INTRODUCTION:

As energy demands increase with corresponding limited sources of energy, a number of current studies focus on possible development of alternative fuels (Cheenkachorn, 2004). One of such alternative fuels for combustion in compression-ignition (diesel) engines is the biodiesel. Biodiesel was established as an alternative fuel after it was discovered that vegetable oils originally used in the diesel engine were problematic (Ramadhas et al., 2005). These engine problems due primarily to high viscosities and low volatilities of the vegetable oil were however, resolved through several chemical processes including transesterification, which gave rise to biodiesel. Vegetable oils as well as animal fats, because of their similar chemical composition therefore form the basic raw materials for the production of biodiesel. These vegetable oil sources contain a high percentage of triacylglycerol, which is the major component for the production of the biodiesel (Pinto et al., 2005).

Melon (a member of the Cucurbitaceae family) is one of the most popular vegetable crops grown in Africa (Schippers, 2005). It is a tendril, creeping, herbaceous, annual crop, which grows well on a sandy free draining soil and is cultivated mainly for its seeds (Schippers, 2005).

Melon seeds are contained in globular, smooth, hard rind fruits that are unedible because of their bitter taste. The seeds are however, edible. When planted, it can be harvested between two and a half to three months and with good management, there can be a seed yield of 350-400kg per hectare (Adekunle et al., 2009).

In Nigeria, there are different varieties or cultivars of melon seed. The two major cultivars are bara and serewe (Oloko and Agbetoye, 2006). Morphologically, bara has large brown seeds with thick black edges thickened towards the apex, about 16 x 9.5mm and is common in the northern and western part of Nigeria (Adekunle et al., 2009). Serewe seeds, on the other hand, are smooth, light brown with a light whitish edge that is not thickened, about 15 x 9mm in dimension. They are mainly found in the eastern part of Nigeria (Adekunte et al., 2009). Most of the studies on the melon seed have centred on the nutritional, agronomical, and some industrial applications of the seed and oil (Njoku et al., 2009).

Melon seeds are rich in protein but more in oil, about 44-53% w/w (Achu et al., 2005). The oil is made up of 71.85% unsaturated fatty acids, which are majorly, linoleic (56.94%) and oleic (14.50%), and saturated fatty acids, which are mainly stearic (13.71%) and palmitic (13.45%) (Oluba et al., 2008). The presence of the unsaturated fatty acids makes melon nutritionally desirable especially in the possible lowering of blood cholesterol (hypocholesterolic effect). Besides the characterization of the melon seed oil, other works on the oil include: lecithin production from melon seed oil (Njoku et al., 2009); the use of melon seed for the control of bruchid damage in cowpea (Okunola, 2003); investigation on some toxicological and nutritional properties of melon seed oil (Njoku et al., 1994); investigation of melon (Citrullus colocynthis L.) seed oil as a potential for biodiesel production (Giwa et al. 2010). This work is a complementary study on the ongoing exploration of melon seed oil as a potential feedstock for biodiesel production.

MATERIALS AND METHODS:

Collection and processing of Sample:

The melon (Citrullus lanatus) seeds were obtained from Kogi State of Nigeria, and identified by Mr Ozioko of Bioresources Development and Conservation Programme (BDCP) Centre, Nsukka, where a voucher specimen is also maintained. The seeds were washed first with distilled water and then, with normal saline (0.9% NaCl) to clean up and remove possible mycotoxins. This was followed by the dehulling of the wet melon seeds. Thereafter, the seeds and their hulls were sun-dried for about 8 hours, and the hulls willowed out. The dehulled melon seeds were then ground using mechanical grinder before oil extraction.

Extraction of Oil from the Melon Seed:

This was done by the Soxhlet extraction method as reported by the AOAC (1980). The percentage (%) yield of the melon seed oil was determined after triplicate run and the mean value reported.

Physico-chemical Analysis of the Melon Seed Oil:

The physico-chemical properties of the oil were determined using the standard methods of AOAC, 1975. These properties include kinematic viscosity, relative density, acid value, iodine value, peroxide value, saponification value.

Production of Biodiesel from the Melon Seed Oil:

The Freedman method, (1984) was used for the production of the biodiesel. Methanol containing 1% sodium hydroxide (NaOH) was mixed with the melon seed oil in the mole ratio of 6:1 (v/v) and refluxed at 60^oC for 1 hour. The refluxed mixture was poured into a separating funnel and left overnight to allow for separation by gravity. A bilayer was then introduced by adding petroleum-ether followed by distilled water into the refluxed mixture. The glycerol phase was run-off the separating funnel and the biodiesel phase washed with saturated sodium hydrogen carbonate (NaHCO₃) and thereafter, dried over anhydrous sodium sulphate (anh.Na₂SO₄). The petroleum-ether was removed using rotary evaporator operated at 80°C.

