The contribution of fruits, seeds and vegetable of some plants in Nigeria to minerals, vitamins and amino acids in human nutrition is limited due to the presence of anti-nutrients which render some of the essential nutrients and protein unavailable for human nutrition. There is a geometric increase in demand for vegetable oils in Nigeria for both domestic and industrial purposes. Ikhuoria et al. (2008) attributed increasing demand for vegetable oils and fats in Nigeria, for both domestic and industrial purposes, in recent times, to the nutritional needs of the teaming population and the increasing number of industries that require oils and fats as their primary raw material. It is therefore vital to determine the nutritive values of some of these products from tropical plants in Nigeria, since some of these products have been widely accepted as a dietary constituent among peasants in Nigeria and other African countries and as a source of vegetable oils. Oyolu (1980) stated that vegetables will continue to remain the primary source of proteins, minerals and vitamins in African countries.
African pear (Dacryodes edulis) is classified by most agriculturists as underutilized tree crop because the mineral composition of this plant has not been fully investigated. As a traditional food plant in Nigeria, it has a potential to improve nutrition, boost food security and support sustainable land care. Ikuhoria et al. (2008) stated that Nigeria being tropical country, has wide variations in climatic conditions and therefore has a wide variety of domestic plants that produces oil, but lack information on the composition and utilization of the many and varied oil seeds. Oil composition depends on fruit origin and ripening conditions (Gadet et al., 2005) as well as the variety. D. edulis is consumed traditionally in Nigeria raw, roasted or boiled in hot water and is eaten alone, or used in garnishing fresh maize (Arisa and Lazarus, 2008). It is sometimes used as butter to eat bread. The leaves, bark, stem and roots of African pear are used as local medicine against diseases (Jirovetz et al., 2003; Annabelle et al., 2004; Ikhuoria and Maliki, 2007).
Previous studies have shown that oil of fruits of D. edulis is a rich source of amino acids and triglycericides (triacylglycerols) with saturated fatty acid content of 50.85% and an unsaturated fatty and content of 49.14% (Lam et al., 2006). Lam (1985) and Omoti and Okiy (1987) have long proposed African pear (D. edulis) oil as a source of vegetable oil. The seeds of African pear have been found to contain reasonable amount of oil. The composition of this oil shows that it has both domestic and industrial potentials (Gunstone and Norris, 1982; Arisa and Lazarus, 2008).
The preferential habitat of D. edulis is shady, humid tropical forest like Nigeria. It is also found in other African countries like Cameroon, Sierra Leone, Uganda, Malaysia, Liberia and Zaire. There are two varieties of Dacryodes edulis in Nigeria-D. e. var. edulis and D. e. var. parvicarpa. The fruit of D. e. var. edulis is larger and the three has stout, ascending branches. D. e. var. parvicarpa has smaller fruit and slender, dropping branches.
Although, the consumption of D. edulis fruit has been widespread in Nigeria, particularly in the south eastern states of the country, the mineral and anti-nutrient composition of different varieties of this fruit has not been thoroughly investigated. It has also been proposed by some authors that the fruit can serve as a source of vegetable oil, but the variety in which oil can be extracted in commercial quantity has not been investigated. This study is therefore aimed at determining the oil content, anti-nutrients and minerals composition of the two varieties of Dacryodes edulis grown in Nigeria. This will help boost food security and improve nutrition, as well as providing information on food chemistry of the two varieties of D. edulis oil.
MATERIALS AND METHODS
The method of acquisition and handling of materials is very significant in any research, as this to a greater extent including other factors determines the accuracy of the result obtained thereof (Ekpa and Isaac, 2008). The two varieties of African pear-D. e. var. edulis and D. e. var. parvicarpa were purchased at Uyo Main Market, along Etuk Street, Uyo, Akwa Ibom State-Nigeria. All the reagents and other materials were of analytical grade. Distilled water was used throughout the whole analytical procedure.
