Some traditions are kept by laws, others by recipes. One of the oldest ties between Spain and the British crown isn't diplomatic - it's sweet: the bitter oranges from Seville that have travelled to London for more than a century to become marmalade on the royal table.

The story began at the start of the 20th century, in the time of King Alfonso XIII, when Queen Victoria Eugenia longed for the traditional British marmalade made from bitter oranges. Since then, interrupted only during the Civil War and the Second World War, the custom has endured for more than 120 years. Every winter, the oranges from the gardens of the Alcázar in Seville are picked and sent to the British representatives.

The secret lies in the fruit itself. The bitter orange, Citrus aurantium, is prized precisely because it's no good to eat fresh - the sour flesh, aromatic peel and natural pectin make it ideal for a marmalade that sets on its own, with no additives. What looks on the table like an ordinary preserve is in fact the result of centuries of experience in knowing which fruit serves which purpose.

"A unique bond between Seville and the British crown, maintained for over 120 years," is how the Spanish embassy in London describes the tradition. For a Balkan reader, used to homemade preserves and winter stores passed down from grandmother to granddaughter, the point is familiar: sometimes the most lasting ties between people and nations aren't written into treaties - they're tasted at breakfast.