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IS THERE any fruit more useful than a lemon? The citric acid in its juice can provide a touch of sharpness that improves a vast range of dishes. The fragrant oil concentrated in the peel of lemons has an aroma so appealing that we add it to cleaning products. It is also a good source of vitamin C: once its power to prevent scurvy was discovered in the 1750s, it led to a boom in demand for lemons.

The family history of citrus fruits is complex and uncertain, but genomic studies suggest that lemons are a , another citrus fruit that resembles a large, knobbly lemon. From South Asia, they made their way to Europe and North Africa in the Middle Ages.

The fruits are usually green when harvested. As they ripen, the green chlorophyll pigment breaks down, revealing the carotenoid pigments that make them yellow.

Lemon juice contains about 2.5 per cent sugar, but its sweetness is masked by the sourness from citric acid, which makes up about 5 per cent of the juice. The aroma of lemons comes largely from a family of volatile compounds called terpenes, including limonene, citral and pinene. We associate lemons with sourness so strongly that a non-sour lemon extract can .

Although the flavour of the fresh fruit is so useful in cooking, it develops into something even more wonderful when lemons are preserved in salt. This has been done in North Africa for at least 800 years: the is from a 12th-century Egyptian physician named Ibn Jumay.

As in other preserves, such as sauerkraut and kimchi, salting inhibits the growth of microbes that cause spoilage, instead favouring harmless salt-tolerant bacteria and yeasts. Over time, the peel softens and loses its bitterness.

Most lemons found in shops are coated in wax to extend their shelf life. The wax is safe for consumption, but it is best to use unwaxed lemons for preserving, or to scrub off the wax.

Slice off the very top and bottom of the lemons. Make two cuts lengthwise in each fruit so that it is almost split into quarters, but leave these attached at one end. Mix the salt and sugar, then stuff this into the lemons and pack them into a jar, with as little space as possible. Leave at room temperature. After one or two days, once some fluid has been drawn out of the lemons, you should be able to squash them into the jar a bit more. If needed, add some juice from more lemons to ensure they are fully submerged.

They are ready to eat after about a month. After that, you can keep them in the fridge for up to a year. Rinse off the brine before using, then chop finely and add them to any dish that would benefit from a lemony kick.

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