Techniques and Factors Influencing Fermentation in Orthodox Tea

Fermentation is widely regarded as the most defining step in orthodox black tea manufacture. It is the stage where the leaf transitions from green and grassy to the rich, amber-coloured, aromatic product we recognise as black tea. Despite its name, tea fermentation is not a microbial process in the traditional sense – it is an enzyme-catalysed oxidation, driven entirely by the tea leaf’s own biochemistry. Getting this step right determines whether the final cup delivers brightness, briskness, and flavour, or falls flat with a dull, over-fermented character.

What fermentation actually means in orthodox tea 🔗

The term “fermentation” is technically a misnomer, retained mostly out of historical habit. What actually happens is enzymatic oxidation. When the leaf is rolled during manufacture, its cell walls rupture, bringing two previously separated components into contact: polyphenol oxidase (PPO), the primary enzyme, and catechins (polyphenolic compounds). In the presence of atmospheric oxygen, PPO catalyses the oxidation of catechins into new compounds – chiefly theaflavins (TF) and thearubigins (TR).

Theaflavins contribute brightness, briskness, and the golden tone in the liquor, while thearubigins provide body, depth, and the reddish-brown colour characteristic of black tea. An optimum TF:TR ratio of approximately 1:10 is widely considered necessary for a quality cup. Volatile aroma compounds – including linalool, geraniol, and methyl salicylate – also form or concentrate during oxidation, giving different teas their floral, fruity, or malty notes.

Fermentation technically begins at the rolling stage, when the cells are first ruptured, but it continues in a dedicated fermentation phase after roll-breaking and sifting. The roll-breaker and sifter cool the leaf, aerate the mass, and separate it into portions that will ferment at a reasonably uniform rate. The sorted leaf, now called dhool, is then laid out under controlled conditions for the bulk of the fermentation process.

Methods of fermentation in orthodox tea manufacture 🔗

Orthodox tea factories use several physical arrangements for fermentation, each suited to different scales of production and levels of environmental control.

Floor fermentation 🔗

Fermentation on a cement floor is the oldest and most widely used method. The dhool is spread over the floor or racks at a thickness of 2.5-3.75 cm for orthodox tea. The floor must be dry, free from stale juice deposits, and washed daily with a suitable detergent to prevent bacterial contamination. The thermal mass of concrete helps stabilise temperature, making it suitable for large-scale, consistent production. While floor fermentation requires significant space and strict hygiene management, its simplicity and capacity make it a practical choice in many factories.

Trough fermentation 🔗

In trough fermentation, aluminium troughs are placed on saddles to facilitate uniform air distribution. Air pressure is typically maintained at 2 inches water gauge, and the quantity and pressure of airflow can be adjusted, giving this method a greater degree of control than floor fermentation. A standard trough can hold up to 16 kg of pressed leaves. Because the airflow is regulated, trough fermentation is particularly well-suited to managing variations in ambient conditions across seasons.

Trolley fermentation 🔗

Trolley fermentation uses mobile carts fitted with multiple perforated trays stacked vertically. Each tray holds a thin, even layer of dhool, allowing air to circulate from all directions. The mobility of the trolleys is a practical advantage – batches can be repositioned within the fermentation room to optimise exposure to conditioned air. This method offers good aeration and is commonly used in medium to large operations where flexibility is needed.

Rack fermentation 🔗

Rack systems use fixed, multi-tiered shelving units with shallow beds of dhool on each level. The vertical arrangement maximises space efficiency while maintaining consistent air circulation across all tiers. Because the racks are stationary, they integrate well with environmental control systems, making them a preferred choice in modern, automated tea processing facilities. Drum fermentation, while more common for CTC manufacture, is also used in some orthodox operations in South India, where the rotating drum ensures continuous exposure of the leaf to conditioned air, producing teas that are generally brisker than those from floor fermentation.

Batch fermentation: the standard approach 🔗

Regardless of the physical method chosen, orthodox tea manufacture relies on batch fermentation to maintain quality consistency. Each batch groups leaf plucked on the same day, of similar grade, and processed under identical parameters. This separation ensures that differences in leaf maturity, moisture content, or degree of rolling do not cause uneven oxidation within a single lot. There is no fixed fermentation time for orthodox tea – it depends on the type of tea, degree of rolling, withering, and standard of plucking. Skilled tea makers judge the end point primarily by colour and nose assessment: the dhool shifts to a characteristic copper-bronze tone, and the grassy smell gives way to a pleasant, floral-fruity aroma.

Factors influencing fermentation quality 🔗

The quality of the finished tea depends heavily on how well the following parameters are managed throughout the fermentation phase.

