Elemental and Proximate Compositions of Sesame Seeds and the Underlying Soil from Tsegede, Ethiopia

The proximate compositions and elemental contents of sesame (Sesamum indicum) seeds and the underlying soil from different cultivation areas of Tsegede, Ethiopia, were investigated. The ash, protein, fiber, fat, and carbohydrate contents of the sesame seeds were determined following standard methods. Essential major (Ca and Mg) and trace metals (Fe, Zn, Cu, Mn, and Ni) in the sesame seeds and the underlying soil were determined by using flame atomic absorption spectroscopy. The sesame seeds contained high levels of fat (52.9 ± 1.5%), followed by protein (23.5 ± 0.9%). The seeds contained 525 ± 1 and 453 ± 38 mg/kg of Mg and Ca, respectively. Iron was the most abundant (37.8 ± 1.4 mg/kg) of the trace metals, followed by Zn (14.6 ± 2.2 mg/kg) and Cu (7.26 ± 0.84). Manganese and Ni were found only in minute amounts. The concentrations of the trace metals varied significantly across the different cultivation areas. Similar to the sesame seeds, iron was found in higher amounts (212.6 ± 2.6 mg/kg) in the underlying soil followed by Zn and Cu, which were both 28.8 mg/kg. The amounts of the trace elements, Fe, Cu, and Zn found in the soils were about 2 to 6 times higher than that found in the sesame seeds. Whereas, the concentrations of Mg and Ca present in the seeds were comparable with that determined in the soils. Correlation analysis indicated that the Fe and Cu contents of the sesame seeds are negatively influenced by the amounts of Mg and Ca present in the soil. Ca in the seeds was also negatively associated with the Mg levels in soil.


Introduction
Sesame (Sesamum indicum) is a plant that belongs to the family Pedaliaceae and the genus Sesamum. Te genus Sesamum consists of about thirty-six species, of which the most commonly cultivated is Sesamum indicum [1]. Sesame is cultivated for its edible seeds, which are relatively rich in oil and protein [2].
Sesame is one of the most important oil seed crops in the world [3]. Te crop is cultivated both in the tropic and temperate zones of the world [4], where it is grown mostly for the edible oil extracted from its seeds. Sesame is a source of high quality edible oil with high preservative qualities [5]. Te oil is also used in the production of perfumes, skin conditioners, and hair creams [3,6].
Soil is the material that is found on the Earth's surface and consists of mineral, organic matter, water, and air [7].
Te fertility of the soil has great contribution to sesame production and productivity. Te fertility of a soil is determined by both its physical properties and its constituent chemicals that are essential for plant growth [8,9]. In seed crops, like sesame seeds, nutrient imbalances may manifest themselves in quality characteristics of the fruit and its production [9]. Diferent soil types have diferent nutrient levels which are blended together in difering amounts. Soil properties like pH, moisture, and essential element content greatly infuence plant growth [8,10]. Essential elements are needed for both plants and animals as they form part of various tissues and act as catalysts in a wide range of metabolic processes [11].
Ethiopia ranks among the top six world producers of sesame seeds, and it is the second biggest export earner to the country after cofee [12]. Sesame is produced in many parts of Ethiopia, where Tsegede district stands as one of the main producers of sesame in the country.
Te composition of the sesame seed is dependent mainly on genetic and environmental factors [13]. Diferent researchers have reported diverse results about the proximate composition of sesame seeds grown in diferent countries [14,15]. Tis study aimed at determining the physicochemical characteristics of white sesame seeds, and the underlying soil, in Tsegede district, Amhara region, Ethiopia.

Experimental
2.1. Samples. Tsegede district is found in Amhara regional state, which is on the Northwest parts of Ethiopia between 147°55′ 29.46″ N latitude and 37°32′ 573″ E longitude ( Figure 1). White sesame seeds and soil samples were collected from three diferent localities of Tsegede district (Soroka, Kisha, and Kola Zana) ( Figure 1). Te localities were selected based on their commercial importance. From each locality, three samples of sesame seed were collected from three diferent farmlands. Soil samples were collected at the depths of 15 cm from the sesame growing farmlands. From each farmland, fve samples of soil were collected randomly, by walking diagonally and from rectangular angles, and mixed together thoroughly to produce a bulk sample. All samples were stored in polyethylene plastic bags.

