Mafura seed oil : improvement in oxidative stability

Abstract

Trichilia emetica seeds are processed for their oil by rural communities in the South of Mozambique. Traditionally, the oil is extracted by boiling the seed pulp in an aqueous medium for 5 h and recovering the clear yellow oil. The extended heating period and the use of water for oil extraction may reduce the oil quality. Mafura oil was extracted using a laboratory method that mimics the traditional method. Acid value (titrimetry), iodine value (Wijs method), saponification value (titrimetry) and fatty acid composition (gas chromatography) were determined in comparison with sunflower and olive oils. Oxidative stability (OS), (peroxide and anisidine values) of mafura oil stored at (65oC) over a 14 day period was also determined in comparison with sunflower and olive oils. The effect of refining, solvent extraction (hexane and ethanol) and the effect of incorporation of freeze dried crude phenolic extract (CPE) from red condensed tannin sorghum bran (at 1000 and 2000 ppm) compared with tertiary-butylhydroquinone (TBHQ) (200 ppm) on OS of mafura oil during storage was determined. Whole grain, bran and CPE were analysed for total phenolic content (TPC), (Folin-Ciocalteu assay) and antioxidant activity, (ABTS radical scavenging). Phenolic composition of the CPE was determined using High Performance Liquid Chromatographic. Traditionally extracted crude mafura oil (CMFO) had the highest acidity. Crude and refined mafura oils had the lowest iodine values and the highest saponification values compared with sunflower oil and the olive oils. CMFO was rich in palmitic acid, sunflower oil in linoleic acid and the olive oils were rich in oleic acid. CMFO was the least stable during storage as shown by high anisidine values. Solvent extraction (but not refining) improved the OS of CMFO. Mafura oil treated with CPE was more oxidatively stable than the control and this seemed to be dose-dependent. CPE at 2000 ppm was less effective in inhibiting the formation of primary oxidation products than TBHQ, however, it was as effective in reducing secondary oxidation. The CPE had higher TPC and antioxidant activity than whole grain and sorghum bran. The CPE contained phenolic acids and high levels of (+)-catechin and (-)-epicatechin. The ability of the CPE to improve the OS may be related to the action of the phenolic compounds as primary antioxidants in scavenging free radicals. This study shows that the traditional method of mafura oil extraction decreases its OS due to extensive heating in an aqueous medium. The CPE may potentially be used as a natural antioxidant to prevent oxidation of mafura oil due to the antioxidant properties of the phenolic compounds present in the sorghum bran.