Determination of primary and secondary oxidation products in vegetable oils with gold nanoparticle based fluorometric turn-on nanosensor: A new total oxidation value

Abstract

Oxidation of fats and oils gives rise to lipid hydroperoxides as primary oxidation products (POPs) and carbonyl compounds as secondary oxidation products (SOPs). Here, we report the sensitive detection of POPs and SOPs (oxidation products) involving direct and indirect redox reactions with iodide ions, respectively, using the fluorophore probe (fluorescein-5-isothiocyanate-gold nanoparticles; FITC-AuNPs). Iodide released the fluorescent FITC from FITC-AuNPs in the turn-on mode. In case of SOPs, an indirect iodide determination was made through the use of chloramine-T as carbonyl reagent. To various vegetable oil samples, deep frying process (at 160 °C, simulating frying conditions) and soft oil oxidation (at 100 °C, simulating cooking conditions) were carried out for a period of time (1, 5 and 10 days). Total content of POPs varied between 4.00 and 5.70 g TBHP (t-butylhydroperoxide)/kg oil, and the amount of SOPs was found as 1.12–4.11 g t-2-nonenal/kg oil in non-oxidized oil samples. The proposed method was validated against FOX and p-anisidine value (p-AV) assays. Sensory analyses (taste, color, smell and appearance) results of non-oxidized and deep-fried oil samples were consistent with those of the proposed FITC-AuNP nanosensor. Finally, a new total oxidation (TOD) value (expressed as iodide equivalent meq/kg oil) was proposed to determine the overall oxidation state and quality of the oils. Our proposed TOD value is not only mathematically precise with respect to its units (i.e. POPs and SOPs are added up in the same equivalent units to obtain TOD) but also is in better accordance with the results of sensory analysis.