1. Introduction

Groundnut, or peanut (Arachis hypogaea L.), is a species in the legume family Fabaceae, native to Mexico, South and Central America ^[1]. In Nigeria, among the leading producing states of groundnut is Niger state ^[2]. In this Northern part of Nigeria, groundnuts are processed into various products which include groundnut paste popularly known as peanut butter, groundnut cake, salted groundnut and groundnut soup. Groundnut has contributed extremely to the development of the Nigerian economy through the sales of seeds, cakes, oil and haulms ^[3]. A substantial part is eaten in the cooked form, while fewer people indulge in eating the seeds raw ^[4]. Therefore, good nutrition is very essential to humans. Healthy population promotes development of the people through the relation between food, nutrition and health which needs to be reinforced in order to satisfy the needs of the increasing population ^[5].

However, the seeds serve as an important diet in many countries. Owing to its richness in high quality dietary protein and oil it helped to reduce malnutrition in the developing Countries ^[6] and help to supplement the nutrients of the basic carbohydrate food of the needy who cannot afford enough protein food of animal origin Achu et al. ^[5]. Apart from this, cake made from groundnut could be used in infant food formulations. Groundnut provides considerable amounts of mineral elements like phosphorous, calcium, magnesium and potassium and vitamin E, K, and B group to supplement the dietary requirements of humans and farm animals ^[7]. Groundnut contained 44 to 56% oil, 22 to 30% protein, and 9.5 to 19.0% total carbohydrates as both soluble and insoluble carbohydrate ^{[8, 9, 10, 11]}. According to Aremu et al. ^[12] seed proteins possess the essential functional properties for their successful utilization of some food products. Functional properties are essential physicochemical characteristics which affect the behaviour of proteins in food systems during manufacturing, storage and preparation. Some of these properties can be as a result of the thermodynamically favorable protein water interactions such as wettability, swelling, water retention, solubility, foaming and emulsification ^[13]. In previous study as reported by Yu et al. ^[14] the functional properties of defatted groundnut flour such as emulsification, bulk density, viscosity, and water and oil absorption were essential in food processing and formulation of food product.

A survey of the literature showed that research had been done on the proximate and mineral analysis of groundnut seeds, but there is little or no information as regards the chemical composition and functional properties of groundnut from a market in Minna, Niger state. It is on this basis that this research was conducted so as to establish the base line data in the chemical compositions, minerals and functional properties of groundnut seeds in this region.

2. Materials and Methods

Groundnuts (Arachis hypogaea L.) seeds were purchased from Mobile markets, Minna, Nigeria and were transported with polythene bag to the laboratory. The seeds were undecorticated, pre-treated by washing with distilled water and then divided into three portions: the first part was air dried, second part sun dried and the third part was toasted in an oven for 3 hours at 105^oC. The toasted seeds and the other two portions of sample were grounded into a paste to pass through mesh sieve.

The standard AOAC ^[15] methods were used to determine proximate composition of the groundnut samples.

The wet-ashing method was employed for the digestion of the seed sample; 1 g of sample was digested with 20 cm³ of concentrated HNO₃ and perchloric acid (1:1) and thereafter transferred to a 100 cm³ volumetri flask. It was diluted to volume with distilled water and stored in sample bottle. The mineral element contents were determined using an atomic absorption spectrophotometer (Perkin–Elmer model 703) and a flame photometer as described by Onyeike and Acheru ^[16].

The weighed sample was transferred into 10 cm³ graduated. The bottom of the cylinder was gently tapped on a laboratory bench for several times until no further diminution of the sample level was observed after it was filled up to the calibrated mark. Bulk density is expressed as:

Emulsifying capacity (EC) was determined according to the method of described by Pedroche et al. ^[17] with little modification. 1.0 g of each sample was homogenized at a speed of 160 rpm for 1 min at room temperature in 25 cm³ distilled water. The protein solution was mixed with 25 cm³ of vegetable oil followed by homogenization at 160 rpm for 1 min and finally, the emulsion was centrifuged for 5 min. Emulsifying activity was determined by:

Emulsion stability (ES) was measured by re-centrifugation followed by heating at 80°C for 30 min. Emulsion stability is expressed as follow:

Foaming capacity (FC) and stability (FS) were determined by the method described by Sze-Tao and Sathe ^[18] with certain modification. 1 g of each sample was isolated in 50 cm³ of distilled water. The solution was stirred at a speed of 160 rpm for 10 min. The blend was immediately transferred into a 100 cm³ graduated cylinder. The volumes were recorded before and after stirring.

