The Role of Drying in Orthodox Tea Manufacture: Methods and Factors

Drying is the final critical step in orthodox tea manufacture – and arguably the one that seals the fate of the entire batch. After the tea leaves have been withered, rolled, and fermented, the fermented leaf mass (called dhool) carries a moisture content of around 55%. Before the tea is fit for grading, storage, or sale, that moisture must be brought down to approximately 3%. This is not just about removing water. The drying stage arrests enzymatic oxidation, locks in the chemical profile of the made tea, and directly determines key quality indicators like theaflavins (TF) and thearubigins (TR) – the two pigments that define a black tea’s color, taste, and market value.

Why drying matters so much in orthodox tea 🔗

Orthodox black tea processing involves four core steps: withering, rolling, fermentation, and drying. Research published in Foods (2023) confirms that the quality of black tea is heavily influenced by drying – specifically the temperature used and the method applied. The drying step performs two functions simultaneously: it removes moisture to extend shelf life and storage stability, and it halts the enzymatic oxidation that began during rolling and fermentation.

During fermentation, the enzymes polyphenol oxidase (PPO) and peroxidase convert tea catechins into theaflavins and thearubigins. This transformation must be stopped at precisely the right point. According to the same review, as drying temperature rises, the enzymes become increasingly active at first, accelerating reactions – but once temperatures reach the point of enzyme denaturation, the reactions stop entirely. This narrow window is what makes drying both powerful and risky.

If the tea is under-dried, residual moisture allows continued enzymatic activity, increasing astringency and making the tea unstable for storage. A study on ECP dryer optimization in Sri Lanka notes that under-drying leads to a dry, puckering mouthfeel, while over-drying damages volatile aroma compounds and can produce a burnt taste.

The Endless Chain Pressure (ECP) dryer: how it works 🔗

The Endless Chain Pressure (ECP) dryer is the most widely used drying equipment in orthodox tea factories across India, Sri Lanka, and East Africa. As described by PCD Industries, in this machine the moist fermented leaf is uniformly spread at the feed end through a Feed Distributor, which also acts as a ball breaker to separate clumped dhool. The leaf is then carried through three temperature zones on perforated trays, and hot air forced through specially designed perforations dries the leaf progressively at each stage.

Research on ECP dryer optimization explains that the machine uses an endless chain conveyor to transport tea leaves from top to bottom, while hot air is blown from the bottom upward. This creates a deliberate temperature gradient: lower temperatures at the top, higher temperatures at the bottom. The design ensures that high-moisture tea entering at the top encounters cooler air, while low-moisture tea at the bottom is exposed to higher heat. This countercurrent arrangement is intentional – it prevents case hardening, a defect where the outer surface of tea particles dries out and forms a crust while the interior remains wet.

Temperature settings in the ECP dryer 🔗

In the tea industry, the ECP dryer typically operates with hot air at temperatures around 120°C. The study at the Houpe tea factory in Sri Lanka reports that the standard operating temperature at that factory was 121°C (250°F), and tested variations between 115°C and 127°C to determine the optimal setting. The drying is completed in three stages over the three temperature zones, with the leaf spending a controlled amount of time in each zone. PCD Industries notes that the drying curve is optimized by fixing the travel ratios between different circuits, with a “first-in, first-out” principle to ensure homogeneous processing and hygienic output.

Impact of drying on theaflavins and thearubigins 🔗

Theaflavins (TF) and thearubigins (TR) are the two most important quality-determining compounds in black tea. TF is responsible for the golden color, brightness, and astringency of the tea liquor, while TR is reddish-brown and contributes to the body and mouthfeel. The balance between these two compounds directly determines the flavor index and commercial valuation of the made tea.

According to the Foods review, increasing fermentation time before drying causes TR to rise while TF declines – making the timing of drying entry critical. Once inside the dryer, research on drying temperature effects in black tea shows that theaflavin and thearubigin levels respond differently to heat. Thearubigin levels remain relatively stable across a wide temperature range (70-120°C), but increase significantly at 130°C. Theaflavin content, on the other hand, tends to decrease with rising temperature and then partially recover at very high temperatures.

This is why intermediate temperatures – around 85-105°C for initial drying – tend to produce better quality outcomes. A study comparing four initial drying temperatures found that 85°C and 105°C outperformed 65°C and 125°C for overall quality, with the 105°C treatment yielding the best sensory scores due to superior shape, aroma, taste, and a favorable TF-to-TR ratio. Drying at excessively high temperatures (such as 125°C or above) can degrade polyphenols and generate off-flavors.

Chemical reactions triggered during drying 🔗

Drying is not a chemically inert process – several important reactions occur simultaneously. As reviewed in Foods, high drying temperatures degrade chlorophyll into pheophytin, which is what gives black tea its characteristic dark appearance. Polyphenols interact with proteins at higher temperatures to form complex compounds, which reduces astringency. Carbohydrates and amino acids react under heat to generate new flavor compounds through the Maillard reaction. This reaction compensates for the decline in essential oil content that occurs during drying, contributing to the characteristic toasty, complex flavor of well-fired orthodox tea.

