Phenolic Acids and Their Derivatives in Tea: Chemical Diversity, Processing Changes, Sensory Contributions, and Health Implications—A Comprehensive Review

ABSTRACT

Phenolic acids are increasingly recognized as underexplored yet influential determinants of tea quality, sensory identity, and health functionality. This review integrates chemical, biochemical, sensory, and technological perspectives to construct the first comprehensive, cross‐tea‐type framework for understanding phenolic acid dynamics. We consolidate current knowledge on their structural diversity, biosynthetic regulation, and metabolic fluxes, and map how processing‐specific transformations across the six major tea categories reshape free and bound phenolic acid pools. By linking these molecular changes to sensory outcomes—color formation via metal coordination and oxidation, aroma generation through thermal and enzymatic degradation pathways, and taste modulation driven by ortho‐diphenolic structures—we highlight overlooked mechanistic connections. The review further synthesizes emerging evidence on phenolic acid–mediated biological activities, including antioxidant and anti‐inflammatory effects, while evaluating recent advances in analytical chemistry and molecular biology that refine quantification and elucidate pathways. Comparative analysis reveals distinct phenolic acid trajectories: green tea preserves 85%–90% of total phenolics with elevated free forms; white tea maintains balanced free/bound ratios through gentle withering; and black and dark teas undergo extensive oxidation and polymerization, reducing free acids but generating complex pigments. By integrating these insights, we identify leverage points in processing—such as fixation temperature and fermentation control—that offer new opportunities to optimize both product quality and functional potential. The review concludes by outlining critical research gaps and proposing a forward‐looking agenda to accelerate mechanistic, translational, and application‐oriented studies on phenolic acids in tea.

1. Introduction

Tea is one of the most widely consumed beverages worldwide and continues to attract scientific and commercial interest owing to its distinctive flavor profile and well‐documented health benefits (J. Y. Yu, Zhang, et al. ²⁰²³). Its sensory qualities and bioactivity arise primarily from a diverse suite of secondary metabolites—including phenolic acids, flavonoids, tannins, saponins, amino acids, polysaccharides, alkaloids, and terpenoids—that collectively shape the beverage's aroma, taste, mouthfeel, and functional properties (Y. Chen, Lai, et al. ²⁰²³; Luo et al. ²⁰²⁴; C. Wang, Han, et al. ²⁰²²). Among these constituents, phenolic compounds hold a particularly central role. Within this group, phenolic acids are notable for their dual contributions: they are key determinants of tea's color, aroma, and taste, and they also function as bioactive agents with a broad spectrum of physiological activities.

Phenolic acids can be defined by the presence of a phenolic ring and a carboxylic acid group, and they occur mainly as hydroxybenzoic acids (C₆–C₁ skeleton) and hydroxycinnamic acids (C₆–C₃ skeleton). As a key secondary metabolite in tea leaves, phenolic acids are mainly synthesized in tea plants through the shikimic acid pathway and the phenylalanine pathway. Their accumulation and distribution are complexly regulated by various factors, including tea varieties, ecological environments, cultivation management, and biological and abiotic stresses (Z. B. Liu, Vincken, et al. ²⁰²²). However, the key step determining the final composition and content of phenolic acids occurs during tea processing. During the production of the six major tea categories (green tea, yellow tea, white tea, oolong tea, black tea, and dark tea), critical processes such as fixation, rolling, fermentation, and drying lead phenolic acids to undergo various complex transformation pathways including enzymatic oxidation, nonenzymatic oxidation, hydrolysis, and polymerization reactions (Gui et al. 2023; M. F. Sun, Jiang, et al. ²⁰²²). These dynamic changes form the chemical basis for the formation of tea quality: they regulate the intensity of astringency, bitterness, and umami to constitute the taste framework of tea infusion; as one of the aroma precursor substances, transformation products of phenolic acids are key contributors to the formation of diverse aromas such as floral, fruity, or smoky notes; in addition, phenolic acids can also interact with metal ions and oxidative pigments to affect color development (Ito and Yanase 2022; Z. D. Jiang et al. ²⁰²³; Y. Shi et al. ²⁰²²; J. Zhou et al. ²⁰²²).

Beyond their sensory contributions, the health benefits of phenolic acids represent another core area of significant scientific progress. As the most representative bioactive substances among plant‐derived phenolic compounds, phenolic acids, such as chlorogenic, caffeic, p‐coumaric, ferulic, and vanillic acids, have been extensively studied and confirmed to possess multiple significant physiological effects, including antioxidant, anti‐inflammatory, antibacterial, anticancer, and cardiovascular protective properties (Z. D. Jiang et al. ²⁰²³; Rashmi and Negi ²⁰²⁰; M. F. Sun, Jiang, et al. ²⁰²²; C. N. Zhao et al. ²⁰¹⁹). Novel emerging technologies and innovations, including encapsulation techniques, chemical modification, and synthesis of phenolic acid derivatives and phenolic acid‐protein conjugates, are the new trends in the field (Giordano et al. 2025; Hu et al. ²⁰²⁶; Kuo et al. ²⁰²⁵; Islam et al. ²⁰²⁶). Furthermore, novel food delivery systems of tea polyphenol nanoparticles (Song et al. 2025), edible packaging materials (Benlloch‐Tinoco et al. ²⁰²⁵), functional beverages (Benlloch‐Tinoco et al. ²⁰²⁵; Farrell et al. ²⁰²⁴), emulsions (Z. Tan, Yang, et al. ²⁰²⁵), novel ingredients (Sánchez‐García et al. ²⁰²⁵), edible liposomes (Liang et al. 2025), and hydrogels (Z. Wang, Ni, et al. ²⁰²⁴) represent state‐of‐the‐art breakthroughs in food science research.

This work builds on our previous studies on the chemistry and biological activities of Camellia sinensis teas that focus primarily on tea catechins and other flavonoids, and our research clarifying the effects of tea constituents on human taste perception (L. Zhang, Cao, et al. ²⁰²⁰; L. Zhang et al. ²⁰¹⁹). Phenolic acids, as an important class of secondary metabolites in tea, occupy a distinctive position in both tea flavor quality and human health functions. However, unlike catechins, which account for 70%–80% of total tea polyphenols and have been extensively studied, phenolic acids have long been regarded as subordinate components of tea polyphenols, and their independent chemical identity and functional contributions have not received attention. Between 2021 and 2025, the number of research papers focusing on tea phenolic acids has surged dramatically, marking a substantial surge in research activity. Nevertheless, these findings remain highly fragmented, scattered across journals in plant chemistry, flavor chemistry, food processing, and clinical medicine. Against this backdrop, the present review shifts its analytical focus to phenolic acids as a unifying framework, integrating the latest evidence on their structural diversity, biosynthetic origins, and metabolic regulation in tea, while systematically tracing their dynamic transformation trajectories during the processing of major tea types, thereby advancing the understanding of tea quality and its health‐promoting properties. By linking biochemical pathways with sensory science, we reveal how phenolic acids underpin key quality attributes and shape color, aroma, and taste, and further discuss how these compounds impact human health through diverse biological mechanisms. This integrated perspective interconnects plant metabolism, processing chemistry, sensory performance, and functional outcomes, charting new theoretical and practical directions for future research and innovation in tea science.

2. Phenolic Acids in Tea

2.1. Chemical Structures of Phenolic Acids

Phenolic acids are a class of secondary metabolites commonly found in plants and play a crucial role in the physiological and quality development of tea (Robbins 2003) (detailed information on common phenolic acids in tea is provided in Table S1). As shown in Figure 1, phenolic acids can be divided into two main categories based on their carbon skeleton structure: C₆–C₁ (hydroxybenzoic acids) and C₆–C₃ (hydroxycinnamic acids) (Kumar and Goel 2019; Shahidi and Yeo ²⁰¹⁸; M. F. Sun, Jiang, et al. ²⁰²²). Hydroxybenzoic acid compounds are derived from the benzoic acid core structure, with key compounds including gallic acid, known for its potent antioxidant activity (de Sousa Ferreira et al. ²⁰²²). The phenolic hydroxyl group of gallic acid is substituted with a methyl group to form a derivative, 3‐O‐methylgallic acid. Ellagic acid is also one of the main phenolic acids in tea, which is formed by the condensation of two gallic acid molecules. m‐Digallic acid is linked by a meta‐position ester bond or a carbon–carbon bond. Theogallin is formed by the combination of gallic acid and quinic acid through an ester bond. As an important flavor‐contributing compound in tea, it particularly contributes to the umami taste. As important phenolic acid derivatives in tea, hydrolyzable tannins, including gallotannins (e.g., 1‐galloylglucose, 1,6‐digalloylglucose, 1,4,6‐trigalloylglucose, 1,2,3,4‐tetragalloylglucose) and some ellagitannins (e.g., strictinin, 1,2‐Di‐O‐galloyl‐4,6‐O‐(S)‐hexahydroxydiphenoyl‐β‐d‐glucopyranose), also have a benzoic acid structure, with glucose as the core. Their hydroxyl groups are esterified with one or more gallic acid molecules.

FIGURE 1.

Chemical structures of phenolic acids in tea. Created with BioRender.com.

Hydroxycinnamic acids, which contain cinnamic acid as their core structure, are phenylpropanoid metabolites. Typical representatives, p‐coumaric acid and caffeic acid (3,4‐dihydroxycinnamic acid), are key intermediates in the shikimate pathway and act as precursors for the synthesis of flavonoids or lignin. Ferulic acid, containing one methoxy moiety, presents better structural stability but less antioxidant activity compared to caffeic acid, containing ortho‐hydroxyls. Sinapic acid, with symmetric dimethoxy groups on the benzene ring, extends the conjugated system. Meanwhile, some hydroxycinnamic acid derivatives, such as p‐coumaroylquinic acid and caffeoylquinic acid, could be formed by the condensation of p‐coumaric acid and quinic acid. Furthermore, multiple caffeoylquinic acids have been identified in tea, primarily including chlorogenic acids formed by esterification of one molecule of caffeic acid with one molecule of quinic acid, as well as isochlorogenic acids formed by condensation of two molecules of caffeic acid with one molecule of quinic acid (Afnan et al. 2022; J. Y. Liu, Poojary, et al. ²⁰²³; Rashmi and Negi ²⁰²⁰).