For the determination of foam stability, foam volume changes in the graduated cylinder were recorded at 25 min of storage.

Fat absorption capacity was determined using the method described by Lin and Zayas ^[19] with minor modifications. 1 g of each sample was weighed into 20 cm³ pre-weighed centrifuge tube and thoroughly mixed with 5 cm³ vegetable oil. The emulsion was incubated at room temperature for 25 min and then centrifuged for 15 min and the supernatant was carefully removed, and the tube was reweighed. FAC was determined by:

Where is the weight of the sample (in gram), is the weight of the tube plus the sample (in gram), and is the weight of the tube plus the sediment (in gram).

The modified procedure as described by Rodriguez-Ambriz et al. ^[20] was used to determine the water absorption capacity. 1 g of each sample was weighed into 20 cm³ pre-weighed centrifuge tube. 10 cm³ of distilled water was added in to the tube under continuous stirring with a glass rod at room temperature for 25 min. The tube was centrifuged for 20 min and the amount of added distilled water resulting in the supernatant liquid in the test tube was recorded. WAC expressed as grams of water per gram of sample, was calculated by:

Where is the weight of the sample (in gram), is the weight of the tube plus the sample (in gram), and is the weight of the tube plus the sediment (in gram).

Results are expressed as the means and standard deviations of three separate contents. The data were statistically analyzed by 2-way analysis of variance (ANOVA). Standard deviations were compared by Duncan’s multiple range tests at 5% level of significance (P ≤0.05).

3. Results and Discussion

Results of the proximate compositions of the groundnut seeds are shown in Table 1.

Table 1. Proximate Composition (%) of Undecorticated Groundnut Seed

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The moisture content of the raw groundnut seeds was significantly high compared to the toasted and the sundried groundnut since it was not exposed to any heat. The moisture contents of the groundnut seeds were lower when compared to the reported work of Ibeabuchi ^[21] on the proximate and functional properties of raw and fermented bottle gourd seed (Lagenaria sicerari). As a result of this, it would enhance storage stability by preventing mould growth and also suitability and adaptability for further use in food formulation. Ash content is an indicator for mineral elements in seed. The ash contents (1.51-2.33 %) of the groundnut seed flours are higher than 1.38-1.48 % of Arachis hypogaea seed flour reported by Ayoola and Adeyeye ^[22]. The increase in the ash content of toasted groundnut may be as a result of pretreatment since volatile compounds are lost. According to Pomeranz and Clifto ^[23] ash contents of seeds should be in the range 1.5-3.5 % for suitable consumption. In this study, the ash contents fall within this range thus it can be recommended for human consumption and animal feeds.

The protein contents in both the raw and sun dried groundnuts were low while in the toasted groundnut, the heat applied showed a synergistic effect. The carbohydrate content was higher in the toasted and the sundried than in the raw groundnut. The crude protein contents (24.48-26.37 %) are greater than that of benniseed (16.4–18.1 %) as reported by Yusuf et al. ^[24]. Therefore, the seeds can be used as alternative sources of protein in diets. High fiber content in the samples is an advantage, and hence this may have potential uses as food supplement. The coefficient of variation (CV) % of the groundnut samples range from 0.08 to 0.99 % indicating the results were not critically varied.

