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.
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 ...
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