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Article

Phenolic Acids and Flavonoids in Selected Commercial Organic and Conventional Tea Products Characterized by Different Degrees of Leaf Fragmentation

by
Renata Kazimierczak
1,
Klaudia Kopczyńska
1,
Ewelina Hallmann
1,2,
Alicja Ponder
1,
Małgorzata Żebrowska-Krasuska
1,
Martyna Jakubowska
1,
Adrijana Filipović
3,
Marija Banožić
3,
Ana Mandić
3 and
Dominika Średnicka-Tober
1,*
1
Department of Functional and Organic Food, Institute of Human Nutrition Sciences, Warsaw University of Life Sciences, 02-776 Warsaw, Poland
2
Bioeconomy Research Institute, Agriculture Academy, Vytautas Magnus University, Donelaicio 58, 44248 Kaunas, Lithuania
3
Faculty of Agriculture and Food Technology, University of Mostar, 88000 Mostar, Bosnia and Herzegovina
*
Author to whom correspondence should be addressed.
Appl. Sci. 2026, 16(7), 3283; https://doi.org/10.3390/app16073283
Submission received: 5 March 2026 / Revised: 25 March 2026 / Accepted: 26 March 2026 / Published: 28 March 2026
(This article belongs to the Special Issue Advancements in Food Nutrition and Bioactive Compounds)
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Versions Notes

Abstract

Tea (Camellia sinensis L.) infusion is the second most commonly consumed drink in the world after water, valued for its sensory qualities and health-promoting properties. Tea contains a range of chemical compounds that give it specific nutritional and refreshing properties. These include alkaloids, polyphenolic compounds, carbohydrates, amino acids, enzymes, and aromatic compounds. The content of individual compounds in tea leaves is impacted by factors such as the variety, region, and cultivation method, as well as specific processing operations. The aim of the present study was to investigate the content of bioactive compounds in a selection of organic and conventional tea infusions characterized by different degrees of leaf fragmentation. The analysis of selected phenolic acids, catechins, quercetin, and caffeine in black tea and black Earl Grey tea infusions was conducted using high-performance liquid chromatography (HPLC). The study confirmed that the chemical composition of tea infusions is significantly impacted by the type of tea, cultivation practices, and form of the leaves, and revealed some previously underexplored interactions between the leaf fragmentation and cultivation system effects. From a consumer or product design perspective, organic loose-leaf Earl Grey teas appear to offer the most favourable balance of catechins, and flavonoids whereas conventional bagged black teas provide higher phenolic acid content.

