Tea, derived from the leaves of the Camellia sinensis plant, is one of the world’s most widely consumed beverages. Its characteristics are largely attributed to polyphenols, a diverse group of plant-based compounds that constitute a significant fraction of the tea leaf’s dry weight. These compounds are responsible for the astringency, color, and aroma, and they form the chemical basis for the biological activity associated with tea consumption. The concentration and chemical structure of these polyphenols vary depending on the tea plant cultivar and the manufacturing process employed. This variation dictates the unique properties of different tea types.
Key Polyphenol Compounds in Tea
The largest and most studied group of polyphenols in tea are the catechins, a class of flavanols found in high concentrations in the unprocessed leaf. These monomeric compounds include four primary structures: epicatechin (EC), epigallocatechin (EGC), epicatechin gallate (ECG), and epigallocatechin gallate (EGCG). EGCG is often the most abundant catechin and is noted for its chemical structure, which includes a gallate moiety that contributes to its reactivity. In fresh, unoxidized tea leaves, catechins can account for 60% to 80% of the total polyphenol content.
The chemical profile of tea also includes smaller amounts of other polyphenolic classes, such as flavonols, including quercetin and kaempferol. Catechin structures are susceptible to enzymatic transformation during processing. This reactivity leads to the formation of larger, more complex polyphenols, which alter the final composition of the brewed beverage. These resulting compounds, primarily theaflavins and thearubigins, are distinct from catechins and are responsible for the color and body of extensively processed teas.
How Tea Processing Affects Compound Levels
The distinct categories of tea—green, oolong, and black—are defined by the controlled enzymatic oxidation that occurs after plucking the leaves. Green tea production bypasses oxidation by applying heat immediately after harvest, which inactivates the polyphenol oxidase enzymes. This minimal processing preserves the native catechin structure, resulting in a product that retains a high concentration of EGCG and other monomeric flavanols. The lack of oxidation explains the pale infusion and the characteristic astringency of green tea.
Black tea production involves a complete, intentional oxidation phase where the leaves are crushed or rolled, allowing enzymes and oxygen to interact fully with the catechins. The polyphenol oxidase catalyzes the conversion of catechins into colored, polymeric compounds. This process depletes the original catechins, transforming them into theaflavins (reddish-orange dimers) and thearubigins (larger, reddish-brown polymers). Black tea is characterized by its high content of these oxidized polymers, which contribute to its darker color and fuller body.
Oolong and white teas represent intermediate stages in the oxidation spectrum, resulting in a mixed polyphenol profile. Oolong tea is semi-oxidized, with processing controlled to halt the enzymatic reaction before the extent seen in black tea. This yields a composition containing both unoxidized catechins and some theaflavins and thearubigins. White tea is minimally processed, often just withered and dried, and is considered unoxidized or very lightly oxidized, meaning its profile is closer to green tea.
Mechanisms of Biological Activity
The biological activity of tea polyphenols is determined by their chemical structure, which allows interaction with cellular components and signaling molecules. A primary mechanism is their antioxidant capacity, involving the direct scavenging of reactive oxygen and nitrogen species (ROS/RNS). Polyphenols act as reducing agents, donating hydrogen atoms to neutralize reactive molecules like hydroxyl radicals and superoxide anions, preventing oxidative damage to lipids, proteins, and DNA.
Polyphenols also engage in metal chelation, a significant aspect of their cellular protection. The presence of hydroxyl groups, specifically the 3′,4′-dihydroxy catechol structure on the B-ring of catechins, enables binding to pro-oxidant transition metals such as iron and copper ions. By forming inactive complexes, polyphenols prevent these metals from catalyzing the formation of new free radicals, shutting down a major source of oxidative stress.
Polyphenols modulate cellular signaling pathways, influencing the body’s inflammatory response. They interfere with the activation of transcription factors, such as the nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). NF-κB is a protein complex that controls DNA transcription and regulates immune and inflammatory responses. By inhibiting NF-κB activity, tea polyphenols can downregulate the expression of genes responsible for producing pro-inflammatory signaling molecules.
Maximizing Extraction and Bioavailability
Optimizing the final concentration of polyphenols involves controlling the extraction process, governed by water temperature and steeping time. Higher water temperatures and longer steeping times increase the yield of polyphenols extracted from the tea leaf material. For instance, total phenolic content increases significantly with steeping times extending beyond five minutes. While a higher temperature, such as 100°C, maximizes extraction, prolonged exposure to high heat may lead to the degradation of less stable catechin compounds.
Once extracted, the bioavailability of polyphenols is influenced by consumption practices. Catechins, particularly EGCG, are unstable in the non-acidic environment of the small intestine, leading to degradation before absorption. Adding a source of acid, such as lemon juice or ascorbic acid (Vitamin C), stabilizes these compounds; citrus juice has been shown to increase the recovery of catechins in simulated digestion models by more than fivefold. Milk addition is a complex factor, as milk proteins (caseins) can form complexes with tea polyphenols, potentially inhibiting absorption.