## 3 Results and Discussion
### Effect of Breed and Finishing Diet on Volatile Compounds
#### 3.2.5 Ketones  
As shown in Table 4, six ketones were detected. As regards the breed effect, significant differences were observed among BU and JN foals. Concretely, total ketones were shown to be significantly the most abundant (\(p<0.01\)) in the BU group compared with the JN one (21.86 _vs._ 18.90 AU \(\times\) 10\({}^{4}\)/g fresh meat, respectively). These dissimilarities could be related to the fact that some ketones, including certain lactones, have a metabolic origin [56], and that their synthesis can be influenced by breed [71]. Moreover, our outcomes were shown to be largely lower than the amounts detected in GM \(\times\) HB and GM foals [25; 26; 27]. On the other hand, considering the individual ketones, out of six volatile compounds identified, only two substances (2,3-pentanedione and butylactone) were significantly affected by breed (\(p<0.05\)). Among them, 2,3-pentanedione represented the predominant ketone in BU foals, showing the highest amounts (\(p<0.001\)), whereas butylactone showed greater (\(p<0.05\)) amounts in JN foals. The presence of higher quantities of butylactone in JN could be justified by its significantly higher levels of C18:2_n_-6 (Table 2), since recent studies found that the lipid oxidation of this fatty acid can generate this type of lactone [64]. On the whole, this family of compounds corresponded to small percentages in relation to the total volatile compounds content, being the seventh most abundant group for both breeds (Figure 1). As can be seen, the type of finishing diet enormously affected both the total ketones content and the individual amounts, except for 2,3-pentanedione. Indeed, D1 foals showed higher (\(p<0.001\)) total ketones content than the D2 group (22.80 _vs._ 17.67 AU \(\times\) 10\({}^{4}\)/g fresh meat, respectively). On the other hand, a similar trend was found in recent studies [54; 64], where authors observed that animals finished with commercial concentrates reported higher values than those reared under an extensive production system. Similarly, all singular ketones showed higher values in the D1 group in comparison with the D2 one (\(p<0.05\)), apart from 2,3-pentanedione. Among them, acetoin represented the most plentiful compounds in D1 foals, corresponding to 31.24% of the total ketone content. According to several studies [72; 73; 74], the production of acetoin could be related to the microbial carbohydrate metabolism. Moreover, this volatile compound has a very low odor threshold, being easily perceived by olfactory analyzers, and is characterized by a buttery, cream-like and sweet aroma [75]. Hence, the presence of different contents could affect and generate modifications at several levels in the aromatic profile of the meat [76]. However, this ketone was not detected in previous studies [25; 27], investigating the volatile profile of raw foal meat from animals fattened with concentrates; whereas, it was found in fresh lamb meat reared under intensive and extensive systems, but their values were hugely higher than ours [54]. In addition, D1 foals, as commented, showed higher contents of two ketones correlated to lipid degradation, 2-heptanone and butylolactone [64; 77; 78; 79]. In particular, some authors [77] reported that 2-heptanone could be originated by the oxidation of linoleic acid (C18:2\(n\)-6). Recent studies [54; 80; 81], in fact, observed a high presence of this ketone in concentrated-fed ruminants and related this outcome to the elevated content of C18:2\(n\)-6 in concentrate feeds. For this reason, this volatile compound is defined characteristic of animals fed on commercial concentrates [54; 76]. In our study, D1 and D2 groups reported similar values of this fatty acid (\(p>0.05\)) (Table 2), probably due its high contents in the concentrates employed both in D1 and D2 (Table 1). Thus, this ketone cannot be considered a direct and appropriate biomarker of the D1 group.