Fermented or oxidized infusions: the secrets of a process inspired by black tea

Let's start by correcting a centuries-old confusion. In the world of tea, "fermentation" has long been used to describe the transformation of black tea. The problem: this term is technically incorrect.

Fermentation, strictly speaking, involves living microorganisms—yeast or bacteria—that transform one substance into another. This is the principle behind cheese, wine, or kombucha (which is made from infused tea, with added live cultures).

Oxidation, on the other hand, requires no microorganisms. It's a chemical reaction that occurs when plant material comes into contact with oxygen in the air. The same phenomenon that causes a nail to rust or a cut apple to brown.

However, the vast majority of so-called "fermented" teas—black, oolong—do not involve any microorganisms. They are simply leaves that oxidize. The scientifically correct term would be "enzymatic oxidation," not fermentation.

(A small exception to keep in mind: dark tea, particularly certain pu-erh, does involve true microbial transformation.)

How does oxidation work, concretely?

It all starts with a damaged leaf. When a fresh leaf is crumpled, rolled, or bruised, its cell walls break. The compounds that were carefully separated inside the cell are then mixed together and exposed to oxygen.

This triggers a chain reaction, driven by two enzymes naturally present in the leaf: polyphenol oxidase (PPO) and peroxidase. These enzymes transform catechins—polyphenols abundant in the fresh leaf—into theaflavins, then into thearubigins. These new compounds give black tea its coppery to dark red color and its fuller-bodied taste.

In parallel, chlorophyll transforms into pheophytins and pheophorbides (responsible for the brownish-black color of dried leaves), while lipids, amino acids, and carotenoids degrade to create many of the aromas of the finished tea.

The most telling analogy remains that of the apple: cut it, and it browns in a few minutes upon contact with air—exactly the same enzymatic mechanism. Cook it for a pie, and it won't brown in the same way: the heat has destroyed the enzymes responsible for this reaction. This is precisely what distinguishes a green tea (heated early to block oxidation) from a black tea (where oxidation is allowed to run its course).

A detail that often surprises people: even "unoxidized" teas are never 100% unoxidized. White and yellow teas, in particular, undergo slight natural oxidation simply because their more minimal processing does not completely deactivate the enzymes involved. The line between "oxidized" and "unoxidized" is much blurrier than generally thought.

(Want the complete details of the 6 types of tea and their differences? We already broke it down in our previous article — Tea: understanding the art of transformation.)

And herbal infusions, in all of this?

The vast majority of herbal infusions never undergo this process. They are harvested, dried, and that's it—the goal is to preserve the plant as is, not to transform it.

However, there are two well-known exceptions: rooibos and honeybush, two plants originating from South Africa, traditionally processed by complete oxidation which gives them their characteristic amber-red color and their soft, round aromatic profile.

Fun fact: these two plants can also be processed using the reverse method, like green tea (heated early to block oxidation). This results in green rooibos or green honeybush—same plant, same origin, but a completely different profile depending on the processing choice. The most concrete proof that oxidizing a plant is neither an accident nor a manufacturing shortcut, but a deliberate choice.

Why traditional herbalism generally does not seek to oxidize its plants

This is the most interesting question, and its answer lies in a simple idea: traditional herbal tea and oxidized infusion do not pursue the same goal.

In classical herbalism, the aim is to dry and stabilize the plant in a state as close as possible to its original state—to preserve its characteristic compounds, not to transform them.

In an oxidized infusion, like tea, the logic is reversed: the goal is deliberately to transform the leaf. To develop new aromas, soften overly pronounced vegetal notes, gain roundness, achieve a deeper color, and get closer to the mouthfeel of a true tea.

These are not two opposing philosophies, where one is right and the other is wrong—just two different intentions for the same raw material.

Oxidation means neither "better" nor "worse." It means "different."

What happens to the benefits during oxidation?

This is probably one of the most interesting questions.

Oxidation does not mean that all interesting compounds in the plant are simply "destroyed." Instead, it leads to a transformation of the chemical composition.

In black tea, for example, a significant portion of the catechins present in the fresh leaf is transformed into other polyphenolic compounds, particularly theaflavins and thearubigins. These molecules contribute to the sensory characteristics of black tea and also possess antioxidant properties studied by research.

But this does not mean that one can simply say that "the benefits remain the same."

