Tree nut oils: chemical characteristics, oxidation and antioxidants

Miraliakbari, Homan (2005) Tree nut oils: chemical characteristics, oxidation and antioxidants. Masters thesis, Memorial University of Newfoundland.

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Abstract

The chemical compositions, antioxidant activities and oxidative stabilities of tree nut oils were examined. The oils of almonds, Brazil nuts, hazelnuts, pecans, pine nuts, pistachios and walnuts were extracted using two solvent extraction systems, namely hexane and chloroform/methanol. The chloroform/methanol system resulted in higher oil yield for each tree nut type examined. Pine nuts had the highest oil content while almonds had the lowest. The lipid class compositions of tree nut oils were analysed using thin layer chromatography-flame ionization detection and showed that triacylglycerols were the predominant lipid class present. Smaller amounts of sterols, sterol esters, phospholipids and sphingolipids were also present. The fatty acid compositions of tree nut oils were analysed using gas chromatography, showing that oleic acid is the predominant fatty acid in all samples except pine nut and walnut oils, which contained high amounts of linoleic acid. The sterol and stanol content and compositions were analysed using gas chromatography; β-sitosterol was the predominant sterol compound present in all samples, with lower amounts of campesterol, stigmasterol, Δ5-avenasterol, 22-nordehydrocholesterol, 24-methylenecholesterol, cholesterol, cholestanol and β­sitostanol also present. The tocopherol compositions were analysed using high performance liquid chromatography, showing that α- and γ-tocopherols are the predominant tocopherol isomers present; however δ- and β-tocopherols were also detected in some samples. -- The antioxidative components of tree nut oils were extracted using a solvent stripping process. The minor component stripped oils that remained after the solvent stripping process were analysed for their chemical compositions as described above, and were also used in the oxidative stability studies. The lipid composition of the tree nut oil extracts showed that they contained phospholipids, sphingolipids, sterols and tocopherols. The total phenolics contents of tree nut oil extracts were analysed and showed that chloroform/methanol extracted oils had higher amounts of phenolic compounds than their hexane extracted counterparts. The antioxidant activity of tree nut oil minor component extracts were assessed using the Trolox equivalent antioxidant capacity, 1, 1-diphenyl-2-picrylhydrazyl radical scavenging capacity, β-carotene bleaching test, oxygen radical absorbance capacity and photochemiluminescence inhibition assay; results of these studies showed that extracts of chloroform/methanol extracted oils possessed higher antioxidant activities than extracts of their hexane extracted counterparts, while the extract of chloroform/methanol extracted pecan oil possessed the highest antioxidant activity. -- The oxidative stability of non-stripped and stripped tree nut oils were examined under two conditions, namely accelerated autoxidation and photooxidation. Progression of oxidation was monitored using tests for conjugated dienes, peroxide value, para-anisidine value and headspace volatile analysis. Primary products of oxidation persisted in the earlier stages of oxidation while secondary product levels increased dramatically during the latter stages of oxidation. Hexanal was the major headspace aldehyde formed in all oxidized samples except walnut oil, which contained primarily propanal. Results showed that chloroform/methanol extracted oils were more stable than hexane extracted oils in both the accelerated autoxidation and photooxidation studies. Oils of pecan and pistachio were the most stable while oils of pine nut and walnut were the least stable.

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