© K.O. Omeje et al., Published by EDP Sciences, 2019

1 Introduction

Fats and oils (lipids) are one of the important macromolecules of the living organisms. Its importance span from energy generation, through membrane formation and maintenance to the biosynthesis of other essential compounds in the body. Any kind of changes in lipid metabolism can result in modification of membrane composition and subsequently in changes in its permeability (Orsavova et al., 2015). They consist of mixtures of organic molecules, which are mainly triacylglycerols, diacylglycerols, monoacylglycerols, free fatty acids and other minor components such as phospholipids, phytosterols, tocopherols and tocotrienols and hydrocarbons (Hamm et al., 2013). They could be classified as saturated (no double bonds), monounsaturated (one double bond), and polyunsaturated fatty acids (multiple double bonds) (Enechi, 2001).

Plant seeds have been used since antiquity as sources of vegetable oil (Adebayo et al., 2012). Vegetable fats and oils are lipid materials derived from plants, which are solids and liquids at room temperature, respectively (Adebayo et al., 2012). Some major oilseeds as enumerated by Ononogbu (2002) are soybeans, groundnuts, cottonseeds, sunflower, rapeseeds, oil palm, and coconut. These plants have been developed to maximize their oil production capacity, as they are traditional and economic products of most tropical and sub-tropical countries (Ononogbu, 2002).

They are some unexploited source of phyto-oil, which could be alternative to the conventional plants. One of such plants is African star apple (Chrysophyllum albidum). There is dearth of information to any usefulness of the seed of African star apple (Chrysophyllum albidum).

African star apple (Udara in Igbo) belongs to the family of Sapotaceae which comprises of about 800 species (Ehiagbonare et al., 2008). It is an evergreen tree and can grow up to 40 m high and about 2 m in girth. It has a straight and long fluted bole with small buttress at the base (Adebayo et al., 2012). The fruit when ripe is ovoid to sub-globose, and contain three to five seeds which are not eaten. The seeds are dark brown or blackish, obliquely ellipsoid to obovoid, up to 2.8 cm long and 1.2 cm wide; its coat are hard, bony, shiny and dark brown and when broken reveals white coloured cotyledons (Adebayo et al., 2012).

African star apple (Chrysophyllum albidum) is one fruit of great economic value in tropical Africa due to its diverse medicinal and food uses (Adebayo et al., 2012). In recent times, the plant has become a crop of commercial value in Nigeria (Oboh et al., 2009). The seeds are been discarded in discriminately after the consumption of the succulent fruit. In this research, oil was extracted from the unexploited seed of African star apple (Chrysophyllum albidum), characterized and the fatty acid profiled using gas chromatography-mass spectroscopy. The extraction of oil from this agro waste would lead to the eradication of the waste from the environment, thereby creating a clean environment. Also, the oil produced could be applied in various aspect of industries. Being a non-edible material, it is a good oil feed stock since there would not be any debate as regards the creation of food scarcity due to its usage in oil production. Similarly, the oil could be used in the production of biodiesel because it is rich in fatty acids.

2 Materials and methods

2.1 Apparatus

Gas chromatography-mass spectroscopy (GC-MS-QP2010 plus Shimadzu, Japan), n-hexane and other chemicals were products of Sigma-Aldrich, USA.

2.2 Plant sample collection and preparation

Seed of African star apple (Chrysophyllum albidum) were picked from its natural habitat in Ikem village of Nguru community of Nsukka LGA, Enugu state, Nigeria. The seeds were de-shelled and cotyledons were sun dried for twelve days. The dried cotyledon was ground using electric blender.

2.3 Seed oil determination using soxhlet extraction

Milled sample weighing 20.86 g was placed in a thimble before adding the solvent (hexane: chloroform) in a ratio 50:50 ml in the flat bottom flask. The set-up was heated at 67 °C for 4 h. After the extraction processes, the oil residue was exposed to the atmosphere and the solvent allowed to evaporate and oil extracted was quantified.

2.4 Gas chromatography-mass spectrometry analysis

Gas chromatography-mass spectroscopy (GC-MS-QP2010 plus Shimadzu, Japan) system, is a very efficient technique commonly used for the identification and quantification of fatty acids in substances (Fig. 1). The unknown organic compounds in the complex mixture found in the oil were matched with the National Institute of Standards and Technology (NIST) library.

Fig. 1 Chromatogram of African star fruit seed oil.

3 Results and discussion

Oil was extracted using normal hexane and chloroform in the ratio 50:50. The percentage oil yield from the seed of African star cherry was 23.80%. The properties of the oil extracted are shown in Table 1. Figures 2 and 3 show the picture of African star apple seed and the ground seed respectively. The oil had a pleasant sweet smell, with honey-like colour and was semi solid at room temperature. Acid value of the oil was 17.41 ± 0.43 mg/KOH/g, peroxide value was 57.74 ± 2.77 meq/kg⁻¹ and 8.75% free fatty acid. Also, iodine value of 29.00 ± 0.16 mg/100g and saponification value of 236.34 ± 6.80 mg/KOH/g were obtained for the oil. The percentage oil yield was low when compared to those reported earlier by researchers on oil seed such as B. parkii, (34.0%), L. lanceolata (40.0%), S. setegera (33.0%) and S. birrea (42%) and in agreement with the value B. sapida (26.0%) but high when compared to the value of D. microcarpum (7.42%) (Kyari, 2008), African star fruit (10.71%) (Adebayo et al., 2012).

