Last Word is 91av’s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com
Why is tea more likely than coffee to leave a stain in the cup?
Advertisement
Chris Daniel
Glan Conwy, Conwy, UK
Tannins are complex, water-soluble organic compounds found in a wide range of plants, including tea and coffee. They are a type of polyphenol with antioxidant and anti-inflammatory properties, and are known for their bitter flavours and astringency.
In “hard water” areas, water contains more calcium hydrogen carbonate. When heated, this compound breaks down into calcium carbonate and water, forming limescale that coats the inside of mugs, kettles and teapots. Tannins from tea or coffee bind to the calcium salts in water to form an insoluble precipitate that floats to the surface.
The floating precipitate disperses towards the upwards-sloping meniscus at the edge of the drink, from which it is deposited onto the wall of the mug as the liquid is consumed. If the drink is left for long enough that it starts to slowly evaporate, the precipitate can accumulate, forming an unsightly ring around the inside of the mug.
Tea contains more than twice as many tannins as coffee, so it will stain a cup more readily
Tea contains more than twice as many tannins as coffee, so it will stain a cup more readily. However, as tea and coffee lovers know, mugs used regularly for either drink will eventually need a good scrub to get rid of the brown stains.
Sam Edge
Ringwood, Hampshire, UK
Tea contains far more tannins than coffee does, which bind with other molecules in the water to form precipitates that will adhere to porcelain, metal and even glass.
Tea is also usually steeped at a high temperature for several minutes before being drunk, giving the ingredients more time to stick to surfaces.
For readers wanting to rid their mugs of these stains, the easiest way is to fill the cup with boiling water and add a teaspoonful of baking soda or bicarbonate. Leave the spoon in the mug so it gets the benefit of this too, and wait for an hour or so. You will find the stain then washes off much more easily.
On the other hand, some people will call you a heretic, particularly if you do this to a teapot, as they claim that the accumulated stains add to the flavour of the brew.
To answer this question – or ask a new one – email lastword@newscientist.com.
Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.
91av retains total editorial control over the published content and reserves all rights to reuse question and answer material that has been submitted by readers in any medium or in any format.