## 4 Discussion

Table 5: Effect of aging time on Thiobarbituric Acid Reactive Substances (TBARs), hydroperoxides, protein carbonyls, superoxide dismutase, catalase, and glutathione peroxidase of donkey meat steaks aged for 14 days under vacuum conditions (\(n=10\) samples for each aging time). However, all of these VOCs are poorly represented in our study. [PERSON], et al. [53] reported that toluene and ethylbenzene, the last one significantly decreased by ageing in our work, were most likely derived from amino acid degradation than lipid oxidation. The most represented aromatic hydrocarbon was the furan, 2-pentyl-, showing more than 85% of the total aromatic hydrocarbons, and it probably affects the statistical differences of the entire VOCs family. In similar studies that were conducted on horse meat vacuum aged, [PERSON], et al. [54] and [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [25] observed that VOCs formation and trend changed depending on muscle considered. In fact, vacuum aging in _Semimembranosus_ muscle only affected toluene, whereas in _Longissimus thoracis_ muscle the vacuum aging modified the benzene and 3-carene content. But more interesting is the total absence of the furan, 2-penthyl- that is, instead, the most produced in _Longissimus thoracis_ donkey meat. As said before, usually aromatic hydrocarbons derived from the lipid oxidation [49; 50], but we observed a decreasing trend of total aromatic hydrocarbons VOCs. First of all, the lipid oxidation would be slightly depressed under vacuum aging [45], probably also for the low permeability of the film, and this can explain why aromatic hydrocarbons did not increase during the aging process. The results reported about oxidation products, as TBARs and hydroperoxides, strengthened this hypothesis. They remain constant during the aging period, as reported in horse meat [25], confirming that probably lipid oxidation is slightly depressed for both the short period and the packaging type [45; 55], also considering the film low permeability. The total amount of aldehydes was not significantly affected by ageing. On the contrary, the values of many aldehydes individually changed, following a heterogeneous trend. First of all, we must consider the limited number of animals that are involved in the trial. This aspect may have influenced the large variability of results with no statistical differences. However, the hexanal that represented about the 85% of the total aldehydes and also the most abundant VOC generate did not show any change during the vacuum aging. This result agrees with studies that were conducted on horse meat vacuum packaged [25; 54], but also with other studies conducted on same species (horse) in different storage conditions [56] and in different species and storage conditions as beef [57], pork [58], and lamb [59], in which researchers reported that hexanal is the most abundant VOC. Hexanal, which gives the meaty, grassy, and fatty odours to the meat, as the most of aldehydes, derived from phospholipids and polyunsaturated fatty acids oxidation [46; 60; 61], and the constant trend seems to validate the hypothesis of a reduced oxidative activity in meat vacuum stored. The authors reported a positive correlation between aldehydes formation and degree of oxidation, particularly with TBARs [62], and the absence of variation of both oxidative parameters and aldehydes seems to consolidate this hypothesis. Linear hydrocarbons identified are the most numerous, although some authors pointed out their unimportant role in meat flavour [49], but not the most abundant, differently from that found by some authors in beef meat [45]. As observed in horse meat [25; 54], the heptane,2,2,4,6,6-pentamethyl- and the undecane are the most abundant, representing around 20% and 10% of total linear hydrocarbons in cooked donkey meat, respectively.