## 3 Results and Discussion
### Hierarchical Cluster Analysis (HCA) and Volatile Profiles Development
#### 3.3.2 Changes in the Concentration Profiles of Heterocyclic Compounds  
Heterocyclic compounds containing the heteroatoms' oxygen and nitrogen were by far most predominant volatile compounds accounting for 160.5 \(\mathrm{\SIUnitSymbolMicro g}\)/g in S-240 (Figure 4). The species and overall intensity increased significantly (\(p<\) 0.05) at the post stage of the dehydration process, particularly furrans produced by the thermal breakdown of sugars and pyrazines derived from the amino acids' thermal degradation, both of which indicate the occurrence of the Maillard reaction [28]. These MRPs are flavor enhancing, providing typical earthy, peanut, and roasted odors to NCS [10]. Since the formation of furrans only occurred at high temperature (\(>\)100 \({}^{\circ}\)C), volatile furrans were mostly detected in the post-stages of the dehydration process, except furfural (chocolate odor), 2,3-dihydrobenzofuran, and 2-pentyl-furan (earthy, fruity odor), which were detected in the initial stages. Further, 2-furanmethanol that has bready odor was detected from S-180 and experienced a sharp increase towards the end (S-180 to S-240, 0.54 to 9.92 \(\mathrm{\SIUnitSymbolMicro g}\)/g), which contributed to the overall NCS aroma. Meanwhile, 5-methyl-2-furanhanol, 2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one (DDMP), and 2-methyl-furan were all detected from S-210 (S-210 to S-240, 0.83 to 2.10, 1.81 to 3.23, and 0.30 to 0.27 \(\mathrm{\SIUnitSymbolMicro g}\)/g, respectively) which have pleasant bread, sweet-caramel, and chocolate characteristics, respectively; 5-hydroxfurral (S-240, 1.52 \(\mathrm{\SIUnitSymbolMicro g}\)/g, buttery and caramel odor) and dihydro-2-methyl-3(2H)-furanone (S-240, 0.1 \(\mathrm{\SIUnitSymbolMicro g}\)/g, caramel odor) were identified towards the end of the dehydration process. Moreover, 2-vinyl-furan (S-240, 0.21 \(\mathrm{\SIUnitSymbolMicro g}\)/g), 2-n-butyl-furan (S-240, 0.54 \(\mathrm{\SIUnitSymbolMicro g}\)/g), 2,2\({}^{\prime}\)-bifuran (S-240, 3.98 \(\mathrm{\SIUnitSymbolMicro g}\)/g), and 3-phenyl-furan (S-240, 4.07 \(\mathrm{\SIUnitSymbolMicro g}\)/g) were also detected in the final dehydration process and were good alternatives of spicy odor to complement the predominant sweet-caramel odor. Pyrazines, including 2,6-dimethyl-pyrazine (roasted nutty odor), 2-methyl-pyrazine (nutty chocolate odor), 2,3-dimethyl-pyrazine (nutty and peanut butter odor), 2-ethyl-5-methyl-pyrazine (nutty and roasted coffee odor), 3-ethyl-2,5-dimethyl-pyrazine (nutty roasted odor), ethyl-pyrazine (nutty odor), and 2-ethyl-6-methyl-pyrazine (nutty odor), were all identified in S-240 (content range from 0.46 to 3.55 \(\mathrm{\SIUnitSymbolMicro g}\)/g), while cocoa roasted odor scented 2,6-dimethyl-pyrazine was detected from S-210, and developed obviously towards the end (3.55 \(\mathrm{\SIUnitSymbolMicro g}\)/g). Overall, the aroma contribution of the formed heterocyclic compounds to NCS are similar to previous studies [6; 10; 29]. 3.3.3. Changes in the Concentration Profiles of Ketones  
Ketones are the main sugar-derived compounds together with furans, and are generated at a relatively lower temperature, and so were identified throughout the heating and dehydration process. Seventeen ketones were found as differential aroma compounds, as shown in Figure 3 and Supplementary Table S1. Ketone concentration had a slight increasing tendency throughout the process (Figure 4), which is consistent with the results of [PERSON] et al. (2016) [10]. S-210 and S-240 have higher concentrations of ketones than the other samples. Among those identified differential ketones, immature apple scented 2-octanone was the only ketone that existed throughout the process, and the concentration almost tripled from S-0 to S-210 (4.07 to 11.70 \(\mathrm{\SIUnitSymbolMicro g}\)/g), followed by a remarkable decrease in the final 30 min (S-240, 1.47 \(\mathrm{\SIUnitSymbolMicro g}\)/g).