## 3 Results
### Commercial Fragment Sesame Oil
#### 3.3.2 Colorimetric Sensing  
Typical scanned images and average color difference maps of the commercial hot-pressed and small-milled samples are shown in Figures 6a and 6b, respectively. Most of the receptors could be activated after 30 min of exposure. Similar to sesame oil standards, hot-pressed samples generally produced stronger signals than small-milled samples, especially in receptor Nos. 5, 7, 8, and 11. This trend could also be observed in hot-pressed and small-milled sesame oil products from the same brand (Figure 6c). According to the color fingerprints of the chemical standards in Section 3.1, these four receptors were sensitive to small-molecule acids and aldehydes, especially acetic acid. The results obtained by the colorimetric sensor thus suggested a higher content of acids and aldehydes in hot-pressed sesame oil, which also agreed with GC-MS analysis. Interestingly, small-milled samples were found to induce greater color change in receptor No. 14 (indicated by a dark-teal dot on the color difference map). Greater differences were observed in receptor Nos. 3-8, 10, 11, and 14, which consisted of pH-sensitive, aldehyde-sensitive, and solvatochromic indicators (Figure S2). Data collected by these receptors was used for PCA. The total variance explained by the first three principal components was 84.5% (Figure 6d). The clustering behavior of fragrant sesame oil samples suggested a dependency on processing technique, as hot-pressed samples and small-milled samples formed two distinct clusters. Similar to the finding in Section 3.2, hot-pressed and small-milled samples were largely differentiated by PC1. Among the 28 hot-pressed samples, 10 samples were found to have color difference maps with weaker signals compared with the rest, and these samples were separated from the majority by PC2. Among the abnormal hot-pressed samples, the sample from Brand I was a blended sesame oil product (containing both refined and fragrant sesame oil). Hence, the remaining abnormal hot-pressed samples were also suspected of containing refined oil. The refining process could affect the lignan content in sesame oil. Research has found that refined sesame oil tends to have lower lignan content [7]. The lignan contents of the abnormal hot-pressed samples were thus analyzed by reversed-phase liquid chromatography (RP-LC) as described in Section 2.8 (Figure 7). Brands D, G, H, and J were found to have significantly lower lignan contents compared with the reference group, which suggested that the samples from these brands might contain refined oil. However, Brand B had a lignan content comparable to those with normal color difference maps. Thus, Brand B might contain either a low amount of refined oil or might be produced by technique(s) other than hot pressing and small milling. Figure 5: The VOC profiles of the hot-pressed and small-milled samples from (**a**) Brand A, (**b**) Brand C, and (**c**) Brand O. For products from the same brand, the hot-pressed sample always contained more volatile acids and aldehydes (indicated by the arrows). Figure 6: **(a)** Typical scanned images of the colorimetric sensor array after exposure to hot-pressed (HP) and small-milled (XM) samples. Differences in the color fingerprints could be easily observed the naked eye. **(b)** The average color difference maps of commercial hot-pressed and small-milled samples. Compared with small-milled samples, hot-pressed samples generally produced more significant color changes, especially in sensing unit Nos. 5, 7, 8, and 11. **(c)** The color difference maps of hot-pressed and small-milled samples from Brands A, C, and O. For products from the same brand, the hot-pressed sample always produced stronger signals. **(d)** PCA results of 53 commercial samples; hot-pressed samples with normal color difference maps, hot-pressed samples with abnormal color difference maps, and small-milled samples are marked by blue, grayscale, and red symbols, respectively.