Phenolic acids can exist in free form or be bound to various molecules, such as monosaccharides, organic acids, and plant‐based polymers (Robbins 2003), which account for about 10%–15% of the total polyphenols and approximately 5% of the total dry matter in fresh tea leaves. From the perspective of chemical structure, simple phenolic acids are centered around a benzene ring. The aromatic ring of C₆–C₁ hydroxybenzoic acid‐type phenolic acids is directly linked to a carboxyl group. In contrast, C₆–C₃ cinnamic acid‐type phenolic acids contain an acrylic acid structural unit, with their benzene rings often bearing substituents such as hydroxyl or methoxy groups (J. X. Chen, Yang, et al. ²⁰²⁰).

These structural differences impart phenolic acids with distinct physicochemical properties and biological activities. First, the presence of phenolic hydroxyl groups confers a certain level of acidity (pK _a ≈ 4–5) to the molecule, and the antioxidant capacity varies depending on the substitution positions (ortho‐diphenol, para‐monohydroxy, and meta‐hydroxy). Notably, the ortho‐diphenol structure, by combining the two mechanisms of single‐electron transfer (SET) and hydrogen atom transfer (HAT), and with its unique electron delocalization and bond energy characteristics, becomes the decisive factor in the antioxidant capacity of phenolic acid compounds (Ciardullo et al. 2024; Y. Ma et al. ²⁰²³; Panda et al. ²⁰²⁵). For instance, gallic acid substituted at the 3, 4, and 5 positions exhibits the strongest radical scavenging activity. Phenolic acids with a higher number of phenolic hydroxyl groups generally exhibit greater water solubility and lower lipid solubility. Second, the carboxyl group affects the molecule's solubility and ionization state, which, in turn, influences its transport and distribution within tea plants (J. Y. Liu, Poojary, et al. ²⁰²³). Moreover, the conjugated system formed by the acrylate side chain enhances molecular stability, whereas substituents such as methoxy groups regulate the compound's lipophilicity and bioavailability. These structural characteristics collectively determine the physiological roles of phenolic acids in tea plant growth, development, and stress responses, as well as their interactions with other components, such as catechins and metal ions, ultimately affecting the sensory qualities and clinically proven health benefits of tea.

2.2. Biosynthesis and Metabolism of Phenolic Acids in Tea Plants

2.2.1. Biosynthesis Pathways

As shown in Figure 2, the biosynthesis of phenolic acids in the tea plant predominantly relies on the coordinated interplay between the shikimate and phenylpropanoid pathways (Anantharaju et al. 2016; Martinović et al. ²⁰²³; Zhong et al. ²⁰²⁴). These two cascading metabolic routes systematically convert primary metabolites into hydroxybenzoic acids and hydroxycinnamic acids (Martinović et al. 2023). The shikimate pathway converts glycolysis‐derived PEP and pentose phosphate pathway‐derived E4P into chorismate via DAHP and shikimic acid, serving as the sole route for the biosynthesis of aromatic compounds (Almeida et al. 2024; L. Zhao et al. ²⁰²²). Chorismate‐derived phenylalanine enters the phenylpropanoid pathway, where PAL is the rate‐limiting enzyme for phenolic acid synthesis, whose elevated activity is commonly regarded as a hallmark of stress responses, catalyzing the deamination of phenylalanine to cinnamic acid, which is hydroxylated to p‐coumaric acid by C4H (Mishra and Baek 2021; A. A. Zhang, Ni, et al. ²⁰²³). Activated by 4CL to form p‐coumaroyl‐CoA (O. Chen, Deng, et al. ²⁰²¹; Guo et al. ²⁰²⁴; Ji et al. ²⁰²¹), this intermediate yields various hydroxycinnamic acids (e.g., caffeic acids, ferulic acids, p‐coumaroylquinic acids, chlorogenic acids) through modifications, while hydroxybenzoic acids form via β‐oxidation or side‐chain shortening of cinnamic acid derivatives (Gacnik et al. 2021; Y. Y. Sun, Luo, et al. ²⁰²²; Q. Xu, Li, et al. ²⁰²⁴).

FIGURE 2.

Major biosynthesis pathways of phenolic acids in tea.

Differential expression of 4CL genes in tea plants modulates carbon flux partitioning among downstream metabolic branches. Overexpression of Cs4CL concurrently affects the accumulation of phenolic acids, lignin, and flavonoids. The intermediate p‐coumaroyl‐CoA occupies a pivotal branch point in phenylpropanoid metabolism: in addition to serving as a substrate for hydroxycinnamoyl transferase (HCT) to generate phenolic acids and certain acylated flavonoids, it can also undergo condensation with malonyl‐CoA, catalyzed by chalcone synthase (CHS), to yield chalcone, the essential precursor for flavonoid backbone formation (G. Xu et al. ²⁰²⁵). Therefore, elevated PAL activity does not necessarily equate to the directed activation of flavonoid or phenolic acid pathways; rather, the actual routing of carbon flux is determined by the expression and enzymatic regulation of downstream branch‐point enzymes. HCT and CHS reside at critical branch points between the phenolic acid and flavonoid pathways, and their transcriptional expression and catalytic efficiency jointly modulate carbon flux partitioning into these two branches, thereby contributing to plant adaptive mechanisms under abiotic stresses such as low temperature and ultraviolet (UV)‐B radiation (X. Wang, Chai, et al. ²⁰²³).

2.2.2. Metabolic Regulation

The metabolism of phenolic acids in tea plants can be regulated by both biotic (e.g., pests and diseases) and abiotic (e.g., temperature, salt stress, light, etc.) stresses (Jia et al. 2024). Under abiotic stress, the phenylpropanoid pathway in tea plants is globally activated, leading to the coordinated accumulation of defensive metabolites, including phenolic acids, lignin, and flavonoids. These changes collectively enhance cell wall stability and antioxidant capacity. However, the associated reallocation of metabolic resources and alterations in tissue structure result in reduced leaf tenderness and diminished nutritional quality (X. Y. Liu et al. ²⁰²⁶). The optimal air temperature for tea plant growth is 18°C–30°C. New shoot growth is significantly inhibited when the temperature persistently exceeds 35°C or falls below −6°C in small‐ and medium‐leaf tea cultivars (X. Q. Tan et al. ²⁰²³; Xiang et al. ²⁰²¹). Moderate to high temperatures (30°C–38°C) promote the accumulation of phenolic acids and flavonoids by upregulating the expression of PAL, C4H, and 4CL, thereby strengthening antioxidant defense. In contrast, extremely high temperatures (>38°C–45°C) actively suppress the flavonoid pathway through the CsHSFA‐CsJAZ6 module, redirecting carbon flux from flavonoids toward phenolic acids and lignin biosynthesis (X. Zhang, Li, et al. ²⁰²³). Similarly, moderately low temperatures (4°C–10°C) activate the transcription factor CsABF2 via the ABA signaling pathway, which upregulates CsPALa and CsMYB6c, thereby enhancing the expression of phenylpropanoid pathway genes and the accumulation of phenolic compounds (Huang et al. 2026). However, extremely low temperatures (<−6°C) cause membrane lipid peroxidation and photosynthetic inhibition, ultimately blocking phenolic acid synthesis. Meanwhile, the carbon flow is redirected toward osmotic adjustment substances, such as soluble sugars and amino acids, leading to an imbalance in the phenolic acid‐to‐lignin ratio. Salt stress treatment significantly increases transcription of the tea plant CsHCT gene. Overexpression of CsHCTs in tobacco and Arabidopsis results in increased accumulation of phenolic acids and lignin, decreased levels of flavonol glycosides, and enhanced stress tolerance (Y. Chen, Yi, et al. ²⁰²¹). Under salt stress, overexpression of CsF3H promotes the accumulation of flavan‐3‐ols. In contrast, the key enzyme genes in the lignin branch (CsCCR, CsCAD) are simultaneously upregulated, thereby enhancing salt tolerance (Gaude and Jalmi 2025). Light intensity and quality can modulate PAL gene expression, thereby affecting the amount of phenolic acid synthesized. Furthermore, appropriately increasing light intensity can promote the accumulation of phenolic acids in tea leaves (Yue et al. 2021). For example, under high‐light‐intensity conditions, the levels of amino acids and their derivatives, as well as alkaloids, in tea leaf tissues significantly increase, while the content of flavonoids decreases. Pre‐irradiation with UV‐B before low‐temperature exposure induces coordinated accumulation of flavonoids and phenolic acids via the CsHY5 pathway, thereby enhancing cold tolerance. These findings suggest that light and temperature signals act synergistically to regulate phenolic acid metabolism (Ke et al. 2025; Z. W. Zhou et al. ²⁰²⁵). UV‐B treatment substantially downregulates the expression of MYB repressors CsCPCa and CsMYBL2b, relieves inhibition of CsPAL and Cs4CL, and promotes phenolic acid accumulation (P. H. Li et al. ²⁰²²). Under high‐altitude conditions, the expression of key upstream enzyme genes involved in phenolic acid biosynthesis in tea plants, including CsPAL, CsC4H, and Cs4CL, is upregulated, and the accumulation of phenolic acids exhibits pronounced structure‐dependent patterns. Specifically, carbon flux is redirected from the anthocyanin branch toward nonpigmented flavonoids (e.g., flavonols, flavanols, and flavanones) and structurally complex phenolic acid derivatives, such as galloylated esters and caffeoyl glucosides, which exhibit enhanced antioxidant and UV‐protective properties (B. Wang et al. ²⁰²⁶). Glufosinate stress inhibits glutamine synthetase (GS) activity in tea plants, leading to disruption of nitrogen metabolism and the accumulation of toxic ammonium. This, in turn, suppresses photosynthesis and triggers a reprogramming of carbon flux through the phenylpropanoid pathway. The metabolic shift involves upregulating key enzymes, including CsHCT, CsCAD, and CsPER, thereby preferentially diverting carbon flow toward the biosynthesis of phenolic acids and lignin (H. Yu, Li, Wu, et al. ²⁰²⁴). Targeted metabolomic analyses have further demonstrated that infection by Exobasidium vexans systemically activates the phenylpropanoid pathway in tea plants, leading to pronounced accumulation of phenolic acids, including 2,4,6‐trihydroxybenzoic acid, and galloylated proanthocyanidins (X. Zhou, Hu, et al. ²⁰²³). At the molecular level, the specific upregulation of key structural genes, such as Cs4CL, together with the concomitant accumulation of critical phenolic acids and their derivatives, namely, p‐coumaroyl‐CoA, caffeoylquinic acid, and feruloyl‐CoA, has been shown to enhance the resistance of tea plants to the tea aphid (Toxoptera aurantii) (J. Fan et al. ²⁰²⁵).

