Impact of Fermentation on Tea Flavor and Caffeine Content

The fermentation process in tea production plays a significant role in shaping the flavor, color, and aroma of the tea. During this process, tea leaves undergo enzymatic oxidation, which leads to a transformation in their chemical structure. This oxidation is responsible for developing the complex flavors and darker hues associated with fermented teas, particularly black and oolong varieties. The oxidation process intensifies the tea’s richness and depth, contributing to the unique characteristics that differentiate various types of teas. However, fermentation has a relatively minor impact on the caffeine content, which remains relatively stable throughout the process.

Research has demonstrated that although fermentation induces numerous chemical changes within the tea leaves, caffeine levels do not significantly decrease. This is evident when comparing fully fermented black tea to non-fermented green tea. Despite their differences in taste and color, both types of tea retain similar caffeine concentrations. This stability of caffeine content is primarily influenced by the inherent qualities of the tea plant, such as the species or cultivar, and the part of the plant used (e.g., buds or older leaves), rather than the fermentation process itself. In fact, the caffeine content is largely determined during earlier stages of processing, such as withering and rolling, before fermentation even begins.

Although certain fermentation methods, like aerobic fermentation, may cause a slight reduction in caffeine, the overall changes are negligible. These minimal changes occur due to the oxidation of polyphenols rather than direct alterations to caffeine molecules. For tea drinkers seeking to lower their caffeine intake, relying on fermentation to reduce caffeine levels is not an effective strategy. Instead, choosing naturally low-caffeine teas like white tea, which is made from young tea leaves and buds, or caffeine-free herbal teas is a better approach. Herbal teas, in particular, contain no caffeine, making them an excellent alternative for individuals sensitive to caffeine or those looking to limit their consumption.

Ultimately, while fermentation enriches the sensory experience of tea, its effect on caffeine content is minimal, underscoring the importance of selecting the right tea variety for desired caffeine levels.
Impact of Fermentation on Tea Flavor and Caffeine Content

Theabrownin in tea

Tea pigments have several components: theaflavins, thearubigins and theabrownin.

Theabrownin is the main bioactive component derived from the process of fermentation of black pu-erh tea.

It was formed during oxidation of polyphenols of tea leaves. It was reported that the cellulases and pectinases produced by microorganisms, together with polyphenol oxidase, play an important role in theabrownin formation.

The tea pigment theabrownin gives pu-erh its characteristics brown color and plays an important role in the sensory properties of the final product but its pharmaceutical-like properties are not well understood.

Theabrownin are characterized by high molecular weight and complex structure. They dissolve easily b water and not in organic solvents like ethyl acetate.

Animal studies showed that theabrownin has a significant blood lipid-lowering in hyperlipidemic rats. It was hypothesized that TB would regulate the activity of key enzymes involved in lipid metabolism and accelerate the catabolism of exogenous cholesterol.
Theabrownin in tea

Drying of tea leaves

Drying achieved by blowing hot air through tea leaves, arrests oxidation by inactivating enzymes, resulting in color enhancement and the final balance of tea aroma and reduces the moisture content of the leaves to less than 5%.

In Keemun black tea processing, the leaves are subjected to drying after fermentation at 110 °C– 120 °C for 10 – 15 min. The moisture will drop below 20%.

After spreading the first dried leaves, another drying is carried out at 70 °C – 90°C for 45 –60 min to moisture below 4%.

Drying is usually done by a drying machine such as the multi-band drier or fluid bed drier.

The temperature is sufficient to inactivate the polyphenol oxidases. The sap released during rolling and fermentation solidifies during drying on the fine little hair on the surface of the leaf.

During drying, aroma substances are formed and the coppery-red color is changed to black (hence ‘black tea’).

Following the drying the leaves are sorted and graded to yield a commercial product.
Drying of tea leaves 

Taste of tea

Besides the aroma, the taste quality is also one of the key criteria used by tea tasters to describe the quality of tea liquor.

Green tea, oolong tea and black tea all have their own distinctive flavors. Its taste is at first bitter and astringent and then sweet.

The balance of astringency, bitterness and brothy taste is important to the characteristic taste of tea; catechins and caffeine are counted as the major contributors to astringency and bitterness.

The major catechins in green tea are EGCg, ECg, EGC, and EC, which are classified as either gallate-type catechins (EGCg and ECg) or non-gallate type catechins (EGC and EC) by the presence of galloyl group on the oxygen atom at the C3 position.

Human perceive the taste of the gallate-type catechins to be more bitter astringent than that of the non-gallate type catechins.

Black tea and green tea are different in flavor due to the fermentation process during production of black tea, in which catechins are oxidized and polymerized to form theaflavins; and consequently the astringent taste of green tea is replaced with more complex astringent taste.

Theaflavin constituting about 0.3 – 1.8% of black tea dry weight and 1- 6 % of the solid in tea infusion. It contributes significantly to the bright color and brisk taste of tea infusions.

