How fermentation, the microbiome and the skin continue the chemistry of tea.

Tea is not finished when it leaves the teapot. Its compounds continue to change—first through fermentation, then through digestion and the activity of our intestinal microorganisms. Some of the resulting metabolites may eventually influence tissues far beyond the digestive tract, including the blood vessels, immune system and skin.

In this final part of our tea series, we follow Camellia sinensis beyond the familiar infusion: into kombucha and post-fermented tea, through the microbiome and, finally, into modern dermatology and cosmetic formulation.

When tea becomes kombucha

Kombucha begins with an infusion of tea, usually black or green, to which sugar and a culture of yeasts and bacteria are added. The floating layer traditionally called a “tea mushroom” is not a mushroom. It is a cellulose-rich structure created by the microbial community—a symbiotic culture of bacteria and yeasts, commonly shortened to SCOBY.

During fermentation, yeasts break sucrose into simpler sugars and produce small amounts of ethanol and carbon dioxide. Acetic-acid bacteria then convert much of the ethanol into organic acids. The drink becomes sharper, less sweet and chemically more complex.

Its final composition depends on the original tea, the microorganisms within the culture, the quantity of sugar, temperature, oxygen exposure and length of fermentation. Two drinks sold under the same name may therefore be quite different preparations. [1]

What fermentation changes

Fermentation does not simply preserve tea or add bacteria to it - it remodels the drink. Microbial enzymes act on compounds already present in the infusion. Some larger polyphenols are broken into smaller forms, some compounds are released from bound forms, and others are altered or gradually degraded. The result may contain a different balance of polyphenols—and potentially different biological activity—from the original tea.

Fermentation time matters: a younger kombucha generally retains more sugar and tastes softer, while a longer fermentation is usually more acidic and less sweet, but it is not necessarily more medicinal. With prolonged fermentation, desirable tea compounds may also begin to deteriorate.

Kombucha is therefore best understood as transformed tea. It retains characteristics inherited from the original leaf while acquiring organic acids, microbial metabolites and, in an unpasteurised product, living microorganisms.

Is kombucha probiotic?

Possibly—but not automatically. A probiotic is not simply any living microorganism. It is a particular strain, present in an adequate quantity, with a demonstrated health benefit. Kombucha may contain living yeasts, lactic-acid bacteria and acetic-acid bacteria, but their identity and abundance vary between cultures and finished products. Pasteurised kombucha may contain few or no viable organisms.

The beverage may nevertheless retain useful products of fermentation even when living microbes are absent. These include organic acids, transformed polyphenols and microbial metabolites, sometimes described as postbiotic compounds.

Kombucha is consequently more complex than “tea with probiotics.” Its activity arises from the interaction between the original plant, the fermenting culture and the chemistry produced by fermentation.

Tea and the intestinal microbiome

The relationship between tea and the microbiome is reciprocal. Not all tea polyphenols are absorbed in the small intestine. A considerable proportion travels onwards to the colon, where intestinal microbes break larger molecules into smaller metabolites. Some of these metabolites may be more readily absorbed and may possess biological activity of their own.

At the same time, tea compounds can influence the intestinal environment. Research is examining their effects on microbial composition, intestinal-barrier integrity, inflammatory signalling and the substances produced by gut bacteria.

This has been described as a component–microbiota–metabolite–host relationship: tea affects the microbes, the microbes transform the tea, and the resulting compounds interact with the person consuming it. [2]

The effect will not be identical in everyone. Each person brings a different microbial ecosystem to the encounter, meaning that two people may transform—and respond to—the same tea differently.

Green-tea and black-tea kombucha are not identical

Green and black tea come from the same plant, but their leaves have already undergone very different transformations before kombucha fermentation begins. They therefore provide the microorganisms with different starting materials and produce kombuchas with different chemical and potentially therapeutic emphases.

