Take two London chefs, stir in Perigord truffles, vintage Dom Perignon, and Kiki the excitable pig. Photographs by Benoit Rajau
I've eaten truffles many times over the years, but I must confess that I've never really seen the point of them. Restaurants often seem to use them more to boost prices than flavours, and their musky, fugitive taste has never truly come through for me. But standing in a field of oak trees in the wilds of Quercy, south-west France, nibbling a morsel of fresh truffle nosed out of the earth a few minutes earlier by an excitable pig, I finally understand what all the fuss is about.

Its browny-black flesh threaded with white veins, the fungus has an aromatic, feral taste with a dark undertow carrying something unmistakably sexual. Kiki, the truffle-hunting pig, is obviously getting it big-time, too. He butts the earth, frenziedly snuffling and scrabbling, while his owner hauls him to one side, then bends and brushes aside the reddish topsoil to reveal a loamy fist-sized nugget, which he adds to the haul in his basket.

Three days into my truffle trip and I'm beginning to share Kiki's taste for the fragrant fungus, though I'm not yet tempted to emulate the American food writer who, according to local legend, was so certain she'd be able to sniff out a truffle herself that she got down on all fours and snuffled around in the dirt for several hours.

Legends like this tend to spring up around the truffle. It's the most mysterious of foods, partly because of its colour - black organisms aren't normally edible - and partly because, in this age of factory farming and genetic modification, it's one of the few remaining foodstuffs that can't be cultivated. For it to appear, three conditions must be exactly right: climate, soil, and the trees in whose roots the truffles grow. But even then there's no guarantee of success.

In an attempt to penetrate the mystique of the elusive black diamond, I've been sent on a three-day road-trip in the company of two London-based French chefs who are participating in some of London Restaurant Week's special events.

Both men have a particular interest in all things truffled. Michel Bourdin, venerable head chef of The Connaught, is a veteran of a dozen such expeditions and an enthusiastic ambassador for the black truffle of Perigord. He is currently offering a special truffle-themed menu, which includes his personal pride and joy - a creme brulee which uses truffle in place of the more customary vanilla.

Pascal Aussignac has never been on a truffle-hunt before, though he was born in Toulouse, bordering French truffle country. He came to Britain only six months ago, as chef and co-owner of Club Gascon, the hottest new restaurant in London, and his contemporary renderings of regional specialities have swiftly secured him two nominations in the Carlton London Restaurant Awards.

When we rendezvous at Waterloo, the two chefs couldn't present more different images. Michel is grave and jowly in houndstooth checked jacket and matching flat cap, while Pascal is cute and impish in designer casuals and trainers. That morning, he's been profiled in a newspaper under the heading "Sex on a Plate", and I find myself secretly hoping that rumours of the truffle's aphrodisiac qualities haven't been exaggerated. Michel and Pascal have never met before, and it's fascinating to eavesdrop on their rapprochement during the journey, as their shared passion for food breaks down the barriers of age and professional wariness.

Our first stop en route for Perigord is Epernay, in the heart of Champagne country. There, Moet et Chandon take us on a tour of their labyrinthine cellars and treat us to a memorable eight-course feast in a local hotel, to demonstrate how well Champagne accompanies truffles. We begin with a creamy soup of wild mushrooms topped by substantial slices of fresh truffle, whose powerful taste and surprising crunchiness are different from the slimy slivers I've experienced hitherto. Michel explains that the crisp texture is unique to fresh truffles and that I am probably used to tasting the preserved or tinned variety, which have a meatier taste, softer texture and less distinctive aroma.

I get the chance to taste the difference later in the meal, when we're each served a whole preserved truffle, wrapped in cabbage and accompanied by melting foie gras. From now on, I resolve, it's fresh or nothing for me. The partnership of truffles and Champagne works surprisingly well - but then it's hard to imagine anything that wouldn't go well with vintage Dom Perignon.

The next morning we continue south to truffle country - Cahors in the Lot valley - where we meet a man whose family name has been synonymous with truffles for three generations. Jacques Pebeyre's company has been supplying many of the top kitchens in France and Britain since the turn of the century. He still lives in the handsome townhouse where he was born, and where he and wife Monique welcome us to a family meal, in homespun contrast to last night's epicurean wonders.

The smell of truffles hits us as soon as we walk in the door - sweet and slightly rank, with a distinctly uric note. The elegant Monique complains that during the height of the season, she has walked into shops and been embarrassed to hear fellow customers wondering where the smell was coming from. Seemingly unfazed by having to cook for two celebrated chefs, she sets about preparing a classic omelette aux truffes. She's left the sliced truffles to infuse the beaten egg mixture for several hours, and the result is dark and erotic, like glimpsing an old friend dressed up in stockings and suspenders.

Over the next course of truffled Toulouse sausages, I ask Monsieur Pebeyre about the truffle's legendary aphrodisiac properties. "In my experience, there's nothing in it," he says. "But put them into a wonderful meal, with wine and attractive company, and people are bound to get inflamed." My neighbour, a British filmmaker working on a documentary about truffles, sheds further light on the question, explaining that there's a testosterone-like hormone in the scent, which is what makes the pigs so excited. "It's the same smell as sperm," he adds, sotto voce, making it rather difficult to keep up the polite truffle-related chit-chat for the rest of the meal.

Next morning we're due to go out on the truffle hunt. We've been warned that if it's raining the owner won't risk taking his pig out, in case it catches a cold, but luckily, the day dawns cloudy and dry. We rendezvous at M Pebeyre's factory - although factory is too grand a word for the back-street workshop where he processes and despatches the annual harvest, as his father and grandfather did before him. At any one time, the workshop may house up to pounds 100,000 worth of truffles, and Pascal's jaw drops at the sight of mounds of them, heaped up in wicker baskets. Soon he's bounding about like Tigger between the old-fashioned wooden trestle tables, sniffing and sampling.

