In the first part of this series, we talked about the different chemical compounds in tea.
In this post, we will look at classifying tea according to its method of processing that in turn affects its chemical composition.
Today, all types of teas can more or less be classified according to the Chinese system- into:
- Green tea (绿茶)
- Yellow tea (黄茶)
- Dark tea (黑茶) often referred to as post-fermented tea
- White tea (白茶)
- Wulong tea or Oolong (乌龙茶) aka Cyan tea (青茶)
- Red tea (红茶) otherwise known in Western nomenclature as black tea
Then, there are teas known as modified teas- teas that are originally produced in the 6 different methods above but undergo additional steps.
These include:
- Scented tea (花茶)
- Compressed teas- usually dark teas that are pressed into shapes of cakes, bricks among others
- Sheng Puer- technically processed according to green tea methods but naturally aged and oxidized
What determines the classification of tea is how it is oxidized- or ‘fermented’[1]- and not merely how much it is oxidized.
The oxidation in question refers to the oxidation of flavanols (黄烷醇)- i.e. ECGC, ECG etc- into other compounds- notably theaflavins and thearubigins.
This in turn gives the wet leaves and the liquor- with the exception of oolong tea- the color after which it is named.
In other words- the wet leaves of green tea are green, ‘red tea’ is red and so forth.
Green Tea (绿茶)
Green tea is “un-oxidized”, that is not to say that there is absolutely nil oxidation but rather there is no deliberate act of oxidation- i.e. no withering, zuoqing, menhuang, wodui, wohong and rolling takes place after shaqing.
The stages of production are-
ii) Shaping aka Rolling (揉捻)
iii) Drying (烘干)
While the freshly picked leaves are spread out prior to the shaqing production in certain teas, they are kept indoors and well-spread to minimize oxidation in a step known as tan fang(摊放) which is distinct from the process known as ‘withering’ (萎凋) where sunlight and heat induces the biochemical changes.
Key Process
Shaqing or literally translated as ‘kill-green’ is a process where the leaves are heated to destroy the polyphenol oxidase that induces oxidation.
What is unique in green tea production is that shaqing is performed at the initial stages to ensure the oxidation is minimized and the flavanols are preserved.
Typically there are 2 main methods-
i) Steaming- used in Japanese green teas and lower grade Chinese teas
ii) Pan-frying- used in medium to high grade Chinese green teas
Mechanization makes it possible for mass production with steaming machines- albeit with a grassier taste- otherwise it would be dependent on the skills of the producers.
The key is swiftness and evenness as oxidation increases with temperatures, if the leaves take too long to be heated to 85-90⁰C where enzymes are destroyed, oxidation is induced. If the leaves are heated too long, it becomes reddish and loses its briskness.
Yellow Tea (黄茶)
Yellow tea is defined as “slightly oxidized”. While there a few different styles of producing yellow tea, the general process is as follows:
i) Shaqing (杀青)
ii) Menhuang (焖黄)
iii) Shaping/Rolling(揉捻) (optional- not performed for bud teas)
iv) Drying (烘干)
Key Process
The key distinguishing process is menhuang, which is generally performed by covering the heated leaves (after shaqing) with a damp cloth. The heat and humidity induces oxidation but as this stage is done after shaqing and the polyphenol oxidase are destroyed, oxidation is minimal.
The result is a ‘yellowing’ of the leaves, akin to stewing.
Dark Tea (黑茶)
Dark tea or post-fermented teas are produced in this manner:
i) Shaqing(杀青)
ii) Shaping/Rolling (揉捻)
iii) Wodui (渥堆)
iv) Drying (烘干)
Key Process
The key distinguishing process is Wodui where the leaves are heaped in a controlled environment with sufficient heat and humidity to induce the biochemical process.
