## 4 Discussion
### Volatile Flavor Compounds  
Lamb meat is very popular with many consumers because of its unique flavor. The flavor of lamb meat depends on the chemical composition of volatile (aroma) and nonvolatile (taste) compounds, and the quantity and balance of flavor molecules are critical to the acceptability of meat flavor [61]. Volatile compounds such as aldehydes, alcohols, ketones, phenols and others have been implicated as responsible for or contributing to lamb meat flavor [62; 63]. Factors affecting volatile flavor compounds mainly include the genetics, age, sex, and cooking methods, especially diet [64]. A number of researchers have studied the volatile differences between pasture- and grain-based feeding systems, and a considerable number of volatile compounds were identified and reported associated with lamb meat flavor or can be used to discriminate between animals from different feeding systems [20; 61; 65; 66]. In the current study, 28 volatile compounds were identified, and the odor descriptors were described according to previous studies [5; 67; 68]. Aldehydes were the highest percentage of volatile compounds and indicated a relatively lower odor threshold, which were considered as a critical composition of volatile flavor [69]. The indoor feeding lambs had a higher concentration of saturated aliphatic aldehydes such as pentanal, hexanal, heptanal, octanal, and benzaldehyde, but the concentration decreased with the combination of grass feeding and concentrate feeding. Higher concentration of these aldehydes may increase the undesirable odor production [21]. The alcohols were also an important member of volatile flavor with a higher odor threshold than aldehydes and relatively lower contribution to flavor [70]. Alcohols such as 1-octene-3-ol (green, mushroom, earthy, oily) and 2-octenol (green, fatty, citrus) decreased with grazing time limit in this study, which contributed to the lamb meat characteristics. Ketones have an obvious milk or fruit flavor with a relatively higher odor threshold but were not found in the indoor feeding lambs. Moreover, esters indicated limited contribution to meat flavor because of the relatively higher odor threshold and were not detected in the G2 and G0 groups. Additionally, most of the heterocyclic compounds were derived from complex reaction processes, such as lipid oxidation and Maillard reaction [71]. 2-pentylfuran which has a roast and buttery aroma showed a higher concentration in lamb meat from pure grazing or indoor feeding than in pasture combined with concentrate feeding. These results suggested that the categories of volatile decreased with the limited access of lambs to pasture grass, but the quantity of aldehydes and heterocyclic compounds increased in the indoor feeding lambs. However, the influence of feeding systems on volatile compounds were inconsistent among different studies. Some studies reported that a concentrate-based diet could increase the levels of aldehydes, ketones, and lactones and provide a more intense mutton aroma [72; 73]. On the other hand, the increased content of aldehydes, ketones, phenols, and indoles were reported based on the grazing system by previously research [65; 74]. Moreover, no differences in lamb meat flavor attributes (taste and odor/aroma) were found when the lambs were fed with different diets [75]. Additionally, the different extraction methods used in these studies also indicated a huge influence on the volatiles that were recovered as each technique has a bias towards certain chemical classes. The mechanisms by which the type of feeding systems or diets can affect the lamb meat flavor are complex and multiple. In its simplest form, dietary composition may affect the final flavor of lamb meat by direct transfer of specific plant-derived compounds into the meat or they may undergo degradation during thermal processing to form new flavor-active compounds [68].