Water and soil nutrients also have important effects on the growth and metabolism of tea plants (Shao et al. 2025; M. J. Wang, Sun, et al. ²⁰²⁴; Xie et al. ²⁰²⁵). Nitrogen deficiency can increase CsPAL activity in plants, thereby elevating phenolic acid levels. Moreover, the synthesis and accumulation of hydroxybenzoic acid‐type and hydroxycinnamic acid‐type phenolic acids may be regulated in different ways. Different irrigation methods and nitrogen application rates also affect the formation of phenolic acids in tea. Appropriate increases in the nitrogen application rate will increase the concentrations of ferulic acid, p‐coumaric acid, and vanillic acid (D. Y. Ma et al. ²⁰¹⁵). The expression levels of CsPAL and Cs4CL are low in young leaves, and the content of phenolic acids is also relatively low; as leaf age increases, activators such as CsMYB58/63 are upregulated, and chlorogenic acid and caffeic acid gradually accumulate. Young leaves mainly rely on flavonoid–phenolic acid “soluble defense” under low‐temperature stress, while mature leaves enhance structural resistance through lignification (H. Yu, Li, Wu, et al. ²⁰²⁴; H. Zuo et al. ²⁰²⁴).

As intermediate products of the phenylpropanoid metabolism, phenolic acids (e.g., caffeic acid, ferulic acid) exhibit lignification‐promoting effects, of which ferulic acid is a characteristic representative that serves as the direct precursor of lignin monomers (M. H. Han et al. ²⁰²¹). Their accumulation catalyzes lignin synthesis via enzymes such as peroxidase (POD), thereby indirectly reducing leaf tenderness and retention (J. W. Li et al. ²⁰²⁴; H. B. Yang, Jia, et al. ²⁰²⁴). Lignin, a major component of secondary cell wall thickening in plants, accumulates during leaf maturation, leading to leaf hardening and fiber formation. In tea, young leaves exhibit low lignin content. Still, as leaves mature, lignin synthesis (driven by the phenylpropanoid pathway) increases significantly, enhancing cell wall rigidity and diminishing tenderness retention (Z. Zhao, Wang, et al. ²⁰²⁵). With the phenylpropanoid pathway, substrate competition exists between phenolic acid and lignin synthesis (N. N. Liu, Wang, et al. ²⁰²³). Environmental stressors (e.g., drought, high light intensity) or hormonal signals (e.g., jasmonic acid) may upregulate this pathway, simultaneously promoting the accumulation of both phenolic acids and lignin (W. B. Jiang et al. ²⁰²⁴; H. R. Liu, Duan, et al. ²⁰²⁴). Jasmonic acid is a core hormone that enables plant responses to biotic stresses (e.g., insect feeding and pathogen infection) and abiotic stresses (e.g., drought, salt stress, low temperature, and heavy metals). The expression of CsPAL and CsTAT (tyrosine aminotransferase, a key enzyme in the tyrosine metabolic pathway) is regulated by hormones, including methyl jasmonate. Tea varieties with strong leaf tenderness retention traits (such as Fuding white tea) usually exhibit low CsPAL activity (W. Y. Zhang et al. ²⁰²¹), thereby reducing lignin and phenolic acid precursor synthesis and delaying leaf hardening.

2.3. Changes of Phenolic Acids During Tea Processing

2.3.1. Green Tea Processing

As shown in Figure 3, during green tea processing, fresh leaves undergo fixing, rolling, and drying (J. Yu, Li, Lin, et al. ²⁰²⁴; Y. Q. Zhang, Wang, Wu, et al. ²⁰²⁵). The core of the processing is to inhibit enzymatic oxidation through high‐temperature fixation, maximizing the preservation of the natural components in tea leaves. In fresh leaves, phenolic acids are mostly bound, with a relatively low content of free phenolic acids (Gui et al. 2023). The dynamic transformation of phenolic acids during processing demonstrates significant process‐dependent characteristics, exerting a notable influence on the final product quality (Gui et al. 2023). During the fixation stage, heat treatment significantly affects the transformation dynamics of phenolic substances. Studies have shown that the hydrolysis of ester‐type catechins (such as epigallocatechin gallate [EGCG]) results in a significant increase in the levels of specific free phenolic acids (e.g., gallic acid). In contrast, the variation trends of phenolic acids with different structures show noticeable differences (e.g., the content of p‐coumaric acid decreases) (R. Li, Qu, et al. ²⁰²⁵; Y. Yang, Ye, et al. ²⁰²⁴). High‐temperature fixation treatment has dual effects: on one hand, it can rapidly inactivate endogenous enzymes such as polyphenol oxidase (PPO) and POD; on the other hand, it promotes the hydrolysis of ester‐type catechins through thermodynamic effects, and in particular, the degradation of EGCG directly leads to an increase in the content of free gallic acid (Hua et al. 2025; Lai et al. ²⁰²²; R. Li, Qu, et al. ²⁰²⁵; Z. Yang, Zhang, Song, et al. ²⁰²⁵). This transformation pattern of phenolic substances is closely related to parameters. Moderate‐heat‐treatment conditions can maximize the ratio of ester‐type to non‐ester‐type catechins, thereby improving the flavor quality of green tea (Y. Yu, Zhu, et al. ²⁰²³). Subsequently, the rolling process disrupts cell structure through mechanical force, thereby enhancing the uniform distribution of phenolic acids and inducing slight oxidation, thus reducing the content of reactive phenolic acids, such as caffeic acid (J. Yu, Li, Lin, et al. ²⁰²⁴). The drying phase of phenolic acids is based on their thermal stability.

FIGURE 3.

The processing procedures of the six major types of teas.

The drying stage exhibits selective retention of phenolic acids, with gallic acid, which has high thermal stability and antioxidant capacity, retained at up to 90%. Caffeic acid, due to its catechol structure, is more prone to thermally induced isomerization and oxidative degradation (Luan et al. 2023). During the drying stage, it can be converted into other derivatives, with a loss rate of about 10%–15%. Throughout the processing, total phenolic acids retain a relatively high retention rate under high‐temperature conditions (Y. Liu, Chen, et al. ²⁰²³). The degradation of some bound phenolic acids compensates for the loss of free phenolic acids. Moderate heat treatment helps reduce the degradation of nonvolatile components, thereby stabilizing total phenolic acid content and improving tea sensory quality (H. C. Zhou et al. ²⁰²⁴).

These transformations directly determine the key quality attributes of green tea: while retained phenolic acids impart a fresh taste, increased free phenolic acids enhance antioxidant capacity, and interactions with other components contribute to the formation of its distinctive fresh aroma (A. Sun et al. ²⁰²³). Optimizing critical processing parameters (such as fixed temperature and drying methods) can precisely regulate the pathways of phenolic acid conversion, providing scientific evidence for quality improvement (Y. Wei et al. ²⁰²⁴). Changes in the types and amounts of phenolic acids during processing not only affect sensory characteristics but also influence the biological activity and stability of green tea products, highlighting the importance of process control in determining final product quality and functional properties.

2.3.2. White Tea Processing

White tea, a typical representative of lightly fermented teas, undergoes relatively low fermentation, typically 5%–10% (J. M. Zhang et al. ²⁰²⁴; H. Zuo et al. ²⁰²³). It primarily achieves specific chemical transformations in the tea leaves through two key steps: natural withering and low‐temperature drying (P. Y. Wang, Zhao, et al. ²⁰²⁴; Ye et al. ²⁰²⁵). This unique processing method enables white tea to retain high levels of original catechins and promotes the conversion of specific amino acid components during withering. The technique can also modulate the metabolic pathways of phenolic compounds by controlling fermentation intensity, thereby shaping the characteristic chemical profile of white tea (Sanlier et al. 2018).

During the withering process, which lasts 48–72 h, the endogenous enzyme system in tea leaves gradually becomes activated. The continuous action of β‐glucosidase and esterase promotes significant hydrolysis of bound phenolic acids (e.g., gallic tannins and chlorogenic acid esters), thereby increasing the free gallic acid content. At the same time, residual PPO mediates slow enzymatic oxidation at low activity levels, selectively converting chlorogenic acid and other caffeic acid derivatives into dimeric forms. During sun exposure, photoperoxidation further promotes the conversion of hydroxycinnamic acids (such as p‐coumaric acid), reducing their content. Under natural withering conditions, dehydration continues to occur in tea leaves. Along with respiratory decomposition and enzymatic oxidation, polyphenol content gradually decreases (T. Wang, Wang, et al. ²⁰²³). The total content of polyphenols and catechins in white tea leaves declines slowly during the initial stage of withering and rapidly thereafter (S. Zhou, Zhang, et al. ²⁰²³). The levels of phenolic acid substances also show a downward trend. However, the content of tea pigments, such as theaflavins (TFs), thearubigins (TRs), and theabrownins (TBs), increases, which is beneficial for the formation of the color and infusion color of white tea (Dai et al. 2017). Temperature‐controlled withering can accelerate the rate of change in phenolic acid content in white tea, thereby enhancing withering efficiency (Q. C. Chen, Shi, et al. ²⁰²⁰).

The unique phenolic acid profile of white tea is formed through a low‐temperature drying process (Lai et al. 2022). During processing, the levels of free phenolic acids—such as gallic acid and p‐coumaric acid—significantly increase, while bound forms, including isomers of p‐coumaroylquinic acid, undergo hydrolysis and transformation (Y.‐H. Chen et al. ²⁰²²). Studies indicate that the retained phenolic compounds contribute to the fundamental flavor characteristics of the tea infusion, with free gallic acid being a key constituent; its elevated content is associated with enhanced biological activity in the tea. Meanwhile, the transformation of flavonoid glycosides during drying may confer potential aging properties to the tea (Z. Wang, Liang, et al. ²⁰²⁴; S. Zhou, Zhang, et al. ²⁰²³).

Compared with processed tea varieties, phenolic acid transformation in white tea processing exhibits the following significant characteristics: it avoids intense oxidative polymerization reactions (e.g., the formation of TFs in black tea), thereby maximizing the retention of the original structure and functional activity of phenolic acids. This gentle and precise chemical conversion is the material foundation for white tea's clarity, sweetness, smoothness, and freshness, as well as the source of its unique health benefits (Q. C. Chen, Shi, et al. ²⁰²⁰). At the same time, it provides abundant precursors for quality evolution during subsequent storage processes.

