## 3 Results and Discussion
### AHC Analysis of Aldehyde LOP Variables  
Agglomerative hierarchal clustering (AHC) analysis of only the three major aldehyde LOP variables (Figure 3) indicated that total _trans_-2-alkenal and alka-(_trans_,_trans_)-2,4-dienals were correlated; these aldehydes are derived from the peroxidation of PUFA (linoleoyl- and linolenoylglycerols), the latter from these acylglycerol FA sources alone. Additionally, 2-heptenal isomers arise from the degradation of alka-2,4-decadienals, together with acetaldehyde, hexanal, acrolein, butenal, 2-heptenal and 2-octenal, amongst other aldehydes [55]. This AHC analysis also indicated that these correlated aldehyde concentrations were distinguishable from that of total _n_-alkanals (data not shown), which are known to arise from the peroxidation of MUFAs as well as PUFAs. However, the substitutional status of linoleate, linolenate and perhaps further PUFAs at the glycerol backbone of triacylglycerlycerols [i.e., _sn_-1(3) and/or -2 positions] is also a key determinant of the nature and concentrations of secondary aldehyde LOPs arising from their autoxidation [17]. However, performance of a related AHC analysis of all seven aldehyde LOP variables provided further details regarding the clustering of aldehyde LOPs (Figure 4). Indeed, two major clusters were notable, the first (cluster A) containing more structurally complex aldehyde classes (4,5-epoxy-_trans_- and 4-hydroxy-/4-hydroxy-_trans_-2-alkenals, along with alka-(_cis,trans_)- and alka-(_trans_,_trans_)-2,4-dienals), while the second comprised more structurally simpler aldehydes, specifically _cis-_ and _trans_-2-alkenals, and _n_-alkanals (cluster B). From the dissimilarity truncation limit indicated (dotted line in Figure 4), the  
Figure 3: AHC monoclustering heatmap diagram displaying, from left to right, increasing concentrations of the three major aldehyde LOPs (_n_-alkanals, _trans_-2-alkenals and alka-_trans_,_trans_-2,4-dienals) as a function of LSSFE time-point for canola (red), coconut (blue), extra-virgin (green) and sunflower oils (yellow); there were n = 6 replicate samples analysed per time-point. The dataset was glog-transformed and autoscaled prior to analysis. Transformed aldehyde concentrations are shown in the right-hand side _y_-axis: deep blue and red colourations represent extremes of low and high levels, respectively. The left-hand side of the plot shows results arising from a preliminary AHC analysis of these aldehyde LOP variables monitored. Abbreviation: (_E_,_E_)-2,4-alkadien represents alka-(_trans_,_trans_)-2,4-dienals. second major cluster was split into two statistically significant sub-clusters (B1 and B2), the first containing _cis_-2-alkenals alone, the second a _trans_-2-alkenal and _n_-alkanal combination. Although below the dissimilarity threshold value and therefore not significant, cluster A was further split into two sub-clusters, the first (A1) featuring a combination of 4.5-epoxy-_trans_-2-alkenals and 4-hydroxy-/4-hydroxy-_trans_-2-alkenals, the second (A2) both alka-(_cis,trans_)- and alka-(_trans,trans_)-2,4-dienals. The more complex aldehyde LOPs present in cluster A, which arise from PUFA and not MUFA peroxidation, include 4-hydroxy-_trans_-2-alkenals generated from reduction of their corresponding 4-hyperoxy- derivatives in linoleoylglycerol-rich oils, and 4.5-epoxy-_trans_-2-alkanals such as 4.5-epoxy-_trans_-2-decneal, which is known to be derived from alka-(_trans,trans_)-2,4-decadienal [13; 22]; the latter dienal arises from fragmentation of linoleate's 9-hydroperoxide [14; 15; 16; 17; 18]. Moreover, it is also conceivable that some alka-(_cis,trans_)-2,4-dienals may at least partially be formed from the thermally induced isomerism of their corresponding _trans,trans_-isomers, and this may explain the co-clustering of these two \(\alpha,\beta\)-unsaturated aldehydes (sub-cluster A2) [13].