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Figure 1. Graphical Representation of Proximate Composition of Groundnut Samples

From “Table 2” and “Figure 2”, the toasted groundnut does not lead to reduction in levels of mineral contents but rather increases in the levels since volatile and organic matter are lost by heat which corresponds with the observation of Ayoola and Adeyeye ^[22]. Therefore, groundnut is a good source of oil protein and minerals which can be used in diets to prevent against some mineral deficiencies

Table 2. Mineral Composition (mg/100g) of Undecorticated Groundnut Seed

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Na, K, Cu, Fe and Mn contents were found to be higher than conophor nut, cashew and bean seed ^{[25, 26]}. Potassium was the highest mineral element. The highest value of potassium agreed with the study that potassium was the most major mineral in Nigerian agricultural products ^[27]. The Na/K ratios obtained in raw, toasted and sundried samples are 0.12, 0.13 and 0.12 respectively. The values were less than 1 which signifies that the groundnut seeds flour would probably be a good source for electrolyte balance and controls high pressure in the body ^{[28, 29]}. The raw, sun dried and the toasted seeds contained appreciable amounts of calcium. Calcium is of greater amount when compared to Abelmoschus esculentus reported by Ndangui et al. ^[30]. This is very important to health of humans and is required for formation of bones and teeth, formation of blood clot, and body mechanisms ^[31]. The intake of the seeds flour which contains zinc as a micro element will play a role in the body in the healing of wound. The presence of phosphorous in the samples indicates that the soil nutrient where the samples were planted is rich in the mineral. So therefore, intake of the mineral would help in bone growth, kidney function, cell growth and in maintaining the body's acid-alkaline balance ^[32]. The absence of lead in the samples indicated that the investigated samples were free from the toxic metal.

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Figure 2. Graphical Representation of Percentage Mineral Content of Groundnut Samples

The toasted groundnut showed decrease in its functional properties compared to the raw and sundried seeds (Table 3). The fat absorption capacity (FAC) of raw groundnut was higher than that of toasted and sundried groundnut. This suggests that raw groundnut may have more hydrophobic proteins ^[33].

The water absorption capacity (WAC) (201.7 %) of raw groundnut seeds was the highest. The high water absorption capacity may be due to the high polar amino acid residues of raw groundnut proteins have strong affinity for water molecules ^[34]. The foam stability of toasted seeds decreased and other functional properties showed similar trends. Foam stability is dependent on protein content and environmental temperature. The higher the protein content the higher the foam stability ^[35]. Heat processing diminished the nitrogen solubility of proteins by denaturation and reduced their foaming capacity ^[36]. High temperature and prolong toasting time would not be desirable for processing the seeds where water or oil absorption are importance. Heat has been reported to inactivate the anti-nutritional factors ^[37] and also increase the antioxidant activity ^[38] seed flour. The application of heat during the pretrearment of the toasted and sundried groundnut seeds may contribute to the decreased in their foaming capacity and foaming stability leading to protein denaturation. Similar trends have been reported by Rahma and Mustafa ^[39] for peanut flour.

Table 3. Functional Properties of Undecorticated Groundnut Seed

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The emulsion activity shows the ability and capacity of a protein to help in the formation of an emulsion and is related to the protein’s ability to absorb to the interfacial area of oil and water in an emulsion ^[40]. The emulsion stability normally reflects the ability of the proteins to impart strength to an emulsion for resistance to stress and changes ^[41]. Raw seed flour has the highest emulsion capacity (31.50 %) because the seeds contain low amount of carbohydrate and this may be attributed to the high fat content in the groundnuts. This may be of great important in food emulsion such as yogurt, mayonnaise and ice-creams because of it high emulsion capacity.

The bulk densities of groundnut seed flour of raw, toasted and sundried were found to be in the range of 0.72-0.80 g/cm³. The bulk density values in this study for the groundnut seed flour are not comparable with 0.54 – 0.57 g/cm³ for different chickpea varieties as reported by Kaur et al. ^[42]. Higher bulk density is advantageous since it helps to reduce the paste thickness which is an essential factor in child feeding ^{[43, 44]}. However, the groundnut seed flours could be use as food supplement for children.

4. Conclusion

The undecorticated groundnut seeds contained high level of oil and protein this makes it a potential source of edible oil. The sources of protein and essential minerals can serve as various food and economic purposes. Functional properties of the groundnut seeds flour could be a good replacement in food formulation. This work has helped us know the nutritive values of groundnut from a part of Nigeria which can be considered as a good source of protein and promote the incorporating groundnut to improve protein level in reducing malnutrition in the developing countries. However, more studies should be carried out to investigate the possibility of using the seeds as an ingredient on other food products in order to increase the application of such value added food ingredient.

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