Development of a spectroscopic method to determine the content of free radical scavenging compounds and oxidation products in thermally oxidised oils

A simple spectroscopic method using 2,2-diphenyl-1-picrylhydrazyl (DPPH) was developed to simultaneously monitor the contents of free radical scavenging antioxidants (FRSs) and oxidised lipid products during oxidation through selection of proper solvents. Validation of the DPPH method was conducted in a stripped oil matrix, and practical applicability of the method was tested with heated oils. The absorbance of DPPH in isooctane simultaneously reflected the changing amounts of both FRSs and oxidised lipid products, whereas the absorbance in methanol mainly reflected changes in FRSs. Total polar materials (TPMs) were found to be representative of oxidised lipid products, which reacted with DPPH in isooctane better than in methanol. Validation parameters including accuracy, precision, linearity, limit of detection and limit of quantification were determined using α-tocopherol as a model for FRSs and TPMs for oxidised lipid products. The DPPH protocol in isooctane and in methanol is useful for assessing the degree of oxidation in heated oils through comparing the contents of FRSs and oxidised lipids.