Nuclear Magnetic Resonance Spectroscopic Analysis of the Evolution of Peroxidation Products Arising from Culinary Oils Exposed to Thermal Oxidation: An Investigation Employing 1H and 1H-1H COSY and TOCSY Techniques

Abstract

Scientific warnings on the deleterious health effects exerted by dietary lipid oxidation products (LOPs) present in thermally stressed culinary oils have, to date, not received adequate attention given that there has been an increase in the use and consumption of such oil products in everyday life. In this study, high-resolution 1H nuclear magnetic resonance (NMR) analysis was used to characterize and map chemical modifications to fatty acid (FA) acyl groups and the evolution of LOPs in saturated fatty acid (SFA)-rich ghee, monounsaturated fatty acid (MUFA)-rich groundnut, extra virgin olive, and macadamia oils, along with polyunsaturated fatty acid (PUFA)-rich sesame, corn and walnut oils, which were all thermally stressed at 180 °C, continuously and discontinuously for 300 and 480 min, respectively. Results acquired revealed that PUFA-rich culinary oils were more susceptible to thermo-oxidative stress than the others tested, as expected. However, ghee and macadamia oil both generated only low levels of toxic LOPs, and these results demonstrated a striking similarity. Furthermore, at the 120 min thermo-oxidation time-point, the discontinuous thermo-oxidation episodes produced higher concentrations of aldehydic LOPs than those produced during continuous thermo-oxidation sessions for the same duration. On completion of the thermo-oxidation period, a higher level of triacylglycerol chain degradation, and hence, higher concentrations of aldehydes, were registered in culinary oils thermally stressed continuously over those found in discontinuous thermo-oxidized oils. These findings may be crucial in setting targets and developing scientific methods for the suppression of LOPs in thermo-oxidized oils.

Ultrasound-Assisted Extraction of Blackberry Seed Oil: Optimization and Oil Characterization

“…All the spectral data confirm the fatty acid profile of the BB seed oil, determined chromatographically. In addition, upon examining the regions at 9.41–10.10 ppm (corresponding to protons from aldehyde groups), 8.00–8.50 ppm (corresponding to the resonances from the hydroperoxide functions), 3.85–3.89 ppm (alcohols, resulted from hydroperoxides breakdown), and 2.40–3.60 ppm (assigned to the protons from the oxirane rings of epoxides) [ 62 ], the 1 H-NMR spectra clearly show that no primary (hydroperoxides) or secondary (such as epoxy fatty acids, alcohols, and aldehydes) oxidation products exist in the BB seed oils (neither obtained by Soxhlet nor by UAE) in detectable amounts, indicating that the ultrasound treatment had not initiated peroxidation processes. The absence of significant oxidative processes in both the cases of Soxhlet- and UAE-obtained BB seed oil was confirmed by the low peroxide values of 4.23 and 4.31 meq active oxygen/kg oil, respectively ( Table 3 ).…”

High-resolution 1H NMR analysis of continuous and discontinuous thermo-oxidative susceptibility of culinary oils during frying at 180℃

“…Notwithstanding, the susceptibility of PUFA-rich culinary oils to oxidation at room and storage temperatures [ 5 ], along with variable cooking temperatures of 70 and 100 °C [ 6 ], 180 °C [ 7 – 12 ], 160 °C [ 13 ], 170 °C [ 14 ], 190 °C [ 15 – 17 ], and 250 °C [ 7 , 18 ] will clearly negate any possible health benefits offered by PUFA-rich culinary oils in view of. Even so, lipid oxidation products (LOPs), which are chemical oxidation and fragmentation products derived from the oxidation of monounsaturated fatty acid (MUFA)-rich and PUFA-rich culinary oils, have been shown to also be detectable in unheated culinary oils [ 9 , 10 , 19 ], albeit at much lower levels than those found in the thermally stressed products.…”

“…As part of the creation of a directory for the thermo-oxidative dynamics of several culinary oils of varying unsaturation degree, the current study is built on previous investigations conducted on the continuous and discontinuous thermo-oxidation of coconut, olive, rapeseed, and sunflower oils by [ 9 ]; which was the first scientific report on this subject, and also subsequently on ghee, groundnut, extra virgin olive, macadamia, sesame, corn, and walnut oils [ 12 ]. According to both studies, susceptibilities of the culinary oils to thermo-oxidation, as manifested by chemical changes to triacylglycerol acyl chains and the evolution of LOPs following a 300 min (continuous) and 480 min (discontinuous) thermo-oxidation duration, were in order of PUFA-rich sunflower > MUFA-rich olive and rapeseed > saturated fatty acid (SFA)-rich coconut oils [ 9 ], and PUFA-rich sesame, corn, walnut > MUFA-rich groundnut, extra virgin olive, macadamia > SFA-rich ghee oils [ 12 ]. However, at the end of 120 min thermo-oxidation duration, the discontinuous episodes led to higher levels of aldehydic LOPs than those reported for continuous thermo-oxidation in both studies [ 9 , 12 ].…”

“…According to both studies, susceptibilities of the culinary oils to thermo-oxidation, as manifested by chemical changes to triacylglycerol acyl chains and the evolution of LOPs following a 300 min (continuous) and 480 min (discontinuous) thermo-oxidation duration, were in order of PUFA-rich sunflower > MUFA-rich olive and rapeseed > saturated fatty acid (SFA)-rich coconut oils [ 9 ], and PUFA-rich sesame, corn, walnut > MUFA-rich groundnut, extra virgin olive, macadamia > SFA-rich ghee oils [ 12 ]. However, at the end of 120 min thermo-oxidation duration, the discontinuous episodes led to higher levels of aldehydic LOPs than those reported for continuous thermo-oxidation in both studies [ 9 , 12 ].…”

“…On reconsideration of the scientific conclusions drawn by [ 9 , 12 ] for the extensive varying degrees of unsaturation status of the culinary oils studied, this report aims to investigate the chemical changes to the acyl chains of fatty acids, along with the resultant evolution of LOPs in continuous and discontinuous thermo-oxidised coconut, rice bran, rapeseed, cold press rapeseed, grape seed, and hemp seed oils. Furthermore, the research questions originally outlined in [ 9 ] have also now been addressed in the current study by the use of culinary oils that were not investigated by [ 9 , 12 ]. It should also be noted that the coconut and rapeseed oil brands that were reported by [ 9 ] were different to those investigated in this study.…”