Chemistry of Tea and Coffee

On the whole, you can divide people into two groups: coffee drinkers, and tea drinkers. (A shadowy third group – those who don’t like hot drinks – are to be regarded with deep suspicion.) Imbibing these plant-infused hot beverages has become deeply entrenched in the culture and lifestyle of people the world over.

Around 2.5 million tons of dried tea is manufactured every year.1 Whether you take yours with a dash of milk in a fine china cup and saucer in a leafy garden in rural England, or prefer it black and thick with sugar from a glass on the streets of Cairo – or even eschew black tea in favour of green tea with lemon – the leaves have come from the tea shrub, a variety of Camellia.

The compounds that produce the familiar aroma, flavor, and taste include polyphenols, amino acids, methylxanthines, and volatile organic compounds.2 Hundreds of volatile substances in tea leaves make up the flavor and aroma. Many of these aromatic compounds do not exist in the fresh leaves, but are derived from other substances during processing,2 such as leaf withering, fermentation, and drying.3 Each step is designed to achieve optimal oxidation of catechins and produce tea with good flavor and color.3 Polyphenols (catechins) and flavonoids are metabolites produced by the plant as a defense against predators. These make up 39% of the dry weight of fresh tea leaves,3 and in your mug of tea they deliver astringency as well as purported anti-oxidant and chemoprotective benefits.2,3,4 Amino acids such as theanine affect alpha brain wave activity and promote relaxation,2 perhaps explaining why we put the kettle on ‘for a nice cup of tea’ when someone is shocked or upset.

Both tea and coffee contain caffeine – a methylxanthine that delivers a characteristic bitter taste and famous stimulant effect2 by blocking the action of adenosines, natural tiredness triggers in the brain.5 Tea is often said to contain more caffeine than coffee, but this is by weight – and tea is often diluted much more than coffee. Variety and brewing method can affect how much caffeine is extracted into the final drink.

Both drinks also require brewing in hot water at the points of consumption, with an infusion time of 3–5 minutes, depending on water temperature and particle size, but this is where the similarities end.

Over 2 billion cups of coffee are consumed every day.6 Whether you take your fix at home or on the run, black, white or laced with syrups, the basic ingredient is a plant extract. In contrast to tea, coffee products originate from a seed rather than a leaf. Chlorogenic acids make up 8% of the composition of raw coffee.5

Once extracted from the fruit, the seeds (or beans) are roasted at high temperatures. The temperature and length of roast affects the final taste and acidity. Roasting is the real key to unlocking the flavor since it causes hundreds of interactions between carbohydrates, proteins, and lipids.7 This includes the Maillard reaction between sugars and proteins, which in the coffee bean produces melanoidins – accounting for nearly a third of the final product’s composition and its distinctive flavor.5 Caramelized sugars give butterscotch flavors and aromas, 3-methylbutanal lends a fruitiness, methylpropanal hints at floral notes and spice, while acetaldehydes are pungent.8 Finally, brewing affects the taste in your cup: the polarity of each aroma and flavor compound impacts how prominent it will be in the final brew,9 and coffee to water ratio, water quality and temperature all dramatically change the taste,8 explaining why there are so many different devices and techniques aimed at delivering the perfect cup – drip, filter, instant, cold brew, French press.

1 Ho & Zhu. The Chemistry of Tea. In: Caffeinated Beverages. ACS Symposium Series 754;Chapter 32:316–326.
2 https://www.worldoftea.org/tea-chemistry/
3 Ho et al. Chemistry of Theaflavins: The Astringent Taste Compounds of Black Tea. In: Challenges in Taste Chemistry and Biology. ACS Symposium Series 867;Chapter 8:125–138.
4 Jhoo. Antioxidant and Anti-Cancer Activities of Green and Black Tea Polyphenols. In: Antioxidant Measurement and Applications. ACS Symposium Series 956;Chapter 15:215–225.
5 http://www.compoundchem.com/2014/01/30/why-is- coffee-bitter- the-chemistry- of-coffee/
6 Ponte. The ‘Latte Revolution’? Regulation, Markets and Consumption in the Global Coffee Chain. World Development 2002;30:1099–1122.
7 Segall. Comparing Coffee and Tea. In: Caffeinated Beverages. ACS Symposium Series 754;Chapter 3:20–28.
8 https://www.acs.org/content/acs/en/pressroom/reactions/videos/2016/coffee-chemistry.html
9 http://www.compoundchem.com/2015/02/17/coffee-aroma/