Abstract: Peanut (Arachis hypogaea L.) oil from seeds of six varieties; boro red, boro light, mokwa, ela, campala and guta as well as oil from three geographical zones in Nigeria; northern, eastern and western were investigated. Gas chromatography analysis showed high concentrations of oleic and linoleic acids in the oil samples. Capric (0.0%) and Lauric (8.1%) acids were absent and highest, respectively in the mokwa variety and hence diagnostic. More so, the comparative chemical analysis of peanut oil from the three zones and some selected refined vegetable oil; sunola, grand, olive and corn oil, indicated that western and grand oil had high iodine value 1.74±0.1 and 2.63±0.1, respectively, compared to others. The northern oil had high acid and fat value than the others (4.49 and 133%, respectively). Furthermore, the saponification value of the local vegetable oil was found to be significantly higher than the refined vegetable oil (P < 0.05), the eastern oil having the highest (140.25mgKOH/g). However, the peroxide values for both the local and refined oil were less than the standard peroxide value (10mEqKg-1) for vegetable oil deterioration. Minerals were present and no rancidity was observed in all the samples. In conclusion, the groundnut oil from Nigeria may have a higher shelf life, and serve as a useful substitute in nutrition and industrial applications.
Introduction
Arachis belongs to the family, Fabaceae, widely cultivated in the tropics and warm temperate regions of the world. The members are well known for their nitrogen-fixing bacteria associated with the root nodules (Woodland, 2000)
Edible oil from plant sources are of important interest in various food and application industries. They provide characteristic flavors and textures to foods as integral diet components (Odoemelam, 2005) and can also serve as a source of oleochemicals (Morrison et al., 1995). Oleochemicals are completely biodegradable (Kifli and Ahmad, 1986) and so could replace a number of petrochemicals. In Nigeria, the major sources of edible oil are groundnut also called peanut (Arachis hypogaea L.) and oil palm (Elaeis guineensis). These oils are used mainly as cooking oil and for the production of soap, margarine, and cosmetics (Ong et al., 1995). Peanut is an important source of edible oil for millions of people living in the tropics. In Nigeria, 1917 tons of peanuts are being produced annually (Ergül, 1988). Peanuts are among the oldest oil crops in Nigeria and are mostly consumed as snack, after roasting (Bansal et al., 1993; Jambunathan et al., 1993). Vegetable oil had made an important contribution to the diet in many countries, serving as a good source of protein, lipid and fatty acids for human nutrition including the repair of worn out tissues, new cells formation as well as a useful source of energy (Gaydou et al., 1983; Grosso and Guzman, 1995; Grosso et al., 1997, 1999). Oil quality and its stability are therefore very important for the consumers and application industries (Jambunathan et al., 1993). Thus, this study investigates the chemical properties of groundnut oil from Nigeria with the objective of evaluating the nutritive and industrial suitability.
Materials and Methods
Plant and oil materials:
Seeds of six varieties of arachis hypogaea: Boro Light, Boro Red,
Mokwa and Campala, Guta and Ela (Fig.1), as well as refined
vegetable oil such as grand and sunola were procured from Mile 12 market, Lagos
while olive and corn oil were purchased from Bodija, Ibadan. Locally produced
vegetable oils (groundnut oil) were obtained from western (Ilishan-Remo), eastern
(Ubakala) and northern (Kaduna) zones of Nigeria.
Preparation of samples for analysis: The seeds were stored under dry and cool condition prior to analysis. Extraction was carried out with soxhlet apparatus using n-hexane as solvent. The peanuts were dried at room temperature, peeled and crushed. 100g of each variety was measured, tied in a cellulose thimble and inserted into the soxhlet apparatus. The apparatus was left to run for 6h and the solvent was eliminated (evaporated) in vacuo using the rotary evaporator. Optical density was measured at 470nm using a SpectrumLab 752S UV-visible spectrophotometer.
Fat extraction: Fat extraction was carried out by Soxhlet method according to Pearson°s (1981).
Chemical analysis:
Acid value: Acid value was determined by titre metric method of Pearson
(1970). 5g of the oil sample was weighed and 75ml of hot neutral alcohol was
added with a few drops of phenolphthalein. The mixture was shaken vigorously
and titrated with 0.1M NaOH solution with constant shaking until the pink colouration
remains permanent.
| Fig. 1: | Varieties of Arachis hypogaea |
Acid value was calculated using the formula:
Iodine value: Iodine value was determined according to the titre metric method of Pearson (1970). 2g of oil sample was weighed into a dry glass stopper bottle of 250ml capacity and 10ml of carbon tetrachloride was added to the oil. About 20ml of Wij°s solution was then added and allowed to stand in the dark for 30 min. 15ml of (10%) Potassium Iodide and 100ml of water was added and then titrated with 0.1M Sodium thiosulphate solution using starch as indicator just before the end point. A blank was also prepared alongside the oil samples. Iodine value was calculated from the formula:
Peroxide value: Peroxide value was evaluated according to AOAC (1984). 2g oil sample was weighed into a tube and 1g of powdered Potassium iodide with 20ml of solvent mixture (glacial acetic acid and chloroform) was added. This was then placed in boiling water for 30s. The content was poured into a flask containing 20ml of 5% iodide solution. The tube was then washed with 25ml of distilled water and titrated with 0.002N Sodium thiosulphate solution using starch as indicator. A blank was prepared alongside the oil samples. Peroxide was obtained using the formula:
Saponification value: The Saponification value was determined according to the titre metric method of Pearson (1981). 2g of oil sample was weighed into a conical flask and 25ml of alcoholic Potassium hydroxide was added. Solution was heated in boiling water for 1h. 1ml of 1% Phenolpthalein was added and titrated with 0.5N HCl. A blank was prepared alongside the oil samples. The value was calculated by the formula:
| Saponification value = 56.1 N (A -B)/W |
where N = Normality of HCl acid used, A = Volume of H2SO4, for blank, ml, B = Volume of H2SO4, for sample, ml, 56.1= Equivalent weight of potassium hydroxide, W = Weight of oil used (2g)
Qualitative test for rancidity in oil and fat: The test for rancidity of oil was carried out according to Pearson (1981). 10ml of the oil samples was placed in a 100ml test tube vigorously mixed with 10ml of 0.1% Phloroglucinol solution and 10ml of concentrated HCl for 20s.
