# Effects of Three Different Withering Treatments on the Aroma of White Tea
## 3 Results and Discussion
### Formation Mechanism of Key Odor-Active Compounds between Different Withering Treatments  
Odor-active compounds play a key role in the overall aroma of white tea, and an understanding of their formation mechanism can improve regulation of the aroma release mechanism during white tea processing to obtain tea of higher quality. Most volatile compounds obtained in the study can be classified as fatty acid-derived volatiles (FADVs), amino acid-derived volatiles (AADVs), volatile terpen (VTs), carotenoid-derived volatiles (CDVs), and glycosidally bound volatiles (GBVs) [14], which are produced through the shikimic acid and terpene pathways or from the oxidation of fatty acids and carotenoids. Linalool, geraniol, and \(\beta\)- myrcene have a more pronounced trend as key odorants in SWT and, very interestingly, are all VTs. VTs, primarily C\({}_{10}\) and C\({}_{15}\) compounds, are the major contributors to the floral aroma of tea [36; 37]. Isopentyl diphosphate (IPP) and its allyl isomer, dimethyl allyl diphosphate (DMAPP), are two common C\({}_{5}\) terpenoid precursors [38]. These two precursors are synthesized by the cytoplasmic mevalonate (MVA) and plastid methylerythnitol phosphate (MEP) pathways [39]. The key identified  
Figure 4: Differences in concentration of key odor-active compounds among white teas. On the left are odor-active compounds with floral odor, and on the right are odor-active compounds with green odor. Data are shown as the mean \(\pm\) standard deviation (\(n=3\)). Different letters on the columns represent they have significant difference (\(p<0.05\)). (SWT: sunlight withering-treated white tea; WWT: withering tank withering-treated white tea; IWT: indoor withering-treated white tea). potent floral odorants are linalool, _\(\beta\)-myrrene_, and geraniol are VTs. [PERSON] reported that the relative content of monoterenes and sesquiterpenes in oolong tea increased significantly after SWT because of the significant upregulation of genes involved in the MEP and MVA pathways [7]. Linalool and geraniol are GBVs; [PERSON] revealed that treatment with red light withering could increase the activity of glycosidase and thus significantly improve the aroma of black tea [6]. GBV synthetases, as well as hydrolases, can strongly promote the formation of white tea aroma, and they actively participate in aroma regulation during the withering process [9]. Therefore, we speculate that the primary reason for the strong floral aroma in SWT is that sunlight exposure during withering results in an increase in VTs due to the key synthetic genes of the MEP and MVA pathways being significantly upregulated, thus enhancing the activity of related hydrolases and producing more floral odor-active compounds. Most FADVs have a fresh or green odor [9]. The odor-active compounds _hexanal_, _pentanal_, _(Z)-3-hexanol_, (E,E)-2,4-heptadienal, and (E)-2-nonenal with green odor were among the FADVs were identified in higher concentrations in WWT. Thus, the freshness and green quality of white tea can be attributed to the increase in FADVs. Saturated and unsaturated C\({}_{6}\) and C\({}_{9}\) aldehydes and alcohols are also key to the production of fresh, green odor-active compounds, such as _hexanal_ and _(Z)-3-hexanol_[7]. Various enzymes are involved in the production of these odor-active compounds. For example, lipoxygenase (LOX) catalyzes the production of (Z)-3-hexanal to _(E)-2-Hexanal_ (leaf aldehyde) and (Z)-3-hexanol_[40] (leaf alcohol). Hydroperoxide lyases (HPLs) can cleave hydroxides to generate C\({}_{6}\) aldehydes. In potato plants, downregulation of HPL causes an increase in LOX activity and a decrease in C\({}_{6}\) volatiles [41]. The more pronounced grassy note of WWT is mainly due to the increase of _hexanal_ and (Z)-3-hexen-1-ol, which may be caused by the increase in AADVs and changes in enzyme activity caused by the heating and blast treatment in the withering-tank, which leads to the acceleration of the synthesis or retardation of the degradation of C\({}_{6}\) or C\({}_{8}\) compounds.