Recent Advances in N-Ethyl-2-Pyrrolidinone-Substituted Flavanols (EPSFs), Novel Bioactive Components in Tea: Chemical Structure, Formation Mechanism, and Potential Health Benefits

N-Ethyl-2-pyrrolidinone-substituted flavanols (EPSFs) are a newly discovered class of flavanol derivatives in tea and have emerged as a focal point in tea studies in recent years. Over 50 EPSFs and their analogs have been identified in fresh tea leaves and various tea products. These compounds are characterized by N-ethyl-2-pyrrolidinone substitution at either the C-8 or C-6 position of flavanols, and they exist as chiral enantiomers (R- or S-configuration). EPSFs are formed from theanine and flavanols through biosynthesis in fresh tea leaves, microbial-mediated synthesis during postfermentation of dark tea, chemical synthesis during thermal processing, and accumulation over prolonged storage periods. The contents of these compounds in teas are closely related to tea processing and storage; thus, they hold promise as potential markers for monitoring the degree of thermal processing (baking and roasting) and storage age of tea. Moreover, EPSFs exhibit a range of significant biological activities, as substantiated through both in vitro and in vivo studies. Nonetheless, the current status of EPSFs has not yet been systematically reviewed. This article aims to provide a comprehensive review of recent advancements in EPSFs from years 2005 to 2024, with a focus on their chemical structures, formation mechanisms, analytical methods, relationships with processing and storage, and bioactivities, as well as techniques for EPSF enrichment in teas. Furthermore, this review discusses future prospects of EPSF-containing functional foods and nutritional health products to illuminate potential applications of EPSFs in both the tea industry and the healthcare sector.