## 3 Results and Discussion
[10] describes that alterujo and orujo, when stored under uncontrolled environmental conditions over time, generate VOCs that increase their concentration as a result of the chemical and biological transformations that occur in these residues. Aldehydes, in particular hexanal and octanal, increased their concentration over time for both alterujo, as a direct influence of oxidative processes of the decomposition reactions of the hydroperoxides formed by the slow self-oxidation of oleic acid as well as the self-oxidation of lipoxygenase through the formation of hydroperoxide precursors 13-LOOH (linoleic acid hydroperoxide) [28; 29]. It is important to note that the thermal degradation of the polysaccharide molecules has an impact on the compounds presented in the emissions. A number of compounds comes from the thermal drying itself but also an important group of compounds comes from the aforementioned degradation, in particular, acetic acid, acetaldehyde, furane and so forth. Interestingly, octane was the only VOC that showed a decreasing trend both in alperujo and orujo. [PERSON] et al. [29] described that this compound is recognized as a marker of oxidation in oils since it  
Figure 3: Monthly concentration of target VOCs found in (**a**) alperujo and (**b**) orujo drying emissions. In these graphs, bars of the groups represent each month where the experimentation was carried out between July to December from left to right. appears in high concentrations at the beginning of the process. In addition, [PERSON] et al. [28] indicated that octane is formed from hydroperoxide precursors 10-OOOH (oleic acid hydroperoxide) [28]. Thus, octane degradation can be a consequence of oxidative deterioration of oleic acid over time as described by [PERSON] et al. [30]. In a 33-hour experiment, it was observed that the main volatile decomposition products found in oxidized olive oils were produced from monohydroperoxides of oleic, linoleic and linoleic unsaturated fatty acids that decreased their concentration over time, that is, in 98.2% at 0 h, 80.8% at 21 h and then 49.2% at 33 h. Considering that the odorants are emitted from open reservoirs during waste storage and that initial humidity of wastes is almost constant during the 3 rd and4 th months (Table 1), results indicate that a proper alternative to reduce the impact of exhaust gases from the oil mill waste drying from an emissions perspective is to process them as soon as they are produced to minimize the overall emission of odorants from the storage and drying. Finally, the compounds found in the outlet of the trommel may be considered similar to the original species before the drying [31] and hence similar to the species found by [PERSON] et al. in previous work [3]. Odor concentration measured in the gaseous emissions of a treatment process can be complemented with the individual odor impact values, which can help in the identification of the major odor contributors found in a complex odorous gas mixture. Table 6 shows the odor impact value (OIV) per each VOC, which is obtained by dividing the average concentrations of each VOC quantified by its odor threshold according to literature data (Table 4). Results show that aldehydes and aromatic compounds, mainly phenol and octanal, are the ones that present the highest OIV, implying that these families of compounds are the precursors of the odors when the alperujo and orujo are dried in the drying trommel over time. A study conducted by [PERSON] et al. [24] shows that the high concentrations of aldehydes in olive oils (rancid, fusty, winey-vinegary, mustiness-humidity) are mainly produced by the oxidation of unsaturated fatty acids, while the presence of acids that appear in the final part of the oxidative process are the product of the oxidation of the previously formed aldehydes.