## 2 Results and Discussion
### Identification and Quantification of Key Aroma Compounds of 15 Brands of EVOOs  
Eleven compounds were identified as the key aroma compounds in alternative brands of EVOOs by SAFE-AEDA (Table 2). FD value reflects the flavor contribution degree of each compound. Higher value indicates greater contribution to EVOO flavor. Among them, four compounds [hexanal (FD: 16 to 64), (_E_)-2-hexenal (FD: 2 to 64), 1-octen-3-one (FD: 4 to 32), and (_E_)-3-hexenol (FD: 0 to 8)] were the joint key aroma compound in the 15 brands of EVOOs, and the former two compounds contributed to the flavor greatly with the highest FD values, which was in accordance with the low odor thresholds of aldehydes. In particular, (_E_)-2-hexenal in all the Spain EVOOs had high FD (64 in PL, BDS, OL, and YGY/16 in BLN), suggesting its great contribution to Spain samples. The flavor character of hexanal and (_E_)-2-hexenal were grass, fat, apple, or green note represented the flavor of EVOOs to a great extent. In addition, the mushroom/metal note of 1-octen-3-one and moss/fresh note of (_E_)-3-hexenol also impacted the whole flavor of EVOOs significantly. The other three kinds of compounds, 2-octanone (FD: 0 to 16, does not exist in BLN, soap or gasoline note), (_E_)-2-heptenal (FD: 2 to 16, does not exist in PL, fat or almond note), and (_E_)-2-Nonenal (FD: 0 to 16, does not exist in BDS, cucumber and green note) affected the EVOO flavor notably. These three brands of EVOOs were both from Spain. Nonanal (FD: 0 to 8) and 2-nonanone (FD: 0 to 4) existed in most of Italy and Greece EVOOs, but they both did not consist in the Spain samples. It indicated that the EVOOs from Spain were lack of the flavor of fat, citrus, green, milk, or soap. Acetic acid (FD: 0 to 4, 16 in ALCF) and methyl benzoate (FD: 2 to 4, 8 in YGY) only existed in several brands of EVOOs from these three countries, and they had little effect to the flavor of EVOOs. Among these key aroma compounds identified by SAFE-AEDA, 1-octen-3-one could not be quantified because it had no peak in total ion chromatogram. 2-Octanone could be detected only in four samples by SAFE, so it could not be quantified by SPME with external standard; nonanal, 2-nonanone, (_E_)-2-nonenal and methyl benzoate could only be detected in several samples with the weak contribution (lower FD). Therefore, they were not quantified precisely. Except for the compounds above, all the other five compounds were quantified by external standard. Besides, (_Z_)-3-hexenyl acetate and 6-methyl-5-hepten-2-one were also identified as the key aroma compounds by SPME with internal standard, although they could not be sniffed by SAFE. Hence, all these seven compounds were quantified precisely (Table 3). \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Compounds**} & \multicolumn{11}{c}{**FD**} \\ \cline{2-13}  & **PL** & **BDS** & **OL** & **BLN** & **YGY** & **OLWL** & **OS** & **ALCF** & **MNN** & **AN** & **XBK** & **DMDN** & **MSWN** & **AGL** & **DNLE** \\ \hline Hexanal & 32 & 32 & 16 & 64 & 16 & 16 & 64 & 32 & 32 & 64 & 32 & 32 & 64 & 32 & 32 \\ (E)-2-Hexenal & 64 & 64 & 64 & 16 & 64 & 16 & 8 & 8 & 16 & 16 & 64 & 8 & 4 & 4 & 2 \\ 2-Octanone & 4 & 2 & 4 & - & 4 & 2 & 4 & 16 & 2 & 4 & 0 & 4 & 8 & 4 & 4 \\ 1-Octen-3-one & 8 & 8 & 4 & 4 & 16 & 8 & 32 & 16 & 16 & 8 & 4 & 32 & 16 & 16 & 16 \\ (E)-2-Heprenal & - & 2 & 2 & 4 & 4 & 4 & 8 & 4 & 8 & 8 & 4 & 8 & 4 & 16 & 8 \\ (E)-3-Hexenal & 8 & 4 & 4 & 4 & 4 & 4 & 8 & 0 & 2 & 2 & 0 & 0 & 4 & 4 & 4 \\ Nonanal & - & - & - & - & - & 2 & 2 & 2 & 0 & 4 & 8 & - & 4 & 2 & 0 \\ 2-Nonanone & - & - & 0 & - & - & 4 & 4 & 0 & - & - & - & - & 2 & 0 & 2 \\ Acetic acid & 4 & 2 & 4 & - & 4 & - & 4 & 16 & 2 & 2 & 2 & 4 & 0 & - & - \\ (E)-2-Nonenal & 16 & - & 2 & 0 & 2 & 4 & 2 & 2 & 2 & 4 & 4 & 2 & 4 & 2 & 0 \\ Methyl1 & & & & & & & & & & & & & & & & \\ benzoate & & & & & & & & & & & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Key aroma compounds identified in 15 EVOOs by FD.