Sample Preparation.
Te sesame seeds were cleaned manually to remove foreign matters, immature, and damaged seeds. Te soil samples were air-dried left open in the air. Te samples were then powdered and sieved through 0.5 mm mesh size and stored in polyethylene plastic bags at room temperature until analysis.

Determination of Sesame Seed Weight.
A total of 2000 sesame seeds were counted manually and weighed using digital balance. Te average weight of a sesame seed was calculated by dividing the measured weight by 2000.

Determination of Proximate Composition of Sesame.
Te proximate compositions (moisture, ash, fat, fber, and protein) of the seeds were determined following standard methods (AOAC, 2010). Briefy, for moisture content determination, a 1 g sample was weighed in crucible and dried at 120°C for 6 h in oven. Te moisture content was calculated as the loss in weight of the dried samples. Te moisture content was used to calculate analytical results on the dry weight basis. Te total ash content of samples was determined by the method of furnace incineration. For this, about 2 g of fnely ground sample was converted to ash by heating in air at 600°C. Te resulting ash was weighed and expressed as percentage to the weight of original sample. Te crude fat content was determined after Soxhlet extraction of samples with 75 mL of hexane for 6 h at 40°C. At the end of the extraction, hexane was removed using rotavapor at 40°C, residue was dried at 100°C for 1 h, cooled, and weighed. For the determination of crude fber content, 2 g of sample was extracted with 200 mL of 0.255 N H 2 SO 4 for 30 min, fltered through muslin cloth and washed with boiling water. Te residue was further extracted with 200 mL of 0.313 N NaOH for 30 min, fltered through muslin cloth, and washed successively with 25 mL of hot 1.25% H 2 SO 4 , twice with 50 mL of distilled water and 25 mL of alcohol. Te residue obtained was dried at 130°C for 2 h, cooled in a desiccator, and weighed. Protein content was determined following the Kjeldahl method, while carbohydrate content was determined by subtracting the sum percentage of crude protein, fat, ash, moisture, and fber from 100%.

Determination of Moisture Content of Soil.
A 1 g soil sample was weighed, dried at 105°C for 24 h kept in an oven, cooled in a desiccator, and weighed again. Te weight loss after drying was obtained by subtracting the weight of the dry sample from the original. Te moisture content was used to calculate analytical results on the dry weight basis.
2.6. Determination of Soil pH. A 5 g of air-dried soil sample was weighed into a 100 mL beaker, 15 mL distilled water was added and the suspension was stirred vigorously for 20 min. Te suspension was allowed to stand for about 30 minute for the suspended clay to settle out from the suspension. Te pH value and temperature was then read and recorded immediately. Te pH meter was calibrated with pH bufer 4, 8, and 11.

Sample Digestion.
In order to analyze metals present in the sesame seed and soil samples, wet digestion method using HNO 3 and HClO 4 solutions in diferent proportions were tested and the one that consumed smaller reagent volume with minimum digestion time and temperature to produce clear and colorless solution was selected for the digestion of samples.
For sesame seed, a 1 g powder sample was frst mixed with 6 mL of HNO 3 (70%) and heated at 160°C for 3 h in a conical fask placed on a hot plate. Ten after, 5 mL of HClO 4 (30%) was added and heated until 2 mL of the volume remained. After cooling, 10 mL of distilled water was added, fltered through Whatman No. 42 flter paper, and diluted to 50 mL by adding distilled water.
For soil, a 1 g powdered sample was weighted into a conical fask and mixed with 6 mL HNO 3 and 6 mL HClO 4 . Te mixture was heated on a hot plate at 180°C for 3 h. After cooling, the digest was fltered through Whatman No.42 flter paper and diluted to 50 mL by adding distilled water. Each sample was digested in triplicates. Tree blank solutions were also prepared following the same digestion procedure as the samples. During the measurement of Ca and Mg, lanthanum chloride was added to each sample and standard solution.

