Fatty acid composition and rheological characteristics
of sunflower oil (SO), cottonseed oil (CO) and
palm olein (PO) during deep frying for 4–16 h were
investigated. In the different oils, linoleic acid decreased
while palmitic, stearic and oleic acids increased in the bath
oil during frying. Total phenolic content (TPC) in different
oils increased during frying and strongly correlated with
frying time. As frying progressed, it was observed that the
rate of increase in TPC was relatively slower in PO than in
the SO and CO. The shear stress versus shear rate results
was fitted to Newtonian, Bingham and Herschel–Bulkley
rheological models. The flow behaviour of fresh and fried
SO, CO and PO was recorded at 25 C. Fresh oils showed
Newtonian behaviour with correlation coefficients greater
than 0.99 at 25 C and slight non-Newtonian behaviour
after frying. Palm olein showed higher increase in viscosity
in comparison to CO and SO. Rheological parameters of
vegetables oils showed great changes, wherein the highest
change in viscosity was recorded after 16 h of frying. Palm
olein had higher flow behavior parameters than SO and
CO. The increase in frying time caused an increase in K,
gB, s0HB, s0B, and g values, while the n values decreased
with increasing the frying time. The viscosities of oils were
plotted against the levels of C18:1 and C18:2 wherein
highly positive correlations were found between them
(R2 = 0.99). The temperature dependence of viscosity was
studied by using the Arrhenius relationship and the
activation energy indicates the sensitivity of viscosity to
changes in temperature. It could be concluded that the
rheological parameters can provide an overall estimate of
oil quality during frying. |