Some compounds decrease, others appear, some are transformed or degraded. The final chemical profile depends on the plant and the process used.

Rooibos provides a good example: oxidation particularly transforms aspalathin and modifies its phenolic compound profile. Some of the products resulting from this transformation have different antioxidant activity from the initial compound.

Therefore, it is more accurate to speak of transformation rather than systematic loss or gain.

One last important clarification: the exact chemistry of thearubigins—which represent a significant fraction of the compounds in black tea—is still imperfectly understood. Their structure is complex and heterogeneous, and research continues to study their formation and properties.

How to oxidize plants at home? An experiment inspired by black tea

After understanding what happens during tea oxidation, a natural question arises:

could the same principle be applied to other plants?

Yes, some leaves are particularly well-suited. This is notably the case for rooibos, honeybush, or willowherb, known as Ivan Chai in the tradition of Eastern European infusions.

The idea is not to "ferment" the leaves, but to reproduce certain steps in black tea production: withering → bruising → oxidation → drying.

1. Harvest

Choose fresh, healthy, and correctly identified leaves, ideally harvested in dry weather.

Not all plants are suitable for this type of transformation. Before any experimentation, ensure that the chosen plant is intended for food or infusion use.

2. Wither the leaves

Spread the leaves in a thin layer on a clean cloth or rack, in a location away from direct sunlight and well-ventilated.

The goal is not to dry them completely, but to let them lose some of their water so that they become more pliable and easier to work with.

In black tea production, withering prepares the leaves for the next step: rolling or bruising.

The duration depends on many factors—the plant used, leaf thickness, ambient humidity, and temperature. It is therefore better to observe the texture of the leaves rather than follow a fixed time.

3. Bruise or roll the leaves

This is where the oxidation process truly begins.

Roll, crumple, or knead the leaves between your hands to break some of their cells.

This action brings enzymes and plant compounds into contact with oxygen and triggers oxidation reactions.

You can work the leaves by hand or use a rolling pin. The goal is to bruise the leaves evenly, without necessarily reducing them to a pulp.

4. Allow oxidation to develop

After rolling, the leaves are placed in a clean container. This step allows oxidation to develop in contact with oxygen in the air.

The leaves can be covered with a clean, slightly damp cloth, but never soaked. The goal is to maintain a sufficiently humid atmosphere to prevent the leaves from drying out too quickly, while still allowing air to circulate. An ambient temperature of about 18 to 25 °C is generally suitable for this phase.

During oxidation, enzymes naturally present in the plant cells act on different compounds in the leaf. This transformation gradually manifests through changes in color, texture, and especially aroma.

Initially, the leaves may release very vegetal and herbaceous notes. Then, as oxidation progresses, the fragrance can evolve towards floral, fruity, honeyed, or slightly sweet notes, depending on the plant and the composition of its leaves.

For some preparations inspired by the black tea process, this phase can last several hours, sometimes 12 to 24 hours, but this duration should not be considered a universal rule. It depends notably on the plant used, the thickness of the leaf layer, its humidity, temperature, aeration, and the desired degree of oxidation.

It is therefore preferable to regularly monitor the evolution of the leaves rather than relying solely on a timer. Every few hours, they can be gently mixed to promote more homogeneous oxidation and to check that they are not starting to dry out. If necessary, the cloth can be slightly re-moistened.

Your nose as a guide

The smell is one of the best indicators of this transformation. The fragrance can gradually evolve, first towards herbaceous notes, then floral, before becoming more fruity, sweet, or sugary, sometimes with nuances evoking ripe fruits or candies.

When the aromatic profile reaches the desired level, it's time to move on to the next step: drying, which stops the enzymatic evolution and stabilizes the leaves.

⚠️ Beware of excessive humidity and overly prolonged oxidation. The leaves should not remain soggy or show any abnormal odor, slimy texture, or signs of mold. The goal is to promote controlled transformation, not uncontrolled microbial fermentation.

This is a stage where observation, smell, and experience are as important as the indicated time. With practice, the evolution of the fragrance becomes a true benchmark for determining the right time to move on to drying.

5. Dry to stop oxidation

When the desired level of oxidation is reached, it is time to move on to drying.

Unlike green tea, where a heat fixation step occurs quickly to prevent oxidation, black tea does not undergo fixation before drying. Oxidation is first allowed to develop to the desired level, then drying occurs to significantly reduce moisture and stabilize the leaves.