There are 2 main schools of thoughts on the actual process-
i) Oxidation- While the polyphenol oxidase are destroyed in shaqing, catalase peroxidase remains and triggers the effect
ii) Fermentation- Microbes like Penicillus glancum, Aspergillus niger & Aspergillus glaucum cause the fermentation and decomposition of the tea leaves
That should be best left to a separate post- if it is at all forthcoming.
White tea (白茶)
White tea is the more simplistic- but not necessary the simplest- tea to process. It undergoes 2 processes:
i) Withering (萎凋)
ii) Drying (烘干)
Key Process
The most important stage in the production of white tea is withering.
Through heat, the oxidation process is induced. But as there is no rolling, the cell structures are intact and it is a minimal oxidation. Withering increases the amino acid content and is key for increasing the sweet and brisk quality of the tea.
Cyan tea (青茶)
Production of oolong tea is the most demanding and hardest to mechanize. That partially explains why so few regions in the world are able to produce quality oolong tea- China & Taiwan (naturally) and Taiwanese owned productions in Thailand, Vietnam and New Zealand.
i) Withering (萎凋)
ii) Zuoqing (做青)
iii) Shaqing (杀青)
iv) Rolling/Shaping (揉捻)
v) Drying (烘干)
Key Process
The key process is “zuo qing” which is literally translated as “making green”.
There are 3 main styles which we will not cover here but the fundamental principal is that by bruising the leaves on the sides, the cell structures are partially destroyed and oxidation is induced.
After bruising, the leaves are set down to allow a natural, gradual oxidation to take place and the process is repeated.
This bruising- whether rattling, stirring or tossing- is not haphazard and random. In fact by varying the frequency etc, different fragrances can be achieved. This is also distinct from ‘rolling/shaping’ in purpose, execution and biochemical process.
When the desired level of oxidation is achieved, the leaves are quickly sent to undergo shaqing to ‘fix’ that level of oxidation.
Red tea (红茶)
Red tea is generally the simplest tea to produce and easiest to mass produce. It undergoes the following steps:
i) Shaqing(杀青)
ii) Shaping/Rolling (揉捻)
iii) Wohong (渥红) traditionally referred to as ‘fermentation’
iv) Drying (烘干)
Key Process
The key process is wohong, a stage where the oxidation that has already begun in the withering and shaping stages are further induced. The polyphenol contents are oxidized into theaflavins and thearubigins, among other chemical compounds, giving red tea its liquor and wet leaves color.
Comparing all 6 categories
This table is extracted from Professor Yang Wei Li et al from Hunan Agricultural University’s research in 2001 where they picked tea leaves from the same location, cultivar, picking requirement and processed them according to the 6 different methods.
Category
Amino acid (%)
Polyphenols (%)
Soluble Carbohydrates (%)
Soluble substances (%)
Raw leaves 1.592 23.59 11.78 45.6
Green tea 1.475 22.49 9.97 44.4
Yellow tea 1.361 16.71 10.57 27.6
Dark tea 1.375 15.51 9.45 24.7
White tea 3.155 13.78 12.50 31.9
Oolong tea/ Cyan tea 1.425 12.78 9.06 27.9
Black tea/ Red tea 0.970 7.93 8.06 23.9
Table extracted from 白茶 edited by 叶乃兴 and published by中国农业出版社
As you can see, due to shaqing being performed at the very first stage of processing, green tea retains most of the original form of the raw leaves.
The minimal processing of white tea also boosts its amino acid content and soluble Carbohydrates.
However, in the next part of this series, we will look at the limitations of using processing methods and other factors that determine the chemical content and hence the health benefits.
See here for other frequently asked tea related questions
See here for more articles related to tea and health
Information on processing of tea leaves has been culled from 制茶学 edited by安徽农学院 and published by中国农业出版社 & 茶学概论 edited by周巨根 & 朱永兴 et al and published by 中国医药出版社
[1] The chemical process we now know as oxidation was erroneously referred to as fermentation in the past. Many tea producers & researches continue to use “fermented” out of habit or to avoid confusion for the older generation rather that out of ignorance.
