In one form or another, teas are some of the most popular and widely consumed drinks in the world. In fact, when grouping teas together, tea is the second most popular drink on Earth, coming second only to water!  Alas, anyone who has ever put the kettle on for a group of friends or coworkers knows for certain that not all teas are created equally.
When discussing the science behind tea, it is important that to be precise when talking about a drink with so many different styles. Many herbal teas such as Camomile, Dandelion Root Tea, or fruit tea infusions have different sources of origin than black tea and green tea, and thus have different compounds in them. These herbal infusions are perhaps better labeled as tisanes, but in truth this name is not often used, and they are still widely grouped in alongside true teas in much of the Western world. In common parlance, even a hot infusion of raw fruit and hot water (such as ginger and lemon) often will be called a tea.
Because of this massive overstretching and variation in the meaning of the word ‘tea’, we must first clarify by what tea is before we can discuss the finer details such as tannin content. In this article, we will be focusing on the true teas, black tea and green tea. We’ll take a look at common questions about tea, tannins, and the wonderful chemistry contained within every one of our morning brews. Without further ado, let’s get started.
(If you are in a hurry, you can simply click here to read a quick summary in the conclusion instead.)
Which Plant Does Tea Come From?
Camellias are popular ornamental shrubs grown widely for their varied and colourful winter flowers throughout much of the Northern Hemisphere. Many growers and tea drinkers alike do not realise however, that it is the leaves of a member of this genus, Camellia sinensis, that are used for Black Tea and Green Tea. Camellia sinensis is an evergreen plant that grows as a large shrub or small tree. It has delicate white flowers that are much more subtle than many ornamental Camellia varieties.
The native range of Camellia sinensis spans from Southern China to Northern Burma, and it was thought originally that the plant grew exclusively in this range. This is revealed even in the plant’s name, as the word ‘sinensis’ is a taxonomical label that translates to ‘Of China’. In China, the history of cultivating Camellia sinensis for tea drinking dates as least as far back as 451 BC, with some estimates pushing this date even further back to around 2737 BC. China is the geographical and cutural birthplace of tea, and is still the largest producer and exporter of tea in the world today. 
Plants very much adapt to their environment over succesive generations. Even amongst Camellia sinensis plants, the leaves in different regions have been found to create teas with ever so subtle differences in taste, and different levels of tannins.  These differences are produced over time by plants adapting to their environment, and can produce quite distinct differences over a long enough timeline. Such differences would become apparent in 1823, during the British occupation of India, when a Scottish Major named Robert Bruce found Camellia sinensis plants growing naturally in the Assam region of Northeastern India.  Assam is situated around the Brahmaputra River just South of India’s border with China.
Due to natural variation, this plant found in the Assam region grows slightly differently to it’s Chinese counterparts, with larger, wider leaves as standard. These differences would lead to the plant being formally be classified as ‘Camellia sinensis var. assamica’ to distinguish it from other forms. The shorthand Camellia assamica is sometimes also used.
It is this chance discovery in the Assam jungle that would pave the way for changes in drinking habits across much of the world, and certainly here in the tea obsessed United Kingdom. Assam tea production skyrocketed, and the region’s name is still synonymous with tea to this day. Whilst many Camellia sinensis hybrids have been cultivated, it is the ‘assamica’ variety and it’s associated hybrids that are used largely in the production of black tea and green tea, largely due to it’s historical lineage and the larger yields provided by it’s broader leaves.
Does Tea Contain Tannins?
The leaves of Camellia sinensis are naturally high in the polyphenolic plant compounds commonly known as tannins. Tannins are astringent polyphenols that naturally occur in plants, and are responsible for the dry feeling in the mouth that people may experience after drinking a strong tea or red wine. It is thought that their bitter taste may serve a natural protective function, deterring browsing animals looking for some leaves to eat. 
Tannins are compounds produced upon the degradation and decomposition of organic matter and vegetation. It is only through this process of decomposition that the tannins emerge, which is why we don’t make teas using fresh, green Camellia sinensis leaves. In this sense, tannins can be thought of more as the product of natural reactions, rather than being held in place in a fresh leaf. In the leaves of Camellia sinensis, precursors of tannins exist the form of phytochemicals called Catechins. Catechins constitute around 25% of a fresh Camellia sinensis leaf. 