Factors that influence drying efficiency and quality 🔗

The outcome of drying is not determined by temperature alone. Several interacting factors govern how effectively moisture is removed and how well quality is preserved.

Moisture removal rate 🔗

The rate at which moisture is removed must be controlled carefully. Studies on orthodox broken-type tea drying found that after the first few minutes of drying, internal mass transfer resistance – not surface evaporation – fully controls the process. This means the rate at which water moves from the inside of the tea particle to its surface becomes the limiting factor. This is why a gradual, multi-stage approach to heat application (as in the ECP dryer’s three-zone design) is more effective than simply exposing the leaf to maximum heat at once.

Air volume and airflow 🔗

Research on tea drying control systems from Wageningen University found that drying rate in thin-layer conditions is unexpectedly dependent on air flow velocity. This makes proper fan performance and duct design critical to consistent drying. Insufficient air volume reduces evaporation capacity and can result in uneven drying across the tray. The ECP dryer’s countercurrent design – blowing hot air from bottom to top against the direction of leaf travel – optimizes this airflow interaction.

Drying duration 🔗

Duration is as important as temperature. The Sri Lankan ECP optimization study highlights that outdated dryers and poor maintenance can lead to suboptimal drying durations, resulting in either under- or over-firing. The typical total drying time in a standard ECP dryer varies with loading rate and inlet moisture content. Research cited in a tea drying review indicates that ECP drying consumes around 3.5-6 kWh of thermal energy per kg of made tea, and this energy demand is closely tied to the duration the tea spends in the dryer.

Loading rate 🔗

The thickness of the layer of dhool spread on the trays – or the loading rate – affects how evenly hot air penetrates the bed. Studies on fluid bed drying of orthodox tea identified an optimal loading range for consistent moisture reduction. Overloading slows down drying and risks uneven moisture distribution in the final product, while underloading can lead to over-drying and loss of volatile compounds.

Alternative drying methods: beyond the ECP dryer 🔗

While the ECP dryer dominates the industry, other drying methods are being researched and used in niche applications. Comparative research from IIT Kharagpur evaluated tray drying, radio frequency (RF) drying, and a hybrid ECP-RF approach. RF drying of orthodox tea takes approximately 55-60 minutes, compared to around 150 minutes for conventional tray drying. Notably, RF drying occurs at lower temperatures, which better retains volatile flavor compounds and produces a higher aroma index.

However, energy comparisons show that the ECP dryer requires only 53% of the energy demanded by the RF dryer, making it significantly more cost-effective for large-scale factory operations. Other contemporary methods such as solar vacuum and greenhouse drying have been studied for CTC tea and show promise for eco-friendly, low-temperature drying that better preserves aroma compounds, though they are not yet as fast as conventional hot-air methods.

What happens if drying goes wrong 🔗

The consequences of improper drying are highly visible in the final product’s cup quality. As documented in the Foods review, under-dried tea has elevated astringency and poor shelf stability. Over-dried tea – exposed to excessive heat beyond the moisture removal point – undergoes further thermal degradation of polyphenols, amino acids, and volatile compounds, resulting in a burnt or flat taste. Research from Wageningen University notes that excessive mechanical handling of the dry tea after firing can also knock off leaf hairs and dried fermentation products, causing the tea to appear grey rather than its expected black color – a defect assessed during quality inspection called bloom loss.

Getting drying right is therefore not just about hitting a moisture target. It is about preserving everything that was built during fermentation – the theaflavin brightness, the thearubigin body, and the aroma compounds – while stopping the clock on further enzymatic change. In that sense, the dryer is the last line of quality control before the tea reaches the sorter and the market.

What do you think? Given that drying temperature so precisely shapes the TF-to-TR ratio – the key indicator of black tea quality – should factories invest in continuous real-time monitoring systems rather than relying on periodic cup-tasting to assess drying outcomes? And with newer methods like RF drying showing better aroma retention at lower temperatures, is the tea industry’s continued dependence on ECP dryers a matter of economics, tradition, or genuine technical advantage?

References

1. https://pmc.ncbi.nlm.nih.gov/articles/PMC10743253/
2. https://www.mdpi.com/2624-7402/5/4/122
3. https://pcdindustries.com/portfolio/tray-type-conventional-dryer-ecp-dryer-2/
4. https://www.sciencedirect.com/science/article/abs/pii/S0260877412005857
5. https://www.researchgate.net/publication/309252237_A_machine_vision_system_for_estimation_of_theaflavins_and_thearubigins_in_orthodox_black_tea
6. https://www.mdpi.com/1420-3049/30/2/361
7. https://pmc.ncbi.nlm.nih.gov/articles/PMC11857630/
8. https://www.academia.edu/123327531/Drying_characteristics_of_orthodox_broken_type_tea
9. https://edepot.wur.nl/197258
10. https://www.academia.edu/8084085/Tea_Drying
11. https://www.thepharmajournal.com/archives/2022/vol11issue12/PartCA/12-3-558-871.pdf