Chemical Composition and Fatty Acid Profile of the Lipid Fractions of Selected Nigerian Indigenous Oilseeds

Abstract

Seeds of Cucumeropsis edulis, Hevea brasiliensis, Hura crepitans, Jatropha curcas, Khaya ivorensis and Citrus sinensis were evaluated for their proximate composition, chemical properties and fatty acid profiles. The moisture contents of the seeds ranged from 5.3 to 9.0% while the oil content was between 32.5% for Cucumeropsis edulis and 48.3% for Khaya ivorensis. The ash content ranged between 3.6 and 5.0%. Protein content ranged from 15.8 to 45.9% while the fibre value was in the 8.2–20.5% range. Carbohydrate content varied between 2.4 and 15.4%. The seeds were rich in potassium (349.0–468.6 μg/g of seed) and magnesium (83.7–150.19 μg/g of seed). The extracted oils had saponification value between 202.0 and 210.4; iodine value, 75.6 to 148.2; peroxide value, 4.7 to 13.3; and unsaponifiable matter content, 0.7 to 1.7%. The major fatty acids identified were oleic (14.6–72.1%) and linoleic (10.6–96.3%). Hura crepitans oil was characterized by a high content of linoleic acid (96.3%). The fuel value (H_μ) of the extracted oils ranged from 8276.6–8433.2 kJ/kg.

Keywords:

• Chemical composition
• Indigenous oilseed
• Proximate analysis
• Fatty acid methylesters
• Nigerian

INTRODUCTION

The tropical countries are particularly blessed with a variety of rich vegetations having great, diverse and highly beneficial economic potentials for the populations. One area of such economic potentials is the availability of indigenous non-conventional oil-bearing seeds that may themselves serve as food for humans or feed for animals. However, the extraction and commercial uses of these seeds have not been given serious attention.[1] Oils extracted from such seeds may serve nutritional purposes or as part of raw materials for the chemical production of drying oils, cosmetic emulsifiers, soap and detergents, surface coatings, pharmaceuticals, lubricants, surfactants and fat-liquoring of leather in tanneries.[2,3]

Earlier workers indicated that industrial uses of fats and oils are related to their chemical characteristics. For example, the biofuel potential of any oil is dependent on its iodine and saponification values;[4] the index for the use of some plant oils for application in paint manufacturing depends on their iodine values;[5] the saponification value, free fatty acid value and the acid value of an oil strongly influence the quality of any soap made from it.[6] The lower the saponification values, the better the quality of the oil in producing soaps and detergents. It was also stated that for soap making, the required percentage of free fatty acid values should be between 2 and 5%.

It is note worthy that the negative effects of the global climatic changes currently being experienced could be far reaching with respect to adequate production of traditional or conventional oilseed plants. The increased interest in the use of vegetable oil as a replacement for or extender of diesel fuel[7] might be due to the realization that fossil reserves could be exhausted or become shorter in supply and are not renewable.[8] Thus timely efforts directed at looking for alternative sources of food/feed and fats/oils among the less popular cultivars should be considered with renewed impetus.

In the present work, the chemical composition of six non-conventional oilseeds: Cucumeropsis edulis, Hevea brasiliensis, Hura crepitans, Jatropha curcas, Khaya ivorensis, and Citrus sinensis were measured. The fatty acid profile of their lipid fraction was also determined. The general aim was to explore their potential to be used as food/feed and source of raw materials for our industries.

EXPERIMENTAL PROCEDURES

Materials

The six matured indigenous non-conventional oil seeds, namely: Cucumeropsis edulis (Yoruba: egusi itoo), Hevea brasiliensis (para rubber), Hura crepitans (Sound box tree), Jatropha curcas (physic nut), Khaya ivorensis (Lagos mahogany), and Citrus sinensis (sweet orange) worked on were collected from areas in Osun and Kogi States of Nigeria. After air dried in the sun for five days, the seeds were selected and stored in well-aerated cupboard for further processing. The seeds were later hand shelled and further selected to get rid of damaged ones.

Methods

Moisture, oil, ash, protein, dietary fiber, carbohydrates, and minerals were determined according to the methods described by the Association of Official Analytical Chemists.[9,10] The dried seeds were milled using a Kenwood blender. Oils were extracted with n-hexane from the milled flakes using a Soxhlet extractor. Each extraction lasted about five hours.