Before drying, gently untangle the leaves, especially if they have clumped together or formed small balls during rolling. Then spread them in a thin, even layer so that air can circulate easily around the leaves.

At home, you can use a food dehydrator or an oven. The goal is to gradually reduce moisture without burning the leaves or developing overly strong cooked notes.

Try it like this

In a dehydrator: spread the leaves in a thin, even layer and dehydrate them at a low temperature, around 25 to 35 °C, until they are perfectly dry and brittle.

In the oven: you can experiment with hotter drying, around 85 to 100 °C for 1 to 3 hours. This method, closer to the thermal drying principle used in black tea production, quickly reduces moisture and can promote the development of warmer, woody, or slightly roasted notes.

In both cases, the duration is indicative. It depends on the plant, the size and thickness of the leaves, and their water content. Regularly monitor their evolution and adjust the drying time if necessary.

Drying simultaneously reduces residual moisture and stops the enzymatic reactions linked to oxidation, to stabilize the obtained result.

The leaves are ready when they are completely dry, light, and brittle, with no part still damp or pliable. Their color may evolve during drying, but it alone is not a reliable indicator: depending on the plant used, the leaves may become dark brown, brownish-green, or take on other shades.

The objective is to obtain leaves sufficiently dry to ensure good preservation, while best preserving the aromas developed during oxidation.

6. Store the leaves

Once the leaves are properly dried, place them in a clean, perfectly dry, and airtight container.

Store them away from light, heat, and especially humidity. An opaque container or one stored in a cupboard is preferable to best preserve the aromas developed during oxidation.

And most importantly: note your experience!

The plant used, its condition at harvest, withering time, bruising intensity, oxidation duration, drying conditions, and the resulting outcome.

This will allow you to compare your different trials and observe how each parameter progressively influences the color, fragrance, and taste of your infusion.

An experience more than a recipe

It is important to remember that this method is inspired by certain steps in black tea production and is not a universal method applicable to all plants.

Each species has its own chemical composition and can react differently to wilting, bruising, oxidation, and drying.

Some leaves may develop particularly interesting aromas after oxidation; others, on the contrary, may lose some of their aromatic qualities or produce an unpleasant result.

This is precisely what makes the experience interesting: oxidation becomes a new way to explore the aromatic potential of plants.

By gradually experimenting with the different stages and carefully noting your observations, you will better understand how the same plant can reveal very different aromatic profiles depending on how it is processed.

⚠️ A few precautions

• Only use plants that you are certain you can consume.

• Work with clean hands, surfaces, and utensils.

• Avoid leaving the leaves in excessively humid conditions or in an airtight container during oxidation: the goal is to promote enzymatic oxidation, not to create uncontrolled microbial fermentation.

• Always dry the leaves completely before storage.

• If the leaves develop a moldy smell, abnormal texture, or signs of mold, do not consume them.

In summary

• Fermentation generally involves microorganisms; enzymatic oxidation, however, does not require them.

• In black tea, cell disruption allows enzymes to transform catechins and other plant compounds.

• These transformations notably lead to the creation of theaflavins and thearubigins, which contribute to the color, aroma, and taste of black tea.

• Rooibos and honeybush show that this type of transformation extends far beyond just the tea plant.

• A plant can be preserved by drying or deliberately transformed by oxidation: two different approaches, with different results.

• Oxidation does not simply mean "losing" the plant's compounds: it modifies its chemical composition.

• Some plants, like fireweed, can be transformed using a process inspired by black tea to develop a completely different aromatic profile.

Oxidation is neither good nor bad: it is a gateway to a new way of discovering plants.

Sources:

• Enzymatic Oxidation of Tea Catechins and Its Mechanism, scientific review available on PubMed Central.
• Tea Polyphenols for Health Promotion, scientific review on the transformations of tea polyphenols.
• Theaflavin Chemistry and Their Health Benefits, review dedicated to theaflavins and their formation during tea oxidation.
• Oxidation of Tea — RateTea
• Tea Leaves Oxidation — Tea Epicure
• Is Tea Fermented? — Eat Cultured
• Wikipedia, Tea processing
• The 6 Types of Tea Explained, YouTube video

The information presented here is provided for informational and educational purposes, from public sources. It does not constitute medical advice.