2.3.3. Yellow Tea Processing

Yellow tea, a lightly fermented tea, can be classified into three categories based on the degree of freshness of the raw leaves: yellow bud tea, small‐leaf yellow tea, and large‐leaf yellow tea (Y. M. Wei et al. ²⁰²¹). Its unique “yellowing” process endows it with the distinctive qualities of “yellow infusion and yellow leaves,” as well as a distinct transformation pathway for its tea components. The processing of yellow tea involves four critical steps: fixing, rolling, yellowing, and drying (Figure 3), which achieve targeted chemical transformations under humid and hot conditions (Y. M. Wei et al. ²⁰²¹). This process promotes the spontaneous oxidation of tea polyphenols and the conversion of other substances, thereby significantly reducing esterified catechins and degrading chlorophyll, producing the bright yellow color of dried tea. This technological feature ensures that yellow tea retains 60%–70% of its original catechins while enhancing the levels of free amino acids and producing characteristic yellow pigments and methylated phenolic acid derivatives (Y. M. Wei et al. ²⁰²³).

Significant and specific transformations of phenolic acids occur during the yellowing stage (Q. C. Chen, Shi, et al. ²⁰²⁰). The structures undergo substantial modifications. For example, phenolic acids such as caffeic acid and ferulic acid undergo reactions, including esterification and methylation, involving functional groups, such as carboxyl and hydroxyl, during the yellowing process, thereby transforming them from simple phenolic acids into complex derivatives (Q. C. Chen, Shi, et al. ²⁰²⁰). The formation of these polymers not only changes the molecular weight and solubility of the phenolic acids but may also affect their antioxidant activity and other bioactive properties. During the core yellowing stage, the endogenous enzyme system and the microbial communities in tea interact synergistically (Feng et al. 2024). On the one hand, humid and warm conditions activate esterases and glycosidases, thereby significantly hydrolyzing bound phenolic acids (e.g., chlorogenic acid esters and gallic tannins) and increasing the free gallic acid content. On the other hand, extracellular enzymes secreted by microorganisms (such as Aspergillus and Penicillium) catalyze the modification of phenolic acids, producing metabolites such as hydroxyphenylpropionic acid (X. Han, Zhou, et al. ²⁰²³; C. Ma et al. ²⁰²¹). At the same time, phenolic transformation mediated by oxidases contributes to pigment formation, but the specific mechanism remains to be elucidated (L. Liu, Shi, et al. ²⁰²²). Meanwhile, under oxygen‐limited conditions, PPO‐mediated selective oxidation converts catechins and other phenolic compounds into pigment precursors.

2.3.4. Oolong Tea Processing

Oolong tea is a semifermented tea, and its processing involves withering, shaking, fixing, rolling, and drying, with “shaking” being the core step (Figure 3). The unique alternating technique of shaking and airing, which promotes partial oxidation, is essential in creating its distinctive qualities: a “green center with red edges” and a golden‐yellow infusion (Hao et al. 2024; Salman et al. ²⁰²²).

In traditional processing techniques, the “shaking” step induces the transformation of tea metabolites through mechanical force and dehydration, a key step in shaping the aroma and flavor of oolong tea. This process involves two steps: shaking and withering, and gently breaking the polyphenol oxidation at the leaf edges while allowing the leaves to ferment naturally during the resting period. Research shows that during the green‐making process, the content of phenolic acids and aromatic components undergoes dynamic changes (X. F. Lu et al. ²⁰²³). PPO catalyzes the oxidation of chlorogenic acid to form ortho‐quinones. These highly reactive ortho‐quinone intermediates can cause tea browning by mediating the oxidation of phenolic compounds such as catechins or by polymerizing to form high‐molecular‐weight pigments (W. X. Wang, Ho, et al. ²⁰²²).

The increase in caffeic acid is primarily due to the hydrolysis of chlorogenic acid, with oxidation products contributing to the “golden edge” characteristic of oolong tea. Withering is the first step in oolong tea processing, promoting flavor development through the combination of moisture loss and enzymatic reactions. Phenolic acids experience a certain degree of oxidation and transformation influenced by light and temperature. The conversion of phenolic acids at this stage directly affects the oxidation level during subsequent shaking and the taste balance of the final tea infusion.

Withering is the primary critical step in oolong tea processing, which promotes flavor formation through moisture loss and enzymatic reactions (Z. Liu, Chen, Sun, et al. ²⁰²²; Zeng et al. ²⁰²⁴). Phenolic acids undergo oxidation and transformation under the influence of light (especially blue and red light), and the degree of their transformation directly determines the oxidation level in the subsequent shaking stage and ultimately affects the taste balance of tea leaves (L. Chen, Liu, et al. ²⁰²¹; Hua et al. ²⁰²⁴). During the withering process, cell tissue damage leads to the release of endogenous compounds (e.g., phenolic compounds), among which the oxidation of phenolic acids is an important pathway for the transformation of tea aroma precursors.

Fixation involves rapidly halting enzyme activity in tea leaves through high‐temperature treatment to stop fermentation and stabilize the tea's aroma and color. This high‐temperature process prevents the oxidation of phenolic acids and may reduce specific phenolic acids through thermal decomposition. For instance, catechin derivatives and flavonoid glycosides are degraded during fixation, whereas phenolic acids, such as gallic acid, accumulate. Fixation halts enzymatic reactions but preserves some early‐stage oxidation products and complexes, yielding a range of phenolic acids with intermediate concentrations between those in green and black tea. Additionally, unique flavor compounds associated with phenolic acids are generated. Rolling further facilitates interactions between phenolic acids and other components, such as amino acids and sugars. High temperatures during drying promote the oxidation, decomposition, or polymerization of phenolic compounds, thereby shaping the aroma, flavor, and color of oolong tea.

2.3.5. Black Tea Processing

As a fully fermented tea, the production of black tea involves four stages: withering, rolling, fermentation, and drying (Figure 3). The formation and transformation of phenolic acids are closely linked to these four processing steps (Cai et al. 2022; P. H. Yu, Huang, et al. ²⁰²³). The content of free phenolic acids in fresh leaves is relatively low, and they primarily exist in the form of soluble conjugated phenolic acids.

Using Keemun Congou black tea as an example (Figure 4), we examined the changes in phenolic acids during the black tea processing. During the withering process, the loss of moisture due to high temperature induces an increase in salicylic acid synthesis. The change in quinic acid content is particularly notable, as it gradually activates endogenous enzymes (PPO, POD) (Chiang et al. 2022; L. Lu et al. ²⁰²⁴). The rolling process mechanically damages leaf tissue, promoting the release of phenolic acids from vacuoles, thereby enhancing enzyme–substrate interactions, facilitating the hydrolysis of catechins, and significantly increasing free gallic acid content. A large amount of coumaric acid ester is hydrolyzed to produce coumaric acid (Lai et al. 2022; S. Zhang, Wu, et al. ²⁰²³). Fermentation, as the core stage, drives a broad spectrum of enzymatic oxidation reactions, significantly transforming the chemical composition of black tea and thereby affecting its flavor, aroma, color, and nutritional and biological characteristics. Catechins undergo oxidation catalyzed by PPO/POD, with phenolic acids (such as gallic acid and chlorogenic acid) donating phenolic hydroxyl groups that act as hydrogen donors to facilitate electron transfer during oxidation, thereby accelerating the formation of ortho‐quinones (G. Y. Zhang, Yang, et al. ²⁰²⁰). During black tea fermentation, the B‐ring catechol moiety of catechins is oxidized by PPO to generate highly reactive o‐quinone intermediates (Tanaka 2025). Concurrently, phenolic acids bearing o‐diphenolic structures, such as chlorogenic acid and caffeic acid, are similarly oxidized by PPO to their corresponding o‐quinones. These phenolic acid‐derived o‐quinones undergo radical‐mediated oxidative coupling with catechin o‐quinones, yielding mixed oligomers. For example, the o‐quinone of chlorogenic acid (CQA) couples with the o‐quinone of (−)‐epigallocatechin (EGC) to form a dichlorogeniccatechin (DCGC) oligomer bearing a bicyclo[3.2.2]nonane core; the content of this product increases sharply after the withering stage of black tea processing (Lai et al. 2022). In addition, the electron‐rich phenolic hydroxyl groups of phenolic acids can serve as nucleophiles, directly attacking the electrophilic carbon atoms of catechin o‐quinones to form covalent C─O or C─C bonds via nucleophilic addition, thereby tethering phenolic acids onto the growing catechin polymer chains to form mixed oligomers. These oligomeric intermediates can further undergo radical coupling or successive quinone–phenol reactions, ultimately being oxidatively polymerized into TRs. Furthermore, the partial cleavage of ester bonds in galloylated catechins (e.g., EGCG) during fermentation releases free gallic acid, which can either be re‐oxidized by PPO to its o‐quinone and enter the coupling pathways described above, or undergo esterification via its carboxyl group with hydroxyl groups on TR precursors, thereby covalently inserting galloyl moieties into the TR polymer backbone (F. Zhang, Wang, Aaqil, et al. ²⁰²⁵). Among these, caffeic acid serves as the crucial mediator for the formation of the colored product, while ferulic acid facilitates the polymerization of the colored product (Ito and Yanase 2022; X.‐K. Zhang, He, et al. ²⁰¹⁸; J. Zhou et al. ²⁰²²). p‐Coumaric acid is consumed and converted into the metabolite p‐coumaroylquinic acid during fermentation. Meanwhile, the theogallin content initially increases slightly before declining. The primary phenolic acid, chlorogenic acid, and its isomers (e.g., caffeoylquinic acid) are formed during black tea fermentation (S. W. Zhang, Yang, et al. ²⁰¹⁸).

FIGURE 4.

Changes in phenolic acids during the processing of Keemun Congou black tea.

High temperatures during the drying process may further oxidize or decompose phenolic acids, reducing their content (J. Zhou et al. ²⁰²²). High temperatures and oxygen‐rich environments can accelerate the oxidation of phenolic acids. For example, gallic acid can be oxidized to form pigments (X. Fang et al. ²⁰²³). Under high‐temperature conditions, phenolic acids may undergo thermal decomposition, producing small‐molecule organic acids or other compounds (J. Zhou et al. ²⁰²²). Phenolic acids may also react with other components in tea (such as amino acids and sugars) through esterification or condensation, forming new compounds. Changes in phenolic acids during the drying stage significantly affect the final quality of black tea.