Green tea is heated soon after harvesting, limiting oxidation and preserving more of the leaf’s original catechins. These smaller polyphenols—particularly EGCG—have been studied in relation to glucose regulation, vascular function, inflammatory signalling, cellular protection and the skin’s response to ultraviolet damage.

Green-tea kombucha retains this more catechin-led character. Fermentation may release some polyphenols from bound forms and convert others into smaller metabolites, while adding organic acids and microbial products.

For this reason, green-tea kombucha is particularly relevant where the interest is metabolic health, post-meal glucose regulation, low-grade metabolic inflammation, vascular protection or the relationship between oxidative stress and skin ageing. The most relevant recent human studies examining kombucha, inflammation and excess body weight have used green tea.

Black tea is deliberately allowed to oxidise during manufacture. During this process, many of its original catechins combine to form larger polyphenols known as theaflavins and thearubigins. These compounds create black tea’s amber-red colour, deeper flavour and characteristic astringency.

Black-tea kombucha therefore begins with fewer unchanged catechins but more oxidised and polymerised tea compounds. During fermentation, these larger polyphenols may be transformed further, while gallic acid and other smaller phenolic compounds can become more available.

This makes black-tea kombucha particularly interesting in relation to digestive and microbial ecology, mucosal immunity and the metabolism of larger tea polyphenols by intestinal bacteria.

Research into black tea itself has shown changes in intestinal bacterial populations, including increases in certain butyrate-producing bacteria in people with lower mucosal immune activity. Butyrate nourishes cells lining the colon and participates in intestinal-barrier and immune regulation. [3]

Green tea may therefore be preferable when the desired emphasis is metabolic regulation, inflammatory balance, vascular protection or skin ageing. Black tea may offer a more appropriate polyphenol foundation when the interest is mucosal health, microbial metabolism and the production of short-chain fatty acids.

For general microbiome support, however, neither can be described as universally superior. The quality and quantity of tea, the microorganisms within the culture, fermentation time, residual sugar and storage conditions may influence the finished drink as much as the colour of the starting leaf.

Comparative research confirms that green- and black-tea kombuchas retain different phenolic profiles and display different patterns of antioxidant, antibacterial and cellular activity. The starting tea can also influence the yeast community that develops during fermentation. The SCOBY does not erase the identity of the tea; it transforms two already distinct botanical preparations. [4,5]

Kombucha, insulin, the microbiome and inflammation

Kombucha brings together three biologically active elements: compounds inherited from tea, microorganisms responsible for fermentation, and the new metabolites those organisms produce.

Tea polyphenols can be transformed both during fermentation and later by intestinal bacteria. Organic acids and other fermentation products alter the environment through which the drink passes, while viable microorganisms may interact temporarily with the resident microbiome.

Human research is now beginning to show measurable effects.

In one controlled crossover study, kombucha consumed with a carbohydrate-rich meal reduced the immediate glucose and insulin responses compared with the same meal accompanied by soda water. The glycaemic index of the meal fell from 86 to 68, while its insulin index fell from 85 to 70. The authors proposed that the effect may have involved several elements acting together: organic acids, tea polyphenols, tannins and living microorganisms. [6]

A small study in adults with type 2 diabetes found that 240 ml of kombucha taken daily with dinner for four weeks was associated with a fall in average fasting glucose. Analysis of the drink identified lactic-acid bacteria, acetic-acid bacteria and yeasts, together with lactic and acetic acids. [7]

A larger ten-week study followed 59 adults with excess body weight. Both groups followed the same energy-restricted diet, while one group also drank 200 ml of green-tea kombucha each day.

Kombucha did not produce additional weight loss, but it was associated with a reduction in lipid accumulation product—a marker calculated from waist measurement and blood triglycerides.

Both groups showed reductions in the inflammatory messengers IL-1β and IL-8. However, IL-6 increased in the diet-only group and not in those drinking kombucha, suggesting that green-tea kombucha may have moderated part of the inflammatory response during the intervention.