1. Introduction

Tea (Camellia sinensis L.) is categorized based on various criteria, including geographic origin, species, type, cultivation and harvesting methods, as well as the duration and techniques used for fermentation and drying. Globally, tea is distinguished as Chinese, Indian, Ceylonese, Brazilian, Japanese, or Russian, with further classifications based on specific cultivation regions [1]. For instance, Assam tea from the north Indian province of Assam is known for its astringent, dry taste, intense aroma, and dark color. In contrast, Darjeeling tea, originating from the mountainous northern Indian district of Darjeeling, is known for its delicate and aromatic profile, producing light golden infusions. Ceylon tea from Sri Lanka is characterized by a distinctive aroma and pleasant, slightly astringent flavour, which occupies a flavour spectrum between that of Assam and Darjeeling tea [2,3,4]. Additionally, Nilgiri tea, from the hills of the Madras state, is regarded as one of the most refined teas in South India [5,6]. Tea can also be differentiated according to leaf form. Whole-leaf teas, often referred to as Flowery, Golden, or Tippy types, consist of intact leaves and buds. Broken teas contain smaller, cut leaves that facilitate enhanced infusion. Fannings, which are very fine particles resulting from the cutting process, are considered by-products of higher-quality teas and yield dark infusions that brew swiftly, albeit with a lower quality profile. Dust teas represent the most finely ground fraction of tea leaves, while granulated teas may be made from the lower-quality Fannings and Dust, pressed with rice water, though high-quality leaves can also undergo this treatment through meticulous rolling processes [7,8,9,10]. The classification of tea can also be based on the degree and duration of fermentation, leading to three principal types: green, oolong (or red), and black tea. Green tea is unfermented and is produced by drying fresh leaves that have been steamed post-withering to inactivate oxidizing enzymes. This results in a light straw-green color and a slightly more bitter taste compared to black tea. Oolong tea undergoes partial fermentation, resulting in a unique flavour profile that bridges the characteristics of green and black teas. Black tea is fully fermented, characterized by a dark color and an astringent-bitter flavour, and constitutes the main type traded on global markets [11]. To enhance the sensory attractiveness of black tea, natural aromatic plant substances are often added. One of the most popular flavoured varieties is Earl Grey, accounting for approximately one-fourth of global tea sales. Its characteristic flavour results from the addition of bergamot oil obtained from the aromatic peel of Citrus bergamia [12].
Tea can be cultivated using conventional or organic production systems. Organic production offers several environmental benefits, including enhanced biodiversity, restoration of natural biological cycles, and improved soil fertility through practices that bolster ecological balances [13,14]. Organic farming standards, including those for tea cultivation, prohibit the use of synthetic fertilizers, pesticides, genetically modified organisms, and sewage sludge, instead requiring soil fertility enhancement through crop rotation, organic fertilization, and erosion control, alongside mechanical, biological, or thermal pest and weed management [15]. East Asia and the Pacific dominate the organic tea market, which grew from USD 0.98 billion in 2022 to USD 1.1 billion in 2023, with an estimated compound annual growth rate of 11.9%. Continued growth is expected due to increasing consumer awareness of product quality and sustainable production practices [16].
Tea contains a wide variety of chemical compounds responsible for its nutritional and refreshing properties, including alkaloids, polyphenolic compounds, carbohydrates, amino acids, enzymes, and aromatic substances. The health-promoting properties of tea are primarily attributed to the presence of more than 4000 bioactive compounds, among which alkaloids and polyphenols are of particular importance [17,18]. Polyphenols constitute a large group of compounds with antioxidant and anti-inflammatory properties, including phenolic acids (hydroxybenzoic and hydroxycinnamic acids), flavonoids (flavones, flavonols, flavanones, flavanols, isoflavones, and proanthocyanidins), stilbenes, and lignans [19]. These compounds play a key role in determining the quality, flavour, and functional properties of tea, accounting for approximately 20–35% of its dry mass [18,20]. The polyphenol content in tea leaves varies naturally depending on climatic conditions such as sunlight and humidity, as well as the age of the harvested leaves, with young leaves containing significantly higher levels of polyphenols than older ones. The type and properties of phenolic compounds are also influenced by the production process. Green tea primarily contains catechins, while black tea is rich in tannins, which are formed from catechins through biochemical transformations occurring during fermentation [21].
Importantly, the concentration and profile of these bioactive compounds can be influenced by the cultivation system. Organic and conventional practices differ in nutrient management, pest control, and soil fertility, which may modulate the synthesis of polyphenols, catechins, and alkaloids in tea leaves. Consequently, the potential health benefits of a tea infusion may be partially shaped by the agricultural methods employed, linking compositional differences to biologically relevant effects in humans [22].
Scientific studies indicate that polyphenolic compounds exhibit a wide range of health-promoting activities. Regular consumption of properly prepared tea infusions may reduce the risk of numerous diseases due to their antioxidant action, which includes inhibiting the formation of reactive oxygen and nitrogen species, chelating transition metal ions such as iron and copper that catalyze radical reactions, and scavenging free radicals capable of damaging cellular lipids, proteins, and nucleic acids [23]. Research suggests that tea polyphenols may contribute to the reduction in risk and progression of cardiovascular diseases [23], type 2 diabetes [24], and cancer [23,25]. They have also been shown to participate in fat metabolism and body weight reduction [26], and to support bone health by stimulating osteoblast activity and inhibiting osteoclast-mediated bone resorption, thereby reducing the risk of osteoporosis [27].
Similar to coffee, tea contains the alkaloid caffeine, referred to as theine when isolated from tea leaves. The caffeine content in tea primarily depends on the degree of fermentation, with higher levels found in more fermented teas. Caffeine stimulates the central nervous system, shortens reaction time, increases alertness, enhances cardiac performance, and dilates coronary and cerebral vessels, improving brain perfusion and oxygenation. It also exerts bronchodilatory effects, accelerates metabolic processes, and facilitates the elimination of toxic substances from the body. However, excessive caffeine consumption may cause nervousness, irritability, and anxiety, and at high doses may lead to seizures and vomiting. Chronic intake of large amounts may impair glucose tolerance and reduce insulin sensitivity [28,29].
The degree of tea-leaf fragmentation is a well-recognized determinant of extraction behaviour, as smaller particles provide a larger surface area, greater disruption of cellular structures, and shorter diffusion pathways, which together accelerate the release of soluble components during brewing [30,31,32]. Previous studies have shown that finely fragmented teas or powders typically exhibit enhanced extraction of total phenolic compounds due to the increased solvent accessibility of surface-associated phenolics [30,33]. At the same time, different classes of phenolic compounds vary in their sensitivity to mechanical processing and oxidation. While phenolic acids such as gallic and chlorogenic acid are relatively stable and readily extractable from fragmented material [30], catechins and gallated catechins (e.g., EGC, EGCG) are more prone to oxidative degradation during leaf breakage and may be better preserved in intact or coarsely fragmented leaves [30,32]. In addition, intact leaf structures may facilitate the gradual extraction of compounds located deeper in the tissue layers, such as caffeine and certain flavonoids.
To capture these mechanistic and compound-specific differences, our study intentionally included both loose-leaf teas (coarsely fragmented) and bagged teas (finely fragmented), representing the range of commercially available leaf forms consumed by the public. This approach allowed us to assess how leaf structure interacts with the production system (organic vs. conventional) and tea type (black vs. Earl Grey) to shape the final bioactive profile of the infusion.
Most prior studies on organic vs. conventional tea focus on single variables (cultivation type alone, or brewing time alone), thus the presented simultaneous three-factor approach (tea type × cultivation × leaf fragmentation) adds value. Moreover, while the influence of processing factors, such as leaf fragmentation, on tea infusion quality is acknowledged in prior literature, a critical gap persists in market-representative data for commercially available, fragmented leaf teas under real-world brewing conditions. Thus, the present study’s incremental novelty over prior studies lies primarily in: (a) the simultaneous inclusion of leaf fragmentation as a third factor; and (b) the use of commercially available products rather than experimental farm samples.
It is hypothesized that tea products from the organic production system would generally exhibit higher concentrations of phenolic compounds and caffeine than those from the conventional system, with the effects of tea-leaf fragmentation being compound-class-specific.

2. Materials and Methods

2.1. Research Material

The research material consisted of 24 commercially available tea products representing two tea categories: traditional black tea and black Earl Grey tea. For each category, 12 samples were analyzed, comprising an equal number of “loose-leaf” and “bagged” products, as well as an equal representation of organic (EU organic certification labels) and conventional production systems. “Loose-leaf” teas consisted of coarsely fragmented, whole or partially broken leaves purchased in bulk from retail suppliers. These leaves were not contained in bags and retained much of their original leaf structure. “Bagged” teas consisted of finely fragmented teas enclosed in standard filter-paper sachets, commonly sold as single-serving products. The teas originated from major tea-producing regions, including Assam (India), Ceylon (Sri Lanka), and Zhejiang (China), ensuring representation of the most common geographical sources found in consumer markets. All products were purchased in their final retail packaging, in the retail market in Warsaw, Poland, stored under dry and dark conditions, and analysed within their declared shelf life.