During a few minutes infusion, catechins from tea leaves are not fully extracted. The preparation method will affect the extraction efficacy and consequently the content of polyphenols in the cup.
Taste of tea

Fermentation of tea

Among all the teas with various degree of fermentation, the ones with higher degree of fermentation have better storage stability.

In consequence, black tea has the best storage stability, followed by Oolong tea, Pouchung tea and white tea.

The most important reaction in tea fermentation is the oxidative chemical reaction included by the polyphenol oxidase on the polyphenols in tea leaves. This subsequently causes the formation of different colors, aromas, and tastes in tea with various degrees of fermentation.

The reactions result in the formation of two groups of coloring compounds – the yellow group (theaflavins) and the red group (thearubigins). During fermentation tea phenol reduced over 90%.

In partially fermented tea, the fermentation process generally involves a withering or all shaking step to induce oxidation in the tea leaf, followed by blanching to terminate the process.

In the manufacture of fully fermented tea, the tea leaves are withered, rolled and fermented thoroughly, followed by drying or complete the process.

The rolling and crushing of the leaves frees essential oils and cellular constituents that promote the enzymatic activity of polyphenoloxidase.

Under traditional chilling techniques a typical 2-3 hrs fermentation may yield a reddish color tea with strong astringent qualities, while 4 to 5 hr fermentation in black teas provide a bright colored tea with a very low quantity of extractives from a similar fermentation.

While green tea retains the thin and refreshing flavors of the leaves, the fermented black tea gives a stronger and thicker flavor.
Fermentation of tea

Fermentation process

Fermentation is actually a process of enzymatic oxidation and the extent of fermentation determines the type of tea produced.

The most important reaction in tea fermentation is the oxidative chemical reaction induces the polyphenol oxidase on the polyphenols in tea leaves.

This subsequently causes the formation of different colors, aromas, and tastes in tea with various degree of fermentation. It also determines the tea shelf life. The longer the fermentation, the longer the tea can last. 

Fermentation process creates a whole set of powerful antioxidants that are present in black tea. Black tea actually contains more complex polyphenols than green tea.

During fermentation also, amino acids also get converted into aldehydes, unsaturated liquids form aldehydes and lower acids. Oxidation of carotenoid to terpenoids, aldehydes and ketones and degradation of chlorophyll to brown and black pigments occurs.

The changes are reflected in the read amber color, reduced astringency and more complex flavor found in black tea beverages.

Some unique antioxidants called biflavonols, thearubigens, and theaflavins.

There are four main theaflavins common to black teas and a second group of minor theaflavins, including the isotheaflavins and neotheaflavins.
Fermentation process

Fermentation of tea

Almost all tea polyphenols are catechins. In black tea, which is produced by fermenting the tea leaves, the same catechins which are contained originally in green tea react with each other during the process of fermentation.

They are three basic types of tea. They differ mainly in the degree of fermentation. Green tea undergoes little or no fermentation and black tea is produced by full fermentation, semi-fermented tea – oolong tea is the product of partial fermentation.

The fermentation process of tea leaves begins with drying, which is followed by twisting and then rolling to separate the leaf cells and release the oils (oil provides the taste). This step initiates fermentation.

Fermentation is caused by the oxidation of enzymes. The dark color of red tea is caused by full fomentation process, where as oolong is half fermented.

During fermentation, the tannin in tea is partly oxidized and the leaf changes color and turns bright coppery red.

Then the leaves are spread out of a cool, damp place, where they absorb oxygen and undergo further fermentation. The final step is drying by hot air to stop fermentation process and give the tea a uniform color, taste and aroma.

The period of fermentation generally extends for 3 to 4 hours. As a rule, the shorter the fermentation the more pungent the liquor; the longer the fermentation the softer the liquor and deeper the color.

Fermentation of tea

Processing of Black and Green tea

Processing of Black and Green tea
Black and green teas result from different postharvest treatment. In the case of black teas, the freshly picked green leaves are spread out on mats and allowed to dry out slowly and wither after which they mechanically rolled to break up the cells and liberate any enzymes which may present.

The rolled leaves are then fermented for several hours, either in baskets or spread out under damp cloths. During this stage the colorless tea tannins are partially oxidized changing to a reddish-brown color and some essential oil is developed.

This process is stopped by subsequent hot-air drying (frying). The whole process determines the flavor, strength, smoothness and color of the resulting infusion and calls for skill and experience to achieve consistent result.

Green teas are not fermented. The freshly picked leaves are steamed which prevent fermentation and blackening during the final drying process. The tea tannins are in consequence, largely in their original state as no essential oil is developed unfermented teas lack the aromatic character of the fermented varieties.

Oolong teas have a half way character; the leaves are allowed to wither and partially ferment before being rapidly dried at a high temperature. This results in a tea having a fine fragrant character.
Processing of Black and Green tea