## 4 Discussion
Although it seems that linear hydrocarbons also derived by autoxidation processes, particularly of long-chain fatty acids, are principally formed after lipids thermal homolyses [63], and therefore after heat treatments, as cooking. Only two linear hydrocarbons showed changes during aging time, pentane, and 2-octene, together representing less than 1% of total linear hydrocarbons, but the total hydrocarbons did not modify during the whole display. Our results can confirm the idea that oxidative processes are strongly reduced, probably due to the low oxygen permeability of the used film. This study highlighted significant differences in three ketones: 2,3-pentanedione; 2-hexanone; and, 2-methyl-3-octanone, representing around 2% of the total ketones. In fact, the total ketones did not change during the aging process. Ketones are considered very important in giving flavour on cooked meat, in fact it is reported that are responsible of sweet, buttery, spicy, ethereal, caramel, cheese and fatty notes [49; 54; 64]. The main substrate responsible of their formation is the intramuscular fat, deriving from fatty acids oxidation, and their formation and production are positively correlated with intramuscular fat content [65]. However, this is not the only route of formation, because they can also derive from Maillard compounds [66] and from lipolytic activity and alkane degradation by microbial metabolism (\(\beta\)-oxidation) [67; 68]. The very probable lack of oxidative processes on intramuscular fat and fatty acids and, consequently, the probable inactivation of microbial activity and the equal cooking method adopted, can easily explain the absence of variation of total ketones compounds during the aging time. However, is really interesting to observe that ketones formed in donkey meat are more abundant than those that are reported in horse meat vacuum packed [25; 54], more than triple, since the acetoin was the most abundant ketones and about four times more than those found in vacuum packed horse meat. The total alcohols showed a constant trend, and 1-hexanol was the most abundant. Although other authors observed in horse meat that this compound is the most abundant [25; 54], they observed a different trend during the vacuum aging, showing increasing values. These compounds also derive from lipid oxidation, particularly from the degradation of oleic and linoleic acid [69], and their liberation depends on thermal treatment [70]. The absence of variation can be due to a probable inhibition of the oxidation processes during the vacuum aging. These compounds seem to give flavour described as resin, flower, and green aroma [51; 56], but they also have a high odour threshold limit, so they are often classified as not important flavour contributors to meat [70]. Carboxylic acids and sulphur compounds did not show changes and were relatively poor generated, unlike what was observed for nitrogen compounds that tend to increase during the vacuum aging. Nitrogen compounds derived by the interaction of sulphur-free amino acids with sugars lead to the formation of these compounds, like e.g., pyrazines [71]. These compounds are particularly important, because of their lower limit aroma threshold and the garlic and onion aroma that they can give to meat. Our results agree with those observed in beef meat during the vacuum aging [64], since the authors noticed an increase in nitrogen compounds, justifying it with a plausible release of free amino acids during the aging process that enhance, in turn, Maillard reactions. This agrees with the protein carbonyls trend that is the result of protein oxidation, which, in turn, is responsible for the greater availability of free amino acids. Similar to that reported in horse meat [25], antioxidant enzymes activity tend to increase during aging process. Superoxide dismutase, catalase and glutathione peroxidase represented the _in vivo_ cell defence system against oxidative damage [72].