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Why does lemon juice make tiny cuts sting so much?

• It’s the acidity. The skin of fingers is especially populated with pain receptors. These are particularly sensitive to pH, and lemon juice at pH 2.3, or even sometimes as low as 2.0, is sufficiently acidic not only to be detected but also to damage tissue by denaturing proteins. This has culinary uses in tenderising meat by hydrolysing tough collagen fibres, in preventing short-term browning of fruit by denaturing the enzymes responsible, and in cutting down the smell of fish by converting amines to salts.

“Lemon juice is sufficiently acidic not only to cause pain but to damage tissue by denaturing proteins”

The pH of a given acid depends on its concentration, which at up to 6 per cent in lemon juice makes it the highest of the citrus fruits. The pH of blood and tissue fluid is around 7.4 – slightly alkaline.

The acidity league table of common foodstuffs runs something like this: lemon/lime juice pH 2.3, white vinegar 2.4, grapefruit 3.0, orange 3.5, yogurt 3.7 minimum, tangerine 3.9, tomato 4.5, and milk around neutral at 6.6.

However, lime and vinegar are less directly handled in western kitchens, unlike lemon juice, which can also be used as a household polish, cleaner and deodoriser, thereby increasing the likelihood of skin contact or surprise squirts in the eyes, whose populous receptors sting like crazy to drive prompt prevention of damage. Pain receptors are equally sensitive to strong alkaline substances, notably soap at pH 10.

Gloves and goggles would help, but if you get juice in your cut, at least be relieved that its antibacterial action will lessen the risk of infection.

Len Winokur, Leeds, UK