Physico-chemical Properties of the Biodiesel:

The physical and chemical properties of the biodiesel were determined using the standard methods of AOAC, 1975 except for additional physical (fuel) properties which include flash point, cetane number and heat of combustion. Flash point (ASTM D93) and heat of combustion (ASTM D240) were determined according to relevant biodiesel test methods. The cetane number of melon seed oil methyl esters was evaluated via the use of the empirical formula (Ramos et al. 2009):

where CN, is the cetane number of the biodiesel, X_ME is the weight percentage of each methyl ester (Oluba et al., 2008), and CN_ME is the cetane number of individual methyl ester (Moser, 2009). All tests were run in triplicate and mean values were reported.

RESULTS AND DISCUSSION:

Table 1. Melon seed oil yield

Feedstock	Oil Yield (%)
Melon seed	48.25 ± 0.826^β
Linseed	33.33*
Soybean seed	18.35*
Palm	44.60 *

^βValue obtained as Mean ± S.D from triplicate analyses

*Akbar et al. 2009

Table 2: Physico-chemical Properties of Melon Seed Oil

Parameter	Melon seed oil*
Kinematic viscosity (mm²/s)	46.79 ± 1.326
Relative density	0.93 ± 0.243
Acid value (mgKOH/g)	2.96 ± 0.421
Iodine Value (mg I₂/g)	126.9 ± 1.865
Saponification value (mgKOH/g)	188.25± 0.325
Peroxide Value (meq/1000g )	0.00

*Values obtained as Mean ± S.D from triplicate analyses

Table 3: Melon seed oil biodiesel yield

Feedstock methyl esters	Yield (wt. %)
Melon seed	84.00 ± 1.468^ß
Beniseed	74.00*
Jatropha seed	76.54*
Tobacco seed	83.31*
Soybean	84.90**
Sunflower	88.00**
Canola	93.5†
Rapeseed	94.50**

^ßValue obtained as Mean ± S.D from triplicate analyses

*Allen et al. 1999; **El-Diwani et al. 2009; †(Leung and Guo, 2006)

Table 4: Physico-chemical properties of the melon seed oil methyl esters.

Parameter	Melon seed oil methyl esters
Relative density	0.88 ± 0.626 ^∆
Kinematic viscosity (mm²/s)	2.596 ± 1.423 ^∆
Acid value (mg KOH/g)	0.36 ± 0.867 ^∆
Iodine value (mg I₂/g)	111.67 ± 2.657^∆
Flash point (°C)	120 ± 0.874 ^∆
Cetane number	55.84
Heat of combustion (KJ/g)	39.3 ± 0.367 ^∆

^∆Values obtained as Mean ± S.D from triplicate analyses

The % yield of the oil from the extraction procedure gave an average of 48.25 (Table 1). This oil content was found to be higher than that of linseed (33.33%), soybean (18.35%) and palm kernel (44.6%) (Akbar et al., 2009).

Table 5: Fuel properties of melon seed oil biodiesel, petrodiesel and other biodiesel fuels

Parameters	Limits		Petrodiesel*	SOB †	SUOB**	MSOB
Parameters	ASTM D6751	EN 14214	Petrodiesel*	SOB †	SUOB**	MSOB
Relative density	-	0.86 – 0.90	0.86	0.89	0.88	0.88 ± 0.626^∆
Kinematic viscosity; 40°C (mm²/s)	1.9 6.0	3.5 – 5.0	4.1	4.2	4.85	2.596 ± 1.423^∆
Acid value (mg KOH/g)	0.5max.	0.5max.	-	0.14	0.40	0.36 ± 0.867^∆
Iodine value (mg I₂/g)	-	120max.	-	-	-	111.67± 2.657^∆
Flash point (°C)	130min	120min	54	171	168	120 ± 0.874 ^∆
Cetane number	47min	51min	42.0	49	55	55.84^ß
Heat of combustion (KJ/g)	-	-	45.5	38.10	45.5	39.30 ± 0.367^∆

SOB = Soybean oil biodiesel; SUOB = Sunflower oil biodiesel; MSOB = Melon seed oil biodiesel.

*Kulkarni et al. 2008; †Ramos et al. 2009; **Rashid et al. 2009.