Ranging in size from tennis balls to powdery pebbles, the truffles are sorted by hand and brushed clean of earth in a special machine designed by M Pebeyre's father. They emerge gnarled and black, like fossilised monkey brains, and are then either dispatched fresh or processed and stored in cans or jars. Three workers pick patiently through piles of shavings, removing dirt and stones with tweezers. It's like prospecting for gold - nothing is wasted.

Gold is an appropriate analogy. Prices have doubled since last year, because a dry summer and autumn frost devastated the harvest - a kilo now costs around pounds 500 to pounds 700 wholesale, and substantially more in shops and restaurants. And production is dropping dramatically year by year, partly because of rural depopulation, partly because truffles are no longer found in such abundant numbers. Opening a padlocked vault, M Pebeyre shows me three baskets of fresh truffles standing forlornly in the corner. "Last year this room was full at this time," he says. "If things continue as they are, I'm frightened that the industry will die out altogether."

Most of the truffles are brought in by company agents who buy from provincial markets, but an old-fashioned set of scales stands by the door, ready to weigh the offerings of people who call by with individual hauls. An exercise book stands beside it, filled with lists of the truffle-hunters' names, and the weight of their offerings. Some of these same names have been recurring in Pebeyre's books for decades. Another hand-written exercise book records the outgoing orders, an impressively starry collection including Alain Ducasse, Guy Savoy, La Tour D'Argent, and The Connaught.

We drive up into hills covered in golden oaks, to the isolated stone farmhouse of the smallholder we're going out hunting with (and smallholder seems a slightly more acceptable description than "paysan", which is how M Pebeyre cheerfully refers to him). Raymond Boisset is one of the few remaining locals who still hunts for truffles using a pig. Ruddy-faced and high-trousered, he has been prospecting for black diamonds on his land for a quarter of a century. He planted his oak trees himself, 40 years ago, and patiently waited 15 years before the first fruits appeared, as the symbiotic magic developed between their roots and the truffle-producing mycelium spores.

He lives alone, apart from Kiki the pig, that is. When we arrive, he herds the handsome russet-haired creature into a crate fixed in the back of his estate car, and we drive the short distance to his orchard. Kiki hits the ground running, and within five minutes he is nosing feverishly under some moss to uncover a hump of truffle. Feet scrabbling and head bucking, he tries to eat it, but M Boisset tugs him away and palms him off with a handful of dry biscuits and a pat on the back. He seems happy enough, though if there really is a sexual component to his excitement, the whole business must be hugely frustrating for him. A little later, he manages to nab half a truffle, demonstrating why truffle-hunters are increasingly relying on specially trained dogs, who are just as efficient as pigs, but are easier to handle, and have no desire to eat their quarry.

Even so, the whole process seems surprisingly easy, and I wonder how M Boisset deters poachers, since his land doesn't seem to be fenced off. "It's private property," M Pebeyre explains. "It takes a while to find truffles, and someone is always going to spot a poacher during that time. In fact, there's probably five or six people watching us now." I wonder what they make of the peculiar tableau we present, as one of London's most distinguished chefs attempts to manhandle a recalcitrant pig on a lead, while a designer-clad younger version grovels in the dirt to pull up a truffle with his hands.

After an hour, we've collected 10 good-sized truffles, about pounds 100 worth, a fair haul for this late in the season. In their earthy coats, the truffles will stay fresh for another week or so, and M Boisset will probably take them to Tuesday's truffle market at Lalbenque, the biggest in the South-west. He lets us keep a handful, and we take them to an auberge at nearby Varaire, where the owner, Mme Conte, conjures up a multi-course feast, starring a plump, yellow chicken served with a truffle-studded wine and garlic sauce.

Over lunch, M Boisset cheerfully announces that he's going to eat Kiki in a year or so, and replace him with a younger, more manageable pig. "He'll be well truffled by then," he laughs. His only concession to sentiment is that he's not going to kill Kiki himself, but will get a neighbour to do it. Truly, we're in la France Profonde.

Crammed into the car on the long ride back to London, Pascal shoves his face in his hands and inhales deeply - the smell of the truffles he's prised from the earth is still upon them. "I cook with truffles, but I never really think about where they've come from." he reflects. "Now when I use them, I will have a flash of pulling one out of the earth with my hands. I'll remember Kiki!"

Fresh truffles are available in season from Harrods, Harvey Nichols, or via mail order from Wild Harvest (0171-498 5397). The Connaught, Carlos Place, London W1 (0171-499 7070), special truffle menus continue until the end of March, pounds 60 to pounds 75. Club Gascon, 57 West Smithfield, London EC1 (0171-796 0600). Thanks to Renault UK and Eurotunnel

Truffle facts

The black or Perigord truffle (Tuber melanosporum) This is the only variety that can be called "truffle" in France, where it's nicknamed "the black diamond". Browny-black and veined with white, it has a powerful earthy taste, and can be eaten raw or used for cooking. In season from mid-November to mid-March.

The white or Alba truffle (Tuber magnatum) Yellowish in colour, they are at least as twice as expensive as their French cousins, and smell and taste even more powerful. Used in Italian cooking, where they tend to be eaten raw, sliced over food. Mid-September to late December.

Chinese truffles (Tuber himalayensis) Look similar to the Perigordian variety, but are significantly cheaper as they don't have the same powerful smell - or taste. Sometimes sold as garnish, but more often used to tart up factory-produced pates. November to February.

Summer truffles (Tuber aestivum) Black outside and white within, these grow in the same areas as the Perigordian truffle, but have less taste, and are occasionally passed off as the real thing by unscrupulous vendors who dye them black. May to November.