## 3 Results and Discussion
### Flavour Characteristic Analysis of Set Yoghurt with E-Nose and E-Tongue  
To further explore the effect of RE on the mouthfeel and acceptability of set yoghurt, the changes in flavour characteristic were evaluated using an e-nose and an e-tongue without the separation of individual compounds. As shown in Figure 5a, the response values of W1W (sulphur compounds), W2S (broad alcohol), W2W (aromatics compounds, sulphur organic, and chlorine compounds), and W5S (broad-range compounds, polar compounds, and nitrogen oxides) sensors were significantly enhanced with the increase in RE concentrations in the set yoghurt after 24 h of low-temperature storage, whereas those of the other six sensors (W1C, aromatic compounds; W3C, aromatic compounds; W6S, hydrogen; W5C, aromatic-aliphatic; W1S, broad methane; and W3S, methane-aliphatic) did not change significantly. Consequently, W1W, W2S, W2W, and W5S were the main contributors to the flavour of set yoghurt with the addition of RE. Other polysaccharides increasing the W1W, W2W, and W5S of the yoghurt samples were reported by previous studies [22; 49]. For the taste analysis of set yoghurt samples with an e-tongue (Figure 5b), sourness was significantly enhanced in the samples added with 0.15% RE, compared with other samples after 24 h of low-temperature storage. No significant differences in sweetness, saltiness, richness, umami, aftertaste-A and -B, astringency, and bitterness among the varied set yoghurt samples were observed. These results indicated that the addition of RE could improve the sour characteristic flavour of the set yoghurt.  
Figure 4: The SEM observation of set yoghurt samples with different concentrations of RE after 24 h of low-temperature storage. Changes in surface morphology were marked with yellow ellipse.  
Figure 5: The aroma and taste characteristics of set yoghurt after 24 h of low-temperature storage. (**a**) The radar graph of response signals in the e-nose analysis of set yoghurt aroma; (**b**) The radar graph of the taste signals of the e-tongue analysis.  
Principal component analysis (PCA) is an important tool to analyze the similarity and distinctiveness of set yoghurt by extracting and displaying the systematic variation in the acquired data [22; 50]. Figure 6a showed that the PC1 and PC2 of the e-nose contributed 76.0% and 13.3% of the variation, respectively, which could better distinguish different samples. For the control set yoghurt, the 0.05%, 0.10%, and 0.15% RE samples were, respectively, distributed in the four quadrants, and the control samples were clearly separated from the RE samples. These results were similar to the effect of rose flower extract on the set yoghurt [51]. The PCA of the e-tongue is shown in Figure 6b, which displayed that 81.5% of the total taste signal variance was described by PC1 (57.2%) and PC2 (24.3%). Set yoghurt containing the 0.15% RE samples showed clear differences from the control and the 0.05% and 0.10% RE samples, while set yoghurt with the 0.10% RE samples had overlaps with the control and the 0.05% RE samples, respectively. Overall, the addition of RE, to a certain extent, had an effect on the flavour characteristics of set yoghurt.