## 3 Results and Discussion
### Evolution of Odour and Taste Properties Analysed Through Electronic Nose and Electronic Tongue  
The electronic nose can simulate the human olfactory system to achieve the quality assessment with simple sample pre-treatment, short detection time, easy-to-repeat, reliable results and so on [30]. Figure 5A displays the response of 10 electronic nose sensors to the odour of cooked marinated pork knuckle via processing. In T2, T3, and T4, the W2W, W2S, and W1S responses were approximately 3, suggesting the presence of alkanes, alcohols, aldehydes, and esters in the gas component. The findings were aligned with the GC/MS analysis, where the inclusion of spices amplified the odour of the cooked marinated pork knuckle during the heating stage. Figure 5B presents the PCA results of the electronic nose in cooked marinated pork knuckle. The PC1 and PC2 account for 97.1% of the total variance in the system (86.3% and 10.8%, respectively). The changes in the overall odour profile of the cooked marinated pork knuckle through processing were characterised by the PCA model. Meanwhile, the result of PC1 donated more effect on interpreting the information of the sample. Based on these, it is possible to identify the variations among samples based on their relative positions on the abscissa. The large horizontal distance between T1 and the other samples suggested that the odour profile changed as the temperature rose. The electronic tongue is also known as artificial taste recognition or taste sensor technology, and the purpose of the study was to assess the variations in the fundamental taste characteristics (umami, saltiness, astringency, bitter, and sour), which are shown in Figure 5C. There were some studies published the synergistic effects between umami-free amino acids; meanwhile, the correlation between umami detected via the electronic tongue and umami nucleotides based on EUC was strong [31]. These findings were also consistent with our study; that is, the value of EUC rose significantly (\(p<0.05\)) with temperature, and the umami quality of cooked marinated pork knuckle was consistently enhanced while being heated. Furthermore, the electronic tongue detected the richness as the lingering taste of the umami substance, and so the richness was constantly increased during heating. Additionally, the concentration of sodium chloride could be intensified due to the heightened water loss, thus improving the saltiness in cooked marinated pork knuckle. As shown in Figure 5D, the saltiness increased due to the accumulation of umami. The reason for this is that umami compounds also exhibited properties of saltiness. The principal components accounted for 90.9% of the total variance, the PC1 component contributed 80.3%, and the PC2 component contributed 10.0%. The taste profile of samples was presented by PC1 and PC2 components. These findings displayed that the flavour quality of cooked marinated pork knuckle became consistent over the various stages. Figure 6 displays the Pearson correlation between taste-active compounds and sensory evaluation. For nucleotides, there was a positive correlation among CMP, IMP, and Hy with umami discovered by the E-tongue; the correlative values were \(p\) = 0.98, \(p\) = 0.42, and \(p\) = 0.63, respectively. However, the In (\(p\) = \(-\)0.65) and AMP (\(p\) = \(-\)0.46) were negatively relevant to umami. Furthermore, the free amino acids presented umami taste, including Asp (\(p\) = 0.67) and Glu (\(p\) = 0.87), were positively associated with umami. Meanwhile the Asp (\(p\) = 0.64) and Glu (\(p\) = 0.58) were positively associated with saltiness. The positive correlation between bitter taste FAAs, including His (\(p\) = 0.29), Leu (\(p\) = 0.96), Phe (\(p\) = 0.87), Arg (\(p\) = 0.46), Val (\(p\) = 0.67), Lys (\(p\) = 0.72), and bitterness discovered by the E-tongue. The Cys (\(p\) = 0.67) was positively correlated with sourness.