2.3.6. Dark Tea Processing

Dark tea is a post‐fermented tea. The processing techniques for dark tea include fixing, rolling, piling, and drying (P. P. Liu, Feng, et al. ²⁰²⁴; S. H. Liu, Huang, et al. ²⁰²⁴). The pile fermentation stage is crucial, as the content of phenolic acids undergoes significant changes (S. Wen et al. ²⁰²⁴). For example, gallic acid and chlorogenic acid may decrease during pile fermentation, while new phenolic acid derivatives might form. Overall, phenolic acids undergo oxidation, polymerization, and degradation during processing, ultimately contributing to the unique flavor of dark tea (Cheng et al. 2021). During pile fermentation, microbial metabolic activities play a vital role in transforming phenolic acids (C. Q. Ma et al. ²⁰²⁴; Zhu et al. ²⁰²⁰). Microorganisms degrade phenolic acids and generate new phenolic derivatives, which are essential for the development of dark tea quality. Wet‐heat effects promote the oxidation and transformation of phenolic acids during pile fermentation, altering the color and flavor of the tea (S. Wen et al. ²⁰²⁴; W. Xu, Zhao, et al. ²⁰²³). Drying processes fix the content of phenolic acids, while aging can further alter their content and types, enhancing the overall quality of dark tea.

During pile fermentation, enzymatic oxidation (primarily mediated by PPO and POD) is the primary mechanism driving phenolic acid transformation. Free phenolic acids are oxidized to form reactive quinones, which further polymerize with components such as catechins to generate TFs and TRs (T. An et al. ²⁰²¹; Lai et al. ²⁰²²; Xiao et al. ²⁰²²). Concurrently, malic acid participates in metabolism, while hydroxycinnamic acid compounds, including chlorogenic acid, undergo oxidative degradation, resulting in a significant decrease in free phenolic acid content, accompanied by the accumulation of complex polyphenols such as TBs and the production of decarboxylation‐derived volatile substances (T. An et al. ²⁰²¹; W. J. Ma, Shi, et al. ²⁰²²; Martínez‐Inda et al. ²⁰²³; J. Shi et al. ²⁰²¹; W. Yu et al. ²⁰²⁵). The subsequent drying process terminates enzymatic reactions while triggering thermal transformation, promoting the isomerization of chlorogenic acid and the further polymerization of active intermediates (L. Chen, Wang, et al. ²⁰²³; M. Li, Xu, et al. ²⁰²⁵). Ultimately, natural phenolic acids are extensively converted into polymeric oxidation products, and this systematic transformation underpins the unique sensory quality and stability of dark tea (J. Shi et al. ²⁰²¹).

2.4. Contribution of Phenolic Acids to the Sensory Quality of Tea

2.4.1. Color

Phenolic acids (such as gallic acid and chlorogenic acid) are key components of tea polyphenols, and their color‐changing properties directly influence the color of the tea infusion (Kaczmarek‐Szczepańska et al. ²⁰²²; Z. Wang, Han, et al. ²⁰²⁵). Free phenolic acids are usually light yellow or colorless. During processing or storage, phenolic acids participate in the formation and stabilization of tea infusion color through multiple pathways, including oxidative polymerization, nonenzymatic browning, pH modulation, and metal complexation. During black tea fermentation, phenolic acids are directly involved in the formation of tea pigments (X. Liu, Chen, Yang, et al. ²⁰²²; Z. M. Xu et al. ²⁰²²; S. Zhang, Wu, et al. ²⁰²³). After phenolic acids bearing catechol structures, such as chlorogenic acid, are oxidized by PPO to their corresponding o‐quinones, they can undergo radical‐mediated coupling with catechin‐derived o‐quinones to form mixed oligomers. These oligomers then serve as precursor intermediates that further polymerize to generate tea pigments. The presence of gallate esters as the dominant form of phenolic acids in TRs directly confirms that phenolic acids are incorporated as structural units in the construction of TRs. Additionally, galloylated catechins (e.g., EGCG) release gallic acid upon thermal or enzymatic hydrolysis, thereby contributing to color formation. The liberated gallic acid can then be incorporated into the tea pigment backbone via re‐oxidation or esterification, thereby being integrated as galloyl moieties within the polymer structure. Oxidation converts them into quinones, among which chlorogenic acid quinone‐mediated oxidation predominates in nonenzymatic browning (X. Liu, Chen, Yang, et al. ²⁰²²; X.‐K. Zhang, He, et al. ²⁰¹⁸). Phenolic acids influence the color of tea infusions by modulating the pH of the liquor. Under acidic conditions, gallic acid inhibits the oxidative browning of catechins, whereas alkaline conditions accelerate oxidative polymerization and promote the formation of tea pigments (L. Chen, Liu, et al. ²⁰²¹; J. Tan, Vincken, et al. ²⁰²⁵). Chlorogenic acid retards the oxidative process by competing for the active sites of PPO or by chelating Cu²⁺ ions in an acidic environment (C. Wang, Dai, et al. ²⁰²⁵; Z. Xu, Yang, et al. ²⁰²³), and its chelating capacity is enhanced by its ortho‐diphenolic structure. During processing, the increase in free phenolic acid content (e.g., gallic acid), coupled with the accompanying drop in pH, acts synergistically to promote the accumulation of tea pigments, whereas during storage, the increase in total phenolic acid content is correlated with the continued oxidation‐driven darkening of the tea infusion (F.‐Y. Fan et al. ²⁰²¹). Furthermore, phenolic acids can form dark complexes by binding with metal ions such as Fe³⁺ and Cu²⁺, accelerating the browning of the tea infusion (Bijlsma et al. 2022); or under alkaline conditions (such as with hard water), the deprotonation of phenolic acids promotes electron transfer, significantly increasing the oxidation rate and thus affecting the stability of the tea infusion color (Figure 4) (S. Zhang, Wu, et al. ²⁰²³).

Multiple sensory evaluations combined with metabolomic analyses have provided quantitative evidence for the contribution of phenolic acids to tea infusion color. A correlation analysis of Oriental Beauty tea confirmed that 5‐p‐coumaroylquinic acid affects both the color and taste of the tea infusion, while gallic acid was also specifically identified as a contributor to the infusion color (Jin et al. 2024). Similarly, sensory evaluation and colorimetric analysis of Liupao tea identified gallic acid and 3‐galloylquinic acid as key color‐contributing compounds.

2.4.2. Aroma

Phenolic acids, key precursors of aroma, can generate a range of volatile compounds through specific reaction pathways. For example, during thermal degradation (roasting or drying), quinic acid undergoes dehydration and decarboxylation to form phenols and furans (2,5‐dimethylfuran). Caffeic acid, via decarboxylation and radical‐mediated reactions under anaerobic or oxidative conditions, yields large amounts of catechol and 4‐ethylcatechol, as well as pyridine and pyrazines. Chlorogenic acid, as an ester of quinic and caffeic acids, produces a combination of these volatiles, including benzoic acid, furfuryl alcohol, and various phenols (Moon and Shibamoto 2010). As a precursor of phenolic acids, phenylalanine can generate key aroma substances such as benzaldehyde and phenylacetaldehyde through the phenylpropanoid metabolic pathway (X. Q. Wang et al. ²⁰¹⁹). Studies have demonstrated that during black tea processing, the concentrations of free phenolic acids—including gallic acid, chlorogenic acid, and p‐coumaric acid—increase markedly. Under oxygen‐enriched conditions, PPO catalyzes the oxidation of these phenolic acids, converting chlorogenic acid and related o‐dihydroxyphenolic substrates into highly reactive o‐quinones (Lai et al. 2022). These electrophilic intermediates can induce Strecker degradation of free amino acids, generating volatile aldehydes that contribute floral, fruity, roasted, and sweet notes, depending on the amino acid precursors. The hydrolysis of the conjugates of phenolic acids and glycosides releases aroma‐active substances (L. Chen et al. ²⁰²⁵). These compounds significantly affect the quality characteristics of tea aroma under different processing conditions (Figure 5). Phenolic acids can indirectly regulate aroma formation by influencing fatty acid metabolism and phenylpropanoid metabolic pathways. During the fermentation of Pu‐erh tea, tannase hydrolyzes gallotannins to release gallic acid and its degradation products. These intermediates subsequently undergo methylation catalyzed by microbial methyltransferases, yielding methoxybenzene compounds such as 1,2,3‐trimethoxybenzene and 1,2,4‐trimethoxybenzene, which are characteristic contributors to the distinctive stale aroma of Pu‐erh tea (Y. Zhao, Xiao, et al. ²⁰²⁵). In green tea, during fixation, phenolic acids undergo thermal degradation due to the inactivation of PPO and other enzyme systems. Critical high‐temperature roasting stages of Lu'an Guapian tea, such as the high‐temperature roasting (final firing) step, are thought to promote the formation of phenylpropanoid‐derived aroma compounds. It is suggested that during this process, the thermal degradation of ferulic acid yields 4‐vinylguaiacol and guaiacol, while caffeic acid degradation produces catechol and 4‐ethylcatechol. These volatile phenolic compounds are likely key contributors to the characteristic roasted and smoky aroma profile of the finished tea (J. Yu, Li, Lin, et al. ²⁰²⁴). Oolong tea is a semifermented tea, and its aroma is formed through enzymatic oxidation and thermal degradation. These aromatic compounds synergize with alcohols, aldehydes, esters, and other volatile components in tea, jointly contributing to its complex, distinctive aroma and enhancing its overall aroma quality (Cairns et al. 2022).

FIGURE 5.

Contribution of phenolic acids to the sensory quality of tea. Created with BioRender.com.

2.4.3. Taste

Bitterness, astringency, sourness, sweetness, and umami are crucial sensory attributes of tea infusions, with their intensity significantly shaping consumer preferences and market acceptance of tea and related products. They also play a vital role in enhancing tea flavor (C. N. Zhao et al. ²⁰¹⁹). Phenolic acids (e.g., quinic and chlorogenic acids) are the primary contributors to the sour taste of tea infusions. Their acidity serves as a key marker of a “vibrant” character in high‐quality tea infusions, enhancing the layered complexity and freshness of the flavor profile (Y.‐H. Chen et al. ²⁰²²). The total phenolic acid content in fresh leaves accounts for approximately 5% of the dry weight, with gallic acid ranging from 0.5% to 1.4%, chlorogenic acid at about 0.3%, and caffeic acid present at considerably lower levels. Phenolic acids dissociate protons in aqueous solution, thereby lowering the pH of the tea infusion and directly modulating its perceived sourness. In addition, anthocyanins and other quinic acids can increase the acidity of tea infusions (L. Zhang, Cao, et al. ²⁰²⁰).

