Is Black Tea Fermented?

The world of tea involves many varieties that vary in taste, color, and strength depending on how they’re processed. Black tea is one of the most popular types of tea in the world, used both as a delicious drink and for its medicinal properties. 

While the term “fermented” is sometimes used to describe the processing of black tea, black tea is not actually fermented.  Fermentation occurs when tea is broken down by bacteria or fungus, which does not happen during traditional black tea processing. 

Oxidation is actually the name for the stage of tea processing that lends the tea the bulk of its flavor and coloring. 

While it’s common to confuse fermentation with oxidation, let’s take a look at what actually occurs in black tea processing and also look at special cases where black tea is actually fermented. 

Oxidation vs. Fermentation in Black Tea


While many growing conditions that go into tea affect the end flavor, tea is also influenced by how it is handled after it’s harvested. One of the most important steps of processing tea after harvest is the process of oxidation, which some mistakenly call fermentation. During this step of processing, tea is allowed to dry out. 

During the drying process, oxidation causes the tea to turn brown in the same way oxygen acts on a cut apple, potato, or avocado. 

Black tea is a type of tea that is allowed to oxidize completely before it is packaged for use. This is in contrast to lighter teas such as green tea. Green tea is roasted soon into the oxidation process to halt fermentation and give the tea a more grassy, herbal flavor. 

If oxidation was allowed to go on indefinitely, the tea leaves would eventually rot. Instead, once the tea leaves have achieved the desired level of fermentation, the leaves are roasted or steamed to halt the oxidation process. 

fermented black tea leaves

Categories of Fermentation/Oxidation

When it comes to processing tea, the levels of oxidation are broken into different categories so that the tea can be accurately labeled and packaged. These are the different categories of semi-oxidized tea:

  • 0-10% oxidation: This category of fermented teas includes the most lightly-oxidized teas such as green tea and oolong tea. Tea without any fermentation at all is known as white tea. 
  • 10-20% oxidation: Teas that are only lightly oxidized are more aromatic, golden-colored teas with a delicate floral profile. These include strains such as jasmine and pouchong tea.
     
  • 20-50% oxidation: These teas present with a light greenish-brown color and a slightly sweeter taste than lightly-oxidized teas. These semi-fermented teas also have a more full-bodied flavor than lighter teas.
  • 50-80% oxidation: These semi-fermented teas are considered heavy teas and have an amber or reddish-orange color. These teas have a sweet, fruity aroma and a smooth flavor.
  • 100% oxidation: Black tea falls under the category of fully-fermented teas. The longer black tea is left to ferment, the stronger its flavor becomes. Black tea is dark red in color and has a malted flavor. 

Over centuries of trial and error, tea producers have learned the precise amounts of oxidation necessary to create each type of tea. As the tea with the highest level of fermentation out of all other types, black tea also possesses the strongest flavor in the tea family.    

Does Fermentation Affect Caffeine Content?

Although there is an often-repeated old wives tale that fermentation affects the level of caffeine in tea, with green and oolong teas having less caffeine than their fermented counterparts, this is mostly untrue.

While teas that are twice-fermented or have microbial agents added to them for biological fermentation do increase slightly in caffeine levels, there is no real difference in caffeine levels between traditional non-fermented and fermented teas.

Rather than being affected by fermentation, the caffeine found in black tea and other varieties of tea is dependent on the following factors: 

  • Varietals
  • Age and size of the tea leaves at harvest
  • Season

The caffeine content of tea isn’t dictated by fermentation—instead, it is mostly dictated by genetics, growing conditions, and the condition of the tea at harvest. 

Fermentation Or Oxidation?

Even though this process is known as fermentation in the tea industry, only a few types of tea undergo the microbial activity that would accompany true fermentation.

While the stage of tea processing where the tea is allowed to brown and cure is known as fermentation, this is actually a misnomer. True fermentation occurs when biological agents such as yeast or bacteria break down organic material to produce carbon dioxide, alcohol, and flavor. 

The stage of the tea processing known as fermentation is enzymatic oxidation, which occurs when the cell walls of the tea leaves break down and the interior of the cells are exposed to the air.

Exposure to oxygen leads to a chemical reaction with organic enzymes inside the leaves known as polyphenols. The most important of these enzymes is polyphenol oxidase

fermentation black tea

Secondary Fermentation in Black Tea

There are types of teas that undergo secondary fermentation after the oxidation stage, and black tea is one of them. When it has been twice-fermented, black tea is known as Pu-Erh tea.

The type of tea used in fermented Pu-Erh tea is a Chinese black tea specifically grown in the Yunnan province of southwestern China.

Black Tea in Kombucha Tea

Another way that black tea is fermented after the processing stage is when it is transformed into kombucha.  Kombucha is created by adding a symbiotic culture of bacteria and yeast to sweetened black tea and then allowing it to ferment.

In kombucha, the sugar that is added to the black tea serves as food for the bacterial colony that is responsible for the fermentation process. 

If kombucha is allowed to ferment long enough, it can be transformed into an alcoholic beverage. Along with its recreational value, kombucha is also known for having medicinal benefits. Here are just a few of the advantages associated with drinking kombucha:

  • Probiotics: The bacteria in kombucha is known as being a good source for probiotics, or beneficial bacteria that can help regulate gut flora in the digestive system.
  • Polyphenols: Like other types of tea, kombucha is full of polyphenols that act as powerful antioxidants in the body. These antioxidants are associated with everything from reduced cancer risk to improved cardiovascular disease.
  • Acetic acid: Kombucha made of fermented black tea is full of acetic acid, which is known for its antibacterial properties. This is thought to help the body defend itself against infections.
  • Cholesterol reduction: Kombucha has been shown to reduce levels of “bad” cholesterol (LDL) and increase levels of “good” cholesterol (HDL) in the blood. This is the result of catechins, soluble fiber, niacin, and glucuronic acid in the kombucha.
  • Blood sugar stability: Scientific studies have shown that in rats kombucha can help stabilize blood sugar and can slow the absorption of carbohydrates into the blood. There has also been a study to show that those who drink kombucha have an 18% less chance of developing diabetes over time.

The fermentation of kombucha is directly responsible for the probiotic bacteria in the drink. Without it, fermented black tea wouldn’t be nearly as healthy for you as it is. 

Processing Plays an Important Role in Black Tea

The way that tea is grown and harvested has a big impact on how it tastes, but it doesn’t affect black tea nearly as much as the fermentation process does.

Whether it’s just through traditional oxidation or as a part of the secondary fermentation process, fermentation is responsible for the flavor, color, aroma, and mouthfeel of black tea. 

If you love tea as much as I do, pin this Is Black Tea Fermented post to your favorite tea-loving Pinterest board and pass it on for others to enjoy! Pinkies up!

is black tea fermented?  glass tea cup with black tea in front of a rainy window