2.2. Phenolic Compounds and Caffeine Extraction and Identification

To conduct chemical analyses, tea infusions of the examined teas were prepared. For this, 1 g tea samples were weighed. Water was boiled (100 °C) and immediately poured over tea samples (100 mL of water for 1 g of tea). After a five-minute brewing period, the infusion was filtered. Such infusion parameters (tea-to-water ratio, brewing time, and temperature) were previously employed in studies evaluating total phenolic content and antioxidant activity of tea beverages [34]. Three replicates (infusions) were analysed for each type of product.
To extract polyphenolic compounds and caffeine, 3 mL of the filtered infusion was measured into a plastic test tube using a pipette, and then 2 mL of 80% methanol was added. The samples were mixed on a vortex mixer and then incubated in an ultrasonic bath for 10 min at 30 °C. After incubation, centrifugation was performed at 6000 revolutions per minute for 10 min at 0 °C. The low temperatures were applied to prevent degradation of heat-sensitive phenolic compounds. Next, 1 mL of the supernatant (without disturbing the sediment) was taken and transferred to an HPLC vial. For analytical purposes, the following HPLC setups were used: two LC-20AD pumps, a CMB-20A system controller, an SIL-20AC autosampler, a UV-vis SPD-20AV detector, and a CTD-20AC oven, all from Shimadzu (Shimadzu, Tokyo, Japan). The analysis parameters used were as follows: two gradient phases involving 10% (phase A) and 55% (phase B) acetonitrile in deionized water at pH 3.00 (acidified with orthophosphoric acid); time program: 1.00–22.99 min—95% of phase A and 5% of phase B, 23.00–27.99 min—50% of phase A and 50% of phase B, 28.00–30.99 min—80% of phase A 80% and 20% of phase B, 31.00–42.00 min—95% of phase A and 5% of phase B. The column used for analysis was Phenomenex Fusion 80-A RP (Phenomenex, Warsaw, Poland) (measuring 250 × 4.6 mm); the total analysis time was 42 min; detection was carried out in the range of 250 nm (for phenolics and caffeine) and 340 nm (for flavonoids); standards from Fluka (Seelze, Germany) and Sigma-Aldrich (St. Louis, MO, USA) with a purity of 99% were used for identification. Five injections of standards for phenolic acids, flavonoids, and caffeine were made from the prepared standard solutions. Standard curves were generated for the analysed polyphenolic compounds and caffeine. The chromatograms were analysed, and individual polyphenolic compounds and caffeine were identified by comparing their retention times to those of the standards [35].
Analytical method validation parameters (limits of detection (LOD), limits of quantification (LOQ), recovery rates, and repeatability (RSD%) are provided in Table 1.

2.3. Statistical Analyses

Statistical analysis of the obtained laboratory test results was performed in the statistical R programme (version number: 4.4.1) [36]. The results of the black tea and Earl Grey tea tests were analysed in relation to two factors: leaf fragmentation (in the form of “bags” and “leaves”) and production system (organic and conventional). Tukey’s test, with a significance level of α = 0.05, was used to determine significant differences between each group of factors. The results regarding the content of the analysed compounds (mean values and standard deviations) are presented relative to 100 mL of tea infusion. Principal component analysis was carried out in the R statistical environment using the “stats” package and scaling the data. Visualization of results prepared by the “ggbiplot” package. For the presentation of principal components, the first two components were selected based on the Scree Plot. Eigenvalues above 1 corresponded to the first three components.

3. Results

3.1. The Content of Polyphenolic Compounds in the Studied Tea Infusions

3.1.1. Polyphenol Content (Sum)

In the analyzed samples, the sum of the identified polyphenols’ content (Table 2, Figure 1) ranged from 18.58 to 23.26 mg/100 mL. The highest values were recorded in the infusions of Earl Grey–conventional–bagged (23.26 mg/100 mL), while the lowest appeared in black teas–conventional–bagged (18.58 mg/100 mL). For black teas–organic samples exhibited higher polyphenol contents (20.63 mg/100 mL in bagged form) compared to conventional (18.58 mg/100 mL in bagged form, 21.51 mg/100 mL in loose leaf). In Earl Grey teas, the trend was less consistent—the bagged conventional samples showed the highest total polyphenols, whereas among loose-leaf variants, the differences were minimal.

3.1.2. Phenolic Acid Content

Phenolic acid content (Figure 2) in the tested tea infusions varied between 7.05 and 11.54 mg/100 mL. The highest concentrations were observed in black loose-leaf conventional (11.54 mg/100 mL), and the lowest in Earl Grey bagged organic (7.05 mg/100 mL). Generally, black teas contained greater amounts of phenolic acids than Earl Grey, suggesting that aromatic additives may have diluted the native phenolic acid fraction.
Among the analysed phenolic acids, gallic acid predominated (4.95–8.35 mg/100 mL), followed by chlorogenic acid (0.87–3.19 mg/100 mL). The highest level of gallic acid was found in black loose-leaf conventional (8.35 mg/100 mL), and the lowest in Earl Grey bagged organic (4.95 mg/100 mL) tea infusions. Meanwhile, chlorogenic acid peaked in black loose-leaf conventional (3.19 mg/100 mL), indicating that less fragmented leaves favour the retention or extraction of this acid (Figure 3).

3.1.3. Flavonoid Content

The total flavonoid content ranged from 8.42 to 14.56 mg/100 mL (Figure 4). The highest values were found in Earl Grey bagged organic (14.56 mg/100 mL), which may reflect differences in processing or contributions from bergamot-derived flavonoids.
Among catechins, epigallocatechin (EGC) (Figure 5a,b) and its derivative epigallocatechin gallate (EGCG) showed relatively high concentrations (3.72–6.02 mg/100 mL and 1.61–4.06 mg/100 mL, respectively) (Figure 5c,d). Then came catechin (Figure 5e,f) and quercetin (Figure 5g,h), the content of which was significantly lower than in the case of other flavonoids. The free catechin concentrations ranged from 1.85 to 4.07 mg/100 mL, with the greatest values in the organic Earl Grey samples. Quercetin concentrations ranged from 0.330 to 0.521 mg/100 mL. The highest quercetin contents were found in black loose-leaf conventional, and the lowest in Earl Grey loose-leaf conventional. The effect of cultivation system (organic vs. conventional) on quercetin content was ambiguous. These results indicate that despite a lower total phenolic acid content, Earl Grey teas may carry a richer flavonoid profile, especially in organic variants.

3.2. Caffeine Content in the Examined Tea Infusions

The caffeine content in the analyzed samples showed a broad range—from 6.14 to 41.95 mg/100 mL. The highest concentrations were observed in Earl Grey loose-leaf organic (41.95 mg/100 mL), while the lowest—in Earl Grey bagged conventional (6.14 mg/100 mL). These data suggest that caffeine content is not reliably predicted by cultivation system; rather, it is strongly influenced by leaf form and tea type. Loose-leaf teas, particularly organic ones, tend to yield higher caffeine content, perhaps due to more efficient extraction from intact leaf tissues (Figure 6). Jaganyi et al. [37] showed that the observed large variation in caffeine content between tea samples is plausibly explained not by a single factor (e.g., organic vs. conventional), but by the combined effects of heterogeneity in raw material quality and grade, differences in particle size and processing (bagged vs. loose-leaf), and variability in extraction conditions. According to the authors, the infusion of caffeine from loose tea is much faster than from tea in a bag, which can be due to the tea bag membrane offering some hindrance. Furthermore, caffeine content in tea infusions depends strongly on leaf morphology and particle size. Tea bags typically contain fannings or dust (finely broken particles), whereas loose-leaf teas consist of larger, intact leaves. While smaller particles can enhance extraction kinetics, they are often derived from lower-grade material with reduced intrinsic caffeine variability compared to whole leaves selected for premium loose-leaf products [38].

