## 4 Conclusions  
The addition of _Strobilanthes tonkinensis_ Lindau (STL) during black tea (BT) processing had a substantial effect on the volatile metabolites (VMs) composition and aroma improvement of BT. We identified 141 VMs in STL firstly with high alcohol, ester, ketone, enyne, alkyne, and alkane contents. STL has a unique aroma of glutinous rice owing to high contents of benzaldehyde, 1-octen-3-ol, isophorone, 2-methoxy-3-(2-methylpropyl)-pyrazine, linalool oxide (pyranoid), 1-dodecanol, \(\alpha\)-ionone, and _trans_-\(\beta\)-ionone in the STL leaves. Different addition methods and different addition concentrations of STL influence BT aroma and flavor considerably, and the addition of 20:1500 \(w/w\) STL to rolled tea leaves was the best method for processing BT with STL (i.e., BoS), with a strong and lasting glutinous sweet aroma and a sweet, mellow, and thick taste. PLS-DA clearly distinguished BT, STL, and BoS, and revealed 28, 26, and 14 differential volatile compounds when comparing STL vs. BoS, BT vs. STL, and BT vs. BoS, respectively. Combining S-plot, Venn, MEV, and OAV analysis, 16 key differential VMs were identified among BT, STL, and BoS. BoS combined the aroma properties of BT and STL and showed the highest contents of hexanal, (E)-2-hexenal, (E)-3-hexen-1-ol, 1-hexanol, 1-octen-3-ol, benzeneacetaldehyde, linalool, phenylethyl alcohol, 2-methoxy-3-(2-methylpropyl)-pyrazine, 1-dodecanol, and nerol. Our systematic study showed the effects of the new exogenous additive STL on the improvement of the BT aroma and value, thus providing sound theoretical support and insights for the development of new BT products as well as enriching our theoretical understanding of the mechanisms underlying aroma formation in BT. However, this study only shows the effects of STL addition on the VMs and aroma improvement of BT. Thus, the effect of STL addition on non-VMs and taste improvement requires further exploration.  
The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/foods11121678/s1](https://www.mdpi.com/article/10.3390/foods11121678/s1)]([https://www.mdpi.com/article/10.3390/foods11121678/s1](https://www.mdpi.com/article/10.3390/foods11121678/s1)), Table S1: List of the all the VMs in BT, BoS, and STL; Table S2: The contents and proportion of different VMs types in BT, BoS, and STL; Table S3: List of the differential VMs among BT, BoS, and STL.  
Conceptualization, [PERSON], [PERSON] and Y.J.; methodology, [PERSON], J.L., W.O. and J.W.; software, J.H.; validation, H.Y. and Y.J.; formal analysis, J.H.; investigation, J.H., J.L. and W.O.; resources, H.Y. and Y.J.; data curation, J.L. and J.W.; writing--original draft preparation, [PERSON] and J.L.; writing--review and editing, H.Y.; visualization, J.L.; supervision, H.Y. and Y.J.; project administration, [PERSON] and Y.J.; funding acquisition, Y.J. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the China Agriculture Research System of MOF and MARA (NO. CARS-19) and Science and Technology Innovation Project of the Chinese Academy of Agricultural Sciences (CAAS-ASTIP-TRICAAS).  
Not applicable.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**Name**} & **OTs** & **Aroma** & \multicolumn{2}{c}{**OAV**} \\ \cline{4-6}  & & **(\#/kg) a** & **Characteristics b** & **BT** & **BoS** & **STL** \\ \hline
86 & Nerol & 300 & Sweet, floral, ner  
Data Availability Statement: The data presented in this study are available in this article and Supplementary Materials.