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Murray Edwards College
University of Cambridge

School Winner: The Chemistry of Tea

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    17 Mar

    Winning Entry Landau Forte College Derby9C Isi Moss (Landau Forte) 2Who doesn't like a cup of tea? 6.2 billion cups of tea are consumed in the UK every year, proof that tea is Britain's most loved beverage. But what's the chemistry behind a brew, and what is it that makes tea so special?

    It is thought that the optimum brewing temperature for tea is 92⁰C, but 58⁰C is the recommended drinking temperature.

    Black tea tends to contain caffeine, which stimulates the central nervous system, hence making you feel more alert shortly after drinking. As well as caffeine, tea is a natural source of fluoride, which promotes healthy teeth. However, there are also other compounds in tea which are less well known, such as polyphenols, which are responsible for much of the taste as well as the characteristic colour of black tea. Surprisingly, there are 180-240mg of polyphenols in a strong mug of tea, and these compounds make up approximately one third of the weight of dried tea leaves. As antioxidants, they can have a positive effect on the blood, by limiting cell damage due to oxygen that can occur due to free radicals. However, this is disputable, as it has been suggested that the protein-polyphenol complexes can be broken down in digestion, thus meaning that it cannot impact the blood concentration of polyphenols. Polyphenols are made up of catechins, and can be split into two main categories: theaflavins and thearubigins. Theaflavins have several potential medical applications. For example, they could work to reduce blood cholesterol level, reduce obesity (by reducing the likelihood of a fatty liver), and could even reduce breast cancer cell migration. They are present in black tea due to the enzymatic oxidation of green tea to produce black tea leaves (a process referred to as fermentation). The percentage of polyphenols that are theaflavins is only 10%, yet they account for the yellowish colour of tea. In contrast, thearubigins are reddish-brown pigments, which also contribute to the distinctive colour of tea. They are weakly ionising acids, and the anions produced are very colourful. Therefore, the addition of lemon to tea can alter the liquid's colour, making it lighter as the citric acid is pH 2.2 as opposed to pH5 of thearubigins. This makes citric acid more strongly ionising than thearubigins, so it suppresses the ionisation of the thearubigins. Due to the opposite effect, tea brewed in alkaline water will be somewhat darker in colour. Similarly, the chemistry of tea changes again when we add milk and sugar. While only 1 in 3 people drink their tea with sugar, 98% of Britons drink their tea with milk. The possible anti-oxidising effect of polyphenols could be reduced by adding milk, as casein proteins in milk bind to them and reduce their anti-oxidising effect.

    So, next time you sit down to indulge, take a moment to remember the chemistry behind this celebrated drink, whether you like your tea milky, black with a slice of lemon, or a builder's brew with milk and two sugars.

    Isi Moss
    Student at Landau Forte
    "As an aspiring Chemistry student, I'm fascinated by the chemical interactions of the world around us. I would love to be a Science Journalist, communicating my passion with others, and working towards putting Chemistry in the spotlight of public interest, alongside the other popular sciences."