## 4 Discussion

Table 4: Effect of black tea with different withering times on DPPH scavenging, inhibitory activity of \(\alpha\)-amylase and \(\alpha\)-glucosidaseae, and inhibition rate of glucose uptake. critical role in the quality and palatability of black tea. An increase in the relative content of amino acids depends on the rate of protein degradation during withering [8]. Aromatic amino acids, i.e., L-phenylalanine, L-tryptophan, and L-tyrosine were increased due to the synthesis of chorismate (their precursors) through the pentose phosphate pathway during withering [22]. Glutamine, a relatively abundant amino acid in tea, decreases with an increase in withering time, possibly because it contributes to ethylamine formation in theanine synthesis [22]. Thus, glutamate and GABA synthesized from glutamine might cause an increase in the relative content of GABA [30]. Additionally, a decrease in chlorophyll pigment during withering reduces glutamate content [31]. A reduction in L-theanine during withering was previously reported [32] and in this research the same result was found. A possible reason is that theanine is more abundant in the root, where it is synthesized and translocated to the leaves through the xylem, and the detachment of leaves from the tea plant reduces the concentration of theanine in the leaves [33]. It was suggested that theanine decomposes slowly during withering [8]. L-theanine is implicated in the unique taste of umami, bitter, and sweet in tea [25; 34]. L-arginine was reduced by reducing sugar through the Maillard reaction, condensation, and Strecker degradation to form volatile compounds that contribute to black tea aroma [34]. The high content of free amino acids in tea contributes to its mellow taste and is involved in aroma formation [29]. Theaflavins, thearubigins, and theabrowins are the primary polyphenols present in black tea [19]. During withering, polyphenol oxidase (PPO) and peroxidase (POD) activity (which significantly (\(p<0.05\)) increased as withering time increased (Figure S1)) oxidize green tea catechins to form the bright orange-red pigments called theaflavins (TF) via catechins ortho-quinone formation [19]. As oxidation proceeded, theaflavins decreased gradually to form red-brown pigments called thearubigins (TR). Later, the thearubigin compounds reacted with amino acids to form dark brown complex compounds called theabrowins (TB) [19]. The conversion of theaflavins to thearubigins might be the reason for its decreasing trend during withering (Figure 2f). This result agrees with a previous study that showed that TFs decrease as moisture content decreases during withering [7]. The most abundant subclass of flavonoid detected was flavonoids (kaempfer, quercetin, and myrecetin), which are spatially distributed in mesophyll cells and present in glycoside form, especially kaempfer O-glycosides, myricetin O-glycosides, and quercetin O-glycosides [3; 24]. However, black tea has a high proportion of quercetin types of flavonoid glycosides, while in green tea, the kaempfer type of flavonoid glycosides is abundant [35]. The same findings were recorded in previous research, where glycosylated bound volatiles (GBVs) were reduced during the withering process with an increase in terpen volatile content [36]. Both oxidative degradation and downregulation of the genes linked to flavanol synthesis greatly contribute to a significant decrease in flavonoid glycosides [22]. The gallic acid increase was caused by the decomposition of gallated catechins. Gallic acid is a vital phenolic acid in black tea that contributes to its antioxidant property [37]. Furthermore, the accumulation of gallic acid was also influenced by the oxidative degradation of phenolic esters [38]. Decreasing shikimic acid content with increases in withering time was thought to be linked to its contribution to the synthesis of L-phenylalanine [39].