Tea fermentation vs oxidation – difference between the two:

Before delving deeper into the topic i.e. tea fermentation vs oxidation, let me clarify the context why we need to break our head over the topic at all! Most of us have a fair idea on the color of different varieties of tea. Black tea has a color which is more near to dark brown rather than its literal adjective – ‘black’. At the same time, green tea, which you will find commonly in the market, comes with a hint of green. But keep in mind, the intensity of color or brightness for that matter, may come down with quality and age of the tea. However, most among us can easily relate to why the color of green tea is nearer to green, for obvious reason.

But the same does not hold good when it comes to the color of black tea. Many among us may wonder, where from black tea gets its dark brown color? At the same breath, we hear answers like ‘the tea leaves go through a process called fermentation and as a result it turns brown’. But the answer seems to be partially correct. The process, manufactures use to make black tea, is oxidation, not fermentation. And, it is oxidation that brings a dark brown color to the tea

tea fermentation vs oxidization - ctc processed tea spread in a conveyor for oxidization

Then, what is Fermentation?

Fermentation is a specific type of microbial metabolism that converts sugar into acids, gases or alcohol. We ferment things with the goal of producing a specific chemical product or some byproducts. The process of fermentation has the following characteristics.

  1. We typically describe fermentation in terms of its major excreted byproducts. Few of the example of fermentation are alcohol fermentation, lactic acid fermentation (used to produce yogurt from milk) etc. Here we focus on the end products and tend to ignore the substrate or the starting materials.
  2. Fermentation can happen with or without oxygen.
  3. Fermentation needs catalytic organisms like bacteria or yeast. It happens in multiple steps.

What is Oxidation?

Oxidation is a bio-chemical, enzymatic activity. In oxidation the host material absorbs oxygen and as a result, transforms. Please see the following points for a better understanding of oxidation.

  1. It can be spontaneous or controlled.
  2. Oxidation can cause positive or negative change.
  3. It is a single step transformations that could be brought about by individual enzymes. We do not need catalysts like bacteria or yeasts to make oxidation happen.


Let us cut an apple or banana and leave the cut side open to the air. The cells exposed to air shall absorb oxygen and soften up. The cut fruit then turn brown. This is a very simple form of spontaneous negative oxidation. If we dry up the apple slices in a controlled manner, say in a dryer, then we can term this as a controlled negative oxidation.

Why and how tea is oxidized?

In fact, spontaneous oxidation process starts as soon as we pluck the tea leaves. It continues throughout the withering phase of white, oolong, and black teas. Manufacturers dry or pan fry the white or green teas sooner than later. Thus, the the tea maker preempts any possibility of further oxidation. They apply heat to stop the activities of the enzymes which is called de-enzyming. Thus the polyphenols do not change or alter in green or white tea and hence the leaves retain their green color. On the other hand, controlled oxidation is one of the most important steps of oolong and black tea manufacturing process.

To produce black tea, withered leaves are subjected to crushing, tearing, cutting (CTC) or rolling operations to achieve faster oxidation. This process damages the cell walls of the leaves extensively. As the oxidation process is started officially, oxygen comes in contact with the exposed bruises in the leaves and the leaves starts to turn brown. With the CTC or rolling operations damaging the cell walls of the tea leaves, the mixing of the polyphenols and polyphenol oxidases also intensifies. This in turn accelerates the pace of oxidation.

Now you can see, this process does not meet any preconditions of fermentation. So, you guessed it right, this is nothing but oxidation, which requires abundant moist and oxygen-rich air. Now we learn that, whatever process the maker uses in producing black tea is oxidation only, and has nothing to do with fermentation! Thus using the term ‘fermentation’ in place of oxidation in this context of tea making is not appropriate.

tea fermentation vs oxidization - a road through a hilly tea garden

Chemistry behind Oxidization:

When the catechins come in close contact with the respective enzymes in presence of oxygen, they produce polyphenols of higher molecular weight. However we can make it possible if we keep the temperature and humidity at an optimum level. As the polyphenols come in contact with oxygen, the unique flavor and color components along with other distinctive characteristics start to manifest. This is exactly what we look for in a cup of black tea.

There are in total six varieties of catechins  found in tea leaves. They are: catechin, epicatechin, epicatechingallate, gallocatechin, epigallocatechin, and epigallocatechingallate.

