Impact of Frying on Fatty Acid Profile and Rheological Behaviour of Some Vegetable Oils

Rheological properties and fatty acid profile of sunflower oil (SO), cottonseed oil (CO) and palm olein (PO) during deep frying for 4, 8, 12 and 16 h were investigated. In the different systems, linoleic acid decreased while palmitic, stearic and oleic acids increased in the bath oil upon frying. The shear stress versus shear rate data was fitted to Newtonian, Bingham and Herschel-Bulkley rheological models. The flow behavior of fresh and used SO, CO and PO was measured at 25°C. All samples at zero time of frying (fresh oil) showed Newtonian behavior with correlation coefficients greater than 0.99 at 25°C and slight non-Newtonian behavior after 4, 8, 12 and 16 h of 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, ηB, τ0HB, τ0B, and η values, while the n values decreased with increasing the frying time. The viscosities of the vegetable oils were plotted against either C18:1 or C18:2 and there were highly positive correlations 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 the viscosity to the temperature change. The rheological parameters and viscosity measurements can provide a good overall estimate of frying oil quality.