The science of the perfect cup of tea © Getty Images

The science of the perfect cup of tea

Milk in first or last? We'll leave that up to you...

We really like tea. A lot. In the UK, we drink approximately 100 million cups per day. But how much thought have most of us given to what goes into our well-earned cuppa? For such a complex and fascinating beverage, the answer is probably ‘not enough’.

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What exactly is tea?

Tea is made by infusing the dried and crushed leaves of the tea plant in hot water. There are many varieties, typically grouped into five main categories: white tea, green tea, oolong tea, black tea and Pu-erh tea. By far the most widely drunk tea in the UK is black tea, whereas green tea is more popular in East Asia.

All teas derive from the same plant, Camellia sinensis, a sub-tropical, evergreen bush native to Asia. There are two recognised varieties: Camellia sinensis var. sinensis (Chinese tea) and Camellia sinensis var. assamica (Assam or Indian tea).

Other drinks that we might refer to as tea (e.g. mint, chamomile, rooibos, fruit ‘teas’) are not actually tea. Unless they hail from the Camellia sinensis plant, they are technically tisanes.

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If all tea comes from essentially the same plant, what defines the different categories? Well, it all comes down to how the leaves are processed.

How tea is made

After they are harvested, the leaves are laid out to dry (called ‘withering’) so they become supple for the next stage: rolling.

The leaves are rolled and shaped, releasing enzymes and oils that alter the leaf’s flavour, and initiating the oxidation process. Exposing leaves to oxygen for a period of time determines the colour, taste and strength of tea. The longer the leaves are left to oxidise, the darker in colour and stronger in flavour they become.

As you might imagine, black teas are highly oxidised whereas white and green teas are the least oxidised. Once the desired oxidation level is reached, the leaves are fired to halt the process and reduce the moisture content to ensure the tea keeps well. Not all teas go through all the stages of the process, and some may go through different stages a number of times.

This all refers to the orthodox process. There is also the non-orthodox process, the crush-tear-curl (CTC) method that originated in the Second World War to increase the amount of tea that could be packed into a chest. After the leaves are withered, they are put into a machine that crushes, tears and curls them, as the name suggests.

The leaves are formed into pellets, then oxidised and dried. It’s a much quicker process than the orthodox method and the resulting pellets are ideal for teabags, of typically black tea.

The aim when brewing tea is to bring out the best flavours and nutrients, while avoiding bitterness. Going by what experts advise, it seems that most of us are making it all wrong. So, what should we be doing?

How do you make the perfect cup of tea?

First, choose our water wisely. Softer water results in a cleaner finish as the minerals in hard water can result in a scummy layer on the tea. Fresher water brings out a brighter, cleaner taste.

Secondly, consider temperature. We tend to use water straight out of a boiling kettle but this might not be the best method for all teas. Different teas require different brewing temperatures to deliver the perfect cup.

And third, get the steeping time right. As tea brews, tannins, amino acids, aromas, and flavours slowly diffuse into the water. How long it takes depends on the compound, tea type and water temperature.

The chemistry of tea

Aromas responsible for smell and flavour dissolve pretty instantly into the water, lighter polyphenols, caffeine and compounds related to mouthfeel and texture take slightly longer, and the heaviest compounds (heavier polyphenols, flavanols and tannins) take the longest time.

The longer tea steeps, the more tannin is released. A moderate amount contributes to the richness of flavour, giving the tea body, but too much produces a bitter taste. Too much heat may also dissolve the tannins and flavour compounds too quickly creating an imbalance, and too little heat may have the opposite effect, resulting in a weak, flavourless brew.

During brewing, a peak is inevitably reached when the tea is at its best and after which the tea becomes bitter and unpleasant. If you prefer strong tea, it’s better to add more leaves to enhance the flavour rather than steep it for longer, which will just increase its bitterness.

In general, darker, stronger teas, like Pu-erh, black and oolong teas, need a higher temperature and longer time to brew than milder, more delicate teas (i.e. green and white). Most teas favour their water under boiling temperature and brewing times range from around 2 to 5 minutes.

The majority of us (around 96 per cent) use teabags, which behave a little differently to loose leaf tea. Commercial teabags are often considered inferior in quality as they typically contain low-grade leaves that have been broken into small pieces (‘fannings and dust’) by the CTC method.

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Unlike loose tea, teabag tea’s ability to infuse is limited as it doesn’t have as much space to expand and release compounds. The processing method and confinement of the bag are thought to result in a less diverse flavour profile. As the leaf pieces are much smaller than whole leaves, they have a greater surface area exposing them to more light and oxygen.

This affects flavour as aromatic oils are more vulnerable to evaporation (and is why tea you’ve had lurking in the back of the cupboard for ages just doesn’t taste that great). The larger surface area also results in a greater release of tannins (risking a more bitter cuppa), so optimum brewing times for teabag teas are typically shorter than for loose leaf teas.

The upside is that they also better release constituents like L-theanine, a unique amino acid that contributes to the relaxing properties of tea. If you’ve ever wondered why tea is considered relaxing whereas coffee is thought of as stimulating, but they both contain caffeine, then L-theanine (found in tea but not coffee) is part of the answer.

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Tea is filled to the brim with interesting, and potentially beneficial, compounds but most of us drink it just because we like it. There are many ways to make the perfect cuppa but, ultimately, the best way to make a cup of tea is all a matter of personal taste.