3.3. Principal Component Analysis of the Content of Bioactive Compounds in Tested Tea Infusions

The obtained results showed that black teas generally contained higher levels of phenolic acids, while Earl Grey teas tended to exhibit higher total flavonoid contents. The organic production system was associated with enhanced levels of catechins and flavonoids in certain samples, though it did not guarantee the highest total polyphenol content in all cases. The degree of leaf fragmentation had a significant influence: bagged (finely fragmented) teas often showed greater extraction efficiency and higher total polyphenols, but loose-leaf teas, especially organic ones, maintained richer catechin profiles and higher caffeine content. Ultimately, the chemical and biological quality of tea infusions is determined by an interplay among tea type, cultivation system, and leaf form. Organic loose-leaf teas appear as a promising option, balancing bioactive content, while bagged teas provide rapid extraction.
The associations of phenolic compound profiles and caffeine contents in relation to tea leaf fragmentation and tea production systems are illustrated in the following PCA graphs (Figure 7). PC1 and PC2 explain 44.1% and 23.7% of the total variance, respectively. Figures show groups representing tea infusion samples from different production systems (Figure 7a), different fragmentation (Figure 7b), and different types (Figure 7c). The PCA analysis revealed a clear differentiation between teas produced under conventional and organic cultivation systems (Figure 7a). Organic teas are associated with higher levels of flavonoids, catechins, and caffeine, whereas conventional teas show a stronger affinity with phenolic acids. The next PCA biplot (Figure 7b) illustrates a pronounced separation between bagged and loose-leaf teas. Loose-leaf teas cluster with flavonoids, catechins, and caffeine, while bagged teas are more closely associated with phenolic acids, indicating a structural effect on extraction efficiency. PC1 and PC2 together account for 67.8% of the total variance. The third PCA biplot (Figure 7c) demonstrates a distinct separation between traditional black teas and Earl Grey teas. Earl Grey samples align strongly with flavonoids, catechins, and caffeine, whereas black teas are associated with higher phenolic acid content. The first two components explain 67.8% of the overall variability in the dataset.
Taken together, the three PCA analyses of the content of bioactive compounds in tested teas grouped by production system (conventional and organic), fragmentation (bags and leaves), and type (black tea and Earl Grey black tea) (Figure 7) demonstrate that the profiles of bioactive compounds in tea infusions are shaped by multiple interacting factors, with tea type, cultivation system, and leaf fragmentation each contributing to distinct compositional trends. Tea type emerged as the strongest differentiating factor, with Earl Grey teas clustering toward flavonoid- and caffeine-rich profiles, while traditional black teas were more strongly associated with phenolic acids. Leaf form provided a second major axis of separation: loose-leaf teas showed greater affinity with catechins and caffeine, whereas bagged teas clustered with phenolic acids, reflecting the influence of leaf integrity on extraction dynamics. The cultivation system exerted a more moderate but still discernible effect, with organic teas trending toward higher flavonoid and catechin levels, and conventional teas aligning more closely with phenolic acids. Across all three models, caffeine, EGCG, and total flavonoids consistently loaded in the same direction, indicating shared sensitivity to both tea type and leaf structure. In contrast, gallic and chlorogenic acids consistently aligned with conventional and bagged samples. The combined PCA patterns therefore confirm that no single factor fully explains the biochemical variability of teas; instead, the final infusion profile results from the tea type (black vs. Earl Grey), agronomic practice (organic vs. conventional), and processing form (loose-leaf vs. bagged). This multidimensional relationship highlights the importance of considering all three determinants when evaluating the nutritional or functional value of tea beverages.

4. Discussion

The influence of processing factors, such as leaf fragmentation, on tea infusion quality is acknowledged in prior literature. However, a critical gap persists in market-representative data for commercially available, fragmented leaf teas under real-world brewing conditions. Most existing studies emphasize extraction technologies or single variables (e.g., cultivation type or brewing time) rather than the combined synergies between leaf fragmentation degree, organic vs. conventional cultivation, and specific bioactive profiles (total polyphenols, catechins, caffeine). This study addresses this gap by providing novel empirical data from market products, revealing previously underexplored interactions—such as higher catechin retention in coarsely fragmented organic black teas compared to finely fragmented conventional ones—which are often not discussed in existing literature focused on single agricultural system-origin samples.

