## 2 Results and Discussion  
The headspace solid phase microextraction (SPME) followed by gas chromatography-mass spectrometry (GC-MS) analysis of pork cheeks sous-vide cooked at different time \(\times\) temperature \(\times\) vacuum combinations resulted in more than 100 volatile organic compounds (VOCs). This VOCs profile was very similar to that previously described for cooked meat [6,7]. The most representative compounds formed from lipids, amino acids and thiamine were selected, and their average levels (area under the peak curve) in each batch are shown in Table 1.  
Selected lipid oxidation VOCs were linear saturated aldehydes (from butanal to nonanal), unsaturated aldehydes (2-decenal and 2,4-decadienal), a ketone (2-octanone), and several furans and furanones (2-methylfuran, 2-ethylfuran, 2-butylfuran, 2-pentylfuran and 5-pentyl-2(5_H_)-furanone). All these compounds have been previously detected in the headspace VOC profile in different types of cooked meats [6,7]. As a general rule, the VOCs derived from fatty acid degradation had a similar trend as affected by time and temperature cooking conditions, with the highest levels in samples cooked at 80 \({}^{\circ}\)C during 5 h and the lowest in samples cooked at 80 \({}^{\circ}\)C during 12 h. Nevertheless, heptanal, octanal, 2-octanone and 2-methylfuran did not follow such a tendency. These results suggest a promoting effect of heating on the degradation of polyunsaturated fatty acids such as linoleic acid, increasing also the amount of free radicals capable of attacking other fatty acids less susceptible to oxidation, such as oleic acid, leading to the formation of other VOCs like heptanal, octanal, 2-octanone and nonanal in a more advance stage of the lipid oxidation process [11]. Some of these carbonyl compounds derived from fatty acid oxidation may also react with the amine groups of lysine, cysteine and glutathione [12] and thus, the decrease in their concentration at longer cooking times at higher temperatures might suggest the implication of these compounds on the formation of other VOCs [12,13]. Accordingly, the greatest decrease in the detected levels from 5 to 12 h in samples cooked at 80 \({}^{\circ}\)C was found for 2-decenal and 2,4-decadienal, which are known to be more susceptible to further reactions than long chain saturated aldehydes such as octanal [12,13]. In fact, previous studies have shown the reaction of 2,4-decadienal with phenylalanine in model systems through further oxidation of the alkaladienal to epoxydecenal [14]. This type of epoxyalkenals has been shown to induce Strecker degradation of amino acids in a similar way to dicarbonyls [15] (Scheme 1). Selected VOCs derived from degradation of amino acids and/or thiamine, included carbon disulfide, 3-methylbutanal, 1-hydroxy-2-propanone, dimethyl disulfide, 1-(methylthio)-propene, 2-methyl-thiophene, 2-acetyl-2-thiazol, 2-pentylthiophene and benzothiazole. Contrarily to lipid derived VOCs, some compounds originated through Maillard reactions and/or thiamine degradation, such as 1-hydroxy-2-propanone, dimethyl disulfide, 1-(methylthio)-propene and 2-methylthiophene, were found in samples cooked at 80 \({}^{\circ}\)C, while they did not reach detectable levels in samples cooked at 60 \({}^{\circ}\)C. Additionally, 3-methylbutanal, an aldehyde formed as a consequence of the Strecker degradation of leucine (Scheme 1), was found at higher concentrations in the samples cooked at 80 \({}^{\circ}\)C than in those cooked at 60 \({}^{\circ}\)C (Table 1). A similar trend was found in chicken meat cooked to different end-point temperatures [16], although in this latter study samples were cooked for very short times. Therefore, it seems clear that sous-vide cooking of pork cheeks at higher temperature showed a positive influence on the development of volatile compounds from Maillard reactions, and more specifically, from Strecker degradation of amino acids.