Development of a validated efficient HPLC-DAD analysis for assessing polyphenol transformation during black tea processing

Tea (Camellia sinensis) is valued for its polyphenolic compounds, which define its sensory and health attributes. Accurate quantification across processing stages is hindered by analytical and extraction challenges. We developed and validated a rapid high-performance liquid chromatography with diode array detection (HPLC-DAD) method for simultaneous analysis of 12 key constituents - gallic acid, theobromine, caffeine, (+)-catechin (C), (-)-epicatechin (EC), (-)-epigallocatechin (EGC), (-)-epigallocatechin gallate (EGCG), (-)-epicatechin gallate (ECG), theaflavin (TF), theaflavin-3-gallate (TF3G), theaflavin-3′-gallate (TF3’G), theaflavin-3,3′-digallate (TF3–3’G), in green and black tea. The method achieved superior linearity (r² > 0.9995), high sensitivity (LOD: 0.03–1.68 µg/mL), strong precision (RSD < 4.68 %), and high recovery, while also resolving co-elution with a 40-min runtime. Extraction was optimized using ultrasonication with 70 % methanol, which outperformed hot water and ISO-standard methods. Applied to black tea processing, the method revealed a 79.1 % reduction in catechins, post-rolling theaflavin peaks, and dynamic fluctuations in gallic acid, caffeine, and theobromine. These changes were associated with enzymatic oxidation, leaching, and cultivar effects. The validated HPLC-DAD method provides a robust tool for tea polyphenol profiling and enables improved understanding of processing-induced transformations. It holds potential for use in quality control, nutritional labeling, and functional food research in tea and other polyphenol-rich systems.