4.1. The Content of Polyphenolic Compounds

The measured concentrations of total polyphenols (18.58–23.26 mg/100 mL) in the tested tea infusions fall within the range reported for brewed black and flavoured black teas in previous research. However, the variation in total polyphenol content observed between sample groups should be interpreted with caution. Total polyphenols represent a compositional parameter rather than a direct measure of biological or antioxidant activity. Moreover, differences in geographic origin, cultivar, and processing conditions may act as confounding factors influencing polyphenol levels. Therefore, observed differences should be cautiously interpreted as reflecting product heterogeneity and not as implying nutritionally or clinically significant differences between the study groups [39,40]. The tea polyphenol content is influenced by a complex interaction among agronomic conditions, processing methods such as fermentation and oxidation, and extraction parameters including leaf size, brewing time, and temperature [41]. Reviews emphasize that processing and extraction variables often exert effects on polyphenol yield equal to or greater than those of cultivation system differences alone [22]. This observation helps explain why the highest total polyphenol concentration in the present dataset occurred in the conventional, bagged Earl Grey samples, as a combination of fine particle size and product formulation can promote more efficient extraction even under conventional cultivation conditions.
However, the nutritional relevance of these concentrations requires further contextualization in relation to typical dietary intake levels. To contextualize the potential nutritional relevance of the measured concentrations, it is important to relate them to typical dietary intake levels and doses reported in intervention studies. Based on the present results, a standard serving of tea (200–250 mL) would provide approximately 37–58 mg of total polyphenols, depending on the sample [40,42].
Intervention studies investigating the health effects of tea polyphenols, particularly catechins, often report beneficial outcomes at daily intakes ranging from approximately 300 to 800 mg of total polyphenols or 200–400 mg of catechins. In this context, a single serving of the analyzed teas would contribute a modest proportion of these amounts, suggesting that regular consumption (e.g., multiple servings per day) would be required to reach intake levels comparable to those associated with physiological effects [43,44,45].
Therefore, while the differences observed between tea types in the present study reflect meaningful compositional variation, they should not be interpreted as directly translating into clinically significant differences in health outcomes. Rather, the results highlight the contribution of tea as one of multiple dietary sources of polyphenols within a broader nutritional context [43].
The comparative analysis of organic and conventional teas indicated that neither system consistently demonstrated superior total polyphenol concentration. Instead, organic samples frequently exhibited higher levels of specific flavonoids such as catechins and epigallocatechin (EGC), while certain conventional samples, notably the bagged Earl Grey, contained higher total polyphenols. This pattern is consistent with broader comparative analyses across plant-based foods, which have demonstrated variable outcomes. This result can be explained by the dominant effect of particle size and extraction efficiency: finely fragmented material in tea bags enhances the release of readily soluble polyphenols, particularly phenolic acids, due to increased surface area and cell disruption. In contrast, loose-leaf teas may better preserve structurally sensitive compounds such as catechins, which are less efficiently extracted under comparable conditions. This finding highlights that technological factors, including leaf fragmentation and formulation, can override the influence of cultivation system on total polyphenol yield. Meta-analyses indicate that organic production systems often yield higher antioxidant capacity and occasionally greater concentrations of selected secondary metabolites; however, these differences depend strongly on species, tissue type, and compound class [46]. Reviews focusing on tea similarly report that organic management can enhance catechin levels and antioxidant potential in some cases, while other studies observe no significant difference or even higher values in conventional systems, depending on site conditions, cultivar, fertilization (particularly nitrogen availability), and harvest management [22,46]. Lower concentrations of readily available nitrogen in organic soils can trigger enhanced carbon allocation toward the synthesis of phenolic secondary metabolites, whereas conventional fertilization practices that favor rapid shoot regrowth may lead to distinct biochemical profiles. The interplay of these factors produces compound- and context-dependent outcomes rather than a simple conclusion favoring organic systems [46,47]. However, in one of the biggest meta-analysis by Barański et al. (2014) [48], based on several hundred scientific papers assessing the impact of the production system on the content of various compounds (including polyphenols, phenolic acids, flavonoids) in fresh and processed plant products, it was shown that organic products had a significantly higher content of polyphenols compared to products from the conventional system.
Leaf fragmentation or particle size emerged as one of the most important determinants of compositional and extraction variability in the studied samples. Bagged teas with finely fragmented material exhibited higher apparent extraction of total polyphenols, particularly in the conventional Earl Grey samples, consistent with reports that smaller particle sizes increase extraction efficiency through larger surface area exposure [30,49,50]. Finely fragmented material, apart from a larger surface area, exposes more ruptured cell structures, leading to the rapid release of readily soluble constituents, particularly phenolic acids such as gallic and chlorogenic acids [41,49]. However, excessively fine grinding may also accelerate oxidation or mechanical degradation of thermolabile molecules, and may promote the extraction of undesired tannins or bitter phenolics [30,41,49]. Empirical investigations indicate the existence of an optimal particle-size window: moderate grinding, typically around 100–180 μm in green tea, maximizes catechin yield and antioxidant activity, whereas extremely fine particles below 50 μm can result in reduced catechin retention and antioxidant performance, likely due to heat and oxidative damage [50]. The observed pattern, where certain bagged teas displayed higher total polyphenols while loose-leaf samples, especially organic ones, preserved a richer catechin profile and higher caffeine content, aligns well with these mechanistic insights. Catechins and gallated catechins are more sensitive to oxidation and mechanical degradation during processing, and previous research indicates that intact leaf structures better preserve these compounds and promote their controlled release from deeper tissue layers during infusion [30,41]. This may also explain the substantially higher caffeine concentrations observed in loose-leaf organic samples, since caffeine is stored in inner leaf tissues that are more effectively extracted when cellular integrity is partly maintained [28].
Analysis of phenolic acid and flavonoid sub-profiles revealed that gallic acid was the predominant phenolic acid, while epigallocatechin derivatives (EGC and EGC-gallate) were the most abundant catechins, which is in agreement with established compositional profiles of black teas [41]. In our study, all analyzed infusions showed significantly higher EGC levels in organic tea compared to non-organic tea. However, we found no differences in catechin, EGCG, and quercetin. Our previous study [51] showed that organic tea contained significantly more catechin than conventional tea, which, in turn, had a higher EGCG content. Han and Yang (2014) [52] and Han et al. (2018) [53] observed higher total flavonoid content and EGC and EGCG content in organic tea. They found no statistically significant difference in the catechin content. Kim et al. (2018) [54] also found significantly higher EGC and EGCG contents in organic tea, whereas no such difference was observed in catechin content. Phenolic acids such as gallic and chlorogenic acids exhibit greater stability to oxidation and fermentation compared with monomeric catechins, whereas gallated catechins are critical contributors to radical scavenging capacity in vitro [41]. Earl Grey samples demonstrated relatively higher total flavonoid contents, especially in the organic bagged variant. However, this increase cannot be attributed exclusively to either the tea base or bergamot-derived compounds. The measured flavonoid content reflects the combined contribution of tea flavonoids and citrus-derived constituents, as well as differences in extraction efficiency associated with particle size and formulation. As the study was conducted on commercially available products, the results reflect the beverage’s cumulative bioactive composition as consumed, rather than source-specific contributions [55].
The role of bergamot in Earl Grey formulations further modulates the overall polyphenolic and flavonoid profile. Bergamot (Citrus bergamia) extracts contain distinctive flavonoid glycosides such as neoeriocitrin and naringin, as well as brutieridin and melitidin derivatives and various volatile terpenes [56]. These components contribute additional antioxidant and bioactive potential to the final infusion, potentially increasing the total flavonoid sum even when leaf-derived phenolic acids are less abundant [55]. Bergamot polyphenols and essential oils have been documented to exhibit antioxidant, anti-inflammatory, and cardioprotective activities in both experimental and clinical contexts [55,56]. Commercial Earl Grey products, however, differ considerably in their flavouring sources—ranging from natural bergamot oils to synthetic aroma compounds—which can significantly affect measured flavonoid concentrations and antioxidant indices. The elevated flavonoid levels in some of the present Earl Grey samples, particularly the organic bagged type, are therefore consistent with either a higher proportion of natural bergamot constituents or variation in extraction efficiency linked to formulation and particle size [56].

