Assessment of the quality of heated oils by differential scanning calorimetry

Differential scanning calorimetry (DSC) was used to follow changes in the thermal characteristics of commercial frying oils (sunflower seed, colza, and groundnut) during heating at 180 degrees C for up to 10 h. DSC tracings of oils scanned from 10 to -80 degrees C were characterized by a single crystallization peak (CP) at -43 to -48 degrees C. Heating of oils resulted in a progressive shift of the CP to lower temperatures and reduced enthalpies of crystallization (EC). The temperature of the CP decreased as much as 27 degrees C, and the EC was reduced by almost 90% after heating for 10 h. These changes in thermal characteristics correlated well with the appearance of polar compounds, the increase in viscosity, and color changes of the oil. DSC is a fast method of analysis, requires small samples (15-60 mg) with minimal preparation, and may be implemented directly in fried products.