## 3 Results and Discussion
### Effect of Storage on Fatty Acids and Volatile Composition of Negroamaro GPF
While, the defined mechanism underlying the increase in esters is unknown [34], so it could be hypothesized that it results from oxidative breakdown of unsaturated fatty acids [35]. The aldehyde content varied from 11.13% at T0 to 15.94 and 20.93% in T6_4 \({}^{\circ}\)C and T6_25 \({}^{\circ}\)C samples, respectively. The large amount of linear aldehydes identified in GPF could result from oxidation degradation of unsaturated fatty acids, especially oleic, linoleic, linoleic [36], while the increase in aromatic aldehydes such as benzaldehyde and phenyl-lactaldehyde resulted from the degradation of aromatic amino acids during [37]. We found that 2-hexenal, a commonly used indicator of food rancidity, decreased during storage; i.e., from 0.41 mg/Kg in T0 to 0.06 and 0.08 mg/Kg in T6_4 \({}^{\circ}\)C and T6_25 \({}^{\circ}\)C samples, respectively. Furthermore, both values were lower than the concentration associated with rancid odors [38]. Moreover, we found no significant difference betweenT6_4 \({}^{\circ}\)C and T6_25 \({}^{\circ}\)C. It could be considered that constant antioxidant activity with a non-significant decrease in PUFA and MUFA contributes to reduce lipid oxidation. While there is limited literature reporting changes in 2-hexenal content GP during storage, the decrease in 2-hexenal could be linked to oxidative breakdown processes. As C6 compounds are products of fatty acid oxidation, we also detected 1-hexanol in concentration varying from 0.26 mg/Kg at T0 to 0.14 and 0.15 mg/Kg in T6_4 \({}^{\circ}\)C and T6_25 \({}^{\circ}\)C, samples respectively. Volatile alcohols, in particular 1-hexanol and 1-octen-3-ol are produced at low  
Figure 3: Total ion chromatogram of Negroamaro pomace flour at initial and final time point (T0 and T6, respectively) at 4 \({}^{\circ}\)C and 25 \({}^{\circ}\)C in the dark. temperature by oxidation of linoleic acid [39,40]. Therefore, it is probable that a decrease in C6 alcohols is linked to a decrease in the level of its precursor. In the light of this, the low percentage of 2-hexenal and the absence of pentanal considered responsible for off flavor, the increase in esters and aromatic aldehydes during the six months of storage suggest a preservation of sensory quality. To identify main volatile compounds that discriminated GPF samples collected at two different times and temperatures, we performed a principal component analysis (PCA) on data obtained by SPME-GC/MS. As shown in Figure 4, two principal components (PC), PC1 and PC2 accounted for all variation; i.e., 60.23 and 39.77%, respectively. PCA allowed us to differentiate the three samples collected at two different times and temperatures, based on the concentration of specific volatiles. PC1, which explained 60.23% of variance, clearly differentiated T0 and T6_4 \({}^{\circ}\)C samples from the T6_25 \({}^{\circ}\)C sample. On the other hand, PC2, that explained 39.77% of variation, separated the samples into three different groups. The T0 sample was located in the negative region of PC2 and positive PC1 for the higher values of C6 molecules. The T6_4 \({}^{\circ}\)C sample located in the positive region of PC1 and PC2, was characterized by an increase of specific esters (hexadecanoic methyl ester, ethyl acetate, ethyl hexanoate), alcohols (2-ethyl-1-hexanol and 1-octen-3-ol) and aldehydes (hexanal, 2,4-heptalenal). Finally, T6_25 \({}^{\circ}\)C sample is described by component negative of PC1 for the high content of long chain esters and ketones. In generally, alcohols, esters and aldehydes are derived from oxidation of unsaturated fatty acids [39,40]. Previous reports have indicated that higher storage temperature might also be responsible for loss of GPF phenol content [41,42]. However, the above results suggest the lack of any significant effect of temperature the quality of GPF.