Learn the science behind:
After boiling a pot of water and dunking in a fresh tea bag, that bright ,transparent water starts to turn into a golden yellow or brown. Fragrant smells start to waft into your nose. The warm cup is perfect to heat up your hands on colder days.
While enjoying this wonder that is tea, your food science mind might start to wander. Do you really know what’s in that tea bag? And how do those tea leaves manage to harness so much flavour and smells?
Tea starts out as a plant
Long before the tea in your tea bag hits the water, it starts out as an actual leaf on a plant. Likely this plant grows somewhere in a tropical area, where tea plants thrive. The official name for the plant is Camellia sinensis. It’s an old species, it likely originated in eastern Asia thousands of years ago.
Nowadays, tea has spread around the world. India and China are the largest producers by far, but other major producers are Kenya, Turkey, Sri Lanka and Vietnam. Tea has crossed the oceans as well and is now also grown on the American continent. In far smaller quantities tea is grown outside of the tropical regions. Even the British, lovers of tea, grow a little bit of tea (in the estate Tregothnan).
Tea cultivates and varieties
There is just one tea plant species, Camellia sinensis, but within that species, there are a few variants and hundreds of different tea plant cultivars. These are all genetically related. Over the years, humans have cultivated them in different regions and ways to generate specific desirable characteristics. Each of those cultivars makes a slightly different tea.
What’s more the location where the tea plant grows as well as when harvest took place or the general growing conditions all impact tea quality.
Transforming tea leaves into tea bags
You will have a hard time finding fresh tea leaves outside of the tea growing areas. After they’ve been plucked from the tea plants by hand, the fresh leaves, like any other leave, wither and spoil quickly. Processing makes them very stable over time and, just as important, brings out those characteristic tea flavours.
After the leaves have been harvested they are withered, or wilted. This happens naturally by leaving the tea in a warm dry area. Tea producers can speed it up by leaving the leaves in warmer, covered, areas or by flowing air along the leaves.
The goal of withering is twofold. First of all, it starts the drying of the leaves. The moisture content drops from 70-77 to 60-65% during this step. This may not sound like a lot, but it does change the structure of the leaf quite a lot. It makes the leaves more flexible, making it easier to roll them for instance.
Second, during withering a lot of chemical processes get started. As happens in other leaves and vegetables when they get older, tea leaves (partially) loose their green colour due to break down of chlorophyll in this phase (it’s the same process that happens when pickling cucumbers for instance). Also, the polyphenol oxidase (PPO) enzyme is activated. PPO initiates oxidation and causes the colour of tea to change into a brown colour (PPO is also the enzyme that causes bananas to turn brown). Furthermore, the caffeine content increases and various flavours are formed.
The duration and extent of withering depends on the type of tea that a producer want to make. Some need longer withering times than others. For green teas for instance this step is often really short, you don’t want to lose that green colour, nor do you want the leaves to turn brown.
Once withering is completed the rest of the tea production process can vary quite widely for different types of tea. Not all teas will undergo all of the following steps. The only other process they all share is the final drying process at the end.
You don’t want the enzymes to oxidize too much for some teas. Luckily, these enzymes don’t keep well under high heat. A heat treatment breaks down their structures and stops them from working. This is done in a process called fixing.
This process is called fixing. Fixing can be done by heating the tea in a hot pan, using steam or a variety of other heat treatments. Different types of treatment will give a different flavour to the tea. Pan heating, which is a bit like roasting, gives some more of those roasted notes to the tea. Steam on the other hand brings out the earthiness.
As any plant, tea leaves naturally contain a wide range of different molecules. One group of molecules is very important in creating the final flavour of tea. There are polyphenols. Polyphenols all have a similar base structure in common but aside from that many variations are possible.
In a fresh tea lead the four most common polyphenols are epicatechin, epicatechin gallate, epigallocatechin and epigallocatechin gallate. However, during this next step in processing, called oxidation, these molecules are transformed into a wide range of new molecules.
The oxidation is catalyzed by the PPO enzyme that starts to become active during withering, if allowed to. To occur it needs oxygen, any oxidation reaction needs plenty of oxygen. It’s why leaves are either kept moving in drums or are placed such that there is plenty of access to the air and thus oxygen.
