Abstract

The rheological characteristics of oils isolated from the seeds of Afzelia africana and Detarium microcarpum were studied and compared with rapeseed oil. The effects of shear rate and temperature on the flow characteristics were evaluated. The shear stress- shear rate rheological models: Herschel-Bulkley, Power law, Binghan, Newtonian and Casson were used to determine the flow characteristics. All the oils exhibited non-Newtonian behaviour at shear rates < 10 s^-1as indicated by the presence of yield stress. The viscosities of the oils were in the order: Afzelia oil > rapeseed oil > Detarium oil. The activation energy of viscous flow followed the same order as viscosity of the oils. The most abundant fatty acids in A. africana oil were cis-11-eicosenoic acid (39 .06%), linolelaidic acid (18.38%) and nervonic acid (10.20%) and in D. microcarpum oil cis-13,16-dicosadienoic acid (20.51%) and linoleic acid (20.37%) and undecanoic acid (14.24%). A. africana contained higher amount of unsaturated fatty acids (92.26%) and long chain fatty acids (C ≥15; 96.00%) than D. microcarpum (unsaturated fatty acids, 66.27%; C ≥ 15; 79.41%). A. africana oil with the greater amount of long chain fatty acids had higher viscosity and activation energy of viscous flow.

Key words: Afzelia africana, oil, rheological properties, fatty acid composition, Detarium microcarpum