I live in an area with hard water and have noticed that drinking tea without milk stains my cups. Why is that? And why, if I add lemon, does it solve the problem?
• Tea contains : mildly acidic, tannic substances with insoluble calcium salts. In hard water, which often contains calcium ions, these form a scum in black tea. You can prevent scum forming by adding citric acid such as that found in lemons. This removes calcium ions by forming covalent, soluble calcium citrate.
“You can prevent scum forming in tea by adding citric acid such as that found in lemons”
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Interestingly, donated blood is also prevented from clotting by adding sodium citrate as the resulting reaction removes free calcium ions to form calcium citrate. Removing these ions prevents the activation of thrombin – an enzyme that catalyses the formation of , allowing blood to clot. Catechins also combine with proteins in milk; this removes them from solution and so prevents scum. This reaction of tannin and protein is the basis for tanning leather.
Luce Gilmore, Cambridge, UK
The term catechins is commonly used to refer to the related family of flavonoids – Ed
• Tea contains considerable quantities of polyphenolic flavonoids, the substances we refer to as tannins. These make a major contribution to tea’s flavour, particularly its astringency, as well as its colour.
The structures of many of these flavonoids, particularly one called , include an aromatic ring with three hydroxyl groups attached to neighbouring carbon atoms in what is called a gallate group.
At the fairly neutral pH of brewed tea, many gallate hydroxyl groups ionise and in turn react readily with proteins in the mouth lining (hence the astringency) and with silicates in the glaze lining a cup (hence the brown stain). Drinkers add lemon or milk to reduce astringency. The lemon juice also lowers the pH, returning the gallate hydroxyl groups to the un-ionised state that no longer attaches to mouth proteins, or glaze. Milk proteins mop up the reactive hydroxyl groups with the same end results.
Tom Coultate, Leighton Buzzard, Bedfordshire, UK
The writer is the author of Food: The Chemistry of its Components (the fifth edition was published by RSC Paperbacks in 2010) – Ed