The acid value (17.41 ± 0.43 mg/KOH/g) of African star apple (ASA) was high when compared to the value of seed oil extracted from soursop (1.82 mg/KOH/g) (Adepoju et al., 2014). Acid values less than 1% were reported by Kyari (2008). Subsequently, Adebayo et al. (2012), reported acid value of 4.50 mg/KOH/g for African star fruit seed oil. Acid value is the measure of percentage content of free fatty acids in a substance, and degree of rancidity (Ononogbu, 2002). It is used as a parameter in determining freshness (Ononogbu, 2002). This implied high content of free fatty acid which could lead to high lipolytic activities, and a reduced shelf life.

Peroxide value (PV) of 57.74 ± 2.77 meq/kg⁻¹ was obtained. Peroxide value is an index of rancidity (Adebayo et al., 2012). The PV was high when compared to that (1.57 meq/kg⁻¹) obtained by Adebayo et al. (2012). In the report of Kyari (2008) on six oil seeds, high PV of 77.5, 95.0, 150.0, 135.0 were obtained for B. parkii,L. lanceolata,D. microcarpum and B. sapida. Also, a low PV was reported by Adepoju et al. (2014) on their work on soursop seed oil. Free fatty acids FFA (8.75%) was obtained in the study. The concentration of FFA in an oil is an important quality parameter. Frega et al. (1999) reported that FFA added to refined oil shortened their induction time. Similarly, Scarpellini et al. (2005) also reported the prooxidant activity of FFA. Hence, it could be opined that high concentration of FFA could lead to short shelf life of oil. Adepoju et al. (2014) obtained FFA value of 0.91 for soursop seed oil. Also, Jatropha seed oil gave free fatty acid content 2.24%. The high concentration of unsaturated free fatty acids showed that the oil could be prone to autoxidation; and high peroxide value suggest the seed oil had high concentration of peroxide and hydro-peroxide which are primary products of autoxidation. The seed oil been rich in unsaturated fatty acids may be beneficial in the formulation of animal feeds and production of biodiesel.

Iodine value (IV) 29 ± 0.16 g/100g was obtained in the work. It is the number of grams of iodine that combines with 100 g of lipids, which shows the degree of unsaturation of the fat or oil (Ononogbu, 2002), the oil is classified as a non-drying oil, and since its iodine value is less than 100. This value is low when compared to 35 mg/100g obtained by Adebayo et al. (2012). Also, the value is lower than all the six values obtained by Kyari (2008). The oil of soursop seed oil gave iodine value of 115.30 g/100g oil as reported by Adepoju et al. (2014). Also, the seed oil of C. albidum was reported to have iodine value of 31.06 g/100g by Osamudiamen and Afolabi (2014). The value was higher when compared to 29 ± 0.16 g/100g obtained in this study.

Saponification value was another physicochemical analysis conducted on African star fruit seed oil. This is the number of milligram of KOH required to neutralize the fatty acids resulting from complex hydrolysis of 1 g of oil or fat (Ononogbu, 2002). Saponification value of 236.341 ± 6.80 mg/KOH/gram was obtained. This value conforms to the values of some vegetable oils used in soap making such as groundnut and coconut. Similarly, it is in agreement with the work of Kyari (2008) on B. sapidia, L. lanceolota, S. setegera and B. sapidia, but higher than the values of D. microcarpum and S. birrea which were 123.3 and 199.3 mg/KOH/g, respectively. Also, the result obtained in this work is higher when compared to the value 199.50 obtained by Adebayo et al. (2012) on ASC seed oil. Similarly, the saponification value is in agreement with that reported for soursop seed oil of 235.46 mg/KOH/g (Adepoju et al., 2014).

Gas chromatography-mass spectrometry analysis (GC-MS) has been reported as an important tool for the identification and quantification of fatty acids (Shibula and Velavan, 2015; Okereke et al., 2017). The spectra of the study revealed eight peaks that corresponded to eight fatty acids and other organic substances present in the oil extracted from the seed of African star fruit. The compounds were confirmed by their retention time, percentage area, molecular weight and formulae, respectively. The major fatty acids revealed were undecylenic acid (40.33%), oleic acid (30.21%) and 9-octadecanal (7.09%). Other identified fatty acids present included n-hexadecanoic acid (7.55%), 13-hexyloxacyctri-dec-10-en-2-one (1.19%), octadecanoic acid (5.28%), hexadecanoic acid (2.37%), undecylenic acid (40.33%), 9-octadecanal (7.09%), and 9, 17-octadecadienal (5.98%) as shown in Table 2. Similarly, Figure 1 showed eight peaks representing different fatty acids, with the concentration of undecylenic acid being highest as depicted by peak 6. The concentration of undecylenic acid detected was higher when compared to the quantity (6.31%) detected in P. americana seed oil as reported by Omeje et al. (2018). Avram et al. (2014) reported the presence oleic acid, linoleic acid and linolenic acid in the seed oil extracted from rapeseed using GC-MS. Gas chromatography analysis of fatty acids present in soursop seed oil revealed 73.42% of unsaturated fatty acid and 26.68% of saturated fatty acid (Adepoju et al., 2014). The physicochemical properties of the African star apple seed oil indicate it possess useful features of industrial oil which could be employed in food and chemical industries.

Fig. 2 African star apple seed (Chrysophyllum albidum).

Fig. 3 The ground seed of African star apple.

4 Conclusion

The percentage oil yield was 23.8%. The extracted oil was liquid at room temperature, pleasant sweet smell with honey-like colour. African star fruit seed oil comprised of 85.74% fatty acids. Some of its physicochemical properties were good for industrial processes. African star fruit seed is non edible and a serious environmental waste which could be a source of oil which has a potential properties for applications in industries and other sectors of human endeavours.

References

All Tables

All Figures

	Fig. 1 Chromatogram of African star fruit seed oil.
In the text

	Fig. 2 African star apple seed (Chrysophyllum albidum).
In the text

	Fig. 3 The ground seed of African star apple.
In the text