Saponification value, free fatty acid content, ester value, iodine value, peroxide value and unsaponifiable matter content were also determined using triplicate analysis of an equal weight of the different seed oils as outlined by the methods of Association of Official Analytical Chemists.[11]

Fuel potentials of the oils were calculated using the relationship:[12]

(1)

where I and S are the iodine value and the saponification value of the oil, respectively. The data obtained for each sample was subjected to t-test for the purpose of comparison. Fatty acid methyl esters (FAME) were prepared from oils by base-catalyzed transmethylation with sodium methoxide as previously indicated.[13]

FAME of the oils were analysed using a PYE UNICAM PY4500 gas chromatograph, fitted with a flame ionization detector (FID). The stationary phase was 15% diethylene glycol succinate (DEGS) coated on 100–120 mesh chromosorb in the HP glass capillary column of 180 cm(L) × 3 mm (i.d.). The temperature of the column was integrated from 120°C at 10°C/min until the column temperature of 250°C was attained. The sample volume was 1 μL and the detection temperature was 280°C. The carrier gas was nitrogen at a flow rate of 0.3 cm per min and a pressure of 24 psi. These conditions were maintained throughout the analysis period. Peaks of the fatty acids were identified by comparing with those of the commercially available FAME. Compound retention time and areas were recorded automatically and from these, percentage by weight of each fatty acid component was obtained.

RESULTS AND DISCUSSION

The results of the proximate analysis of the oilseeds are presented in Table 1. The moisture content of the seeds ranged from 5.3% in Citrus sinensis to 9.0% in Hevea brasiliensis. The values are such that storage problems like decomposition of the seeds might not easily come up if they are stored in a well aerated place. The oil content of the seeds varied from 32.5% in Cucumeropsis edulis to 48.3% in Khaya ivorensis. All the values compared well with the values of the more traditional oilseeds such as olive (25–30%), safflower (30–35%), sunflower (35–45%), rapeseed (40–45%), and groundnut (45–50%).[14] The 44.0% oil content obtained for Hevea brasiliensis was in good comparison with 42.64–50.00% of earlier workers.[15–19] Hura crepitans oil value of 47.8% was quite close to the 48.1% value for cashew nut obtained by earlier workers[20] while the oil content of 48.3% obtained for Khaya ivorensis was higher than the 45.50% oil content reported for Khaya senegalensis.[21] Jatropha curcas oil content (36.7%) was lower than the amount (45.88%) earlier reported[22] for the same seed. However, the oil content of Jatropha curcas and Citrus sinensis (36.7% and 35.5%, respectively) were comparable to the 36.4% value for cotton seed.[14] Also, the 35.6% oil content obtained for Citrus sinensis was in close agreement with the 34.8% value obtained for the same seed[23] by earlier workers.

Table 1 Proximate composition of the seeds ^a.

Seed	Moisture (%)	Oil (%)	Ash (%)	Protein (%)	Dietary fibre (%)	Carbohydrate (%)
Cucumeropsis edulis	6.7 ± 0.1	32.5 ± 1.9	4.5 ± 0.0	45.9 ± 0.0	8.2 ± 0.3	2.4 ± 2.0
Hevea brasiliensis	9.0 ± 0.0	44.0 ± 2.0	5.0 ± 0.0	22.9 ± 0.2	14.4 ± 0.4	4.8 ± 2.1
Hura crepitans	7.2 ± 0.0	47.8 ± 1.5	3.9 ± 0.1	28.4 ± 0.4	10.0 ± 0.3	2.8 ± 1.6
Jatropha curcas	6.6 ± 0.1	36.7 ± 1.1	7 ± 0.2	33.4 ± 0.2	16.1 ± 0.4	2.5 ± 1.2
Khaya ivorensis	7.1 ± 0.1	48.3 ± 1.6	3.6 ± 0.0	15.8 ± 0.1	19.3 ± 0.3	5.9 ± 1.6
Citrus sinensis'	5.3 ± 0.1	35.6 ± 0.1	3.7 ± 0.1	19.5 ± 0.3	20.6 ± 1.5	15.4 ± 3.2
^aAverage of triplicate analysis ± s.d.

The ash content being an indication of the level of inorganics in the sample, varied between 3.6% in Khaya ivorensis and 5.0% in Hevea brasiliensis. These values compared well with those obtained for seed like Bauhinia rufescens and Hildergadia bateri.[24]

Protein levels for the seeds lied between 15.8% for Khaya ivorensis and 45.9% for Cucumeropsis edulis which particularly compared well with such seeds as cashew nut[20] with protein content of 40.90%. Depending on its amount of protein that is bioavailable and the protein quality, Cucumeropsis edulis, therefore, could qualify as a good source of protein.

The dietary fibre content of 8.2–20.6% for these non-conventional oilseeds compared well with values reported for conventional edible legumes[24] and non-conventional legumes.[25] Thus, some of the seeds, if subjected to critical safety evaluation, might be found to be of dietary importance to both man and monogastric ruminants especially capable of lowering the low density lipoprotein (LDL) cholesterol levels of blood plasma. The carbohydrate content of the oilseeds generally is desirable for diabetic patients who should not consume foodstuff of high carbohydrate levels.

