Rheological characteristics and fatty acid compositions of Afzelia africana and Detarium microcarpum seed oils

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 stressshear rate rheological models: HerschelBulkley, 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 -1 as 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.


INTRODUCTION
Afzelia africana and Detarium microcarpum are tropical leguminous plants of the Caesalpiniaceae family.The seed endosperms of these plants are used to thicken soups in sub-Saharan Africa especially Nigeria.The chemical composition of the A. africana seed endosperm (Balogun and Fetuga, 1986;Ejikeme et al., 2010;Adesina and Osobamiro, 2012) and D. microcarpum (Balogun and Fetuga, 1986;Akpata and Miachi, 2001) have been well studied.The polysaccharide constituents have been extracted and characterized (Ren et al., 2005;Nwokocha and Williams, 2012).Seed oil has been isolated from D. microcarpum at a yield of 5 -15% (Akpata and Miachi, 2001;Okorie et al., 2010) and from A. africana at a yield of 15.92 -34.57% (Ajiwe et al., 1995;Ejikeme et al., 2010;Igwenyi et al., 2011).Since the oils are edible they can be used as cooking oil and in food processing.The performance in these processes is determined by the composition and properties.The physicochemical characteristics and fatty acid composition of D. microcarpum oil (Njoku et al., 1999;Okorie et al., 2010;Adesina and Osobamiro, 2012) and A africana oil (Ejikeme et al., 2010;Igwenyi et al., 2011) have been reported.However, we have not found any report on the rheological properties of the oils.In this work we determined the flow characteristics of D. microcarpum and A. africana seed oils at different temperatures using different shear stress-shear rate rheological models and compared them with those of rapeseed oil.We also determined the fatty acid composition of A. africana and D. microcarpum seed oils and correlated the fatty acid composition with the rheological properties to understand how this would influence the performance of the oils in real time application.

Extraction and purification
The A. africana and D. microcarpum seeds were purchased from a local market in Abakiliki, Ebonyi State, Nigeria.The seeds were cracked, soaked in warm water to enable easy removal of the hull.The endosperm was air dried to ambient moisture and then pulverized.The seed oils were extracted separately from pulverized seed endosperm by soxhlet extraction for 8 h with hexane solvent.The oil was concentrated on a steam bath using a Rotavapor, then transferred into a weighed sample bottle and the remaining solvent removed by drying in a convention oven at 60°C.
The oil was treated with excess amount of anhydrous sodium sulphate and centrifuged to remove traces of moisture and the dry oil stored.The yields of the oils were 8.43% for D. microcarpum and 22.9% for A. africana.

Rheological characterization
The rheological measurements were carried out on a controlled stress Rheometer (AR 500, TA Instruments Ltd, USA) with cone and plate geometry (40 mm, 2° steel cone and 53 μm gap).The oil was placed on the peltier plate by means of a spatula spoon, the gap was set and the excess oil trimmed off.The sample was allowed to equilibrate for 30 s at a given temperature before measurement.

Effect of shear rate and temperature on apparent viscosity
The effect of shear rate was determined by performing a stepped flow procedure in the shear rate range from 0.01 to 1000 s -1 at different temperatures of 30 -90°C.The flow characteristics were determined according to the following shear stress-shear rate rheological models (Equations 2 to 6): (4) Nwokocha and Olorunsola 35 Newtonian ( Where  = shear stress (Pa);  = viscosity (Pas);   = shear rate (s -1 ); Y  = yield stress (Pa) and n = rate index.

Temperature ramp
A temperature ramp step procedure was carried out on the oil from 30 to 100°C at a shear rate of 50 s -1 for 10 min.The flow curves fitted to the Arrhenius equation (Equation 7) give an approximation of this behavior.
Where η = viscosity (Pas), c = viscosity coefficient (Pas), equivalent to viscosity at infinite temperature η T b = temperature coefficient (K), equal to Ea/R; T = temperature (K).Thus Equation 6 can be rewritten as Equation 8to enable the calculation of the activation energy, Ea.R = gas constant (8.314J/mol/K)

Fatty acid composition
Fatty acid methyl esters (FAME) were prepared by standard IUPAC method 2.301 (Anonymous, 1979).Sample of the raw oil (1000 mg) was accurately weighed, placed in 50 ml round bottom flask, followed by the addition of 1 M methanolic sodium hydroxide (5 ml).The samples were refluxed at 70°C for 20 min.After cooling, hexane and water (10 ml of each) were added.The mixture was vortex mixed for 15 min and the upper phase (hexane layer) containing the fatty acid methyl esters was recovered and analysed by gas chromatography.The GC was conditioned for 30 min and 0.2 ml of the methylated sample was injected into the capillary column (VF-1 ms, 30 m, 0.25 mm, 0.25 µm; Part number CP8912) of the Varian Chromopack GC (Model-CP3380, USA) (injection temperature 260°C, detector temperature 260°C).The carrier gas was nitrogen (flow rate 30 ml/min) and flowed through the air drier at 571°C, coolable oven at 100°C which increases with time and the Front FID at 260°C.The sample was allowed to run in the GC for about 1 h 37 min.

Data analysis
The rheological data were analyzed using the Thermal Advantage Data Analysis Software version V.5.1.42(TA Instruments, USA).