^∆Values obtained as Mean ± S.D from triplicate analyses; ^ßEmpirically determined

Table 6: Fatty acid composition of melon(Citrullus lanatus) seed oil and the cetane number values of their corresponding methyl esters

Fatty acid	% Composition^a	Cetane number^b
Lauric acid (C12:0)	0.21	67
Myristic acid (C14:0)	0.78	-
Palmitic acid (C16:0)	13.45	86
Stearic acid (C18:0)	13.71	101
Oleic acid (C18:1)	14.50	59
Linoleic acid (C18:2)	56.94	38
Linolenic acid (C18:3)	0.41	23

^aValues from Oluba et al., (2008); ^bValues from Moser, (2009)

The characterization of the oil showed a yellow colour (Table 2) with a viscosity value of 46.79mm²/s and relative density of 0.93, at room temperature. These results were higher than that reported by Giwa et al., (2010) with a viscosity value of 31.52mm²/s and a relative density of 0.905. The acid value of the oil was found to be 2.96mg KOH g^-1. This value is relatively higher than those of the conventional soybean oil (2.67mg KOH g^-1) and rapeseed oil (2.88mg KOH g^-1) (Jordanov et al., 2007), suggesting relatively lower methyl ester yield on transesterification.

The iodine value was found to be 126.9mg Iodine g^-1, a value higher than that reported by Giwa et al. (2010). This result is indicative of a high content of unsaturated fatty acids and susceptibility to autoxidation in contrast to palm oil (50-55mg Iodine g^-1) (Knothe, 2002). Surprisingly however, the peroxide value of the oil was found to be zero (0.00meq/1000g) at the time of testing, suggesting possibly no rancidity. This must have possibly resulted from proper handling of the melon seeds before extraction and limited effect of heat on the oil during extraction since heat favours oxidation of fatty acids increasing the formation of peroxides (Oluba et al., 2008). In this study, the saponification value of the oil was found to be 188.25mg KOH g^-1. This value is slightly lower than that of palm oil (190-209mg KOH g^-1) (Knothe, 2002) and equally lower than that reported by Giwa et al., (2010). The high saponification value of the oil suggests its usefulness in industrial applications.

Transesterification of the oil with methanol using 1% NaOH as catalyst gave biodiesel whose yield was 84% (Table 3). This yield though higher than that of some feedstock’s, was lower compared to the conventional canola methyl esters (93.5% yield) (Leung and Guo, 2006), sunflower methyl ester, rape seed methyl ester (El-Diwani et al. 2009). A possible reason for the low yield could be high concentration of free fatty acid impurities via hydrolysis of the biodiesel (Naik et al., 2008). The value was also lower than that reported by Giwa et al., (2010). It is possible that environmental factors could be contributory to the variation in the biodiesel yield.

The physico-chemical properties of melon seed oil biodiesel is shown in Table 4. The biodiesel was of similar relative density (0.88) with lower kinematic viscosity (2.596mm²/s) than those of petrodiesel, soybean oil and sunflower oil (Bajpai and Tyagai, 2008; Ramos et al. 2009; Rashid et al. 2009) (Table 5). These values were slightly lower than that reported by Giwa et al. (2010) having relative density and kinematic viscosity values of 0.883 and 3.83mm²/s respectively. The acid value was within the standard and lower than that of sunflower biodiesel but higher than that of soybean biodiesel (Ramos et al. 1999; Rashid et al. 2009), and equally higher than that reported by Giwa et al., (2010). Similarly, the iodine value of the biodiesel was found to be lower than the set maximum value (120 mg Iodine g^-1) by the EN 14214 biodiesel fuel standard. When compared to petrodiesel, the flash point of the melon seed biodiesel was higher indicating safe storage and possibly haulage, but lower than that of soybean, sunflower methyl esters (Table 5) and that reported by Giwa et al., (2010) with a flash point value of 142°C. The biodiesel also had an empirically calculated cetane number of 55.84 which was above the minimum values for biodiesel fuel standards and equally higher than that of petrodiesel, soybean biodiesel, sunflower biodiesel and that reported by Giwa et al., (2010) (Ramos et al. 2009; Rashid et al. 2009) (Table 5). The heat of combustion of the biodiesel was higher than that of soybean methyl esters but slightly lower than that reported by Giwa et al., (2010) (DeOliveira et al., 2006) (Table 5). An additional property, the peroxide value, which does not appear in the biodiesel fuel standards was also determined and found satisfactory for the methyl esters; the low peroxide value being indicative of little peroxidative rancidity. From this study, melon seed oil has shown promise as a potential source of biodiesel, and the results complement other studies on the industrial utilization of melon seed as possible industrial vegetable oil. However, attention should be focused on the agronomical, storage and large cultivation of the seed to enable minimal competition on the plant as currently the only major use is on nutrition.

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Received on 02.06.2011 Modified on 14.08.2011

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