Truffle

• 1. Introduction

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  In 1890, the truffle fields of France spread across 750 square kilometres and gave up 2,000 tonnes in a single year. A truffle is the fruiting body of a fungus that lives underground, and the best known belong to the genus Tuber. Nothing on the surface gives it away. Yet farmers planted whole landscapes for it.

  This documentary asks what a buried fungus wants from a tree, and how it sends its spores out into the world without help from wind or rain. It asks how people and animals find something they cannot see, and why it costs so much. It also follows the long and uncertain effort to grow truffles on purpose.

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• 2. An Underground Partnership

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  Beech, birch, hazel, hornbeam, oak, pine and poplar all share their roots with truffle fungi. The arrangement is a trade. The fungus hands over valuable nutrients, and the tree pays in carbohydrates. For the fungus the deal is not optional. Truffles belong to the ectomycorrhizal fungi, which partner with roots, and these cannot live in soil without a host. Many have lost the enzymes that would let them find carbon elsewhere. Truffle fungi, for example, cannot break down plant cell walls, so they cannot digest much dead plant litter. The dependence runs both ways. