more molecules than
oil or fat containing higher molecular weight fatty acids. The principle in
the methods of Jacobs (1958) revealed that the number
of milligrams of KOH required to saponify the oil will be greater in the former
than in the latter case. Therefore, the higher the saponification value, the
more the lower molecular weight fatty acids and the better the quality of the
Iodine number of oil is a measure of its degree of unsaturation and is a useful
criterion for purity and identification. The iodine value indicates that the
oil was non-drying with iodine number lower than 100 (Kochhar,
1986). Non-drying oils are not volatile at room temperature and does not
pose any danger of inflammability. It showed that the oils were not made up
of very low molecular weight fatty acids as non-drying oils are predominantly
composed of oleic and palmitic acids. This result further confirms the result
of the fatty acid profile of the oil. It is also in line with the results of
Adeyeye and Ajewole (1992) for cereals which indicated
the predominance of stearic, oleic and linoleic acids in sorghum, millet, maize
The value 74.10 Wijs (Giod/100 g) for the iodine value was lower than
the maximum value of 115 Giod/100 g reported in the recommendation of quality
standards for rapeseed oil to be used as fuel in internal combustion engines
(Krause, 1998). However, the values were comparable
to the values for edible vegetable oils (Simpson and Conner-Orgazally,
1986) but was lower than 155.00-205.00 for linseed, 135.00-150.00 for safflower,
120.00-143.00 for soya bean and 110.00-143.00 for sunflower (Ranken
and Kill, 1993; Hoffman, 1986) and 69.40-76.50 reported
by Igwenyi et al. (2008) for Raphia vinifera
seed pulp oil which were utilized in the production of biodiesel.
The iodine value confirmed the stability of the oil and points to a low degree
of unsaturation and a further confirmation of its low peroxide value and fatty
acid profile since non-drying oils do not undergo oxidation. Iodine values increase
with the degree of unsaturation as the number of grams of halogen added to the
double bonds of 100 g fat and expressed as equivalent number of grams of iodine.
It is applied in monitoring progress of hydrogenation, degree of fractionation
and for identity characterization of fats (Hoffman, 1986).
The Cetane index value was estimated and found to be within the maximum range
for diesel engines as diesel engines run well with cetane number between 40
to 55 (Owen et al., 1995).
|| Percentage (% area) fatty acid composition of processed seed
of Afzelia africana samples
This is advantageous as a future diesel fuel since it has a high cetane rating
(55) and can be produced as a biofuel. Cetane index is a measure of injection
delay, a time between the start of injection and the start of combustion (ignition).
It follows then that the vegetable oil from Afzelia africana therefore
can ignite easily and quickly which gives it a positive attribute and a potential
diesel fuel for combustion engines (Heywood, 1988).
The estimated value of the heat of combustion of the oil according to the method
of Hall et al. (1978) was about 9209.45 cal g-1.
This showed that combustion of a little volume of the methyl ester of the oil
will yield a tremendous amount of energy to drive the pistons of diesel engines,
there by providing enough power to propel the engine. This is comparable to
the recommendations for oil standards for use in a heavy duty combustible diesel
engine (Krause, 1998; Klopfenstein
and Walker, 1983).
The fatty acid distribution (Table 2) in the seeds of Afzelia
africana expressed as the percentage area of fatty acid methyl esters composition
yielded 41.25% linoleic acid, 12.65% oleic acid, 6.97% stearic acid, 33.65%
palmitic acid, 1.54% myristic acid and 0.69% lauric acid, indicating the predominance
of linoleic, palmitic and oleic acids. These values were however comparable
to 26.50 oleic acid and 45.10 g/ 100 g (41.10%) linoleic acid reported for
Xylopia aethiopica by Barminas et al. (1999).
The results were also comparable to 15.10-24.80% palmitic acid, 29.90-41.80%
oleic acid and 35.90-51.30% linoleic acid reported by Maestri
et al. (1996) for sorghum cultivars from Argentina.
It was observed that the seed oil was predominantly composed of palmitic acid
and oleic acid families. This result showed that the oil would be stable and
this stability is further confirmed by the low level of free fatty acids. This
is attributable to the presence of natural antioxidants in the oil which are
effective in slowing down the rate of oxygen absorption by reacting with the
fatty acid peroxy free radicals. The high saponification value in oil indicates
that an increase in the volatility of the oil. It enhances the quality of the
oil because it shows the presence of lower molecular weight components in 1
g of the oil which can give high amount of energy on combustion. However, there
was a nutritional advantage of the seeds because they contained good level of
the essential fatty acid linoleic acid. The body requires essential fatty acids
for the synthesis of useful substances. The body can however synthesize linolenic
acid and arachidonic acid if provided with linoleic acid (Voet
and Voet, 2004). Essential fatty acids are very important in maintaining
cell membrane structure and in capillary wall integrity and form starting materials
for the synthesis of other unsaturated fatty acid derivatives such as prostaglandins
Afzelia africana contains extractable oil and the physicochemical and fuel properties of the methyl esters showed potentials in biodiesel production. The oil can release high amount of heat on combustion and the cetane index shows that the oil can ignite easily in a combustion engine. The iodine and peroxide values showed increased stability of the oil during storage and transportation. Biodiesel is a safe biodegradable and renewable fuel from biomass, which reduces the emissions of most air pollutants. It is much less polluting than petroleum diesel. They can replace conventional fuels, completely or partially, in the internal combustion engines.
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