FAAS Determination of Elements.
Te concentrations of metallic elements in the digested samples were determined by using fame atomic absorption spectrophotometer (FAAS) (210VGP, Buck Scientifc, USA) equipped with deuterium arc background corrector and air-acetylene fame at diferent operating conditions for each of the elements (Table 1).
Te atomic absorption spectrometer was calibrated using seven point standard solutions, corresponding to each element, in the concentration (mg L −1 ) range of 0.10-2.00 for Mn, 0.50-4.00 for Fe, 0.10-2.00 for Ni, 0.50-3.00 for Cu, 0.10-1.60 for Zn, 0.50-10.00 for Ca, and 0.50-10.00 for Mg. Te average triplicate readings were taken for each standard solution.
2.9. Method Validation. Accuracy, precision, and limits of detection were determined to assess the validity of the methods used for the digestion and analysis of the sesame seed and soil samples. Te precision of the method was evaluated from the relative standard deviation of the results obtained from repeated measurements made on a given sample. Limit of detection of the method was calculated as three times the standard deviation of the blank signals divided by the slope of the calibration equation. Accuracy was determined by spiking the samples with known concentrations of standard solutions. For this, a 1 g powdered sample of sesame seed or soil was fortifed with a standard solution at a concentration level corresponding to 100% of the average measured value for each element and subjected to the digestion and analysis procedure.

Statistical Analysis.
One-way analysis of variance was used to test the efect of the growing region on the mean concentrations of the diferent chemical constituents determined. Statistical analyses of the data were carried out using Excel (Microsoft Excel, 2007). Diferences were considered signifcant when α < 0.05.

Results and Discussion
3.1. Seed Weight. Te mean seed weights of sesame were 5.21 ± 0.02, 5.89 ± 0.05 and 6.25 ± 0.02 g/2000 seeds from Kisha, Kola Zana, and Soroka, respectively. Te measured seed weights are in the range of 4-7 g/2000 seeds reported by Al-Kahtani [16] from Saudi Arabia. Te weight of a sesame seed is, however, signifcantly diferent across the three studied production areas, indicating the infuence of the growing environment on the seed weight of sesame.

Proximate Composition.
Te sesame seeds were rich in fat (average 52.86% dry weight), with considerable amount of protein (average 23.52% dry weight) followed by carbohydrate (average 14.51% dry weight) ( Table 2). Ash and fber were present in lower and comparable proportions, 4.84 and 4.27%, respectively, in the seeds.
Te moisture contents of the sesame seeds varied in the range of 5.43-5.81%, with no statistically signifcant difference, across the three production areas. On the other hand, the levels of the determined dietary components of the sesame seeds varied signifcantly with production origin. Te ash contents of the sesame seeds varied in the range 4.38-5.48%, with signifcantly higher amount of ash found in seeds from Kola Zana (5.48%) than those from the other areas. Crude fber was found in signifcantly higher amounts in seeds from Kisha (4.97%) than that from the other areas (3.88-3.95%). Sesame seeds from Soroka contained significantly higher levels of crude fat and protein while lower carbohydrate than seeds from the other areas.
Te average concentrations of the dietary components determined in the sesame seed samples from the three areas were compared with some reported values for white sesame seeds from diferent countries (Table 3). For the sake of comparison data obtained from the sesame seed samples have been recalculated to the fresh weight basis using the determined moisture content values. Te moisture contents of the sesame seed samples are comparable with those seeds from Congo-Brazzaville (5.7%) [15] and Nigeria (5.2%) [17], while higher than those from Turkey (4.40%) [18] and China (4.71%) [19]. On the other hand, the determined values are lower than that (6.91%) reported by Okoronkwo et al. [20]. Te moisture contents of the samples are all below 6% that International Journal of Analytical Chemistry has been indicated as good condition for sesame seeds at harvesting [21]. Lower moisture content has also been indicated as benefcial to the quality and shelf life of sesame seeds [22].
Te sesame seed samples contained signifcantly higher amounts of ash and crude fber than seeds from Congo-Brazzaville and Nigeria, while comparable in ash content with seeds from Turkey and China. Ash content is an indication of mineral elements that are present in the sesame seeds, while fber in diet is important as it helps to maintain human health by reducing cholesterol level in the body [23].
Te seed samples contained about twice of crude protein and half of carbohydrate than seeds from Nigeria. Whereas, the crude protein and carbohydrate contents of the seeds are comparable with those from Turkey and China.

