Supercritical fluid extraction of Sclerocarya birrea kernel oil
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Sub-Saharan Africa is a treasure chest of natural materials remaining to be explored for commercial applications and as alternative foods to diversify and improve food sustainability. The Marula tree is available in abundance in South Africa and bears a fruit with a highly nutritious kernel containing high oil and protein content. The oil from the kernels has various applications from food to cosmetics. The accepted oil processing practice is required to be a green technology, producing no effluent or using toxic solvents. Therefore, the oil is extracted using an expeller. However, with average 55 wt. % oil in the kernel the extracted oil yield is far from optimal, typically ranging from as low as 7 wt. % to 47 wt. %. The latter is obtained only with proprietary modified expellers. Therefore, an alternative green technology which retains the native characteristics of the Marula oil is needed. Communication with local producers, South African and Namibian, confirmed the need for investigation of an alternative means of extraction of Marula oil from the seed kernels which can improve the yield and potentially the quality of the oilcake. The latter of which is typically adversely affected by the expelling process. A review of various processing technologies available for oil extraction was completed and supercritical fluid extraction utilizing carbon dioxide as the extraction solvent was identified as a potential solution. An overview on supercritical fluid extraction using carbon dioxide (SFE-CO2) of similar materials to the Marula kernels, such as hazelnuts, walnuts and pine kernels indicates that yields similar to that of solvent extraction and of the quality of the oils obtained by cold pressing can be obtained with the technique. The theory, practical applications, and how one can use the system to improve yield from various natural materials were reviewed. It was determined that the two main parameters one can manipulate on supercritical extraction systems to optimize the yield, were pressure and temperature. Subsequently kernels of the Sclerocarya birrea tree, common name Marula, cultivated in South Africa, were obtained for extraction with supercritical carbon dioxide. The effects of pressure and temperature on extraction yield were investigated. The total maximum yield of Marula kernel oil obtained was found to be 54 wt. %, compared to a solvent extracted yield of 52 wt. %, such that a 100 % oil recovery was obtained with SFE-CO2. The optimal conditions were found to be 450 bar and 60 °C as the yield per kg solvent initially was 41 g kg-1 CO2. Following the extractions, the oils were characterized for fatty acid composition using gas chromatography. Quality parameters of a cold pressed sample and a sample obtained at the optimal extraction conditions were determined and compared; and the results indicate that the two oils are of similar composition and quality. Supercritical fluid extraction using carbon dioxide was successfully verified as a potential processing method for the extraction of Marula oil from the kernels. The SFE-CO2 provided an improved yield compared to cold pressing and a quality of oil similar to cold pressed Marula oil. Additionally, after SFE-CO2 processing, the defatted Marula kernels contain high protein content, 69 wt. %, in the form of a pure white powder. Due to the favourable nutritional content the residue may be used for human consumption to create new products such as meat analogues, porridges, and shakes, or can be sold as a high protein powder.
- Engineering