The Role of Withering in CTC Tea Production

In tea manufacturing, every step matters – but few are as foundational as withering. Before tea leaves undergo rolling, fermentation, or drying, they must first be conditioned through a carefully controlled process that changes both their physical structure and their internal chemistry. According to the Tea Research Association, withering is not just a preparatory stage – it is the very foundation upon which tea quality is built. As the saying goes in tea planter circles, “withering makes or mars the tea.”

What is withering in CTC tea manufacture? 🔗

The UPASI Tea Research Foundation describes withering as the first and foremost step in tea manufacture, in which freshly plucked green leaf is conditioned physically as well as chemically for subsequent processing stages. In CTC (Crush, Tear, and Curl) tea production, this step is especially critical because the maceration process that follows is highly intensive – and it demands a leaf that is sufficiently pliable and biochemically primed.

Withering has two distinct but interlinked dimensions: physical withering and chemical withering. Both must occur in the right sequence and proportion to produce a tea with good color, aroma, flavor, and body.

Physical withering: making the leaf ready for rolling 🔗

The primary objective of physical withering is to reduce the moisture content of freshly harvested leaves and make the leaf flaccid – soft and rubbery enough to be processed without tearing or crumbling. Research published in the Journal of Scientific Agriculture confirms that fresh tea shoots carry a moisture content of approximately 70-80% (wet basis), which drops to around 60-70% during physical withering. This loss of moisture causes the turgid, stiff leaves to become flaccid, making them less prone to breakage and damage during the CTC maceration stage.

The reduction in moisture also increases the concentration of sap within the leaf cells, which is important for the biochemical reactions that take place subsequently. The Tea Research Association (TRA) at Tocklai notes that the moisture content of fresh leaf typically ranges between 74-83%, and physical withering aims to bring this down to a level where the leaf is suitably conditioned for twisting, curling, and rolling. Physically, the leaf transitions from a rigid, brittle state to a flexible, leather-like texture.

It is important to note that over-withered leaves harden and can blunt CTC roller blades. Under-withered leaves, on the other hand, carry too much moisture and tend to clog CTC and Rotorvane machines, leading to inefficient processing and quality loss.

Chemical withering: the biochemical transformation 🔗

While physical withering is largely about moisture removal, chemical withering is where the internal chemistry of the leaf undergoes significant transformation. A comprehensive review on withering in black tea processing explains that during chemical withering, complex chemical compounds break down into simpler ones – including volatile flavor compounds, amino acids, and simple sugars. This is the stage that lays the aromatic and flavor groundwork for the final cup of tea.

Rise in amino acids 🔗

One of the most significant chemical changes is the increase in free amino acids. Total free amino acid content gradually rises during withering and only begins to decline after about 23 hours. These amino acids are important precursors to flavor development – they contribute to the tea’s briskness and interact with polyphenols during fermentation to produce characteristic taste compounds.

Increase in caffeine concentration 🔗

A study published in the Oriental Journal of Chemistry found that caffeine levels increase steadily with withering duration from 2 to 18 hours, after which they show a declining trend. This rise is largely attributed to moisture loss – as water content decreases, caffeine and other compounds become proportionally more concentrated in the leaf tissue, contributing to the tea’s characteristic stimulating quality.

Accumulation of organic acids and volatile flavor compounds 🔗

Organic acids accumulate during withering and contribute to the brightness and overall flavor balance of the manufactured tea. Equally important is the behavior of volatile flavor compounds (VFCs). Research in the PMC journal on tea harvesting and processing shows that withering decreases the total content of Group I volatile flavor compounds (which impart inferior aroma) while increasing Group II volatile flavor compounds that contribute positively to aroma quality.

Enzymatic activity and chlorophyll changes 🔗

Chemical withering also increases enzymatic activity, particularly that of hydrolytic enzymes. As moisture content drops, reduction in moisture enhances the activity of certain hydrolytic enzymes, which in turn increases water extract values – directly linked to the strength and body of the final brew. Additionally, around 15% of chlorophyll degradation occurs during withering, affecting the color and aroma profile of the processed tea. Lipids also begin to break down into simpler molecules, and catechin levels show a slight decline as the process advances.

The relationship between physical and chemical withering 🔗

UPASI Tea Research Foundation lays out two non-negotiable conditions for good withering: first, storage of fresh leaf for a minimum of nine hours is absolutely essential to allow chemical changes to take place, regardless of whether a physical wither is also desired. This minimum nine-hour period is referred to as the chemical wither. Second, physical withering is necessary for efficient fermentation. Both processes must occur together, not independently, to achieve the desired balance of color, flavor, aroma, and taste.

