## 3 Results and Discussion  
### Hr \({}^{1}\)H NMR Case Study: Time-Dependent Tracking of Aldehyde LOPs in CFOs Exposed to LSSFEs  
In the first phase of the current study, the wide range of analytical advantages offered by high-resolution \({}^{1}\)H NMR technologies for the analysis of LOPs in used or unused frying oils is highlighted: this is presented as a case study involving the heating of a series of four CFO products with differing acylglycerol contents and degrees of unsaturation, according to laboratory-simulated shallow frying episodes (LSSFEs) conducted at a recommended frying temperature (180 \({}^{\circ}\)C). The levels of individual aldehyde classifications, and total aldehydes, formed in CFOs exposed to the LSSFEs described increased with increasing oil PUFA contents, and also in a sigmoidal time-dependent fashion. For sunflower and canola oils, \(>\)80% of these were of the more highly toxic \(\upalpha\),\(\upbeta\)-unsaturated class, which included acrolein, _trans_-2-alkenals [(_E_)-2-alkenals], alka-_trans,trans_- and alka-(_cis,trans_)-2,4-dienals [alka-(_E_,_E_)- and alka-(_Z_,_E_)-2,4-dienals, respectively], together with combined 4-hydroperoxy/4-hydroxy-, and 4,5-epoxy-_trans_-2-alkenals (Figures 1 and 2). Moreover, relatively low concentrations of these aldehydes and their CHPD precursors were also detectable in newly purchased,unheated corn and sunflower oil products (Figure 1b,c, respectively). Primary lag-phases for aldehyde generation, which increased with increasing SFA and MUFA, and decreasing oil PUFA contents, also decreased with increasing peroxididative susceptibilities of the oils tested, as expected [53], which were calculated to be 62.6, 38.5, 12.0 and only 2.0 for the sunflower, canola, extra-virgin olive and coconut oil products tested, respectively. An intermediate lag-phase was also observed at a heating time-point of ca. 20-30 min for MUFA-rich oils, particularly canola oil. For PUFA-rich sunflower oil, which generated the highest \({}^{1}\)H NMR-detectable levels of both \(\alpha\),\(\beta\)-unsaturated and saturated aldehydes, the maximal mean concentrations of _trans_-2-alkenals, alka-(_trans_,_trans_)-2,4-dienals and \(n\)-alkanals remaining in this frying medium at the extreme 90 min heating time-point were ca. 23, 11 and 10 mmol kg\({}^{-1}\); respectively: total mean unsaturated and saturated aldehyde levels determined in this heated oil were 47.1 and 10.0 mmol kg\({}^{-1}\), respectively. Corresponding mean (\(n=6\)) 90 min time-point estimates of total unsaturated and saturated aldehydes in the other oils tested were 39.9 and 8.2 (canola oil), 26.1 and 6.9 (extra-virgin olive oil), and only 7.5 and 2.6 mmol kg\({}^{-1}\) (coconut oil), respectively. Of particular interest is the \({}^{1}\)H NMR detection of HNE and HHE in thermally stressed culinary oils--these neurotoxic and potentially carcinogenic secondary LOPs are derived from chemical reduction of their corresponding 4-hydroperoxy-_trans_-2-alkenal precursors. However, since they predominantly arise from the peroxidation of linoleicoyl- and linolenoyl-glycerols, respectively [22], little or none of them are detectable in thermally stressed olive oil products (or other PUFA-deplete oils), since such frying media contain only 5-10% and \(\leq 1\%\) (\(w/w\)) of these triacylglycerglycerol species, respectively. This also serves to explain why much lower levels of similarly toxic alka-(_trans_,_trans_)-2,4-dienals are generated in such MUFA-rich oils when exposed to thermal stressing episodes; this class of \(\alpha\),\(\beta\)-unsaturated aldehydes is only generated from PUFA-derived CHPD species, predominantly decad _trans_,_trans_-2,4-dienal from linoleate hydroperoxide sources [11; 12; 19; 20; 22].