Dependence of viscosity on shear rate and temperature
The shear stress-shear rate flow profiles of A. africana and D. microcarpum and rapeseed oils measured at ) where shear stress did not vary linearly with shear rate, and a linear or Newtonian region (> 10 s -1 ) where shear stress varied linearly with shear rate.At low shear rates, the oils indicated presence of some structures which were broken down as a result of the shearing action.
These are mainly the fatty acid crystals and were responsible for the non-Newtonian flow.At shear rates > 10 s -1 , all the structures had been destroyed and flow became Newtonian (Ennouri et al., 2005;Santos et al., 2005).The shear thinning characteristics of the oils at < 10 s -1 is illustrated in the viscosity-shear rate profiles of A. africana oil (Figure 1b).The analysis parameters    canola oil but less than 4.1 x 10 -2 Pas reported for soybean oil at 30°C (Diamante, and Lan, 2014).The presence of yield stress in the oils observed with Herschel-Bulkley, Binghan and Casson models indicated a range of non-Newtonian behaviour of the 'oil'.The non-Newtonian behaviour at low shear rates indicates the presence of large aggregates of fatty acid crystals in the vegetable oil (Murthukumarappan et al., 2016).The yield stress decreased as temperature increased.Shearing and temperature destroyed this structural state and gave rise to a homogeneous stable suspension.This explains why the flow behaviour index (n) is close to unity at all the temperatures.

Activation energy of flow
The activation energy of viscous flow of the oils was estimated from the temperature ramp (30 to 100°C) at 50 s -1 by applying the Arrhenius equation (Figure 4a) the parameters of flow are presented in Table 4a.The curves gave good fits as indicated by the low values of the standard error of estimates (11.31-14.29).
Afzelia oil had slightly higher E a (23.25 kJ/mol) than rapeseed oil (23 kJ/mol) while Detarium oil (20.88 kJ/mol) was lowest.Correlating E a with the viscosity of the oils, Afzelia oil with the highest viscosity had the highest E a .We also estimated the activation energy of flow, E a ′, at 50.12 s -1 in the Newtonian region of the shear flow profiles for the different oils (Figures 1a, 2 and 3).We calculated the viscosity from the relationship (η = σ/‫)ﻵ‬ and used it to estimate E a ′ from the plot of η versus 1/T, (R 2 = 0.994 -0.998) (Figure 4b, Table 4b).In comparison E a ′ values are less than E a .This is expected because E a covered the entire flow profile and included the energy used to breakdown fatty acid structures in the oils while E a ′ was obtained after Newtonian flow had been attained and was devoid of contribution from the fatty structures.It was observed that the E a ′ for rapeseed oil (21.4 kJ/mol) > E a ′ for A. africana (20.77 kJ/mol).This did not follow the pattern observed with the E a for the two oils.This difference can be explained by considering the yield stress values, σ ɣ , and their contribution to resistance to flow.The σ ɣ for A. africana (5.894E-3 Pa) is more than twice for rapeseed oil (2.042E-3 Pa) indicating the contribution of σ ɣ to E a is higher in A africana.For E a ′, all the fatty structures and σ ɣ had been overcome and the values reflected the activation energy for the oil alone.The activation energies are in the range of values reported for other oils (Giap, 2010).
Table 7 shows a comparison of the fatty acid profiles of  (Giap, 2010).
A correlation of fatty acid composition and rheological properties of the oils indicates that A. africana oil with higher percentage of long chain fatty acids exhibited higher viscosity than D. microcarpum oil.This is in agreement with research findings which established viscosity as being directly influenced by molecular weight (Al-Assaf et al., 2005).Wang and Briggs (2002) have also corroborated this when they reported that oils with high oleic acid content have higher viscosity.
The presence of unsaturation in oils also affects its rheological properties though the relationship was not easily deduced in this study.According to Kim et al. (2010), viscosity decreases with increase in degree of unsaturation of fatty acids in the oil.

Figure 1a .Figure 1a .Figure 1 .
Figure 1a.Shear flow profiles of Afzelia africana seed oil at different temperatures fitted to the best fitting shear stress-shear rate models; b).Viscosity-shear rate profile of Afzelia oil at different temperatures

Figure 2 .
Figure 2. Shear flow profiles of Detarium microcarpum oil at different temperatures fitted to the best fitting shear stress-shear rate models.

Figure 3 .
Figure 3. Shear flow profiles of rapeseed oil at different temperatures fitted to the best fitting shear stress-shear rate models.

Figure 4a .Figure 4 .
Figure 4a.Flow curves of the temperature ramp from 303K to 373K at shear rate of 50 (1/s) for Afzelia africana, Detarium microcarpum and Rapeseed oils.b).Plot of apparent viscosity versus inverse of absolute temperature at shear rate of 50.12 (1/s) for estimation of activation energies of Afzelia africana, Detarium microcarpum and Rapeseed oils

Table 1 .
Analysis parameters of viscosity-shear rate curves of Afzelia africana oil according to different rheological models.

Table 2 .
Analysis parameters of viscosity-shear rate curves of Detarium microcarpum oil according to different rheological models.

Table 3 .
Analysis parameters of viscosity-shear rate curves of Rapeseed oil according to different rheological models.

Table 4a .
Parameters of flow curves of Afzelia africana, Detarium microcarpum and Rapeseed oils subjected to temperature ramp from 30 o C to 100 o C at shear rate of 50 (1/s) fitted to Arrhenius equation

Table 4b .
Analysis parameters of apparent viscosity versus inverse of absolute temperature at shear rate of 50.12 (1/s) of Afzelia africana, Detarium microcarpum and Rapeseed oils.

Table 5 .
Fatty acids present in Afzelia africana seed oil, their percentages and retention times

Table 6 .
Fatty acids present in Detarium microcarpum seed oil, their percentages and retention times.

Table 7 .
Comparison of the fatty acid profiles of A. africana and D. microcarpum seed oils.