What Is a Truffle? A Fungus That Had to Invent a Smell

A truffle is the spore-bearing structure of certain ascomycete fungi that live as mycorrhizal partners on tree roots, principally oak, hazel, beech, poplar and lime. Because it matures underground it cannot disperse spores into the air, and its reproductive strategy depends on being found, dug up and eaten by an animal that passes the spores elsewhere. The aroma compounds are therefore a signal evolved to be detectable through soil at a distance, and they are remarkably potent, with some detectable at concentrations of parts per trillion. Dozens of volatile compounds contribute, with sulphur-containing molecules prominent, and one of them, androstenol, is also found in boar saliva, which was long offered as the explanation for why pigs are so good at finding them, though the picture is more complicated than that neat story suggests. The spores themselves survive digestion, which completes the arrangement.

Only a few species command high prices and they differ substantially:

• •The Italian white truffle from Alba and surrounding regions, the most expensive, with an intense garlicky aroma that is destroyed by heat and is therefore shaved raw over finished dishes
• •The Perigord black truffle from France, Spain and Italy, which tolerates gentle heat and is used in cooking, and which is the species most successfully cultivated
• •The summer or burgundy truffle, milder and far cheaper, which is often what appears in supermarket products
• •The Oregon and other North American species, and the Chinese truffle, which resembles the Perigord visually and is much less aromatic, and whose sale as the more expensive species is a recurring fraud
• •Desert truffles from North Africa and the Middle East, an entirely different group with their own long culinary tradition and much lower prices
• •Almost all commercial truffle oil contains no truffle at all, being flavoured with a synthesised compound that reproduces one aroma note crudely, which is legal where labelled and is why chefs are frequently dismissive of it

Truffles were hunted with pigs, which locate them instinctively and want to eat them, which is the obvious drawback, and the practice has been largely replaced by trained dogs, which find them reliably and can be persuaded to give them up. Hunting is seasonal, secretive and territorial, and productive sites are closely guarded family knowledge. Cultivation, long considered impossible, became practical when it was understood that the fungus must be established on tree roots: seedlings are inoculated with spores in a nursery, planted in suitable calcareous soil, and produce truffles after something like seven to ten years if conditions cooperate, which many plantations never do. That method has made the Perigord black truffle a farmed crop, with substantial production now in Spain, Australia, Chile and the United States, and has moved the season for northern hemisphere consumers by making southern hemisphere supply available. The Italian white truffle has resisted cultivation almost entirely, with only occasional reported successes, which is a large part of why it remains so expensive.

Truffle prices reflect scarcity, unpredictability and a short shelf life, since aroma fades measurably within days of harvest and a truffle is nearly worthless a fortnight after being lifted. That combination makes the trade unusually prone to fraud, including substituting cheaper species, injecting or exposing lesser truffles to aroma compounds, adding moisture to increase weight and selling unripe specimens. Authentication now uses genetic testing in serious disputes. The longer-term pressure is climatic: production in traditional European regions has declined over recent decades, and research relates yields closely to summer rainfall, with warming and drought projected to reduce suitable conditions substantially across the Mediterranean. That is pushing cultivation towards higher latitudes and altitudes and towards irrigated plantations, which changes an activity historically defined by wild foraging into something closer to orchard management.

A truffle fruits underground, so it cannot release spores into the air and instead produces an aroma potent enough to be detected through soil, relying on animals to dig it up and disperse the spores in their droppings. Alba white truffles are the most expensive and lose their aroma when heated, while the Perigord black has been successfully cultivated by inoculating tree seedlings, which takes seven to ten years. Aroma fades within days, most truffle oil contains none, and warming is reducing Mediterranean yields.