Black tea is one of the most popular beverages in the world as well as being extremely popular in the United States. In this short article we look at how it is made because the production of black tea very much depends on specific processes. It is the main source for the beverage tea – in other words our typical cuppa!
The tea is one of a group of infusions and extracts of the leaves Camellia sinensis (L.).
The three largest producers of black tea are Kenya, China, India and Sri Lanka. Black tea accounts for 70 per cent of the world’s tea production based on the Food And Agriculture Organization’s data. Tea is an important economic commodity in these countries.
The most popular types are:
- Assam – soft withered which is bold and malty.
- Darjeeling – delicate hard withered tea which is floral and fruity, many layered and one of the most complex.
- Ceylon tea – said to be spicy and bit brisk.
- Keemun tea which is from Anhui province and is smooth with tobacco notes, floral, fruity and piney.
- Kenyan tea – full bodies and very often used in blended teas.
In China, black tea is often called red tea or Hong cha.
Withering; The First Step In Black Tea Production
From the outset, tea shoots with leaves attached are plucked – these are the soft turgid fresh shoots of tea taken from a tea bush. By definition as in all teas, from black to white, it is the terminal bud and the first two youngest leaves and these are typically picked twice a year.
The moment the tea leaves are put into a bag various reactions start to take place as well as the physical process of wilting which is loss of turgor pressure as moisture begins to be removed. Throughout much of the remaining part of the process there is the continuous removal of moisture.
Withering can lead to up to 50% loss of moisture.
Withering also begins the mix of chemical and physical changes in the leaves and the shoots. It is also the longest step in the process and lasts from between 16 hours and a full 24 hour day.
The most traditional drying method is to use the Sun. Here the green tea shoots are laid out on the floor to optimise sun drying. This method of drying is becoming much more rare because of the lack of control of the outside environment.
The alternative is controlled withering by laying the cut tea leaves on the floor, troughs, stainless steel beds or moving conveyor under controlled temperature, humidity and air-flow conditions.
There are two main types of wither which are sometimes described as soft or hard. Soft wither leaves plenty of water – it is not a full drying process. It means that oxidation processes can take place in the later steps. A softer wither will produce more bold, malty flavours as in Assam tea. A hard wither involves the production of a drier tea leaf.
Leaf Disruption
After withering the tea leaves are disrupted and macerated by being rolled, kneaded, torn or crushed or all of these processes. This is the most aggressive part of the process. Rolling can be conducted by hand which is described more fully later on. There is a process called CTC which is ‘Crush, Tear, Curl’. It can involve using a machine such as a rotorvane. In the production of orthodox tea, great care is taken to minimise disruption of the leaf itself. An orthodox tea should have a fully intact leaf.
In the rolling process, enzymes such as polyphenol oxidase and peroxidases will come into contact with its substrate which are polyphenols. These polyphenols are in the vacuoles whilst the enzymes have been in the cytoplasm of the tea cells.
This prepares the leaves for the final oxidation process.
Fermentation
Fermentation is essentially another word for an extensive oxidation process which produces the unique range of colours and flavours. It is the most critical step in the processing of this type of tea. Manipulation produces so many different types of black tea. It is also a different type of process to that producing other types of tea such as green tea and white tea.
During this process, the leaves change colour from green to coppery brown and the grassy smell is transformed to a floral smell. It is critical that the leaves be allowed to ferment only up to the desired limit and both under and over fermentation result in a poor quality of black tea.
The volatile compounds in the leaves are altered to release theaflavin and thearubigin. The theaflavins are responsible for briskness and brightness of the tea. the thearubigins produce depth and fullness to the brew. Controlling levels of oxidation produces the different levels of flavour and aroma.
Fixation And Kill-Green
Fixation is conducted to stop the oxidation process usually by mild heating which is sometimes called firing. This produces an orthodox form of tea.
Rolling
Rolling is a specific step which is distinct from the processes used in maceration described earlier. It is a source of confusion because referring back to that process, the wilted and fixed leaves are gently rolled to a point where they look wire-like, kneaded or as rolled pellets. In that rolling process, essential oils and other aroma chemicals are released and modified which produces the intense tea flavour.
The more tightly rolled the leaves, the longer they retain their freshness.
Drying
The tea is finished for sale. This will include further solar drying, panning, air drying and baking. It helps stabilise the remaining flavour and ensure it has a longer shelf-life.
Curing
To improve aging and fermentation further, the teas are subjected to various forms of aging and fermentation to make them more palatable. The Chinese Pu-erh tea for example is aged and fermented for years rather like a vintage wine to improve its flavour.
Composition
The extract from the leaves includes the chemical components such as flavanol, caffeine, phenolic substances, fats, amino acids, theaflavin, thearubigin and volatile components. A typical black tea composition contains 8% catechins, 10% flavonol glycosides, 12% theaflavins, and 70% thearubigins (Wiseman et al., 2001; Chaturvedula and Prakash, 2011).
Dried tea leaves contain carotenoids as all leaf products will. The carotenoid content ranges from 36 to 73 mg/100 g dry weight, and is dominated mainly by β-carotene, lutein and zeaxanthin (Ravichandran, 2002). These carotenoids also decline at different rates during the various stages of tea production. In black tea the carotenoid content is as low as 25mg/100g.
Sensory Parameters
Out of the two detectable parameters (colour and smell), smell is very important since a strong, very specific fragrance emanates from the leaves once leaves are optimally fermented.
A new electronic nose-based approach for monitoring of tea aroma during fermentation is proposed (Bhattacharya et al., 2007). The same group has predicted the optimum time for fermentation. They used five different time-delay neural networks (TDNNs), named as multiple-time-delay neural networks (m-TDNN) in a slightly later article. They have used self-organizing maps (SOM) to monitor three different small stages as a means of introducing the technology into black tea fermentation (Bhattacharya et al., 2008) .
One study used a combination of Fourier transform near-infrared spectroscopy (FT-NIR) coupled with a computer vision system (CVS) to determine the degree of black tea fermentation (Jin et al., 2020).
Two mid-level strategies were used to analyze the fusion signals of FT-NIR and CVS. K-nearest neighbors, linear discriminant analysis, and a support vector machine (SVM) were applied for classification modeling. The advantages of FT-NIR and CVS were integrated based on mid-level fusion, and the results obtained were better than those obtained using independent methods for evaluating black tea fermentation
References
Bhattacharyya, N., Seth, S., Tudu, B., Tamuly, P., Jana, A., Ghosh, D., … & Bhuyan, M. (2007). Monitoring of black tea fermentation process using electronic nose. Journal of Food Engineering, 80(4), pp. 1146-1156.
Bhattacharya, N., Tudu, B., Jana, A., Ghosh, D., Bandhopadhyaya, R., & Bhuyan, M. (2008). Preemptive identification of optimum fermentation time for black tea using electronic nose. Sensors and Actuators B: Chemical, 131(1), pp. 110-116 (Article).
Davies, A. P., Goodsall, C., Cai, Y., Davis, A., Lewis, J. R., & Wilkins, J. (1992). Black tea dimeric and oligomeric pigments – structures and formation. In: A. Wilson & N. N. Clifford (Eds.), Plant polyphenols 2. Chemistry, biology pharmacology, ecology (pp. 555–601). London: Chapman and Hall.
Ravichandran, R. (2002). Carotenoid composition, distribution and degradation to flavour volatiles during black tea manufacture and the effect of carotenoid supplementation on tea quality and aroma. Food Chemistry, 78(1), pp. 23-28.
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