Changes were also observed in the salivary microbiome. Microbial richness and overall community differences increased in the kombucha group, while the balance between two major bacterial divisions shifted. The oral microbiome is particularly relevant because it forms the first microbial interface between a fermented drink and the body. [8]

A related analysis of the same intervention examined faecal bacteria and blood metabolites. Green-tea kombucha did not completely reconstruct the intestinal microbiome, but bacterial richness increased at genus level within the kombucha group.

The most striking changes appeared in the blood metabolome. Researchers detected a distinct group of metabolites associated with kombucha consumption, including compounds connected with glutathione metabolism and inflammatory regulation. This suggests that kombucha may influence health not simply by introducing bacteria, but by changing the chemical conversation between food, microbes and human metabolism. [9]

A separate controlled study in healthy adults found more modest but still identifiable effects. Four weeks of kombucha consumption altered the abundance of selected intestinal bacteria and enriched several organisms capable of producing short-chain fatty acids. A bacterium present in the kombucha itself was also detected more frequently in those drinking it.

Circulating IL-6, IL-10 and C-reactive protein did not change significantly in this healthy group. This may indicate that effects on inflammatory signalling are more visible when low-grade metabolic inflammation is already present than when inflammatory markers are normal at the outset. [10]

These findings suggest a broader model of kombucha activity. Its effects may arise from tea polyphenols providing substrates for microbial metabolism; fermentation converting plant compounds into different forms; organic acids influencing the digestive environment; microorganisms interacting with resident communities; and the resulting metabolites communicating with intestinal, metabolic and immune pathways.

Kombucha and post-fermented tea are not the same

Kombucha is produced by fermenting a prepared, sweetened tea infusion. Post-fermented teas—such as certain dark teas and mature pu-erh—are transformed while the leaves themselves are being processed and aged.

In post-fermented tea, microorganisms and plant enzymes gradually alter the stored leaf. Its colour darkens, its flavour becomes softer and earthier, and its polyphenol profile changes. Once brewed, it remains a tea infusion rather than a living culture drink.

These teas have a long association with rich meals and digestion in East Asian food culture. Their microbial transformation may alter the availability and metabolism of tea polyphenols, but the process is fundamentally different from fermenting sweetened liquid tea with a SCOBY.

Tea and the skin

The skin is continually exposed to ultraviolet radiation, pollution, inflammation and oxidative stress. Over time, these pressures contribute to uneven pigmentation, impaired barrier function and the breakdown of collagen and elastin.

Green tea has attracted particular interest because it retains a high concentration of catechins, including EGCG. These compounds are being studied for their influence on inflammatory signalling, oxidative damage and the enzymes involved in degrading the skin’s supporting matrix.

In a controlled study, regular consumption of a green-tea drink improved the skin’s response to ultraviolet exposure, increased microcirculation and produced measurable changes in skin hydration, density and elasticity. Tea does not replace topical sun protection, but the study demonstrated that dietary polyphenols can contribute to the skin’s physiological response to environmental stress. [11]

Tea also has potential when applied in a properly formulated cosmetic product.

A split-face, double-blind clinical study tested green-tea extract carried in microscopic chitosan particles. After eight weeks, the treated skin showed improvements in elasticity, dullness and the appearance of wrinkles compared with the placebo-treated side. [12]

These delivery systems matter. Catechins are chemically delicate: light, oxygen, heat and unsuitable pH can cause them to deteriorate. A well-designed serum or cream must protect the extract, deliver it effectively and remain stable throughout its intended life.

Where tea belongs in cosmetic formulation

Green-tea extracts are particularly interesting for skin exposed to sunlight, pollution, redness and premature ageing. Their antioxidant and inflammation-modulating actions also make them relevant to blemish-prone or excessively oily skin.

Black tea contains fewer unchanged catechins but more theaflavins, thearubigins and other compounds created during leaf oxidation. These provide a different antioxidant and astringent profile and may be valuable in formulations intended to tone, refresh and support environmentally stressed skin.