Preparation of the samples for oil extraction, proximate analysis, minerals and anti-nutrients composition: The ripe fruits of D. e. var edulis and D. e. var. parvicarpa were washed thoroughly with distilled water and split open with a sharp knife to remove the seed from the pulp. The prepared pulp samples were divided into two each, one part for proximate analysis and the other for anti-nutrients, elemental composition and oil content evaluation. For the determination of the latter, the pulp samples were dried in a Gallenkamp hot air oven model OV 160 for 48 h. The dried samples were milled with corona traditional grain mill REF 121 (100 μm mesh size). One hundred gram each of the milled sample were measured and extracted separately using petroleum ether (60-80°C) in a soxhlet extractor. The oil was concentrated by heating in the oven at 60-80°C to ensure complete evaporation of the solvent and latter cooled in desiccator. The percentage oil content for each variety was estimated using the expression:
Proximate composition analysis: Proximate compositions for nutrients were determined using the Onwuka (2005) method. The moisture content of the pulp samples were determined using indirect distillation method employing drying ovens. This was done by drying the samples to a constant weight at 60°C in a vacuum oven and moisture content evaluated following the relationship in Onwuka (2005). Protein was determined using Kjeldahl method and a conversion factor of 6.25. Ash content was determined by ignition in a muffle furnace for 4 h at 525°C. Fibre content for each variety was estimated from the loss in weight of the crucible and its content on ignition. Carbohydrate on the other hand was determined when the sum of the percentages of moisture, ash, crude protein, ether extracts and crude fibre were subtracted from 100. While the calorific values of the samples were calculated by multiplying the value of the crude protein, lipid and carbohydrate by 4, 9, 4 Kcal, respectively and taking the sum of their product.
Anti-nutrient analysis: Anti-nutrients in D. e. var. edulis and D. e. var. parvicarpa-hydrogen cyanide, oxalate, phytate and tannins were determined using standard methods. Hydrogen cyanide, phytate, oxalate and tannins were determined using Onwuka (2005) method.
Elemental composition analysis: The mineral elements-Ca, Na, K, P, Cu, Zn, Cd, Pb and Fe from the two samples were determined based on Onwuka (2005) method using a unicam atomic absorption spectrophotometer.
RESULTS AND DISCUSSION
Proximate analysis: Moisture content of food is of great importance
to every food processor as a number of biochemical reactions and physiological
changes in food depend very much on the moisture content (Onwuka, 2005). The
moisture content of the two varieties of African pear was significantly high
and this could imply short shelf life. The highest moisture content of 61.40%
was observed in D. e. var. parvicarpa (Table 1). The
moisture content of D. e. var. edulis of 46.59% compares well with other
oil crops like fluted pumpkin seed harvested 32 weeks after anthethesis (Akwaowo
et al., 2000), African oil bean seed (Pentadethra macrophylla
Bent) (Ikhuoria et al., 2008).
||Proximate composition (%) of D. e. var. edulis and
D. e. var. parvicarpa
The differences in the moisture content of the two varieties of D. edulis
suggest that D. e. var. edulis is of better quality and more stable than
D. e. var. parvicarpa.
According to Onwuka (2005), moisture content determines the stability and quality of foods. There was no significant difference in crude fibre and ash content of D. e. var. edulis and D. e. var. parvicarpa (Table 1). Also, the crude fibre and ash content were generally low and this will enhance quality control of the extracted oil. The crude fibre and ash content values are important in terms of the suitability of the fruit cakes for the compounding of animal feeds. As a result of the low crude fibre content, the two varieties of D. edulis may be considered unsuitable for compounding animal feeds. However, on the grounds of fairly high carbohydrate content in D. e. var. parvicarp (29.58%) and low ash content of 2.46 (Table 1), it may be considered otherwise. Ikhuoria et al. (2008) also reported that African oil bean seed cake with low ash content and high percentage carbohydrate (38.99%) can be used for compounding animal feeds. Similar report was made by Abighor et al. (1997).
The percentage crude protein and carbohydrate in D. e. var. parvicarpa were higher than that of D. e. var. edulis (Table 1). On the other hand, the caloric value of D. e. var. edulis (724.53 Kcal) was significantly higher than that of D. e. var. parvicarpa of 652.36 Kcal (Table 1). This suggests that more energy is derived from consuming D. e. var. edulis variety of African pear than D. e. var. parvicarpa.
Oil content and mineral composition: African pear (D. edulis)
extracts from the two varieties were liquid at room temperature. This implies
they could be classified as oils. The percentage oil content in D. e. var.
edulis and D. e. var. parvicarpa were evaluated using equation 1
to be 68.29 and 54.68%, (Table 1) respectively. The high oil
content in D. e. var. edulis suggest that processing this variety for
vegetable oil would be more economical. Ikhuoria and Maliki (2007), reported
23.2% oil content for African pear. The researchers did not specify the variety
of African pear from which the oil was extracted. The low value in oil content
reported by the authors could be attributed to the variety of African pear,
the mount of sample from which the oil was extracted (in this case 50 g) and
the method of extraction used.
||Elemental nutrient composition (mg kg-1) of D.
e. var. edulis and D. e. var. parvicarpa
|ND: Not Detected
The oil content varies with varieties as observed in this study.
