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

Abstract

Summary 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.

Effect of Polar and Non‐Polar Compounds from Oxidized Oils on Oxidative Stability in Corn Oil

“…To quantify the contents of FRSs or RSOLs in oil samples, the DPPH absorbance was converted to DPPH loss with units of mmol DPPH/g oil sample using the following equation [8,11] : Samples were prepared in triplicate.…”

“…Polar compounds, which accelerated the rates of lipid oxidation compared to non-polar compounds, as shown in Figure 1 and 2, induced different profiles of DPPH loss in methanol and in isooctane. Song et al [8] reported that only antioxidants, and not RSOLs, could react with DPPH in methanol, whereas both antioxidants and RSOLs could react with DPPH in isooctane. Rising DPPH loss in isooctane in samples with polar compounds at 60 min could be attributed to the formation of RSOLs.…”

“…FRSs or antioxidants in oils with polar compounds were depleted almost completely at 40 min as shown in Figure 3a. [8] Therefore, TPM, which is a recommended parameter for determining the quality of frying oils, could be assessed using modified DPPH methods. Controls and oil with non-polar compounds did not show significant differences, except at 180 min, which also implies that non-polar compounds are neither antioxidants nor prooxidants under current experimental conditions.…”

“…Rising DPPH loss in isooctane in samples with polar compounds at 60 min could be attributed to the formation of RSOLs. [8] Song et al [8] reported that only antioxidants, and not RSOLs, could react with DPPH in methanol, whereas both antioxidants and RSOLs could react with DPPH in isooctane.…”

“…[4] Because each country has different cultural traditions of cooking and dining, recommended levels vary from 24% TPM in France, Germany, and the USA to 30% TPM in Hungary. [7,8] Song et al [8] confirmed that TPMs were RSOLs and reacted mainly with DPPH in isooctane, but not with DPPH in methanol. [6] A modified method using 2,2-diphenyl-1-picrylhydrazyl (DPPH) in isooctane and in methanol has been introduced to monitor the changes in free radical scavenging antioxidants (FRSs) and radical scavenging compounds from oxidized lipids (RSOLs) during lipid oxidation.…”