The enzymes polyphenol oxidase oxidize catechins at first. Then they combine in pairs to form unique and larger molecules called theaflavins. Among the different types of theaflavins, theaflavindigallate is the brightest and briskest followed by theaflavinmonogallates and plain theaflavins. Hence the overall quantity of polyphenols present in the tea does not determines the brightness and briskness of the brew. It is only the extent of formation of theaflavindigallate in the tea that determines these lead characteristics of the black tea.

Then comes the thearubigins. They form as a result of complex reaction of peroxidase on theaflavins. Thearubigins have an even more complex chemistry. The tea maker needs to monitor and supervise the delicate process of oxidization properly. Poor monitoring can affect the process converting theaflavins to thearubigins. Thus, proper oxidation ensures the formation of theaflavins and thearubigins in correct proportion. This in turn influences the brightness, body and color parameter of the output black tea. However,there is one more limiting factors that determines the quality of made tea. It is the inherent composition of catechins and availability of enzymes in the leaves.

Degree of tea fermentation vs oxidation for different tea varieties:

  1. White Tea: It needs small amount of spontaneous oxidation which is around 10%.
  2. Green Tea: Though a tea maker we want to avoid uncontrolled oxidation, some spontaneous oxidation starts automatically as soon as we pluck the leaves. We need not over react to it as it is inevitable.
  3. Oolong Tea: It needs controlled oxidation which can be as high as 80%
  4. Black Tea: As it needs full oxidization and only an experienced tea-master can monitor and control
  5. Pu’erh Tea: In pu’erh, the tea master allows tea oxidation to happen in a spontaneous manner up to the withering process. Once withering is complete, he curtails the exposure to oxygen minimum to facilitate fermentation. To transform the withered and pan-fried leaves or maocha to pu’erh tea, the tea master exposes them to bacteria at first to start the fermentation. Then he does the needful to maintain the fermentation process at required level. The bacteria may live either in the leaves itself. Otherwise, the leaves may contact the bacteria in the tea production room-
  • where the tea master stores the maocha temporarily. Afterwards he compresses the maocha to manufacture raw pu’erh or sheng cha.
  • at the time of fermentation of the piled maocha. The master use this maocha to produce ripe pu’erh or shou cha.
  • in the steam-enriched environment. Here the tea master produces the ripe pu’erh cakes.

Commonly used methods of tea oxidation:

  1. Floor Method: It is commonly done on the factory-floor. The tea master spreads the leaf in thin layers of thickness ranging from 5 to 8 centimeters.
  2. Trough Method: Done on perforated trays that are similar to the withering troughs. This method is a more efficient as the tea master can control the quantity of airflow and the pressure with minimum use of space.
  3. Continuous Fermenting Machine (CFM): It uses the best of both floor and trough methods. At one end of the CFM, the tea master feeds the processed tea leaves through the discharge conveyor. The leaves move slowly over a bed of conveyor belt. At the same time a spreader ensures proper spread and thickness of the leaves across the belt. In CFM the master can vary the parameters like bed thickness, oxidation period, air volume etc. to suit different operating conditions. Here it is possible to maintain a sustainable quality.

Some important points of tea oxidation:

  1. Oxidation needs to be controlled by an experienced tea master. The master should be capable of reading and following the color changes. He monitors the development of aroma & flavor properly and takes action as per the situation.
  2. Oxidation process can run from two to four hours. However there is no hard and fast rule in this regard. Some of the most crucial characteristics  develop during oxidation process. They are color, briskness, brightness, astringency and strength. Each of these characteristics reach their maxima at different points of time. Hence the selection of the optimum time to get the best output is most important.
  3. As oxidation is an exothermic reaction. Due to the fact that it generates heat, it is susceptible to getting out of control too easily. Hence oxidation calls for strict monitoring of ambient conditions like temperature, humidity, airflow etc. The tea master generally maintains the temperature at around 29°C. He does this by blowing cool moist air through the rolled withered leaf known also by the moniker ‘dhool’.
  4. The tea master needs to maintain a ventilation standard. It should be around fifteen to twenty complete changes of air every hour with an optimum level of humidity at 98%.
  5. Another prerequisite for oxidation environment is it should be clean and hygienic. It should be free from stale juice deposits to avert bacterial infection.
  6. The tea master needs to ensure proper rate of oxidation. He should maintain an optimum withering level as hard withering results in slower oxidation.
  7. Mixed leaf and damaged leaf leads to uneven oxidation. Hence segregation of leaf at the time of withering may help in quality improvement of made tea.