The word ‘Tannin’ itself, however, is a slightly misleading label. Tannin is an archaic term used to refer to plant based compounds that could bind to proteins. This was traditionally in the form of tannic acid and similar compounds. The ability of tannic acid bind to proteins is crucial during the process of making leather from raw animal hides, and this use in tanning leather is responsible for the naming convention of these compounds. These tannins are extracted from oak, pine, birch, and willow treas.
Tannic acid similar chemicals are true tannins, but tea does not contain tannic acid. As the compounds in tea leaves are not used in leather tanning and do not resemble tannic acid, the more suitable scientific name for the compounds in our tea is Polyphenols.
How Are Tea Leaves Processed?
To make black tea leaves ready for consumption, the leaves must be processed. Initially, fresh whole leaves are cut from a Camellia sinensis plant, usually the fresh new growth. These leaves are then set aside and allowed to wither and naturally lose their water content. During the withering process, the leaves begin to take in oxygen as their cell walls break down. The leaves begin to oxidise, which is much the same process that happens when a freshly cut apple or banana exposed to the air begins to turn brown.
In the tea making world, this process has traditionally been called fermentation, but it is actually a process of enzymatic oxidation. This difference is subtle but important. Oxidation is the absorption of oxygen by a compound to transform itself into a different compound. The process requires the presence of oxygen in the environment. Fermentation, on the other hand, refers to the production of acids, alcohols and carbon dioxide from sugars in the absence of oxygen. Fermentation also broadly implies the activity of biological catalysts such as bacteria or yeasts, but the process that tea leaves go through does not rely on microbial agents.
The breakdown of the leaf cells that withering causes to the leaves allows air, and therefore oxygen, to reach the chemicals inside the leaves and so be used to power enzyme reactions. To aid this process, tea leaves are often ‘rolled’, either by hand or by automated machines. This gently bruises the leaves further, and evenly distributes moisture on the leaves. This gives the tea leaves shape, and allows more efficient and rapid oxidation to occur.
The withering and rolling processes allow enzymes within the tea leaves such as Polyphenol Oxidase and Peroxidase enzymes to react with catechins and other chemicals also already present in the leaves. These enzyme-driven reactions facilitate the resynthesis of catechins in the leaves such as epicatechin (EC), epigallocatechin (EGC) and epigallocatechin-gallate (EGCG), into new chemicals – Theaflavins, Thearubigins, and Theasinensins.  
What Are The Tannins In Tea?
The polyphenols Theaflavins and Thearubigins are browny-red in colour and are responsible for the deep colouration of black tea. The longer tea leaves are oxidised, the more theaflavins and thearubigins are produced, and the darker the leaves get. Because of this, a darker colour of tea leaves indicates a higher level of theaflavin and thearubigin content.
The length of oxidation is crucial in determining what the final tea product will end up as. We discussed the withering and rolling processes earlier, but to create Green tea, long periods of oxidation are not wanted. In a process that the Chinese tea makers traditionally call ‘Kill Green’ (Shāqīng, 殺青) leaves are heated up, denaturing the enzymes in the leaves and stabilising the oxidation reactions occurring within. This stops any further oxidation, preserving more subtle qualities of the leaves and producing a lower content of tannins in the final product.
When producing traditional green tea in Japan, it is not uncommon for leaves to be heated as soon as they reach the working area. This is done to ensure very minimal oxidation and a very small period of withering, preventing catechins from synthesising in to the more complex polyphenols, and thus preserving catechin content in the tea.
These differences in processing create different chemical profiles in the final product, even though the leaves originate from the same plant. As a result, Green teas have less theaflavins and thearubigins in comparison to the more oxidised Black Tea. The lower levels of tannins present in the final product give Green Tea a lighter colour and less astringent taste.
In the long oxidation periods in the production of black tea, over 75% of catechins in the leaves are transformed into theaflavins, thearubigins, and theasinensins.  Traditionally, it is primarily these compounds that would be referred to as the ‘tannins’ in tea. They give black tea it’s classic astringent quality and taste.
It is important to note that the catechins are not ‘breaking down’ in this reaction, but are instead actually forming more complex molecules. Theaflavins for example, are two catechin molecules bound together. This fusion of molecules is why so many different tannins/polyphenols can be created in the process. The different catechins in the leaves can bind together to form a wide range of many different theaflavins. Tea is chemistry in action!