The regulatory mechanisms by which phenolic acids modulate the bitterness and astringency of tea infusions are particularly complex, exhibiting a dual dependence upon both concentration and molecular structure. The phenolic acid content is significantly positively correlated with the astringency score (r ≈ 0.87) and the bitterness score (r ≈ 0.76) of the tea infusion, and can also act synergistically with other tea constituents, collectively contributing to the overall bitterness and astringency profile (Y.‐H. Chen et al. ²⁰²²). The sensory intensities of representative phenolic acids, including gallic acid, chlorogenic acid, and caffeic acid, are positively correlated with their concentrations (Z. D. Jiang et al. ²⁰²³). Research shows that the bitterness threshold of phenolic acids is affected by the number and position of phenolic hydroxyl groups (Z. B. Zhang, Pan, et al. ²⁰²⁵). Phenolic compounds combine with salivary proteins in the human mouth to form insoluble complexes, reducing oral lubrication and causing astringency (Figure 5). For example, the catechol structure of caffeic acid, compared with the monophenolic hydroxyl group of ferulic acid, can coordinate with proteins, thus producing a more significant bitter taste and astringency. Caffeic acid and chlorogenic acid exert a bidirectional, concentration‐dependent modulatory effect on the astringency of EGCG: they attenuate astringency within the concentration range of 0–0.2 mM, but switch to synergistic enhancement at concentrations exceeding 0.2 mM, whereas both compounds synergistically enhance bitterness at all concentrations examined (Y.‐H. Chen et al. ²⁰²²). One of the mechanisms underlying this modulatory effect is that phenolic acids can interfere with the binding of EGCG to mucin, thereby reducing the turbidity of the EGCG–mucin mixture and attenuating the perceived astringency under certain conditions. However, at excessively high concentrations, the intrinsic astringency of phenolic acids and their synergistic enhancement with other tea constituents become dominant, ultimately intensifying the perception of bitterness and astringency. During black tea processing, increasing the oxygen concentration in the fermentation system promotes the oxidative degradation of phenolic acids, reducing their content in the finished black tea by 10%–30%, thereby significantly mitigating the bitterness and astringency of the tea infusion (L. Chen, Liu, et al. ²⁰²¹). Our previous turbidimetric analyses combined with sensory evaluation identified coumaroylquinic acids as the primary contributors to the astringency of Keemun congou black tea, leading to the isolation and identification of trans‐4‐O‐p‐coumaroylquinic acid (trans‐4‐O‐pCoQA) as a low‐threshold astringent compound (M. C. Wen et al. ²⁰²²). At moderate concentrations, caffeic acid helps balance astringency and umami, imparting a fresh and brisk mouthfeel to the tea infusion, whereas at elevated concentrations, it contributes pronounced bitterness and astringency.

Studies have also confirmed that phenolic acids contribute distinctively to the sweet aftertaste and umami character of tea infusions. The characteristic sweet aftertaste of green tea is attributed to the hydrolysis of galloylated catechins (L. Zhang, Cao, et al. ²⁰²⁰). During brewing or oral processing of green tea, the ester bonds of galloylated catechins, such as EGCG and ECG, are cleaved, releasing gallic acid. At low concentrations, gallic acid synergizes with umami compounds, such as l‐theanine, to activate sweet/umami taste receptors, thereby contributing to the perception of a sweet aftertaste. Furthermore, gallic acid and theogallin have been identified alongside l‐theanine and succinic acid as key umami‐enhancing compounds in matcha, further confirming the central role of phenolic acids in constructing the umami profile of tea infusions. Gallic acid functions as an umami enhancer, amplifying the umami intensity of theanine and enhancing umami perception in the oral cavity (Kaneko et al. 2006). Molecular docking simulations suggest that gallic acid may act as an enhancer, cooperating synergistically with ligands such as myricetin to activate the umami taste receptor (T1R1/T1R3) and induce gate closure of the receptor binding cavity, in a manner analogous to the enhancement of glutamate by inosine 5ʹ‐monophosphate (IMP) (F. Y. Li et al. ²⁰¹³). However, direct experimental evidence from cellular‐level receptor functional assays confirming the binding and activation of T1R1/T1R3 by gallic acid is currently lacking, and this mechanism awaits further validation. Unlike catechins, which primarily present a unitary bitter and astringent profile, phenolic acids function as a multidimensional hub in tea infusions, supporting sourness, modulating bitterness and astringency, and enhancing sweet aftertaste and umami, constituting a key material basis that determines the flavor balance and complexity of tea.

2.5. Bioavailability Considerations in the Antioxidant and Anti‐Inflammatory Actions of Tea Phenolic Acids

2.5.1. Overall Status Quo of Phenolic Acid Bioavailability

Biological activities of C. sinensis teas have been described elsewhere (Alemu et al. 2025; Luo et al. ²⁰²⁴), including by our research group (L. Zhang et al. ²⁰¹⁹). However, these reports highlight the effects of tea consumption (the traditional method), a beverage containing several bioactive compounds, on various human health–related endpoints and biomarkers. Most mechanistic, preclinical, and clinical studies have focused on the major catechin EGCG and, to a lesser extent, on other tea catechins (Chelliah et al. 2025). In other cases, bioactivity evaluation relies only on the pure, native extract, without studying the effects of digestion and microbiota transformation on the outcomes, limiting the physiological importance and translation of results (Mehrabi et al. 2025). An additional challenge limiting the clinical exploitation of tea polyphenols is their suboptimal biopharmaceutical properties. Many of these molecules suffer from poor water solubility, reduced stability under physiological conditions, extensive metabolization during digestion, and restricted ability to cross biological membranes, thus limiting their absorption and distribution. Despite being consistently present in tea infusions in both free and ester‐bound forms, tea phenolic acids have received less attention than catechins and other flavonoids, and their bioavailability and biological activities remain comparatively underexplored.

The bioactivity of tea‐derived phenolic acids in humans depends on multiple factors, including their chemical composition and physicochemical properties, level and frequency of intake, and the presence of other foods during consumption. Additional determinants include the food matrix composition, gastrointestinal stability, phase II metabolism, and interactions with the gut microbiota, which plays a central role in converting tea polyphenols into smaller, more bioavailable phenolic acid metabolites (Lafay and Gil‐Izquierdo ²⁰⁰⁸). In this sense, recent technological innovations have expanded the applicability of phenolic acids in food models. Encapsulation techniques (e.g., nanoemulsions, liposomes, freeze‐drying encapsulation, and biopolymer matrices), enzymatic modification, and fermentation‐assisted biotransformation have been used to enhance polyphenols’ stability and solubility and to control their release during digestion, improving their bioavailability in complex food matrices (X. Fang et al. ²⁰²⁴; C. Li, Li, et al. ²⁰²⁵; Tang et al. ²⁰²⁵). Bioavailability of polyphenols refers to the proportion of ingested polyphenolic compounds that is released from the food matrix, absorbed in the gastrointestinal tract, and metabolized. It ultimately becomes available in the systemic circulation or at target tissues in a form capable of exerting biological activity (Rodrigues et al. 2022).

As well stressed by Theivendren et al. (2025), key limitations in studies on tea‐derived phenolic acid metabolism and bioactivity include the lack of chemically oriented, standardized phenolic acid‐enriched tea preparations, as teas are a mixture of various flavonoids, phenolic acids, and other minor polyphenols, which intrinsically leads to insufficient insights on the isolated effects of phenolic acids on bioactivity compared to those from catechins. Furthermore, limited in vivo studies directly linking circulating phenolic acid metabolites to defined biological outcomes in humans are available. Finally, variability in tea processing, differences in phenolic acid bioavailability, extensive phase II metabolism, and interindividual differences in gut microbiota composition complicate the interpretation of their in vivo bioactivity. Consequently, mechanistic studies of tea‐derived phenolic acids in cellular and animal models are indispensable for defining their molecular targets and structure–function relationships, thereby serving as a prerequisite for rationally designed human intervention studies. For example, recent research has shown that phenolic acids enhance the human cellular antioxidant defense system by activating the Nrf2/ARE pathway, exert anti‐inflammatory effects by inhibiting the nuclear factor kappa beta (NF‐κB) signaling pathway, and regulate the balance of intestinal flora, which provides a new perspective for explaining their systemic health benefits (Afnan et al. 2022; J. X. Jiang et al. ²⁰²¹; Mu et al. ²⁰²³; Y. Q. Zhang, Wang, Wu, et al. ²⁰²⁵).

In vitro and in vivo studies have shown that phenolic acids and their derivatives, such as caffeic acid phenethyl ester (CAPE), can regulate lipid metabolism and oxidative stress, inhibit adipocyte differentiation, reduce fat accumulation, and improve obesity (Mirzaei et al. 2021). At the same time, CAPE can activate the IRS‐1/PI3K/Akt signaling pathway to enhance insulin sensitivity, promote glucose uptake and utilization, and reduce insulin resistance (Z. M. Wu et al. ²⁰²³). In terms of anticancer properties, at least in in vitro assays, some tea phenolic acids, such as chlorogenic and caffeic acids, exhibit low cytotoxicity and can induce apoptosis in cancer cells and inhibit tumor cell proliferation and migration (Tobón‐Vélez et al. ²⁰²⁶; T.‐Y. Wu et al. ²⁰²⁵). The mechanism of action involves regulating cell‐cycle‐related proteins, inhibiting signaling pathways such as PI3K/Akt and AMPK, and thereby affecting cell metabolism and survival.