The mineral composition of the seeds is presented in Table 2. Potassium was the most abundant mineral in all the seed samples, in agreement with values usually found in food products.[26] The generally low sodium content of the seeds is in accord with the report that tropical crops carry subnormal concentrations of sodium, which is a reflection of the low sodium content of the soils.[21] Generally, the seeds contain elements of metabolic importance capable of improving growth and development. Certain elements that were not detected were probably present at a level below the detection limit of the particular instrument (AAS) used.

Table 2 Mineral composition of the oilseeds ^a (μg/g)

	Macro elements				Micro elements			Toxic metals
Seed	K	Na	Ca	Ma	Zn	Fe	Cu	Pb	Cd
Cucumeropsis edulis	349.0 ± 0.2	1.4 ± 0.1	1.8 ± 0.1	142.4 ± 0.2	4.0 ± 0.0	3.4 ± 0.1	0.2 ± 0.0	ND	ND
Hevea brasiliensis	423.9 ± 3.2	2.3 ± 0.1	2.4 ± 0.0	83.7 ± 0.0	4.1 ± 0.1	1.7 ± 0.0	0.9 ± 0.0	ND	ND
Hura crepitans	366.5 ± 2.6	1.4 ± 0.1	34.6 ± 3.0	119.4 ± 0.1	2.4 ± 0.1	1.4 ± 0.1	0.2 ± 0.0	ND	ND
Jatropha curcas	468.6 ± 5.7	3.13 ± 0.0	5.2 ± 0.2	150.1 ± 1.0	3.9 ± 0.1	2.0 ± 0.0	0.8 ± 0.0	ND	ND
Khaya ivorensis	440.6 ± 8.3	3.5 ± 0.0	1.30 ± 0.1	108.1 ± 0.0	4.6 ± 0.2	2.1 ± 0.0	1.4 ± 0.0	ND	ND
Citrus sinensis	447.1 ± 5.4	2.8 ± 0.1	15.8 ± 0.2	84.6 ± 0.1	3.0 ± 0.0	2.2 ± 0.0	0.3 ± 0.0	ND	ND
^aExpressed as: mean ± s.d.; and ND = not detected.

In Table 3, values obtained for some chemical parameters of the oils extracted from the indigenous (non-conventional) oilseeds are presented. The saponification values were between 202.0 and 210.4. These values lie between 190.0 saponification value for palm oil and 219.9 for palm Kernel oil.[27] The oils extracted from the non–conventional oilseeds might also be very useful for soap making, a use in which both palm oil and palm kernel oil have found extensive applications. The acid values were between 0.8 for Citrus sinensis and 16.8 for Hevea brasiliensis. Thus, ester value was highest for Citrus sinensis (207.7) and lowest for Hevea brasiliensis (186.6). The relatively high acid value for oil from Hevea brasiliensis makes the oil undesirable for nutritional applications. Although the iodine value (149.2) for Hura crepitans was higher than that of Hevea brasiliensis (129.6), the oil film of Hevea brasiliensis showed better drying properties than that of Hura crepitans. However, the iodine values of both were consistent with the total unsaturates contained. The fact that Hevea brasiliensis was found to contain linolenic acid (C18:3) to the level of 1.8% whereas none was detected in Hura crepitans might explain the better drying properties of Hevea brasiliensis.

Table 3 Some other chemical parameters of the oils ^a

Characteristics	Cucumeropsis edulis	Hevea brasiliensis	Hura crepitans	Jatropha curcas	Khaya ivorensis	Citrus sinensis
Saponification value (mgKOH/goil)	207.1 ± 0.8	203.4 ± 1.4	202.0 ± 0.0	206.6 ± 1.6	210.4 ± 1.4	208.5 ± 0.8
Acid value	3.2 ± 0.2	16.8 ± 0.2	4.1 ± 0.1	4.3 ± 0.1	3.8 ± 0.1	0.8 ± 0.1
Free fatty acid value (%)	1.6 ± 0.1	8.4 ± 0.1	2.0 ± 0.1	12.2 ± 0.1	1.9 ± 0.0	0.4 ± 0.0
Ester value	204.0 ± 3.8	186.6 ± 1.4	197.7 ± 0.1	202.4 ± 1.6	206.5 ± 146	207.7 ± 0.8
Iodine value (Hanus)	93.2 ± 3.5	129.6 ± 2.9	149.1 ± 1.7	101.1 ± 3.4	75.6 ± 3.4	117.8 ± 3.0
Peroxide value	4.9 ± 0.5	12.4 ± 0.0	9.1 ± 0.5	4.7 ± 0.0	8.8 ± 0.5	13.2 ± 0.2
Unsaponifiable matter (%)	0.7 ± 0.0	1.6 ± 0.1	1.2 ± 0.1	1.3 ± 0.1	1.7 ± 0.1	0.7 ± 0.0
Fuel potential, Hμ (kJ/kg)	8276.58 ± 0.81	8285.21 ± 3.22	8313.24 ± 1.70	8288.44 ± 3.76	8329.32 ± 3.68	8433.22 ± 3.11
^aAverage of triplicate analysis ± s.d.

