## 3 Results and Discussion
### Relationship between Volatile Compounds, Consumer Scores, and Compositional Analyses
(**b**) External preference map of roasted beef samples only (PC2 vs. PC3). Abbreviations: see see Figure 3a. The relationship of the volatile compounds with consumer scores and physicochemical characteristics will be considered by compound class:  
n-aldehydes  
Surprisingly, the significantly higher levels of pentanal and hexanal in the stewed beef were not mirrored by heptanal, octanal, and nonanal, which showed no significant differences (Table 3). The formation of all these n-aldehydes involves the thermal and oxidative degradation of lipids [45]. Hexanal and pentanal are located at a distance from the consumer liking scores on PC1 for stewed and roasted beef (Figures 4a and 5a). This is consistent with the fact that hexanal and pentanal flavors are perceived as unpleasant, rancid, and similar to the aroma of green leaves or vegetables [46]. [PERSON] [47] noted that pentanal concentration was positively correlated with the intensity of off-flavor in grilled ground beef. Heptanal, octanal, and nonanal, derived from the oxidation of linoleic acid, tend to have green, fatty aromas but not especially low thresholds. However, they have been shown to be associated with other products of this pathway such as 2,4-decadienal, 1-octen-3-ol [28,48], and with off-flavors believed to arise from minor volatile components of the same pathway [48]. Thus, these n-aldehydes generally act as characteristic markers for a range of odor compounds and less desirable flavors. Three volatile Strecker aldehydes were reported as follows: 3-methylbutanal, 2-methylbutanal, and benzaldehyde, though this last may also be formed by other pathways. In the case of benzaldehyde, statistically significantly higher levels (\(p<0.05\)) were observed in the stewed than in the roasted beef. In previous studies, a relationship between Strecker aldehydes and acceptability scores for grilled beef was observed [24]. Both 3-methylbutanal and 2-methylbutanal were located to the right of Figure 3b, in the same direction as flavor liking and overall liking, suggesting that a relationship between these compounds and desirable flavor occurs, albeit weakly. A comparison of Figures 4 and 5 confirms that this relationship is only evident for the stewed and not the roasted beef. As mentioned above, this is likely due to the sampling method for the roast beef in which the high temperature-treated surface layers, where the formation of Strecker aldehydes would be favored, were discarded. Thus, these data show that Strecker aldehydes are good markers for flavor liking for grilled and stewed meat, where browning has occurred, as reported previously for grilled steaks [24,28], but not for the center meat of roasted cuts. Additional marker compounds need to be identified for this meat. Ketones  
Heptan-2-one and heptan-3-one were located at the opposite sides of the preference maps to the liking scores for both the stewed and roasted meat (Figures 4 and 5). These compounds were not significantly affected by either the muscle or cooking method but may be differentiating different flavor qualities. These compounds are formed from lipid oxidation pathways and, while they have relatively high odor thresholds and are unlikely to contribute substantially to meat flavor [39], they may be acting as markers for other oxidation products conferring undesirable flavors. 2,3-butanedione does not show any consistent relationship with sensory scores. Sulfur compounds possess low odor thresholds [48] and are important contributors to the flavor profile of cooked beef [49]. Dimethyl disulfide and dimethyl trisulfide were associated with a dislike of flavor by the consumers in Figure 4a,b and Figure 5a. This effect was considerably greater in the stewed meat, presumably due to the very much higher concentrations of these compounds from this cooking method. Dimethyl trisulfide (cabbage-like/sour) and dimethyl disulfide (hay-like) play an important role in meaty and fatty flavor [48].