A comparison of the phenolic composition of old and young tea leaves reveals a decrease in flavanols and phenolic acids and an increase in flavonols upon tea leaf maturation

Old tea leaves (OTL), which are normally discarded as agricultural waste, are potential sources of tea phenolic compounds. However, little details are known about their phenolic composition. In this study, the phenolic profiles of OTL and young tea leaves (YTL) were compared by using ultra-high performance liquid chromatography coupled to hybrid quadrupole Orbitrap mass spectrometry (UHPLC-Q-Orbitrap-MS) analyses, with an untargeted approach. A total of 60 phenolic compounds were tentatively identified. Principal component analysis illustrated distinct differences in overall phenolic profiles between OTL and YTL. In particular, after maturation, flavanols and phenolic acids decreased 1.7- and 3.0-fold, respectively, whereas flavonols increased 1.5-fold. In addition, degalloylation commonly occurred with leaf maturation, as evidenced by a decrease in galloylated catechins and flavanol dimers upon aging. Furthermore, by applying linear discriminant analysis effect size (LEfSe) algorithm, 14 representative phenolic compounds were identified which underwent extensive quantitative changes upon leaf aging, including: rutin, epigallocatechin, epicatechin, epigallocatechin gallate, epicatechin gallate, and epiafzelechin gallate. In conclusion, this study provides detailed insights in the phenolic composition of old and young tea leaves, facilitating the future utilisation of OTL as a new and cost-effective source of tea phenolics.