The level of tea phenolic acids that reach the intestine depends on their solubility and chemical structure, such as the number of phenolic hydroxyls and the position of their substitutions, the nature of substituents, the extent of polymerization, and the degree of glycosylation (if any) (Truong and Jeong 2021). This is evident when the bioaccessibility of tea polyphenols is studied through an in vitro static method of digestion: chlorogenic acid (90%), p‐coumaric acid (59%), gallic acid (32%), and ferulic acid (131%) had comparable or higher bioaccessibility than catechin (37%) in white tea. In green tea, the bioaccessibility of gallic acid (174%), p‐coumaric acid (153%), and ferulic acid (89%) was higher than that of rutin (65%) and catechin (0%) (Gómez‐Mejía et al. ²⁰²²). Thus, it is clear that the chemistry of tea phenolic acids plays a critical role in their bioavailability and, in consequence, bioactivity in humans. A systematic review of 47 human intervention studies estimated the overall bioavailability of hydroxycinnamic acids at approximately 25%, with the maximum plasma concentration of representative C₆–C₃ cinnamic acids—such as caffeic acid and ferulic acid—reaching 0.423 µmol/L and T _max ranging from 2.7 to 4.2 h (Di Pede et al. 2023). In a human trial using black tea as the source of phenolic acids (van der Pijl et al. ²⁰¹⁵), the maximum plasma concentration was 1.2 µmol/L, which is pharmacologically relevant for displaying bioactivities, but nothing was reported about other phenolic acids.

In the literature, bioactivity claims are often overextended from in vitro experiments employing microbial or human‐derived enzymes, cell‐based assays, and chemical bioactivity tests. While these approaches are high‐throughput, cost‐effective, and useful for mechanistic screening, their relevance to human physiology is often limited. Consequently, bioactivity claims based solely on such models should be interpreted with caution and evaluated carefully before extrapolation to in vivo contexts. Direct assessment of the bioavailability of tea polyphenols in humans is challenging due to their structural diversity, extensive metabolism, and pronounced interindividual variability in absorption and biotransformation. Consequently, in vitro digestion and cellular models are widely used as pragmatic and informative tools to estimate polyphenol bioaccessibility and potential bioavailability, providing mechanistic insights that support and guide subsequent in vivo investigations. The biological activity of phenolic acids in humans is governed by their absorption and chemical and biological transformations throughout the gastrointestinal tract (Henning et al. 2013). Research has shown that only 2%–5% of tea polyphenols are absorbed through the intestinal wall (Tenore et al. 2015), where the indigestible polyphenol fraction reaches the large intestine and interacts with the microbiota. Tea polyphenols are modified by ring‐cleavage, reduction, hydrolysis, decarboxylation, demethylation, and dihydroxylation reactions (H. Ma, Hu, et al. ²⁰²²); thus, accounting for these reactions is more physiologically relevant. Several phenolic acids detected in circulation after tea consumption are primarily derived from microbial biotransformation of higher‐molecular‐weight tea polyphenols, indicating that these metabolites may contribute substantially to the biological effects attributed to tea intake (Gómez‐Mejía et al. ²⁰²²). Consequently, assessment based solely on the native phenolic acid content of tea may underestimate in vivo exposure, as a significant proportion of phenolic acids is generated during gastrointestinal digestion and colonic metabolism.

2.5.2. Antioxidant Capacity

The antioxidant capacity of teas is a common analysis in food science (e.g., foods and ingredients) and nutrition (e.g., in animal‐derived organs and tissues, and human plasma) (Figure 6). Flavonoids and phenolic acids found in various C. sinensis teas exert their antioxidant potential via three mechanisms of action: HAT, SET, and transition metal chelation, or any combination of the three (Farrell et al. 2024; Mehrabi et al. ²⁰²⁵). Gallic acid and caffeic acid, among other phenolic acids, possess strong antioxidant capacity in biological media and in real‐world food applications (Deng et al. 2024; C. N. Zhao et al. ²⁰¹⁹). Tea phenolic acids can provide hydrogen atoms to scavenge free radicals such as superoxide anion (O₂ ⁻), hydroxyl radical (·OH), and peroxynitrite (ONOO⁻), and act as metal chelators, thereby protecting cells from oxidative damage (F. K. An et al. ²⁰²⁴; J. X. Chen, Yang, et al. ²⁰²⁰).

FIGURE 6.

Major bioactivities of phenolic acids in tea. Created with BioRender.com.

Considering phenolic acids, ortho‐dihydroxyl groups found in chlorogenic acid and conjugated side chains enable efficient quenching of reactive oxygen species (ROS), mitigating oxidative stress at the cellular and plasma levels. At the molecular level, phenolic acids can regulate the expression of antioxidant enzymes, including heme oxygenase‐1 (HO‐1) and NAD(P)H:quinone oxidoreductase‐1 (NQO1). By activating the Nrf2/ARE signaling pathway, they enhance the activity of the cellular antioxidant defense system (Mu et al. 2023). Their antioxidant mechanisms include direct scavenging of free radicals, chelation of metal ions, and activation of the antioxidant enzyme system. Phenolic acids help reduce the risk of cardiovascular diseases, cancer, and other oxidation‐stress‐related conditions by inhibiting free radical‐induced lipid peroxidation, and they also have potential applications in delaying aging (Afnan et al. 2022; Q. Yang et al. ²⁰²²).

In vivo studies indicate that the intestinal redox state plays a critical role in regulating host immunity and host–microbiota crosstalk through the generation and sensing of redox signals (H. Ma et al. ²⁰¹⁹). Because excessive ROS can disrupt intestinal barrier integrity and alter gut microbial composition, tea has been identified as a functional beverage that can counteract oxidative stress, reduce inflammatory biomarkers, and preserve intestinal barrier function (Tian et al. 2024). For example, C57BL/6 mice were administered a tea polyphenol‐rich extract (483 mg/g EGCG, 196 mg/g ECG, 87 mg/g EGC, 53 mg/g EC, and 3 mg/g gallic acid) by gavage at doses ranging from 100 to 400 mg/kg body weight for 12 weeks. The authors reported that low to intermediate doses enhanced endogenous antioxidant defenses, as evidenced by changes in SOD, GSH, and MDA levels. However, tea polyphenol supplementation did not significantly reduce ROS levels in hepatocytes when compared with the negative control group receiving no tea supplementation (H. Ma et al. ²⁰¹⁹).

In our previous research, a purple tea hydro‐ethanolic extract microencapsulated with sodium alginate exhibited a total phenolic content of 33 g/100 g and inhibited copper‐induced oxidation of human plasma (Farrell et al. 2024). Furthermore, in a two‐dimensional cell culture protocol, the tea extract reduced ROS generation in red blood cells and protected murine astrocytes from chemically induced oxidative stress. Further studies should focus on how phenolic acids differ from other tea polyphenols in terms of their antioxidant capacity in complex mixtures. Given the limited data on the antioxidant capacity of tea‐derived phenolic acids, future research should prioritize in vivo investigations assessing their effects on oxidative stress–related biomarkers, as well as elucidating structure–activity relationships and dose–response profiles for these effects.

2.5.3. Anti‐Inflammatory Effect

Inflammation is a physiological process initiated by the innate immune system in response to infection, tissue damage, or cellular stress, and is characterized by coordinated communication among immune cells, molecular mediators, and local tissues aimed at reestablishing homeostatic balance (Park 2026). Its significance lies in this dual nature: while inflammation is indispensable for host defense and repair, inadequate control or prolonged activation can drive pathological processes, highlighting the need for precise regulation of inflammatory signaling to sustain health and limit disease development. In this regard, C. sinensis teas have been traditionally used to mitigate pro‐inflammatory responses induced by a high‐fat diet or disease (Y. Xu, Ding, et al. ²⁰²⁴). Owing to the chemical complexity of tea polyphenols and the predominance of flavonoids, it remains difficult to assign specific biological effects to phenolic acids alone. As a result, integrated experimental strategies combining cellular‐based assays with animal models are necessary to clarify the mechanisms underlying the anti‐inflammatory effects of tea‐derived phenolic acids. Moreover, chemical interactions among tea constituents, including additive, synergistic, or antagonistic effects, are likely to contribute to the overall health benefits of tea. However, from a mechanistic perspective, this compositional complexity makes it challenging to attribute observed health effects to individual compounds within the mixture.

For instance, in vitro experiments have demonstrated that tea extracts can significantly reduce the expression levels of inflammatory factors, such as tumor necrosis factor (TNF‐α), interleukin‐1β (IL‐1β), and interleukin‐8 (IL‐8) (p < 0.01) in murine RAW 264.7 macrophages (Q. Fang et al. ²⁰²⁵; J. Y. Liu, Poojary, et al. ²⁰²³). Mechanistically, these effects are primarily attributed to modulation of intracellular signaling pathways, particularly the suppression of NF‐κB activation and nuclear translocation by inhibiting NF‐κB signaling (Afnan et al. 2022), which, in turn, downregulates transcription of downstream inflammatory genes.

Phenolic acids are known to exhibit significant biological activity in the regulation of inflammation (Afnan et al. 2022; X. Y. Liu et al. ²⁰²¹) (Figure 6). Studies based on lipopolysaccharide (LPS)‐induced cellular inflammation models and animal inflammation models have shown that tea phenolic acids can exert anti‐inflammatory effects by inhibiting the production of pro‐inflammatory cytokines. For example, in a review of the literature performed by Freitas et al. (2024), ferulic acid was found to attenuate light‐induced retinal inflammation in rabbits by decreasing the secretion of NF‐κB, IL‐8, IL‐1β, and TNF‐α, and improved oxidative stress biomarkers (e.g., SOD, CAT, GPX) and inflammation markers (e.g., TNF‐α) in cadmium‐induced stress in Wistar albino rats. Similarly, p‐coumaric acid decreased levels of inflammatory markers (e.g., IL‐6, IL‐1β) in C57BL/6J mice with intestinal inflammation. These anti‐inflammatory actions are closely linked to phenolic acids’ ability to modulate cellular redox balance and inhibit key inflammatory mediators, making them promising candidates for the prevention and management of inflammatory diseases such as arthritis and colitis (Pannico et al. 2022). In a murine model of ulcerative colitis, gallic acid was shown to exert anti‐inflammatory effects by suppressing the expression of the pro‐inflammatory cytokines TNF‐α, IL‐1β, IL‐17, and IFN‐γ, as well as the inflammatory mediators iNOS and COX‐2 (Pandurangan et al. 2015). Chlorogenic acid and caffeic acid, two phenolic acid derivatives, alleviated DSS‐induced intestinal inflammation by remodeling the gut microbiota composition and thereby indirectly modulating macrophage activation (D. Han, Wu, et al. ²⁰²³).