Peroxide values of the oils ranging from 4.7 in Jatropha curcas to 13.3 in Citrus sinensis are quite comparable to that of ≤ 5 for sesame oil,[28] while unsaponifiable matter content ranged from 0.7% in Citrus sinensis to 1.7% in Khaya ivorensis. Since unsaponifiable matter is essentially made up of hydrocarbons, sterols and higher water–insoluble fatty alcohols[28] the fairly high peroxide value of Citrus sinensis may be explained in terms of its low unsaponifiable matter content; the amount of antioxidants like vitamin E (a sterol) contained was likely below the quantity that could effectively prevent rapid autoxidation of the oil. Other relatively high peroxide values observed may be due to high degree of unsaturation of the fatty acids contained in the oils.

Table 4 shows the results of the fatty acid composition of the oils. Except in Cucumeropsis edulis where a C10 acid was detected to the level of 2.0%, low molecular weight (C10; C12) and high molecular weight (C22; C24) saturated acids were not detected in all the species investigated. Only Cucumeropsis edulis, Hevea brasiliensis and Citrus sinensis contained some quantities of linolenic acid, C18:3, while linoleic acid, C18:2, was the major (96.3%) fatty acid in Hura crepitans making it an interesting source of linoleic acid. The oleic: linoleic (O/L) ratio of 3.3 for Cucumeropsis edulis was particularly desirable for nutritional purposes. The fuel values obtained were lower than the 38.01MJ/L obtained for Avocado Pear seed oil,[4] African pear seed oil (41.95MJ/L), African oil beans seed oil (40.85MJ/L) and pumpkin seed oil (39.99MJ/L), but compared favourably well with the 8291.34kJ/Kg value got for Indian Almond.[29] Thus the oils are valuable in biological systems and as fuels in local lamps for the generation of light and heat.

Table 4 Fatty acid profile ^a of the oils (%)

Seed	Total lipid	C12:0	C14:0	C16:0	C18:0	Other c	C18:1	C18:2	C18:3	O/L ^b Ratio	Total unsaturates	Total saturates
Cucumeropsis edulis	32.5 ± 1.9	2.1	6.7	14.4	3.9	2.0	51.1	15.3	4.5	3.3	70.9	29.1
Hevea brasiliensis	44.0 ± 0.0	—	0.4	8.4	0.3	—	56.5	32.7	1.7	1.7	90.9	9.1
Hura crepitans	47.8 ± 1.5	—	—	1.2	2.5	—	—	96.3	—	—	96.3	3.7
Jatropha curcas	36.7 ± 1.1	—	1.4	14.6	1.3	—	72.1	10.6	0	6.8	82.7	17.3
Khaya ivorensis	48.3 ± 1.6	—	1.7	6.7	32.4	—	14.6	44.6	—	0.3	59.2	40.8
Citrus sinensis	35.6 ± 2.8	—	—	30.4	7.8	—	52.5	—	9.3	—	61.8	38.2
^aValues reported are expressed as weight percentage of total FAME; ^bO/L ratio = oleic: linoleic ratio; and ^cparticular acid could not be confirmed due to lack of standards.

CONCLUSION

All the seeds, with oil yield of not < 30%, can conveniently be classified as oilseeds. Considering the saponification values, they all would serve as raw materials for soap making. Hura crepitans particularly has been shown in this work to have a great potential as food for man or feed for animals, although further work is required to ascertain its safety. With its high content of C18:2, it is capable of lowering blood serum total and low density lipoprotein cholesterol.[30] Also, the generally low free fatty acid, iodine and peroxide values of the oils were an indication of their good storage capabilities at ambient temperatures. The unsaponifiable matter content of not > 1.7% for the oils indicated low level of minor constituents such as cholesterol and tocopherol. Thus, the oils, with other factors duely considered, would not pose cholesterol–related problems such as atherosclerosis and coronary heart diseases for consumers. Generally, all comparative analysis in this study pointed to the greatly untapped economic potentials of the non-conventional oilseeds available in our environment.

ACKNOWLEDGMENT

The authors are grateful to Professor S.R.A. Adewusi of the Department of Chemistry, Obafemi Awolowo University, Ile-Ife, Nigeria, for critically going through the manuscripts and offering useful suggestions.