4.2. The Content of Caffeine

Caffeine concentrations (6.14–41.95 mg/100 mL) also exhibited wide variability, consistent with literature documenting that caffeine levels depend on multiple parameters, including cultivar, leaf maturity, plucking standard, processing, and extraction conditions such as temperature and brew time [22,41]. The integrity of the leaf appears to play a key role: intact loose-leaf teas often yield higher caffeine concentrations during extended infusion due to greater leaf mass and slower diffusion, while finely fragmented or bagged forms may extract caffeine rapidly but less completely within the shorter brewing times typically applied. Additionally, the composition of flavored blends influences caffeine outcomes, since commercial formulations may dilute base tea with low-caffeine carriers or enhance it with high-caffeine cultivars. Consequently, the observed caffeine variability reflects an interaction of leaf form, processing, and formulation, rather than a direct effect of the cultivation system alone. While our study did not reveal any unidirectional differences in caffeine content in the infusions tested depending on the degree of tea leaf grinding, the caffeine content was significantly higher in infusions from organic teas (both bagged and loose-leaf) compared to infusions from conventional teas. Other researchers have reported no correlation between caffeine content and production system [52,53,54]. At the same time, some reports suggest an inverse relationship [51]. The higher caffeine concentrations observed in organic teas can be explained by physiological and agronomic factors. While conventional fertilizers supply higher nitrogen, caffeine biosynthesis can also be stimulated by biotic stress, which may be more pronounced under organic management. Additional influences include leaf maturity, cultivar, and environmental conditions, as well as extraction factors during infusion. Together, these variables likely contribute to the elevated caffeine levels observed in commercially available organic teas [57,58].
Overall, the present results, showing compound-specific effects of cultivation system, strong dependence on leaf particle size, and modulation by aromatization, are consistent with findings from contemporary reviews and experimental research. The structural effects were clearly supported by the PCA results. Bagged teas clustered with phenolic acids, while loose-leaf teas clustered with catechins, total flavonoids, and caffeine, demonstrating that leaf-fragment dimension is a major discriminating factor influencing the chemical composition of tea infusions. Together, the results show that the effect of fragmentation is compound-class specific: fine fragmentation favors the extraction of stable, surface-associated phenolic acids, whereas coarser leaves favor the preservation and release of catechins and caffeine, compounds more vulnerable to oxidative and mechanical stress during processing. Processing technology and particle size have been identified as key determinants of polyphenol extraction and antioxidant activity [30,41], while comparisons between organic and conventional teas yield variable, context-dependent outcomes [22,46]. The inclusion of bergamot-derived bioactive compounds in flavored teas further complicates the interpretation of comparisons [55,56].

4.3. Study Limitations

This study has several limitations that should be considered when interpreting the results. First, the analytical scope of the HPLC panel did not include several key bioactive compounds characteristic of black tea. In particular, the omission of theaflavins and thearubigins represents an important gap, as these compounds are major polyphenolic constituents distinguishing black tea from other tea types and are closely associated with its antioxidant and health-related properties. Similarly, L-theanine, a major amino acid unique to tea and frequently linked to physiological effects and differences between organic and conventional production systems, was not analyzed, which restricts the comprehensiveness of the compositional assessment.
Another limitation concerns the lack of total antioxidant capacity measurements. Although the study emphasizes the potential health-promoting properties of tea and its bioactive compounds, it is limited to compositional analysis without evaluating antioxidant activity. Thus, it does not fully allow for the direct assessment of functional implications of the observed compositional differences.
The heterogeneity of the studied material also represents a limitation. The analyzed tea products originated from different geographic regions (Assam, Ceylon, and Zhejiang), which differ substantially in terms of cultivars, climatic conditions, and agronomic practices. Geographic origin is known to strongly influence tea composition and, therefore, may act as a significant confounding factor that was not fully controlled in the study design. In addition, the tested tea samples were purchased from retail markets without detailed information on production year, harvest season, or batch number, which further limits traceability and introduces additional variability.
For Earl Grey teas, the interpretation of flavonoid differences is limited by the lack of characterization and quantification of bergamot content. Since bergamot-derived compounds may contribute to the chemical profile of the infusion, the absence of information on bergamot concentration introduces an uncontrolled variable that may partially explain observed differences between products.
Some methodological aspects should also be acknowledged. The brewing protocol (1 g tea per 100 mL boiling water for 5 min) was standardized to ensure comparability between samples; however, it should be pointed out that tea preparation practices vary widely across consumers and cultures. Therefore, the obtained results reflect a controlled experimental extraction rather than real-world consumption conditions and should be interpreted within this context.
Furthermore, the limitation is in the relatively small sample size of the study. A larger sample size across all product categories would substantially improve the statistical robustness and generalizability of the findings. In addition, the study included only black tea and Earl Grey tea, which limits the broader applicability of the results to other tea types. Future work with expanded sample sets would further solidify observed trends and enhance statistical robustness. Nevertheless, the significant interactions observed provide meaningful preliminary insights applicable to industry and research contexts.
Finally, although the study aimed to investigate differences between teas from different production systems and degrees of leaf fragmentation available to consumers, it did not include sensory evaluation of the infusions. Sensory analysis would provide valuable insight into the relationship between chemical composition, sensory characteristics, and consumer acceptance, and could help determine whether compositional differences translate into perceptible quality differences. Future studies should therefore integrate chemical profiling with sensory analysis to better understand how consumer choices influence the potential intake of bioactive compounds and perceived tea quality.