Analytical Characteristics of the Method.
Te reliability of the optimized digestion procedure used in the determination of essential metals in the samples using FAAS was evaluated with respect to analytical fgures of merit. Te correlation coefcients (r 2 ) obtained for the calibration curves were in the range of 0.9966-0.9997 (Table 4), indicating a good linear relationship between concentration and absorbance. Te limit of detection of the method ranged 0.09-0.38 mg/kg across the diferent elements.
Te average percentage recoveries for the studied metals in the sesame seed sample spikes ranged between 94.1 and 112.4%. Te precision of the method, which was expressed as the relative standard deviation of three replicate measurements made on spiked samples, ranged from the 0.24 to 14.95% across the diferent elements. Both the accuracy and precision of the method were good enough to allow the quantitative determination of the nutrient elements in the samples.

Concentration of Elements in Sesame Seeds.
Te overall mean concentration of the major elements Mg and Ca found in the sesame seeds were 525 and 453 mg/kg dry weight, respectively (Table 5). Tese values are comparable with the 579.53 mg/kg Mg and 415.38 mg/kg Ca reported for sesame seeds from Congo-Brazzaville by Nzikou et al. [15].
Among the determined trace metals in the sesame seeds, the most abundant was Fe (average 37.8 mg/kg) followed by Zn (average 14.6 mg/kg) and Cu (average 7.26 mg/kg). Te amount of Fe found in the sesame seed samples falls within the range of 35.20-43.10 mg/kg reported by Gebrekidan and Desta [24]. Te determined concentration of Zn also falls within the range   13.92-28.49 mg/kg reported for sesame seeds from Turkey by Cemal [25]. On the other hand, the amount of Cu found in the samples is lower than that (13.5 mg/kg) reported by Obiajunwa et al. [26] for sesame seeds from Nigeria. Te elements Mn and Ni were found only in minute amounts in the sesame seed samples. Te concentration of the trace metals determined in the sesame seeds varied signifcantly with the cultivation regions. Sesame seeds from Kola Zana contained higher levels of Fe and Cu than seeds from the other cultivation areas. On the other hand, sesame seeds from Kisha contained lower levels of Zn and Mn than seeds from the other cultivation areas. Tis may be due to diferences in the environmental growing conditions. Te elemental composition of a plant is generally a refection of the elemental composition of the soil in which the plant was cultivated [27]. However, in this study no signifcant diference was found in the concentrations of Fe and Cu among soils from the diferent cultivation areas. Hence, the observed diferences may be due to other environmental factors, such as air temperature and rainfall. Te accumulation of an element within a plant depends on environmental conditions. Plants of the same species that are grown in diferent geographical locations may exhibit varying elemental profles as a result of the environmental conditions [28].
Te elements Fe, Zn, Cu, and Mn are trace essential elements for humans. Te elements are micronutrients that are required in minute quantities to play vital role in the normal functioning of various physiological and metabolic processes [29]. Zinc is an essential trace element required for the functioning of many enzymes, supporting the immune system, and excess Zn is toxic and interferes with the metabolism of other minerals in the body, particularly Fe and Cu [30]. Copper is essential for a variety of enzymes and also involved in the functioning of the nervous system [31]. Iron plays an important role in the function of hemoglobin, and iron defciency causes anemia and infertility, while its excess can damage tissues of the kidneys, heart, and lungs [32].

