Statistical Approach to the Optimization of Oil Extraction from Beniseed (Sesamum indicum) Oilseeds

Abstract: In this work， solvent extraction of oil from Sesamum indicum (beniseed) was investigated. This process was optimized by applying Box-Behnken design and response surface methodology (RSM). A quadratic model was obtained to predict the oil yield as a function of time， solvent volume and sample weight. Based on the RSM， the optimal condition for the oil extraction was found to be sample weight， 30.05 g; time， 58 min; and solvent volume， 296 mL. The optimized condition was validated with the actual oil yield of 87.79%. Physicochemical properties of the oil showed it to be liquid/golden yellow， specific gravity， 0.88; viscosity， 35.08 cP; acid value， 0.5 mg KOH/g oil; saponification value， 190 mg KOH/g oil; iodine value， 108 g I2/100 g oil; higher heating value， 40.02 MJ/kg; and cetane number， 50.7. Fatty acid profile of the oil indicted it is highly unsaturated (73.01%). Hence， the oil could have important food and industrial applications.

Key words: Sesamum indicum， solvent extraction， response surface methodology， fatty acid， physicochemical properties.

1. Introduction??

Sesamum indicum L. also known as beniseed in Africa， is an important oilseed crop cultivated in many parts of the World. Its seeds compose of 44%-58% oil， 18%-25% protein and 13.5% carbohydrate [1]. Beniseed is used to add texture and flavour to a variety of breads， rolls， cracker and salad dressings. Beniseed oil is odourless and has a pleasant taste. It is used widely as cooking oil and as raw material in the manufacture of paints， black ink， margarine， soap and pharmaceuticals [2]. The best quality of the beniseed oil is comparable to olive oil [3]. Fatty acids compositions of the oil are mostly oleic (32.7%-53.9%)， linoleic(39.3%-59%)， palmitic (8.3%-10.9%) and stearic(3.4%-6.0%) acids [4]. However， the fatty acid composition of this oil varies considerably among the different varieties worldwide [4].

Several methods exist in oil separation from oilseeds and these include pounding the seeds in a wooden mortar， extractions with expeller and solvent extraction method. Domestically， pounding beniseed oilseeds in a wooden mortar and treating the product with hot water is the most widely used but oil produced with this method is usually of a slow process， laborious， low oil yield， the unpleasant odour and bitter taste [5]. Extraction of oil from beniseed oilseed using expeller is not without challenges. The method requires high operating and man-power usage， and the oil obtain is of low value. Extraction with solvent has advantages， which include higher yield and less turbidity as well as relatively low operating cost. Literature materials exist on the use of this method for beniseed oil extraction [6， 7]. Maximum beniseed filtered oil recovery reported was 79.63% [7]. Oil separation by supercritical CO2 has also been exploited. D?ker and co-workers [8] successfully extract beniseed oil using this method. Despite the high purity oil extracted， high operating and investment cost pose a great challenge. Although recovery of beniseed oil using solvent extraction

method has been reported [9， 10] but they were not optimized.

Response Surface Methodology (RSM)， a useful optimization tool has been applied in research to study the effect of individual variables and their interactions on response variables. It has been used extensively in the optimization of extractions of edible and non edible oils from different oil sources such as pumpkin， palm oil， silkworm pupae， Hibiscus sabdariffa， among others [11， 12]. The main advantage of RSM is the ability to reduced number of experimental runs needed to provide sufficient information for statistically acceptable results. Physicochemical properties of oils such as colour， refractive index， acid value， saponification value， iodine value， higher heating value， etc. and other properties， for example， cetane number， API， as well as the fatty acid profile are important quality characteristics use in determining their potential use.

This work focused on oil separation from S. indicum oilseeds via application of solvent extraction method. To optimize the extraction conditions for the process， RSM was applied to determine the effects of three factors (sample weight， extraction time and solvent volume) and their reciprocal interactions on the beniseed oil yield. In addition， the quality of oil extracted was evaluated by carrying out physicochemical and fatty acid analysis with a view to determining its potential applications.

2.2.3 Physicochemical Analysis of the Crude Beniseed Oil

The physicochemical properties of the crude seed oil namely， refractive index， moisture content， relative density， viscosity， acid value， saponification value， peroxide value， specific gravity， % FFA (oleic) and cetane number were determined by AOAC methods. The higher heating value (HHV) and iodine value were calculated using the methods of Demirbas [13] and Wijs， respectively. API and diesel index were estimated by methods reported by Haldar et al. [14].

2.2.4 Fatty Acid Compositions Analysis of Crude Beniseed Oil

Fatty acid composition of the crude seed oil was determined using gas chromatography (HP 6890 powered with HP ChemStation Rev. A 09.01 (1206) Software). Oil sample (50 mg) was esterified for five minute at 95 °C with 3.4 mL of the 0.5 M KOH in dry methanol. The mixture was neutralized using 0.7 M HCl and 3 mL of 14% boron triflouride in methanol was added. The mixture was heated for 5 min at the temperature of 90 °C to achieve complete methylation process. The fatty acids were thrice extracted from the mixture with redistilled n-hexane. The content was

noteworthy that previously reported yields were below the result obtained in this work.

The results of this work demonstrated that RSM with appropriate experimental design can be effectively applied to the optimization of the process variables in oil extraction from oilseeds. This may provide useful information regarding the development of economic and efficient processes using solvent extraction method.

3.2 Quality Characterization of the Crude Seed Oil

3.2.1 Physical Properties of the Crude Seed Oil

In order to evaluate the quality of the crude beniseed oil， the content and compositions of the oil was subjected to physicochemical analysis and the results obtained are shown in Table 4. At room temperature， the oil was liquid/golden yellow in colour with refractive index and moisture content of 1.470% and 0.09%， respectively. Observations on the colour， moisture content and refractive index of the oil agreed with previously published report [10]. Elleuch et al. [17] observed the same value of refractive index for the beniseed oil. The specific gravity of the seed

were determined (Table 4). Cetane number is a measure of the fuel’s ignition delay and combustion quality. Standard specification of cetane number for biodiesel is minimum of 40 [20]. The cetane number of the seed oil (50.73) showed that it have high fuel potential. The cetane number reported for most vegetable oils range from 27.6 to 52.9 [10， 13]. The API， diesel index and aniline point of the oil were comparable with other vegetable oils as reported previously [14]. The transesterification of the oil may improve its fuel properties.

3.2.4 Fatty Acid Profile of the Crude Seed Oil

Gas chromatography analysis of fatty acids present in the seed oil is shown in Table 5. The results indicated that the oil is highly unsaturated. The dominant fatty acids were oleic (43.74%)， linoleic(24.01%)， palmitic (17.8%) and stearic (7.41%). The linoleic observed in this work is lower than reported values [4， 17]. This may be attributed to the variety of the beniseed used. The fatty acid composition of this oil varies considerably among the different varieties worldwide [4]. The total unsaturated fatty acid composition of the oil is 73.01%.

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