Simultaneous extraction and selective separation of catechins, caffeine and theanine from waste tea residue facilitated by citric acid-based deep eutectic solvent

This work prepared and investigated a novel type of deep eutectic solvent (DES) composed of citric acid and polypropylene glycol (PPG) for extracting and separating bioactive compounds from tea waste residue. Under optimized conditions, the DES composed by citric acid and PPG400 showed remarkable effectiveness in extracting epigallocatechin gallate (EGCG, 15.58 g/kg), epicatechin gallate (ECG, 12.85 g/kg), theanine (152.20 g/kg) and caffeine (48.44 g/kg). Results of kinetic study revealed that the extraction of ECG, EGCG, caffeine and theanine from tea residue can be well described by the Fick’s second law. Based on the difference in polarity and acidity/alkalinity among four solutes, two biphasic systems were constructed for the sequential separation of catechins (EGCG and ECG), caffeine and theanine from the DES extracts. Using the first biphasic system composed of DES and ethyl acetate, EGCG and ECG with relatively low polarity were effectively extracted into the ethyl acetate phase (efficiency >95 %), while theanine and caffeine were largely retained in the DES phase. Subsequently, a proper volume of KOH solution was intentionally added to the DES phase to break the hydrogen bonds between citric acid and PPG 400, which resulted in an aqueous two-phase system (ATPS) containing PPG400 and potassium citrate. This ATPS allowed selective partition of caffeine toward the PPG400-rich phase and that of theanine toward the citrate-rich phase. The recoveries of theanine and caffeine were 76 % and 96 %, respectively. Molecular dynamics simulations revealed that EGCG and ECG preferentially interacted with ethyl acetate, while caffeine and theanine are likely to interact with PPG400 and citrate, respectively. This provide insights into the high partition coefficients, extraction efficiencies, and selectivity in the extraction process.