5. Conclusions

The aim of the study was to investigate the content of bioactive compounds in a selection of organic and conventional tea infusions characterized by different degrees of leaf fragmentation. The study provides insights into how the degree of leaf fragmentation interacts with organic/conventional cultivation to influence polyphenol and caffeine content in tea. The identified significant differences offer practical guidance for enhancing functional tea quality, advancing beyond established general knowledge toward targeted applications. The findings of this study confirm that the chemical composition of tea infusions is significantly influenced by the type of tea, cultivation system, and leaf form, but it also revealed some previously underexplored interactions between the leaf fragmentation and cultivation system effects. Black teas generally exhibited higher levels of phenolic acids, particularly gallic and chlorogenic acids, while Earl Grey teas were characterized by a richer flavonoid profile, likely reflecting both the effect of bergamot addition and differences in processing. Organic cultivation tended to enhance the accumulation of certain bioactive compounds, such as catechins and total flavonoids, although this trend was not uniform across all samples. The degree of leaf fragmentation also played a decisive role in compound extraction: finely fragmented, bagged teas facilitated a more efficient release of phenolics (total), whereas loose-leaf teas preserved a more diverse and balanced composition of catechins and caffeine. From a consumer or product design perspective, organic loose-leaf Earl Grey teas appear to offer the most favorable balance of catechins, flavonoids, and caffeine, whereas conventional bagged black teas provide higher phenolic acid content and faster extraction. However, the results regarding caffeine do not suggest a concomitant beneficial effect. It is important to note that higher caffeine levels are not inherently beneficial and may be undesirable for certain population groups (e.g., individuals sensitive to caffeine, pregnant women, or those with cardiovascular disease).
These findings highlight the importance of both agronomic and technological factors in optimizing the nutritional and functional value of tea beverages. These results also support the need for multidimensional evaluation in both scientific research and marketing claims related to tea quality and health potential. This difference reflects variability in total polyphenol content; however, its biological relevance cannot be further inferred without direct assessment of antioxidant activity and consideration of compositional and origin-related factors.

Author Contributions

Conceptualization, R.K., A.P. and D.Ś.-T.; Data curation, R.K., K.K., M.Ż.-K. and D.Ś.-T.; Formal analysis, R.K., K.K., E.H., A.P., M.Ż.-K., M.J., A.F., M.B., A.M. and D.Ś.-T.; Funding acquisition, D.Ś.-T.; Investigation, R.K., K.K., E.H., M.J. and D.Ś.-T.; Methodology, R.K., K.K., E.H., A.P. and D.Ś.-T.; Resources, M.Ż.-K., M.J. and D.Ś.-T.; Supervision, R.K. and D.Ś.-T.; Visualization, R.K., K.K. and M.Ż.-K.; Writing—original draft, R.K., K.K. and D.Ś.-T.; Writing—review and editing, R.K., K.K., E.H., A.P., M.Ż.-K., M.J., A.F., M.B., A.M. and D.Ś.-T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Polish Ministry of Science and Higher Education with funds of the Institute of Human Nutrition Sciences, Warsaw University of Life Sciences (SGGW), for scientific research.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest. The funders had no role in the design of the study, in the collection, analyses, or interpretation of data, in the writing of the manuscript, or in the decision to publish the results.

Abbreviations

The following abbreviations are used in this manuscript:
GAGallic acid
CGAChlorogenic acid
CCatechin
EGCEpigallocatechin
EGCGEpigallocatechin gallate
HPLCHigh-performance liquid chromatography

