Dynamic changes of physicochemical compounds in large-leaf yellow tea during processing

To address the unclear mechanisms underlying the impact of the variations of physicochemical component on flavor quality of large-leaf yellow tea （LYT） during processing， high-performance liquid chromatography （HPLC） and ultraviolet-visible （UV-Vis） spectrophotometry were employed to systematically determine the contents of catechin， caffeine （CAFF）， gallic acid （GA）， and total sugars in LYT from different processing stages. The taste characteristics were investigated based on dose-over-threshold （Dot） determination and principal component analysis （PCA）. The significant differences on contents among these chemical compounds were observed in tea samples from LYT during processing. A significant reduction trend was found on contents in catechin， with epicatechin （EC）， epigallocatechin （EGC）， epicatechin gallate （ECG）， and epigallocatechin gallate （EGCG） in the finished tea undergoing full fire processing （FF-LYT） decreased by 68.34%，83.56%，64.75%， and 65.73%， respectively， compared to fresh tea leaves （FTL）. In contrast， gallocatechin gallate （GCG） content in FF-LYT was increased to 8.21 times of that in LYT. Similar to the cases， GA content was significantly increased in FF-LYT after full fire processing， which was 3.69 and 3.87 times of that in re-yellowing sample （RY‑LYT） and FTL， respectively. The CAFF content remained relatively stable， with the lowest coefficient of variation （CV） of 7.01% in LYT during processing， while GCG （CV=133.32%） and GA （CV=89.45%） showed the highest variability. The CV of total sugar was relatively low （7.75%）， and the obviously reduction on its content was observed in FF-LYT after full fire treatment， decreasing by 13.73% in comparison to RY-LYT. This reduction might be due to the sugars serving as substrates for thermal reactions， leading to the formation of volatile compounds. Moreover， the chemical compounds were crucial for the formation of tea taste. The catechin， GA， CAFF， and sugars had the positive contributions for tea taste due to the Dots greater than one in FF-LYT， indicating that these chemical compounds might be the potential key taste contributors. Among them， EGCG and CAFF have the highest Dot values， reaching to 312.51 and 270.06， respectively， suggested that EGCG and CAFF were the potential key non-volatile components for the bitter and astringent taste of LYT. In addition， the tea samples were effectively distinguished by PCA analysis according to tea processes， especially for the FTL， rolling （Ro-LYT）， and FF-LYT samples， indicated that the significant differences were observed among these samples.