Determination of the presence of mineral oil in fats and vegetable oils: The presence of mineral oil in vegetable oil was carried out according to Pearson (1981). 10ml of the oil sample and 0.5M alcoholic KOH was added into test tube. It was then heated in a boilingwater bath with frequent agitation to ensure complete reaction. 0.5ml of water was added to the hot solution at a time and until 10ml had been added altogether.
Determination of Fatty Acid Methyl Ester (FAME) of groundnut oil: Fatty acid methyl ester of groundnut oil was prepared according to Christies° method (1993). 1 g of the groundnut oil was weighed and made up to the 1.5 ml mark of a test tube using hexane. The hexane solution was then treated with a solution of 2% (v/v) concentrated H2SO4 in methanol to prepare the FAME. The reaction mixture was left overnight at 50oC in a thermostated water bath. 15 ml of saturated NaCl solution was added and allowed to cool after thorough mixture. The lower aqueous methanol layer was allowed to separate and discarded while the upper hexane layer was separated and transferred into a separate dry test tube. The hexane layer was washed with 4 ml of 2% KHCO3 and dried over Na2SO4. Four drops of 0.05% of butylated hydroxytoluene (BHT) in methanol was added to prevent autooxidation of esters. The Gas chromatographic analysis of groundnut oil FAME was determined with Hewlett-Packard 5890 powered with HP.
| Table 1: | Oil yield and physical properties of the six varieties of Arachis hypogaea |
| Table 2: | Percentage composition of fatty acids in the varieties of Arachis hypogaea following Gas Chromatography |
| Table 3: | Chemical analysis of some selected refined and local vegetable oil in Nigeria |
| aData expressed as mean ± standard error, *Indicates statistically significant at P < 0.05 | |
Statistical analysis: Statistical significance was established using One-Way Analysis of variance (ANOVA) and data were reported as mean ± standard error. Statistical analyses were carried out using SPSS for Windows, version 14.0. (SPSS Inc. Chicago, IL. USA).
Results and Discussion
Vegetable oils now constitute a major component of daily diet consumption and its growth in the market is now considered on the basis of functionality, economy, and acceptability. The oil types indicate that boro light (20.8%) variety had the highest oil yield while campala (18.6%) had the lowest (Table 1). The range of oil yield in the varieties (18.6-20.8%) signified suitability for commercial production. Spectrophotometric analysis revealed that ela had the highest absorbance (0.398nm) while boro light had the lowest (0.032nm), Table 1. The gas chromatographic data (Table 2) on percentage composition of the different types of fatty acids present showed that oleic and linoleic acids (2:1) were the highest ranging from 41.7-44.2 and 19.6-20.9%, respectively. Ozcan and Seven (2003) similarly found oleic (48-55%) and linoleic (25.13-35.2%) acids the major components of the COM and NC-7 peanut cultivars from Turkey. Mokwa was unique by having the highest or lowest fatty acid value except lignoceric. The values were diagnostic for Capric (0.0%), Lauric (8.1%), Palmitic (4.85%), oleic (41.67%), linoleic (19.58%), Arachidic (1.18%) and Behenic (1.14%) acids. More so, the chemical analysis of the iodine values showed that grand and western zone oil were higher compared to others (Fig. 2 and Table 3). The high iodine value denotes high degree of unsaturation of the oil caused by the extent of oxidation and degree of heat treatment during oil processing (Kirk and Sawyer, 1991). The study also indicated that the oil from the northern zone was relatively high in total fat and acid value in comparison with oil from other zones (Fig. 2). According to Demian (1990), acid values are used to measure the extent to which glyceride in the oil has been decomposed by lipase and other actions such as light and heat. The determination is often used as a general indication of the condition and edibility of oil. Furthermore, the refined oil had significantly low saponification value compared to locally produced oils (P < 0.05), with the highest value found in the eastern zone oil (Table 3). Denniston et al. (2004) reported that high saponification value indicated the presence of greater number of ester bonds, suggesting that the fat molecules were intact. Similarly, the peroxide value of local and refined oil was less than the standard peroxide value (10mEqKg-1) for vegetable oil deterioration.
| Fig. 2: | Chemical analysis of locally produced vegetable oil from three geographical zones in Nigeria |
Fresh oils have value less than 10mEqKg-1 and values between 20 and 40mEqKg-1 results in rancid taste (Akubugwo and Ugbogu, 2007). The low peroxide value indicated slow oxidation of these oils. According to Demian (1990), the peroxide formation is slow at first during an induction period that may vary from a few weeks to several months according to the particular oil and temperature (Pearson, 1981). There was no rancidity of oil samples in the course of this study while minerals were present in all the samples (Table 3). This study indicated that vegetable oil from the six varieties of A. hypogea and those from the three geographical zones of Nigeria may have a higher shelf life, nutritional value and industrial applications.