Moisture Content and pH of the Sesame Growing Soil.
Te pH of the soil samples were in slightly acidic range with values of 5.26, 5.42, and 5.58 at Kola Zana, Soroka, and Kisha, respectively. Tere was no statistically signifcant diference among the cultivation areas with respect to pH. Te pH of the soil samples are within the range of 5-8 that has been indicated as best condition for growing sesame [33]. Te moisture content of the soil samples were 7.9% at Soroka and 6.7% at both Kisha and Kola Zana.

Analytical Characteristics of the Method.
Te reliability of the optimized digestion procedure used in the determination of essential metals in the samples using FAAS was evaluated with respect to analytical fgures of merit. Te correlation coefcients (r 2 ) obtained for the calibration curves were in the range of 0.9985-0.9991 (Table 6), indicating a good linear relationship between concentration and absorbance. Te limit of detection of the method ranged 0.087-0.377 mg/kg across the diferent elements.
Te average percentage recoveries for the studied metals in the soil sample spikes ranged between 89.7 and 94.2%. Te precision of the method, which was expressed as the relative standard deviation of three replicate measurements made on spiked samples, ranged from the 0.50 to 12% across the diferent elements. Both the accuracy and precision of the method were good enough to allow the quantitative determination of the nutrient elements in the samples.

Concentration of Elements in Soil.
Te average determined concentration of Mg and Ca across the diferent cultivation areas were 507.6 and 418.0 mg/kg, respectively (Table 7). Similar to the sesame samples, the most abundant trace metal was Fe, but with the average concentration of 212.6 mg/kg across the diferent cultivation areas. Copper and Zn were found in equivalent proportions, with average concentrations of 28.8 mg/kg, across the diferent cultivation Table 4: Concentrations of standard solutions, coefcient of determination (r 2 ), limit of detection (LOD), recovery, and the associated relative standard deviation (RSD) values of the method used for the determination of essential metallic elements in the sesame seed samples.  International Journal of Analytical Chemistry 5 areas. Tere was no statistically signifcant diference in the concentration of Fe and Cu among soils from the diferent cultivation areas. On the other hand, signifcantly higher concentration of Zn was found in soil samples from Soroka than that from the other areas. Pearson correlation analysis was performed between the elemental compositions of sesame seeds and the underlying soil (Table 8). Te analysis was applied to assess the relationship between the concentration of a metal present in the soil with that in the sesame seeds, as well as to assess whether a metal present in soil facilitate or interfere with the uptake of another metal. Considering strong correlations, correlation coefcient ≥ |0.9|, increased concentration of Mg in sesame seeds is associated with decreased concentration of Fe and Cu in soil. Te level of Fe and Cu in the sesame seeds is afected negatively by the presence of higher amounts of Mg and Ca in soil. Te amount of Ca in sesame seeds decreases with increasing concentration of Mg in soil.

Conclusion
Te proximate composition and elemental content of sesame seeds varies with cultivation region in Tsegede district, Ethiopia. Te seeds are rich in fat followed by protein and carbohydrate. Te seeds also contain considerable amounts of the essential trace metals Fe, Zn, and Cu. Te concentrations of these elements in the sesame seeds varied signifcantly with the cultivation regions. Whereas, no signifcant diference in the concentration of the elements, except Zn, among soils from the diferent cultivation areas. Te levels of Fe and Cu in the sesame seeds are afected strongly and negatively by the presence of higher amounts of Mg and Ca in the soil.

Data Availability
All data generated during this study are included in the manuscript.

Conflicts of Interest
Te authors declare that they have no conficts of interest. Table 6: Concentration of standard solutions, coefcient of determination (r 2 ), limit of detection (LOD), recovery, and the associated relative standard deviation (RSD) values of the method used for the determination of metallic elements in the soil samples.