There are more than 25 different Theaflavins found in black tea, and these constitute around 2-6% of the solids in the dry matter of a black tea brew. Theaflavins even have a unique chemical aromatic ring called Tropolone in their molecular structure, which is thought to contribute to the characteristic taste of black tea. 
Thearubigins meanwhile can account for up to 30–60% of the solids in a black tea brew, largely due to their high water solubility. Thearubigins have been associated with being responsible for the red-brown colour of tea, as well as the rich ‘body’ of the taste. 
Do Tea Tannins Have Health Benefits?
Due to their phenolic nature, catechins, theaflavins, thearubigins, and theasinensins exhibit strong antioxidant properties. Oolong tea, which is particularly high in theasinensins, has been shown to reduce oxidative stress and DNA damage caused by free radicals. Athletes who consumed oolong tea for 30 days showed significantly reduced plasma levels of malondialdehyde at rest and after exercise. Malondialdehyde is a chemical produced naturally when oxidative stress occurs, and can be used as a marker and a measure of oxidative stress.  
Not to be outdone, theaflavins have also shown similar performance related effects. A randomized, double-blind 2010 study revealed that humans subjected to acute anaerobic interval training experienced beneficial impacts from the intake of theaflavins. The results showed that the consumption of black tea enriched in theaflavins led to better recovery outcomes, and reduced levels of oxidative stress. Furthermore, the theaflavin enriched tea also seemed to provide relief in muscle pain levels caused by long periods of anaerobic exercise. 
Despite the positive health impacts associated with Theaflavins, they actually have a very poor bioavailability, and are very poorly absorbed by humans. Attempts to measure absorption have provided extremely low numbers, with some data indicating that theaflavin absorption is as low as 0.000001%! Due to this low bioavailability theaflavins pass unchanged through the digestive tract to the colon, where they interact with the colonic bacteria strains. It is thought that this interaction with the microbiota of the colon produces secondary chemicals that are responsible for the health benefits associated with theaflavins. 
Whilst the exact mechanism may not yet be known for certain, the consumption of tea tannins, including theaflavins, shows reliable results in studies for their positive health impacts. Theaflavins have even been studied for their anti-cancer action, with some research indicating that theaflavin consumption may help to reduce cancer cell proliferation and improve cancer cell shrinkage. 
Despite these exciting positive health benefits, there is a downside to the tannins in tea, and it’s not just their ability to stain clothes and teeth! The polyphenols in tea are known to disrupt the absorption of iron from meals when tea is consumed at the same time, which we discuss in detail here.
This effect is unlikely to lead to clinical issues in healthy populations with balanced diets, but for individuals with low iron stores or those suffering from anaemia, the decrease in iron absorption caused by black and green tea may deserve more consideration. This effect can be mitigated by not drinking tea at least one hour either side of meals. 
What we call ‘tannins’ in tea are actually flavanoids such as catechins, theaflavins, thearubigins, and theasinensins. Catechins are naturally occuring in the leaves of Camellia sinensis plants, which are used to create teas such as black tea and green tea. The withering and rolling process of picked tea leaves causes oxidation reactions to occur within the leaves. These reactions are powered by enzymes, which convert catechins in to theaflavins, thearubigins, and theasinensins. This process was traditionally called fermentation due to a misunderstanding of the process, but it is actually oxidation driven by enzymes in the leaves.
The longer the leaves are oxidised, the more theaflavins, thearubigins, and theasinensins are produced, giving the final tea a darker colour and more astringent taste. The differences in processing are the reason for the chemical differences between Green tea and Black tea. Green tea has a higher concentration of catechins than Black tea because less oxidation has occurred, meaning less degradation of the catechins in to more complex polyphenols. Black tea has a higher concentration of polyphenols (tannins) due to the longer oxidation period, but less catechins.
The tannins in tea are not well absorbed by the body, but yet still seem to have measurable health benefits and antioxidant activity. This is potentially due to secondary compounds formed as a reaction between theaflavins and bacteria in the lower digestive system. Tea however, does reducein iron absorption when consumed with meals, or just before meals. Those at risk of iron deficiency are therefore advised to avoid drinking tea within an hour either side of meals.
Phew! Who’s thirsty? Time for a well earned cuppa, I say!
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