## 4 Discussion  
In this study, a 14-unit paper-based colorimetric sensor was used to analyze the VOCs in fragrant sesame oil by sensing the headspace gas while the sample was placed in a sealed sample chamber. The headspace gas in the sample chamber was analyzed as VOCs only constitute a small fraction of the total mass, while over 90% of the liquid phase is made up of triglycerides [41]. Moreover, fragrant sesame oil generally has a dark color, as brown pigments are formed by the Maillard reaction during the roasting process, and the oil obtained is not subjected to any further refining process; hence, the pigments will remain in the final product, giving fragrant sesame oil its dark color. All these factors make it less feasible to perform VOC analysis directly on liquid oil. However, simply exposing the sensor to the headspace gas would require a longer time for colorimetric reactions to proceed, and a long sensing duration was reported in most of the existing studies [28; 29; 31]. Heating is sometimes used to shorten the sensing time [30]. In this study, the VOC analysis duration of the colorimetric sensor was shortened without the need for heating, and naked-eye observable color changes could be achieved in 30 min at room temperature. To achieve this, a micro vacuum pump was incorporated into the sensing system to facilitate gas circulation, and a cellulose-based material was selected as the substrate to load indicator dyes, instead of other commonly used hydrophobic polymer membranes. Filter paper, as an easily available cellulose-based material, is hydrophilic. Water plays an important role in many chemical reactions, such as proton transfer between immobilized dye molecules and gaseous analytes, which is necessary for pH indicators to change color. The water molecules adsorbed could thus facilitate various reactions to  
Figure 8: (**a**) The average color difference maps of samples containing various volume percentages of fragrant sesame oil. As the volume percentage of fragrant sesame oil increased, the signals detected by the sensor became stronger; the color difference map of blended sesame oil containing more than 80% \(v/v\) of fragrant sesame oil closely resembled that of the pure sesame oil. (**b**) PCA result. Refined oil samples formed a cluster that was clearly separated from that of pure sesame oil; as the volume percentage of refined oil decreased, the PC1 value became more positive; when the percentage decreased to 20% \(v/v\), the clusters started to overlap. (**c**) PLS regression analysis result. With 9 PLS components selected, an \(r^{2}\) value of 0.9423 could be obtained, suggesting a reasonably strong correlation. occur [42; 43; 44]. Moreover, filter paper is intrinsically porous and readily available, making it ideal for disposable gas-sensing devices. In this study, both GC-MS analysis and colorimetric sensing confirmed the differences in the content of acidic and aldehyde VOCs between hot-pressed sesame oil and small-milled sesame oil. It was found that hot-pressed sesame oil tended to contain more acids and aldehydes. Such a difference could be caused by the unique aqueous extraction step in the small milling process. Volatile acids and aldehydes tend to have higher water solubility, and hence, they can be easily removed from the oil phase by aqueous extraction [15]. Many volatile acids and aldehydes have been identified as off-flavor compounds in edible plant oil [45; 46]. Hence, the presence of such VOCs affects the aroma of fragrant sesame oil. In a study conducted by [PERSON] et al., the sensory evaluation suggested that small milling could produce sesame oil with the most preferable aroma among various techniques [10]. [PERSON] et al. compared small-milled and hot-pressed sesame oil samples, and concluded that small-milled sesame oil had stronger and better aroma intensity than the roasted oil [9]. Such difference in aroma could be attributed to the lower content of volatile acids and aldehydes in small-milled sesame oil [15].