Rheological characteristics of vegetable oils as affected by deep frying of French fries

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 degrees C. Fresh oils showed Newtonian behaviour with correlation coefficients greater than 0.99 at 25 degrees 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, eta(B), tau(0HB), tau(0B), and eta 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 (R-2 = 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.