The antioxidant and anti‐inflammatory activities of phenolic acids, their two core foundational functions, jointly modulate microbial, immune, and metabolic networks, thereby providing a scientific basis for the auxiliary prevention and management of chronic diseases such as cardiovascular disease, diabetes, obesity, and liver disorders (Figure 6). Taking caffeic acid as an example, it can regulate the expression of genes involved in lipid metabolism, lower blood levels of cholesterol, triglycerides, and other lipids, and effectively reduce the risk of cardiovascular diseases such as atherosclerosis. Cinnamic acid, another key phenolic acid, exerts cardiovascular protection by inhibiting inflammatory, oxidative, and apoptotic pathways, modulating genes and enzymes involved in glucose and lipid metabolism, and promoting vasodilation. In terms of glycemic regulation, phenolic acids promote insulin secretion and enhance cellular glucose uptake and utilization, carrying important clinical significance for the prevention and management of diabetes and its complications (H. Ma, Hu, et al. ²⁰²²). Studies have shown that phenolic acids can promote lipid metabolism and improve insulin sensitivity by modulating signaling pathways such as AMPK and PPAR‐γ, thereby exerting therapeutic effects in patients with obesity and metabolic syndrome (S. Z. Wang, Zeng, et al. ²⁰²²). With regard to liver protection, phenolic acids alleviate alcoholic or nonalcoholic liver injury by scavenging ROS through their antioxidant activity and by inhibiting pro‐inflammatory cytokines such as TNF‐α and IL‐1β through their anti‐inflammatory activity. Chlorogenic acid exhibits inhibitory activity against chronic hepatitis B virus (HBV), and its hydrolysis products also demonstrate hepatoprotective and anticancer properties (J. Zuo et al. ²⁰¹⁵).

However, despite these recent scientific advances, the contribution of phenolic acids to biological activity within real food matrices remains underexplored. Foods are complex mixtures of proteins, lipids, carbohydrates, and minerals that can bind, transform, or otherwise modulate the stability and bioaccessibility of phenolic acids. Interactions with the food matrix, processing conditions, and digestive processes may significantly alter their antioxidant and anti‐inflammatory potential relative to purified systems (M. Wang, Li, et al. ²⁰²²; M. Yang, Zhang, and Yang ²⁰²⁵). Therefore, a deeper investigation into how phenolic acids behave, interact, and exert bioactivity in complex food environments is essential for translating their mechanistic potential into practical applications in functional foods and dietary interventions.

To conclude, while catechins are the most abundant and well‐studied phenolic compounds in tea, tea also contains a variety of phenolic acids that contribute to its overall biological effects. A comparison of their activities reveals both similarities and differences. Both catechins and phenolic acids exhibit antioxidant, anti‐inflammatory, antimicrobial, and anticancer properties. However, catechins generally exhibit stronger antioxidant capacity in vitro due to their multiple hydroxyl groups and B‐ring structure. In contrast, some phenolic acids possess potent antioxidant activity, often via mechanisms such as metal chelation and radical scavenging. From the technological point of view, phenolic acids are generally more resistant to heat (e.g., pasteurisation), oxidation, and pH changes than catechins, which easily undergo epimerization and oxidation. Disadvantages of phenolic acids relative to catechins include lower overall abundance in tea leaves and less extensive research base, particularly regarding in vivo efficacy and mechanisms. Catechins and phenolic acids exhibit complementary roles in the physicochemical properties, biological activities, and pharmacokinetic profiles that underpin the health functions of tea: catechins are characterized by high abundance and potent direct antioxidant activity, while phenolic acids display greater stability, higher bioavailability, and involvement in anti‐inflammatory and metabolic regulation, in addition to contributing to both flavor quality and health benefits during processing. Future research should prioritize phenolic acids as an independent research focus and elucidate their specific contribution to the overall bioactivity of tea.

3. Perspectives and Future Trends

Future research on tea phenolic acids should examine the precise regulatory mechanisms governing their metabolic networks. While key enzymes and genes in the biosynthetic pathways, such as PAL, C4H, and 4CL, have been identified, the intricate transcriptional and posttranslational regulation, as well as the crosstalk with other metabolic branches (e.g., flavonoid and lignin synthesis), remains unexplored. Employing multiomics technologies (genomics, transcriptomics, proteomics, and metabolomics) in conjunction with molecular biology techniques will be crucial to elucidate the fine‐tuning of phenolic acid accumulation in response to developmental cues and environmental stresses. Furthermore, uncovering signaling pathways, such as those involving transcription factors like CsHY5 and CsMYBs and hormones like jasmonic acid, that integrate various environmental signals to modulate phenolic acid metabolism represents a fundamental direction for future research. This knowledge will provide a theoretical basis for cultivating tea varieties with tailored phenolic acid profiles through genetic improvement or precision agronomy.

Another critical frontier lies in systematically analyzing the dynamic transformation of phenolic acids during the processing of different tea types and their interactions with other components. Although the general trends across major tea categories are well established, a quantitative and mechanistic understanding of the specific reactions—such as oxidation, polymerization, hydrolysis, and derivatization—under varying processing conditions remains lacking. Advanced in situ analytical techniques and model processing systems should be employed to track these changes in real time. Attention should be paid to how phenolic acids interact with proteins, polysaccharides, and other polyphenols, as these interactions co‐determine the final sensory quality. Additionally, the specific roles of microbial communities in the postfermentation of dark tea and in the yellowing of yellow tea during phenolic acid transformation warrant extensive investigation. Such research will pave the way for intelligent, precise processing and quality control, enabling the targeted enhancement of desired tea qualities.

Finally, translating the documented in vitro biological activities of phenolic acids into validated human health benefits is an imperative yet challenging goal. Future studies must prioritize well‐designed clinical trials to substantiate the antioxidant and anti‐inflammatory effects observed in laboratory settings. Research should focus on elucidating the in vivo mechanisms of action, including bioavailability, metabolism, dose‐dependent behavior, structure–activity relationships, and interactions with the gut microbiota. Exploring the synergistic effects of phenolic acids with other tea constituents, such as catechins and theanine, will provide a more comprehensive understanding of tea's health‐promoting properties in humans. Moreover, mechanistic studies on emerging bioactivities, such as neuroprotection and radioprotection, should be intensified. Concurrently, the safety of long‐term, high‐dose intake of specific phenolic acids warrants comprehensive evaluation. These efforts are essential for developing evidence‐based functional tea products and natural therapeutics, ultimately contributing to public health and the high‐value utilization of tea resources.

4. Conclusion

This review comprehensively examined the chemical structures, biosynthetic pathways, and regulatory mechanisms of phenolic acids in tea plants, as well as their transformations during processing across the six major tea types. Furthermore, the impact of phenolic acids on the sensory quality of tea leaves, the biological activity of tea polyphenols, and their potential health benefits were summarized. In‐depth research on tea phenolic acids helps elucidate the mechanisms underlying tea quality formation, providing a scientific basis for precise processing and quality control. The diverse biological activities of tea phenolic acids offer broad prospects for their development and use in medicine, healthcare, food, and other fields.

During processing of the six major tea types, the content and composition of phenolic acids undergo substantial changes, closely related to the final quality of the tea. For example, during black tea processing, the oxidation and transformation of phenolic acid compounds are crucial for the formation of tea flavor. During the piling fermentation of dark tea, phenolic acids undergo complex biochemical reactions under the influence of microorganisms and heat and moisture, further affecting the color and taste of tea infusion. Additionally, research on the metabolic regulation mechanisms of phenolic acids has provided new insights into optimizing tea processing techniques, thereby improving tea quality and health benefits. However, research on tea polyphenols remains underexplored in several areas. These include elucidating the precise regulatory mechanisms of phenolic acid metabolic networks in tea plants, analyzing the complex interactions between phenolic acids and other components during processing, and investigating the in vivo mechanisms underlying phenolic acid bioactivity and safety. Future studies should move beyond descriptive analyses and adopt integrative multiomics approaches to dissect the regulatory mechanisms of phenolic acid metabolism in tea plants. For instance, combining transcriptomics (RNA‐seq to identify key transcription factors and biosynthetic genes) with metabolomics (LC–MS/MS‐based profiling of phenolic acid metabolites) under controlled environmental stresses (such as drought, low temperature, or UV‐B exposure) can pinpoint co‐expression networks and metabolic bottlenecks that govern phenolic acid accumulation. Such an approach would allow researchers to distinguish between transcriptional regulation and posttranscriptional/modification events. Furthermore, integrating proteomics and metabolite flux analysis could reveal how stress signals are transduced into actual changes in phenolic acid composition. Beyond environmental regulation, future research should also explore the dynamic transformation pathways of specific phenolic acid conjugates during tea processing using untargeted metabolomics, chemometrics, and molecular networking. This will enable the identification of key processing steps that maximize the retention of bioactive phenolic acids while optimizing sensory quality. By adopting these multiomic strategies, the tea research community can move from correlation to causation, ultimately providing precise theoretical guidance for stress‐resilient cultivation and quality‐oriented processing of tea.

Nomenclature

CAPE

caffeic acid phenethyl ester

HAT

hydrogen atom transfer

IL‐1β

interleukin‐1β

IL‐8

interleukin‐8

LPS

lipopolysaccharide

POD

peroxidase

PPO

polyphenol oxidase

SET

single‐electron transfer

TBs

theabrownins

TEM

transmission electron microscopy

TFs

theaflavins

TNF‐α

tumor necrosis factor alpha

TRs

thearubigins

UV

ultraviolet

HCT

hydroxycinnamoyl transferase

CHS

chalcone synthase

GS

glutamine synthetase

ROS

reactive oxygen species

HBV

hepatitis B virus

Author Contributions

Chen Jiang: investigation, methodology, visualization, writing – original draft. Jia‐Ping Ke: methodology, investigation, writing – review and editing. Zekai Wang: investigation, writing – review and editing. Guoping Lai: investigation, writing – review and editing. Xuyang Liu: investigation, writing – review and editing. Zisheng Han: writing – review and editing. Liang Zhang: conceptualization, methodology, supervision, project administration, writing – review and editing, resources. Chi‐Tang Ho: writing – review and editing. Daniel Granato: writing – original draft, writing – review and editing, investigation.

Conflicts of Interest

D.G. declares that he serves as an editor for Comprehensive Reviews in Food Science and Food Safety. D.G. was not involved in the peer review process or in any editorial decision‐making related to this manuscript. The submission and review were handled independently by the Editor‐in‐Chief, and the manuscript remained fully blinded to this editor throughout the process. The other authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.

Supporting information

Supplementary Table S1: crf370553‐sup‐0001‐TableS1.docx