## 3 Results and Discussion
### Response Surface Test Results
#### 3.3.1 Analysis of the Odor Source of Instant Black Tea by Electronic Nose Sensors  
An electronic nose equipped with 10 different semiconductor metal oxide sensors was used to measure the odor of instant tea. The sensors included W1C, W5S, W3C, W6S, W5C, W1S, W1W, W2S, W2W, and W3S. The substances corresponding to the response of each sensor are listed in Table 5. During sample analysis, the main volatile substances in the sample could be identified based on the type of gas detected. Using response surface design, we analyzed 12 experimental points, 1 center point, and a blank control of instant black tea. The analysis examined the types and concentrations of odor compounds in 14 groups of instant tea processed using different methods, as shown in Figure 3.  
Figure 2: Response surface plots of the interaction effects of various factors on the sensory score of instant tea. (**A**) Response surface plane of tea–water ratio and \(\beta\)-cyclodextrintrin content to sensory scores of instant tea; (**B**) three-dimensional response surface of tea–water ratio and \(\beta\)-cyclodextrintrin content to sensory scores of instant tea; (**C**) response plane of extraction temperature and \(\beta\)-cyclodextrintrin content to sensory scores of instant tea; (**D**) three-dimensional response surface of extraction temperature and \(\beta\)-cyclodextrintrin content to sensory scores of instant tea; (**E**) response surface plane of extraction temperature and tea–water ratio to instant tea sensory score; (**F**) three-dimensional response surface of extraction temperature and tea–water ratio to sensory score of instant tea.  
As shown in Figure 3, Group 1 exhibits a higher content of nitrogen oxides, as indicated by its prominent response in W5S. Group 3 shows elevated values in W1C, W6S, and W5C, suggesting a higher content of aliphatic aromatic compounds, benzene series, hydrogenated compounds, short-chain alkanes, and other aromatic substances. Group 6 stands out in W1W, indicating a higher content of sulfides and terpenes. Group 9 is prominent in W2W, suggesting a higher content of organic sulfides and aromatic compounds. Additionally, most samples show relatively low and concentrated response values in the W6S, W5C, and W1C sensors, with some values close to 1, indicating minimal differences among these samples. In contrast, the W3C sensor detects substances with response values mostly greater than 1, except for Groups 2 and 3, which have relatively lower values below 1. The response values for different samples are highly concentrated in the W3S sensor, indicating that the long-chain alkane volatile substances in instant black tea produced by different processing methods do not vary significantly. In the W5S, W1S, W2W, W2S, and W1W sensors, most response values are greater than 1, suggesting that the prominent odor substances in instant black tea powder are primarily alkane aromatic substances, aromatic volatile substances, aldehydes, alcohols, and ester volatile substances. The blank control group (Group 14) shows significantly lower response values for volatile odor substances compared to the other 13 groups, indicating that ultrasonic-assisted extraction and the addition of \(\beta\)-cyclodextrinrain can effectively retain volatile odor substances during processing.  
Figure 3: Radar chart of electronic nose sensor response values for instant black tea powder produced with different processing methods.  
\begin{table}
\begin{tabular}{c c} \hline
**Sensor** & **Determination of the Substance** \\ \hline W1C & Aliphatic aromatic compounds, benzene series \\ W5S & Nitrogen oxides \\ W3C & Amines, aromatic substances \\ W6S & Hydrogenated compounds \\ W5C & Short-chain alkanes, aromatic substances \\ W1S & Alkanes, methane, methyl compounds \\ W1W & Sulfides, terpenes \\ W2S & Alcohols, aldehydes, ketones, aromatic compounds \\ W2W & Organic sulfides, aromatic substances \\ W3S & Long-chain alkanes \\ \hline \end{tabular}
\end{table}
Table 5: Substances measured by sensors.