## 4 Conclusions  
Eight volatile compounds were found as chemical key-markers for different rice grains varieties using HS-SPME GC\(\times\)GC-TOFMS and chemometric analysis. These compounds were effectively extracted using HS-SPME under the following optimised conditions: Amount of the sample (0.5 g), volume of water (5 mL), adsorption temperature (80.73 \({}^{\circ}\)C), pre-incubation time (5 min), and adsorption time (50 min). The validation of HS-SPME ensured acceptable precision and accuracy of the method. In addition, the method developed based on HS-SPME GC\(\times\)GC-TOFMS was successfully applied to evaluate the volatile compounds of four aromatic rice varieties, thus considered as a reliable analytical method for the key-marker compounds in rice grains.  
**Supplementary Materials:** The following are available online, Figure S1: 2-acetyl-1-pyrroline (1) and internal standard 2,4,6-trimethylpyridine (2) in Pandan Leaf. Figure S2: Response surface plots showing the effects of variables (\(x_{1}\), sample amount; \(x_{5}\), adsorption time) on the extraction yield. Figure S3: Relative amount of extracted compounds in different adsorption times.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**DOE**} & \multicolumn{4}{c}{**Extraction Variables**} & \multicolumn{6}{c}{**Extraction Yield (Relative \% to Maximum Yield)**} \\ \cline{2-13}  & \(x_{1}\) & \(x_{2}\) & \(x_{3}\) & \(x_{4}\) & \(x_{5}\) & **PEN** & **HEX** & **PYR** & **2 AP** & **2 PF** & **OCT** & **OCA** & **NON** \\ \hline
1 & 1 & 1 & 1 & 1 & 1 & 22.78 & 3.58 & 3.80 & 18.44 & 21.70 & 3.89 & 16.77 & 7.11 \\
2 & \(-\)1 & \(-\)1 & 1 & 1 & \(-\)1 & 6.05 & 1.36 & 1.66 & 100.00 & 11.28 & 9.80 & 100.00 & 100.00 \\
3 & \(-\)1 & \(-\)1 & \(-\)1 & 1 & 1 & 30.78 & 5.79 & 6.39 & 22.01 & 4.85 & 13.46 & 61.75 & 69.87 \\
4 & \(-\)1 & 1 & 1 & \(-\)1 & \(-\)1 & 21.72 & 1.63 & 2.04 & 62.20 & 9.55 & 13.73 & 59.42 & 22.31 \\
5 & \(-\)1 & 1 & \(-\)1 & \(-\)1 & 1 & 100.00 & 100.00 & 100.00 & 13.40 & 100.00 & 100.00 & 47.52 & 42.92 \\
6 & 1 & \(-\)1 & \(-\)1 & \(-\)1 & \(-\)1 & \(-\)1 & 7.55 & 1.55 & 1.71 & 3.43 & 0.65 & 1.38 & 4.61 & 3.96 \\
7 & 0 & 0 & 0 & 0 & 0 & 10.99 & 8.33 & 8.90 & 54.26 & 16.14 & 25.63 & 32.39 & 43.71 \\
8 & 1 & 1 & \(-\)1 & 1 & \(-\)1 & 28.99 & 11.26 & 11.63 & 2.88 & 15.60 & 6.99 & 5.10 & 3.50 \\
9 & 1 & \(-\)1 & 1 & \(-\)1 & 1 & 3.16 & 0.59 & 0.73 & 63.69 & 3.99 & 2.24 & 17.26 & 25.84 \\
10 & 0 & 0 & 0 & 0 & 0 & 12.66 & 13.01 & 8.85 & 51.67 & 14.53 & 23.84 & 26.91 & 41.25 \\
11 & 0 & 0 & 0 & 0 & 0 & 15.25 & 12.51 & 11.49 & 48.62 & 19.49 & 30.29 & 31.88 & 50.60 \\ \hline \hline \end{tabular}
\end{table}
Table 3: The \(2^{5-2}_{III}\) fractional factorial design for five factors with their observed responses.  
**Author Contributions:** Conceptualization, W.S., T.D., J.N. and M.P.; methodology, T.D. and W.S.; validation, W.S. and T.M.; investigation, T.M. and W.S.; writing--original draft preparation, W.S. and M.P.; writing--review and editing, W.S. and M.P.; supervision, J.N.  
**Funding:** This research was supported by the Ministry of Research, Technology and Higher Education of the Republic of Indonesia through the World Class Professor (WCP) Program with the contract number T/47/D2.3/KK.04.05/2019, conducted at Gadjah Mada University (Indonesia) and University of Cadiz (Spain). This research is also partially funded by the Indonesian Ministry of Research, Technology, and Higher Education under the World Class University (WCU) Program, managed by Institut Teknologi Bandung.  
**Conflicts of Interest:** The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.