Duration 🔗

For orthodox tea, fermentation is generally conducted for 2-4 hours, though this varies with cultivar, degree of maceration, and environmental conditions. Duration is arguably the most critical variable. As fermentation time extends, theaflavin and thearubigin levels build up to optimum concentrations and then degrade if the process is prolonged. Under-fermentation results in a green, grassy flavour; over-fermentation produces a dull, flat liquor with reduced brightness. There is no universal timer – the end point must be judged by the processor based on leaf colour, aroma, and, in modern facilities, analytical monitoring of TF levels.

Temperature 🔗

Temperature directly governs the activity of polyphenol oxidase and peroxidase. Research consistently points to around 28°C as the optimal fermentation temperature for black tea, producing the best balance of aroma, taste, and antioxidant activity. Higher temperatures, particularly above 35°C, accelerate the degradation of catechins and cause enzyme instability, leading to reduced theaflavin formation and lower liquor brightness. At very low temperatures (below 15°C), enzyme activity effectively stalls. The ideal dhool temperature during fermentation is kept below 30°C, with a pH range of 4.5 to 5.0. In practice, the exothermic nature of oxidation means the leaf mass itself generates heat, so adequate ventilation is essential to prevent the temperature from climbing above the desired ceiling.

Relative humidity 🔗

Moisture in the fermentation environment has a direct effect on enzyme activity and leaf surface conditions. Relative humidity should be maintained above 90% throughout fermentation. Research on black tea fermentation shows that humidity below 75% produces a grassy, astringent, and bitter character, while humidity of 85% and above results in a sweet, mellow flavour and enhanced aroma. This is because higher humidity sustains PPO activity, enabling more complete conversion of catechins into theaflavins and thearubigins. At the same time, excessive humidity without proper air movement can encourage unwanted microbial growth, making ventilation management critical.

Leaf thickness (spread depth) 🔗

The depth at which dhool is spread during fermentation has a direct impact on aeration and heat distribution. For orthodox tea, the recommended spread thickness is 2.5-3.75 cm on floors or racks, ensuring adequate aeration throughout the leaf mass. When the layer is too thick, the centre becomes anaerobic – oxygen cannot penetrate, enzymatic activity slows, and fermentation becomes uneven. The outer layers oxidise faster than the core, producing a batch with inconsistent colour and flavour. In trough fermentation, air is pushed through the leaf bed from below, which can compensate for slightly greater depths, but uniform spreading remains standard practice.

Factory hygiene 🔗

Hygiene during fermentation is not merely a housekeeping concern – it has a direct bearing on tea quality and food safety. Residual leaf juice on floors, roller covers, and ball breakers can become a source of bacterial growth that carries over to the following day’s production. The fermentation area must be thoroughly cleaned after every day’s manufacture – ideally with hot water – to eliminate bacterial buildup. Floors must be dry before spreading dhool, since wet surfaces promote contamination. Ultraviolet radiation is also used in some facilities, serving a dual function: killing external bacteria and stimulating polyphenol oxidase activity, thereby improving liquor brightness. Teas produced in India must additionally comply with FSSAI specifications covering quality parameters, pesticide residues, and heavy metals – making factory hygiene a regulatory requirement, not just a best practice.

Recognising the end point of fermentation 🔗

Determining when fermentation is complete remains a skill-based judgement in most orthodox operations. The two primary indicators are colour and nose. As fermentation progresses, the green leaf gradually shifts to a dark copper tone, and the smell evolves from grassy to a distinctive, pleasant aroma characteristic of well-oxidised tea. The temperature of the dhool also rises steadily due to the exothermic reactions and peaks near the optimal end point. Once these signs are observed, the dhool is moved immediately to the dryer, where a blast of hot air arrests the oxidation process and locks in the developed flavour and colour compounds. Research using Response Surface Methodology has suggested that under optimum conditions of approximately 80 minutes, 28.76°C, and 92.3% relative humidity, the best balance of theaflavin, thearubigin, and total liquor colour can be achieved in certain tea varieties – though these figures serve as a scientific reference rather than a fixed rule for every factory or cultivar.

What do you think? Given that fermentation in orthodox tea is judged largely by colour and aroma rather than precise instruments, how reliable do you think this traditional method is compared to sensor-based monitoring systems? And with climate change altering ambient temperature and humidity in tea-growing regions, which of the five factors discussed here do you think will be the most challenging to control in the coming decades?

References

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3. https://www.upasitearesearch.org/orthodox/
4. https://teaworld.kkhsou.in/page-details.php?name=Fermentation&page=b2b9fb46ea583eeef6c732c68
5. https://www.upasitearesearch.org/tea-manufacturing/
6. https://hortherbpublisher.com/index.php/jtsr/article/html/3921/
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8. https://www.mdpi.com/2304-8158/12/8/1726
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