Slow food fans savour the return of the mysterious 'saliva of the stars'

A mysterious substance hailed in ancient times as the "perspiration of the sky" and "the saliva of the stars" has been rescued from oblivion by Italy's Slow Food movement.

The Italians call it "manna". It does not fall from the sky, but grows on trees. August and September are the months to collect it. It is a dense, white, sticky material rather like one imagines ectoplasm to be and gathers on the trunks of a particular type of ash tree. Centuries ago, people believed it was produced caused by the vapours of underground lava streams. Even today, some of its ingredients have yet to be identified.

Today those trees are found only in the "Park of the Madonnas", an hour's drive east of Palermo in central Sicily, and in the summer months the aged cullers welcome help from volunteers in making incisions in the trees and carefully scraping the oozing material into copper pans.

In the Middle Ages, manna was imported from the Orient, bought in the markets of the Levant by merchants from Venice and Florence and prized for its delicate sweetening qualities, though whether it was the same as Sicilian manna is unclear. Doctors in medieval Arabia eulogised it as a medicine for problems of chest and stomach.

Manna made it into the modern world, processed in factories into a laxative called mannitol and used as a sweetener by Italian pastry cooks and sweet makers.

Before the Second World War, Sicily regularly exported hundreds of thousands of kilograms of manna to the Americas. But production of natural manna was nearly killed off when much cheaper synthesised mannitol came on the market, a by-product of cane molasses created during the sugar refining process. To keep it from dying out, the Sicilian authorities began subsidising production in the 1950s. But the cultivators grew older and fewer and the product less pure until finally, last year, the Slow Food movement decided to support it, with the aim of procuring the purest possible product and selling it directly to herbalists and sweet makers. One regular French customer buys it for use in séances. Exactly how is a secret.