## 2 Results and Discussion
### Phenolic Characterization  
Data reported in literature for onion phenolic composition vary due to normal biological variations related to cultivar, growing season, environmental and agronomic conditions and, in some instances, the region of the bulb [1; 23]. HPLC analysis of our hydroalcoholic onion extracts produced two principal peaks identified as quercetin-3,4\({}^{\prime}\)-diglucoside and quercetin-4-glucose by comparison of spectroscopic and literature data; they are the main compounds in our sample, data in agreement with the literature [24; 25; 26]. Their content (Table 4) was higher than that reported in recent researches [1; 23] and, on percentage, ranges between 45.8%-49.3% and 42.4%-42.8%, respectively. The sum of the content of these two molecules accounts for 92% of the total flavonoid content in onion flakes and 88% of the total flavonoid content in onion rings, values even higher than the 80% [27] or 85% [18] and in agreement with the 93% [28] previously reported. On the contrary, quercetin was present in percentage ranging 1%-2% on the total flavonoid content, again in agreement with the previous literature [18; 29]. Considering that in fresh onion the content of flavonoids ranges from 0.185 to 0.634 mg g\({}^{-1}\) and that approx. 90% of fresh onion is constituted by water, our data suggest that the applied drying process leads to only a slight loss of flavonoid, which thus resulted in a concentration approx. 10 times that of the fresh onion. Further analysis was performed to evaluate the total polyphenol content--the values ranged from 1.49 (onion rings) to 1.62 mg\({}_{\text{gallic acid g}}\)-1 (onion flakes), data in agreement to [PERSON] et al. 2017 [30] but lower than those reported in another recent study [31]. The difference in this latter study could be due to the lower drying temperature applied in our study--indeed, it was reported that some phenolic compounds can be liberated from the matrix during the drying process at high temperatures [32]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Compound**} & **1 st D RT** & **2 nd D RT** & **Rings** & **Flakes** & **Rings** & **Flakes** \\  & **(min)** & **(s)** & **(volume)** & **(volume)** & **(\%)** & **(\%)** \\ \hline \hline propyl mercaptan & 6.05 & 0.42 & 1,724,317 & 2,979,593 & 5.0 & 5.8 \\ allythiol & 6.20 & 0.62 & 816,521 & n.d. & 2.4 & n.d. \\
2-methyl-2-butenal & 8.65 & 1.04 & 677,649 & n.d. & 2.0 & n.d. \\
2-dimethylthiophene & 12.85 & 1.06 & 562,705 & 787,731 & 1.6 & 1.5 \\
3,4-dimethylthiophene & 13.80 & 1.24 & 1,170,022 & 6,773,463 & 3.4 & 13.2 \\ methyl propyl disulfide & 14.80 & 1.04 & 416,658 & 2,247,811 & 1.2 & 4.4 \\ methyl 1-propenyl disulfide & 15.15 & 1.28 & 149,034 & 1,541,084 & 0.4 & 3.0 \\ dimethyl trisulfide & 16.40 & 1.72 & 133,334 & 1,679,770 & 0.4 & 3.3 \\ dippropyl disulfide & 21.60 & 0.94 & 11,529,581 & 14,500,819 & 33.4 & 28.3 \\ propenyl propyl disulfide & 21.95 & 1.10 & 1,365,546 & 3,030,564 & 4.0 & 5.9 \\ diethanol disulfide & 23.40 & 1.38 & 740,611 & 3,813,741 & 2.1 & 7.4 \\ unidentified sulfide mw 152 & 23.95 & 1.68 & 254,301 & 2,194,284 & 0.7 & 4.3 \\ decanal & 25.15 & 0.92 & 145,059 & 1,481,756 & 0.4 & 2.9 \\ dippropyl trisulfide & 29.50 & 1.08 & 8,135,894 & 5,312,160 & 23.6 & 10.4 \\ trans-3,5-diethyl-1,2,4-trithiolane & 29.90 & 1.62 & 1,341,744 & 220,925 & 3.9 & 0.4 \\ cis-3,5-diethyl-1,2,4-trithiolane & 30.15 & 1.64 & 2,565,177 & 912,354 & 7.4 & 1.8 \\ propenyl propyl trisulfide & 30.15 & 1.38 & 1,970,506 & 3,730,800 & 5.7 & 7.3 \\
4,6-Diethyl-1,2,3,5-tetrathiolane & 38.40 & 2.22 & 809,467 & n.d. & 2.3 & n.d. \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Main volatile organic compounds detected by HS-SPME-GC\(\times\)GC-TOF in the different onion samples. Data are the mean of three determinations. First dimension retention time in minute (1 st D RT). Second dimension retention time in second (2 nd D RT). Response of the two-dimensional peak (volume).