The effect of extraction methods on the oxidatives stability of marula and moringa seed oil.

Abstract

Marula and moringa seeds are used as a source of food, and the seed oils are used in cosmetics, pharmaceuticals and in medicines. This is due to its high nutritive value and high content of unsaturated fatty acids, which however makes them highly susceptible to oxidation. Due to its high nutritive value, the demand for the seed oil often exceeds industrial supply. Therefore, a timeous and environmentally friendly extraction method, which produces an oil with better oxidative stability, is required. Hence, the reason for this investigation on the effect of screw press, aqueous extraction (37 and 60 °C) and solvent extraction (shaker and Soxhlet) method on the oxidative stability of marula and moringa seed oil. The oxidative stability of the seed oils was determined by carrying out several tests, such as moisture content, acid value and peroxide value (PV). In addition, the anisidine value (AV), radical scavenging activity, conjugated diene (CD) and triene (CT) % were determined by UV-Vis spectroscopy. The fatty acid methyl ester content was determined using gas chromatography-mass spectrometry (GC-MS). The result for the oil yield showed there was a significant difference (P˂0.05) in the different extraction methods. Soxhlet extracted marula and moringa seed oil had the highest oil yield of 53.99 % and 35.20 %, respectively. Aqueous extracted marula and moringa oil at 37 °C had the lowest oil yield of 18.67 % and 12.00 %, respectively. The fatty acid profile showed the presence of palmitic, stearic, oleic, linoleic and behenic acid in moringa seed oils with oleic acid being the most dominant in the seed oils with the different extraction methods. Soxhlet extracted marula and moringa seed oil had a fatty acid composition of 70.70 and 77.61 % respectively, while aqueous extracted marula and moringa seed oil at 37 °C was 72.36 and 79.94 % respectively. The oxidative stability test PV, a measure of the initial oxidation, and AV, a measure of secondary oxidation product, showed there was a significant difference (P˂0.05) for the different extraction methods. Aqueous extracted marula and moringa seed oil at 37 °C had the highest radical scavenging ability compared to the other extraction techniques. The oxidative stability test carried out for 35 days at ambient and different accelerated storage conditions (45 and 65 °C) showed that aqueous extracted seed oil at 37 °C for both seed oils had lower values for the oxidative tests and lower values for the rate of change of PV and AV values thus suggesting a better stability oil. This could be as a result of the presence of the higher monounsaturated fatty v acid which aids stability, and lower polyunsaturated fatty acid present. A comparison of the stability tests for two seed oils showed that moringa seed oil had a better stability. This is probably due to the lower secondary oxidation products present in moringa oil as well as a higher quantity of monounsaturated fatty acid and lower quantity of polyunsaturated fatty acid present compared to that of marula. This study showed that aqueous extracted seed oils have a has a better resistance to degradation and oxidation reduction and shelf life in comparison with the other extraction methods. Also, moringa seed oil showed a better resistance to degradation as compared to marula seed oil