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Figure 1. Comparison of the average polyphenol (sum) content in the examined infusions of (a) black tea and (b) Earl Grey black tea. Data are presented as means ± standard deviations. Bars accompanied by different letters are significantly different at the 5% level of probability.
Figure 1. Comparison of the average polyphenol (sum) content in the examined infusions of (a) black tea and (b) Earl Grey black tea. Data are presented as means ± standard deviations. Bars accompanied by different letters are significantly different at the 5% level of probability.
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Figure 2. Comparison of the average phenolic acid (sum) content in the examined infusions of (a) black tea and (b) Earl Grey black tea. Data are presented as means ± standard deviations. Bars accompanied by different letters are significantly different at the 5% level of probability.
Figure 2. Comparison of the average phenolic acid (sum) content in the examined infusions of (a) black tea and (b) Earl Grey black tea. Data are presented as means ± standard deviations. Bars accompanied by different letters are significantly different at the 5% level of probability.
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Figure 3. Comparison of the average gallic acid content in the examined infusions of (a) black tea and (b) Earl Grey black tea. Data are presented as means ± standard deviations. Bars accompanied by different letters are significantly different at the 5% level of probability.
Figure 3. Comparison of the average gallic acid content in the examined infusions of (a) black tea and (b) Earl Grey black tea. Data are presented as means ± standard deviations. Bars accompanied by different letters are significantly different at the 5% level of probability.
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Figure 4. Comparison of the average flavonoid (sum) content in the examined infusions of (a) black tea and (b) Earl Grey black tea. Data are presented as means ± standard deviations. Bars accompanied by different letters are significantly different at the 5% level of probability.
Figure 4. Comparison of the average flavonoid (sum) content in the examined infusions of (a) black tea and (b) Earl Grey black tea. Data are presented as means ± standard deviations. Bars accompanied by different letters are significantly different at the 5% level of probability.
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Figure 5. Comparison of the average individual flavonoid content in the examined infusions: (a) EGC content in the black tea; (b) EGC content in the Earl Grey black tea; (c) EGCG content in the black tea; (d) EGCG content in the Earl Grey black tea; (e) catechin content in the black tea; (f) catechin content in the Earl Grey black tea; (g) quercetin content in the black tea; (h) quercetin content in the Earl Grey black tea. Data are presented as means ± standard deviations. Bars accompanied by different letters are significantly different at the 5% level of probability.
Figure 5. Comparison of the average individual flavonoid content in the examined infusions: (a) EGC content in the black tea; (b) EGC content in the Earl Grey black tea; (c) EGCG content in the black tea; (d) EGCG content in the Earl Grey black tea; (e) catechin content in the black tea; (f) catechin content in the Earl Grey black tea; (g) quercetin content in the black tea; (h) quercetin content in the Earl Grey black tea. Data are presented as means ± standard deviations. Bars accompanied by different letters are significantly different at the 5% level of probability.
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Figure 6. Comparison of the average caffeine content in the examined infusions of (a) black tea and (b) Earl Grey black tea. Data are presented as means ± standard deviations. Bars accompanied by different letters are significantly different at the 5% level of probability.
Figure 6. Comparison of the average caffeine content in the examined infusions of (a) black tea and (b) Earl Grey black tea. Data are presented as means ± standard deviations. Bars accompanied by different letters are significantly different at the 5% level of probability.
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Figure 7. Principal Component Analysis of the content of bioactive compounds in tested tea infusions grouped by (a) production system (conventional and organic), (b) fragmentation (bags and leaves), and (c) type (black tea and Earl Grey black tea); Gallic Acid (GA), Chlorogenic Acid (CGA), Catechin (C), Epigallocatechin (EGC), Epigallocatechin Gallate (EGCG).
Figure 7. Principal Component Analysis of the content of bioactive compounds in tested tea infusions grouped by (a) production system (conventional and organic), (b) fragmentation (bags and leaves), and (c) type (black tea and Earl Grey black tea); Gallic Acid (GA), Chlorogenic Acid (CGA), Catechin (C), Epigallocatechin (EGC), Epigallocatechin Gallate (EGCG).
Applsci 16 03283 g007aApplsci 16 03283 g007b
Table 1. Analytical method validation parameters for phenolic compounds and caffeine (in mg/100 mL infusion).
Table 1. Analytical method validation parameters for phenolic compounds and caffeine (in mg/100 mL infusion).
Compounds (Standard)mg/100 mL
LODLOQ% Recovery% RSD for Peak Area
Gallic acid1.224.0399.101.31
Epigallocatechin gallate0.973.2098.871.38
Catechin0.872.8799.921.26
Quercetin0.120.3998.931.12
Caffeine4.26 14.06 99.071.13
LOD—limit of detection, LOQ—limit of quantification, RSD—relative standard deviation.
Table 2. Phenolic acids, flavonoids, and caffeine content (mg/100 mL of infusion) in tea of different types (black and Earl Grey), degree of leaf fragmentation, and from organic and conventional cultivation systems.
Table 2. Phenolic acids, flavonoids, and caffeine content (mg/100 mL of infusion) in tea of different types (black and Earl Grey), degree of leaf fragmentation, and from organic and conventional cultivation systems.
SamplePolyphenolsPhenolic
Acids		Gallic AcidChlorigenic AcidFlavonoidsCatechinEpigallocatechinEpigallocatechin GallateQuercetinCaffeine
Black–Bags–Conventional18.58 ± 0.41 *10.17 ± 0.108.31 ± 0.061.85 ± 0.058.42 ± 0.331.85 ± 0.164.28 ± 0.461.85 ± 0.070.44 ± 0.0113.26 ± 0.50
Black–Bags–Organic20.63 ± 0.3310.41 ± 0.607.46 ± 0.502.96 ± 0.1010.22 ± 0.482.15 ± 0.056.02 ± 0.391.61 ± 0.070.44 ± 0.0022.94 ± 0.31
Black–Leaves–Conventional21.51 ± 0.4911.54 ± 0.088.35 ± 0.103.19 ± 0.039.96 ± 0.412.46 ± 0.054.38 ± 0.522.60 ± 0.080.52 ± 0.0114.47 ± 0.17
Black–Leaves–Organic19.40 ± 0.177.48 ± 0.166.60 ± 0.180.87 ± 0.0311.92 ± 0.293.27 ± 0.135.41 ± 0.082.84 ± 0.390.41 ± 0.0017.46 ± 0.34
Earl Grey–Bags–Conventional23.26 ± 0.5610.26 ± 0.448.29 ± 0.421.97 ± 0.1113.00 ± 0.203.31 ± 0.225.41 ± 0.283.80 ± 0.060.49 ± 0.026.14 ± 0.44
Earl Grey–Bags–Organic21.61 ± 0.367.05 ± 0.354.95 ± 0.352.10 ± 0.0214.56 ± 0.404.07 ± 0.115.96 ± 0.484.06 ± 0.030.47 ± 0.0036.63 ± 0.93
Earl Grey–Leaves–Conventional20.31 ± 1.098.82 ± 0.917.36 ± 0.901.46 ± 0.0311.48 ± 0.223.70 ± 0.053.72 ± 0.183.74 ± 0.070.33 ± 0.0010.14 ± 0.73
Earl Grey–Leaves–Organic18.72 ± 0.287.09 ± 0.335.01 ± 0.292.08 ± 0.0811.64 ± 0.173.22 ± 0.184.01 ± 0.213.94 ± 0.260.46 ± 0.0341.95 ± 2.24
p-value **
Type0.0000.0000.0000.0000.0000.0000.1100.0000.0360.000
Fragmentation0.0000.0010.0300.0000.0400.0000.0000.0000.0000.005
Cultivation0.0020.0000.0000.0000.0000.0000.0000.1250.9770.000
Type × Fragmentation0.0000.8300.9550.0750.0000.0000.0000.0000.0000.000
Type × Cultivation0.0020.1500.0000.0000.0020.0020.0050.1200.0000.000
Fragmentation × Cultivation0.0000.0020.8960.0000.0340.0050.1170.1700.1740.003
Type × Fragmentation × Cultivation0.0000.0000.0170.0000.0100.0000.4440.0840.0000.000
* Values in the table are presented as means ± standard deviation. ** ANOVA p-values for factor effects and interactions. Significance level α = 0.05.
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Kazimierczak, R.; Kopczyńska, K.; Hallmann, E.; Ponder, A.; Żebrowska-Krasuska, M.; Jakubowska, M.; Filipović, A.; Banožić, M.; Mandić, A.; Średnicka-Tober, D. Phenolic Acids and Flavonoids in Selected Commercial Organic and Conventional Tea Products Characterized by Different Degrees of Leaf Fragmentation. Appl. Sci. 2026, 16, 3283. https://doi.org/10.3390/app16073283

AMA Style

Kazimierczak R, Kopczyńska K, Hallmann E, Ponder A, Żebrowska-Krasuska M, Jakubowska M, Filipović A, Banožić M, Mandić A, Średnicka-Tober D. Phenolic Acids and Flavonoids in Selected Commercial Organic and Conventional Tea Products Characterized by Different Degrees of Leaf Fragmentation. Applied Sciences. 2026; 16(7):3283. https://doi.org/10.3390/app16073283

Chicago/Turabian Style

Kazimierczak, Renata, Klaudia Kopczyńska, Ewelina Hallmann, Alicja Ponder, Małgorzata Żebrowska-Krasuska, Martyna Jakubowska, Adrijana Filipović, Marija Banožić, Ana Mandić, and Dominika Średnicka-Tober. 2026. "Phenolic Acids and Flavonoids in Selected Commercial Organic and Conventional Tea Products Characterized by Different Degrees of Leaf Fragmentation" Applied Sciences 16, no. 7: 3283. https://doi.org/10.3390/app16073283

APA Style

Kazimierczak, R., Kopczyńska, K., Hallmann, E., Ponder, A., Żebrowska-Krasuska, M., Jakubowska, M., Filipović, A., Banožić, M., Mandić, A., & Średnicka-Tober, D. (2026). Phenolic Acids and Flavonoids in Selected Commercial Organic and Conventional Tea Products Characterized by Different Degrees of Leaf Fragmentation. Applied Sciences, 16(7), 3283. https://doi.org/10.3390/app16073283

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