White tea is often promoted as the most delicate cosmetic tea. Its true value depends less on its pale colour or prestige than on the quality, concentration and preservation of the extract.

Fermented-tea and kombucha-derived ingredients are also appearing in modern cosmetics. Their activity may arise from tea polyphenols, organic acids, fermentation metabolites and bacterial cellulose.

Bacterial cellulose is especially interesting as a cosmetic and dermatological material. It can hold large quantities of water and form close contact with the skin, making it a possible vehicle for masks, dressings and the controlled delivery of active compounds.

A cosmetic-grade fermented ingredient is filtered, characterised, preserved and tested for stability and safety. Its value lies not merely in the word “fermented,” but in knowing precisely what the fermentation has produced and how those compounds behave within the finished formulation.

Can tea support healthier skin ageing?

Ageing cannot be reduced to a single molecule or antioxidant. It involves accumulated ultraviolet damage, chronic low-grade inflammation, changes in circulation, altered cellular repair and the gradual loss of collagen, elasticity and barrier resilience.

Tea is interesting because it touches several of these processes at once. Its polyphenols may help regulate oxidative and inflammatory stress. Some influence enzymes involved in collagen degradation, while others affect pigmentation, microcirculation and the cellular response to ultraviolet exposure. Tea compounds may also be transformed by the intestinal microbiome into smaller metabolites, creating a possible link between regular tea consumption, systemic metabolism and the condition of the skin. [13]

Fermentation adds another dimension. By changing the form of plant polyphenols and producing microbial metabolites, it may alter which compounds are available to the body and how they interact with the gut–skin axis.

Tea should not be reduced to an “anti-ageing antioxidant.” It is more accurately understood as a complex botanical system capable of interacting with circulation, inflammation, microbial metabolism and tissue protection.

Beyond the teapot

A tea leaf may be steamed, rolled, oxidised, aged, ground or fermented. It may be infused and absorbed—or travel onwards to the colon, where the microbiome performs another stage of transformation. ts compounds may then appear in the circulation as metabolites that no longer resemble those originally present in the leaf. They may interact with intestinal cells, immune signalling, blood vessels and skin. In cosmetic science, extracts from the same plant can be stabilised and delivered directly to the tissues for which they are intended.

This is what makes tea far more than a collection of fashionable antioxidants. Its activity depends on the variety of leaf, the way it is processed, the method of preparation, microbial ecology and the individual receiving it. From leaf, to culture, to microbiome, to skin: tea continues to change—and to teach us—long after the water has been poured.

References

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2. Yue R, Wen Y, Zhao S, et al. Tea and Digestive System Health: Integrating Gut Microbiota-Involved ADME and Personalized Nutrition for Gastrointestinal Disorders Intervention. Critical Reviews in Food Science and Nutrition. 2026. doi:10.1080/10408398.2026.2684712.
3. Tomioka R, Tanaka Y, Suzuki M, Ebihara S. The Effects of Black Tea Consumption on Intestinal Microflora: A Randomized Single-Blind, Parallel-Group, Placebo-Controlled Study. Journal of Nutritional Science and Vitaminology. 2023;69(5):326–339. doi:10.3177/jnsv.69.326.
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12. Wisuitiprot W, Ingkaninan K, Jones S, Waranuch N. Effect of Green Tea Extract Loaded Chitosan Microparticles on Facial Skin: A Split-Face, Double-Blind, Randomized Placebo-Controlled Study. Journal of Cosmetic Dermatology. 2022;21(9):4001–4008. doi:10.1111/jocd.14707.
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Disclaimer

This article is provided for educational purposes and does not replace individual medical advice, diagnosis or treatment. Tea, fermented beverages and concentrated botanical extracts may not be appropriate for everyone. Individual health conditions, pregnancy, medication use, caffeine sensitivity and the composition of a particular product should be considered with an appropriately qualified healthcare practitioner.