An important operational point: research findings establish that withering up to 16 hours enhances the quality of processed teas, while extended withering beyond 18 hours deteriorates liquor characteristics and flavor profile. Withering for fewer than 10 hours results in poor biochemical development. The ideal chemical withering duration ranges from 14 to 18 hours; exceeding 20 hours is known to deteriorate tea quality.

Withering trough: design and function 🔗

Modern CTC tea factories rely on specially designed withering troughs to control both forms of withering simultaneously. These are engineered systems, not simple storage containers. As specified by UPASI, a standard withering trough is typically 100 feet long and 6 feet wide, with a plenum chamber 2 to 2.5 feet in height beneath the wire mesh. Green leaf is loaded over the mesh to a depth of 8-9 inches, at a loading rate of 8-9 kg per square meter of trough area.

Airflow is the critical variable. To achieve proper withering, the fan must deliver 45 CFM (cubic feet per minute) of air for every one square foot of trough area. Axial flow fans are used to push air up through the bed of leaves, carrying away moisture and regulating temperature. When atmospheric humidity is high and the hygrometric difference falls below 3°C, hot air is blended with outside air to increase the drying capacity. However, the dry bulb temperature of this mixed air must not exceed 35°C to avoid damaging the biochemical compounds in the leaf.

Wider troughs of 12 to 15 feet with two axial fans are increasingly preferred in modern factories as they provide more even withering across the leaf bed. A well-designed factory plans for an optimum load of 30 kg per square meter of trough area for peak crop conditions. The quality of withering also depends heavily on a well-ventilated withering loft that allows large volumes of fresh air to be drawn by the trough fans.

Uniformity of withering: a key technical challenge 🔗

Studies using Computational Fluid Dynamics (CFD) modeling have shown that temperature and velocity distribution of drying air varies considerably along the length of a withering trough, which contributes to non-uniformity in the degree of wither achieved across the leaf bed. This uneven withering is one of the most significant quality challenges in factory operations. Modified trough designs using convergent-divergent duct configurations have been shown to reduce thermal variation by up to 58% and velocity variation by 60% across the trough bottom, resulting in significantly more uniform withering.

Consequences of improper withering 🔗

The downstream effects of poor withering are serious. Improper withering adversely affects all subsequent manufacturing operations – maceration, rolling, fermentation, drying, and even storage. Under-withered leaves introduce excess moisture into the CTC machine, causing operational jamming and uneven cell rupture. Over-withered leaves increase fiber content in the final product and raise the risk of over-firing and over-fermentation. Both conditions ultimately compromise the cup quality of the finished tea – including color, brightness, and the critical theaflavin and thearubigin balance that determines liquor character.

The Tea Research Association also cautions against pre-withering leaf damage from rough handling. If cell damage occurs before withering begins, the leaf undergoes uneven oxidation, leading to degradation of essential chemical constituents. Leaves with physical damage also wither faster than undamaged ones, making it difficult to achieve a uniform and consistent wither across the entire batch.

Practical indicators of correct withering 🔗

Experienced factory managers assess the degree of wither through both physical and analytical checks. Physically, a properly withered leaf should feel soft and pliable – when a handful is squeezed, it should clump together without springing back. The stalks should be bendable without snapping. Analytically, the moisture content of the withered leaf should be controlled within a narrow range of 2 to 3% variation to ensure consistent output and reliable machine performance at subsequent stages. In well-equipped factories, the out-turn ratio – comparing the weight of withered leaf against made tea – serves as a practical check on withering efficiency and consistency.

Getting withering right is not a matter of guesswork. It demands precise control of time, temperature, airflow, leaf loading, and trough design. When all these variables align, the withered leaf arrives at the CTC rollers in the ideal condition: biochemically enriched, physically pliable, and ready to yield a tea of the highest quality.

What do you think? Given that withering alone can determine the success or failure of the entire CTC tea manufacturing process, how should small-scale tea processors balance the cost of mechanized withering troughs against the need for consistent quality? And with the rise of precision sensors and smart factory systems, do you think real-time monitoring of withering parameters will eventually replace the traditional skill of the experienced tea maker?

References

1. https://www.tocklai.org/tea-manufacture/
2. https://www.upasitearesearch.org/tea-manufacturing/
3. https://www.e-sc.org/articles/SC000021733
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC10743253/
5. http://www.orientjchem.org/vol35no4/biochemical-changes-during-withering-process-of-ctc-black-tea-manufacture/
6. https://www.upasitearesearch.org/ctc/
7. https://www.academia.edu/68446106/A_Review_of_Withering_in_the_Processing_of_Black_Tea
8. https://www.upasitearesearch.org/tea-machinary/
9. https://www.researchgate.net/figure/Flow-chart-for-CTC-tea-manufacturing-process_fig1_266052659
10. https://learning.uonbi.ac.ke/courses/AFT406/scormPackages/path_2/a32_withering.html