law and Herschel–Bulkley models (Eqs. (1) and (2), respectively)
were used to find better dispersion flow curves [27,28].
Power law model : б
=
k(n) (1)
Herschel–Bulkley : б
=
б0 +
k(n) (2)
where б is shear stress, k is consistency coefficient (K-value), is
shear rate, n is flow behavior index (n value), and б0 is yield stress.
Oiling out values were calculated from the ratio of oil volume
over total volume for each sample upon standing. 50 g of
each spread were carefully poured into 100-mL glass cylinders
and stored at 20 ◦C. These spreads were observed periodically
for 90 days, and the stability of each spread was expressed as a
percentage of the total weight of the spread in the tube: Oiling
out = (Moil/Mtotal)
×
100 [22].
2.4. Experimental design and statistical analysis
The rheological properties of PO-based spreads prepared with
various formulations were subjected to analysis of variance
(ANOVA), applying full factorial design, using Minitab software
(version 14 Minitab Ltd., USA). The factors studied were the concentration
of emulsifying (DMG) and stabilizing (XG) agents, PO
and CB. Significant differences (p < 0.05) for these variables, based
on at least three individual measurements, were determined by the
ANOVA procedure. Three dimensional (3D) graphs and interaction
plots also obtained from this software.
3. Results and discussion
3.1. Evaluation of flow curves
Fig. 2 depicts flow curve of different formulation of spreads. All
samples showed shear-thinning behavior, decreasing shear stress
due to increasing shear rate, until the slope of curves (viscosity)
became constant (Newtonian plateau) [23,24]. Increasing the shear
rate caused disruption of aggregates and decreasing viscosity to
an equilibrium value. The aggregates reformed and the viscosity
relaxed back to a higher equilibrium value as soon as the shear rate
was reduced [17]; this event generally is described as thixotropy