Stirring up trouble

Earlier this year, chemistry caused an international incident. Michelle Francl, a chemistry professor at Bryn Mawr College in the US, wrote a book, Steeped, about the science of tea. Controversially, she suggested the following steps to achieve a brilliant brew: use leaves instead of teabags, warm the milk to stop it curdling and, if all else fails, add a pinch of salt. The last suggestion was so inflammatory the US embassy to the UK took to social media to publicly disown it. And while their tongues were firmly in their cheeks, the world’s media loved the opportunity to stir the pot.

Let’s dive into tea’s origins. The UK’s favourite drink is made from the plant Camellia sinensis, which comes from China and other countries in east and south Asia. It doesn’t matter if your tea is green, black or oolong, they all come from the same plant species. The differences stem from how producers process the leaves: they largely don’t disturb green tea, whereas they oxidise black tea at a set temperature and humidity.

There are a lot of different chemicals in tea leaves (too many to go into here) affected by this process, but 10 ingredients – known as catechins – make up about 30–40% of a tea’s dry mass. During processing to prepare tea for consumption, enzymes break down the catechins to create a host of other molecules, each contributing to the different notes and tastes evident in a cuppa.

Today, there are many varieties of black tea. Most teas no longer come from China though. This is due to some rather nasty politics.

The Chinese would buy opium with silver, that the British would then use to buy tea

When tea first reached England, it became so popular that merchants were willing to send their ships on huge ocean voyages to purchase it. The problem was that the Chinese government would only accept silver in exchange, and the British merchants couldn’t afford the asking price.

Wanting to exploit a lucrative market, the British became drug dealers. They harvested opium poppies in India and shipped the opium to China. The local population became hooked – opium is the basis for both morphine and heroin, and people can smoke it as a drug too. The Chinese would buy opium with silver, that the British would then use to buy tea.

Unsurprisingly, the Chinese government weren’t happy about this. They tried to fight back, and lost, having to surrender the island of Hong Kong. Not long after, the British began to steal tea plants and grow them elsewhere, which is why today’s leaves come primarily from India and Sri Lanka.

Milk and sugar … or salt?

But that’s just the tea. What about the rest of the ingredients, such as water and milk? In 2003, Andrew Stapley of Loughborough University, UK, pored over the chemistry of a perfect cuppa. He suggested fresh water to ensure there’s dissolved oxygen, which brings out the flavours, and to avoid hard or mineral water to prevent tea scum.

It’s essential to add the milk first

When it comes to milk, people add this because it’s sweet and offsets tea’s bitterness. However, Andrew warns, it’s essential to add the milk first. This is because proteins in milk denature – lose their shape – in temperatures above 75°C. If you pour the milk into the tea, individual droplets of milk separate and come into contact with the boiling water, making this denaturation more likely. If you put the milk in first, by the time the fluids have mixed together the temperature is likely to be under 75°C and you avoid denaturation.

And Michelle Francl’s pinch of salt? Table salt is an ionic compound made from sodium and chlorine. As the salt dissolves, the sodium ions react with the bitter receptors on our tongue, blocking them and thus making tea’s bitter notes less identifiable. It’s perfectly good chemistry – although you’re going to get the same amount of success by adding a lump of sugar to your brew as well.

While the kettle boils, watch this:

@royalsocietyofchemistry Grab a brew and put your feet up as Ross explains some of the #chemistry of #tea Should you add #salt ? what is tea made from? Find out! #learnontiktok ♬ Late Tears - Muspace Lofi