According to Onwuka (2005), mineral elements can be classified as major elements, trace elements and non-essential and or toxic elements. The concentration of major elements-Ca, Na, K and P, in the two varieties of D. edulis were very high (Table 2). Phosphorus was 4446.04 mg kg-1 in D. e. var. edulis, while concentration of 3674.07 mg kg-1 was obtained from D. e. var parvicarpa. Ca, K and Na concentration in D. e. var. edulis were 3001.07, 433.36, 255.20, 3205.49, 266.65 and 354.37 mg kg-1 were observed for D. e. var. parvicarpa (Table 2), respectively. The minerals (Ca, K, Na and P) are essential to health; their presence in the African pear makes the fruits and the oil from the two varieties viable source of these minerals in the body. Zn content in the two varieties was lower than Fe concentration (Table 2). Fe is needed for blood formation in the body. Cu and Zn, when present even in trace concentrations are important for the physiological functions of living tissues and regulate many biochemical processes. The present of Zn in both varieties of African pear implies the oil can be use to formulate cream that can be used to check skin infections in babies and consumption of these fruits will serve as a source of these nutrients in the body. Pb and Cd were not detected in the two varieties. This implies the two varieties are free from toxic elements. Generally, the varying levels of these minerals may be attributed to the different rates at which the elements are taken up from the soil by the plants and subsequent incorporation into the oil. It can also be due to geographical location of these plants and availability of these minerals in the soil.
Anti-nutrient composition: The most common anti-nutritional factors in fruits and leafy vegetables are oxalic acid, tannins, phytic acid and hydrocyanic acid (Akwaowo et al., 2000). The distribution of anti-nutrients in some vegetable crops and their effects on other nutrients has been reported (Broadhurst and Jones, 1978; Desphande and Cheryan, 1985; Marks et al., 1987; Udosen and Ukpana, 1993). A daily intake of 450 mg of oxalic acid has been reported to interfere with metabolism (Akwaowo et al., 2000). High oxalate levels in food may reduce the bioavailability of such metal as calcium. Phytic acid intake of 4.00-9.00 mg/100 g reduces iron (Fe) absorption by 4-5 fold in humans (Hurrel et al., 1992).
The anti-nutrients composition of the two varieties of African pear were generally
low such that none of the anti-nutrient were above the lethal dosage approved
by standard bodies like National Agency for Food and Drugs Administration and
Control (NAFDAC) in Nigeria. The cyanide content (0.007 mg/100 g) for D.
e. var. edulis and 0.176 mg/100 g for D. e. var. parvicarpa (Table
3), indicates that the two varieties of D. edulis will not affect
human nutrition if consumed in a large quantity. But it should be noted that
the concentration of cyanide is always reduced during processing and as such
there might be a reasonable concentration of cyanide in a raw African pear fruit
which could render the consumption of it in a raw form harmful to health. It
is therefore, advisable to consume the fruit when cooked than raw. Some D.
e. var. parvicarpa has bitter taste because of elevated cyanide content
in this variety. As seen in this research. Total oxalate concentration was 13.20
mg/100 g for D. e. var. edulis and 17.60 mg/100 g for D. e. var. parvicarpa
(Table 3). Munro and Bassir (1969) reported that the lethal
level of oxalate in man is 2-5 g. Pingle and Ramastin (1978) have demonstrated
the ability of soluble oxalates to inhibit calcium, potassium and sodium absorption
due to their insolubility properties. Akwaowo et al. (2000) also reported
that oxalates form insoluble complexes with Ca, Mg, Zn and Fe, thereby interfering
with utilization of these mineral elements. From this study, the concentration
of oxalate both total and soluble does not reach the level that could be injurious
to health. The same was applicable to phytate and tannins with very low concentrations
in both variety of African pears. Though tannins and phytates are also known
to affect human nutrition and metabolism, for example, Akwaowo et al.
(2000) reported that higher intake of tannic acid has been associated with carcinogenic
effect in man, poor protein utilization, liver and kidney toxicity. The same
authors also reported that tannic acid is associated with lower nutritive value
of protein foods. Their concentrations in the two varieties of D. edulis
did not exceed lethal levels; therefore the consumption of the fruits will not
pose any harmful effect to health. Phytic acid intake of 4-9 mg/100 g is said
to decrease Fe absorption by 4-5 fold in humans (Hurrel et al., 1992).
None of these effects could be noticed when these fruits are consumed because
the levels of antinutrient does not reach lethal dosages. It is however, documented
by Dunu et al. (1986) that most of these toxicants are eliminated during
processing and cooking; hence it is advisable to consume cooked African pear
and not raw.
||Anti-nutrient composition (mg/100 g) of D. e. var. edulis
and D. e. var. parvicarpa
Evaluation of the chemical composition of the two varieties of African pear shows that D. e. var. edulis has a high percentage of oil than D. e. var. parvicarpa and the mineral element and anti-nutrient compositions of these fruits suggest that they can serve as a source of essential elements and vegetable oil of high quality and high nutritive value.