The reactions that occur next are complex and highly diverse. A lot of different reactions all occur simultaneously. Out of those reactions, the most commonly formed new polyphenol is theaflavin (an appropriate name for a polyphenol in tea!). The newly formed polyphenols give the tea its red/brown/black colour as well as a lot of its flavour. Those bitter flavours from especially black teas are made at this point.
If a tea manufacturer doesn’t want the full oxidation to occur they can stop it at anytime by heating the tea, just as is done in the fixing step.
Rolling & bruising
Withering the leaves makes it easier to roll them. But, they aren’t actually rolled until the rolling step. This can happen right after withering, or after oxidation, depending on the tea type. Rolling & bruising the tea releases some of the oils and moisture from the tea leaves. As a result the final tea made from these leaves is more aromatic. If you roll or bruise the leaves before oxidation, it also makes the molecules more accessible for one another, speeding up oxidation.
Any tea needs to be fully dried before it is ready to be shipped out to its customers. The final drying will ensure that all chemical reactions are stopped. Also, it ensures that no micro organisms can grow in the tea leaves. A tea that is insufficiently dried can get musty, create off-flavours and in the worst case might not keep long enough.
You dry tea by continuing to drive off moisture, often using a combination of heat and air flowing along the tea. The heat speeds up water evaporation and the air ensures any evaporated water is whisked away from the tea.
Other possible process steps
There are a lot of teas and as a result, a lot of different manufacturing processes. In some cases tea is fermented using micro organisms. Others are aged (just like wine!) to develop their flavours over time. Yet others are wrapped in sheets of cloth.
In this whole process tea leaves can be processed whole. Or they can be cut into smaller pieces along the way using a crush, tear, curl (CTC) process. Smaller pieces release their flavour and colour more quickly when brewing tea. Unfortunately, they also do so during storage. Which type you choose in the end will likely how you brew your tea.
White, green and black tea
Not every tea type goes through every process. The most commonly known tea types: white, green and black tea are each processed quite differently.
Green tea is withered for a short period of time, preventing oxidation from occurring. After that it is immediately fixed and dried. White tea undergoes a longer withering process, but isn’t oxidized either before it is dried. Black tea is oxidized after withering and rolling, which explains its dark colour. Just as the other teas it is fixed and dried before final packaging.
Ways in which teas can differ
There are a lot more different types of tea, each made up of their own combination of process steps, leaves, etc. It’s why buying tea, especially in a store dedicated to tea, can be quite an overwhelming experience. There are just so many different types of tea! To summarize, here are the most important ways in which teas can differ:
- Tea cultivar or variety: these are actually different plants and as such may differ in flavour
- Origin: the same plant, grown in a different area can give a different tea
- Flavours & mixes: a lot of teas are made with the same tea varieties but are blended together with flavours, herbs, etc. A lot of the variation you see in tea stores actually comes from this!
- Herbal teas: not all ‘teas’ are actually made with tea. A lot of herbal teas might not contain a single tea leaf in their blends. These ‘teas’ are may only contain a mix of spices, herbs, etc. (such as mint tea or cinnamon tea).
- Tea processing type: how a tea leaf is processed impacts its flavour and colour, white, green and black tea can all be made from the same leaf, they’re just processed differently.
Deb, Saptashish & Pou, K R Jolvis. (2016). A Review of Withering in the Processing of Black Tea. Journal of Biosystems Engineering. 41. 365-372. 10.5307/JBE.2016.41.4.365.
FAO, World tea production and trade – Current and future development, 2015, link
Gebely, Tony, Nuances of tea classification, Tea Epicure, link
Harbowy, M.E., Balentine, D.A., Tea chemistry, Critical Reviews in Plant Science, 16(5):415-480, 1997, link ; provides an overview of all polyphenols in tea (as known in 1997)
Heiss, M.L., Heiss, R.J., The Story of Tea: A Cultural History and Drinking Guide, 2011, link
Hoh, E., Mair, V.H., The true history of tea, 2009, link
Tanaka, T., Juono. I., Oxidation of Tea Catechins: Chemical Structures and Reaction Mechanism, Food Sci. Technol. Res., 9(2), 128–133, 200, link ; explains the chemical reactions behind the polyphenol transformations
Tea in the city, Introduction to tea cultivars, link
Teabox, How is tea processed and classified, Jan 31, 2017, link
Tregothnan, About, link ; the first British tea growing estate
World Atlas, The world’s top 10 tea producers, link