A study of the evolution of the physicochemical and structural characteristics of olive and sunflower oils after heating at frying temperatures

The evolution of the density, viscosity, oil/water interfacial tension and structure of vegetable oils after heating at frying temperatures was studied to explore the possibility of reusing waste vegetable oils as solid agglomerants for different purposes. Commercial olive and sunflower oils were heated at 150 and 225 degrees C in the time interval of 1-15 days to achieve a wide range of alteration degrees. Structural changes in the oils were monitored by FT-IR and H-1 NMR. Significant variations occur in the physicochemical and structural characteristics of these oils, which may affect their agglomeration capability, when they were heated at frying temperatures for periods above two days. Under these conditions, the viscosity of the oils increased very quickly, whereas their unsaturation degree decreased noticeably. Decreases in the relative intensities of the NMR spectra signals of the unsaturation-related protons were observed, those corresponding to the diallyl protons occurring much faster. Of the two vegetable oils studied, sunflower oil was found to be more sensitive to thermal treatment, undergoing greater changes in its properties, especially in viscosity, which may show a marked increase. (c) 2005 Elsevier Ltd. All rights reserved.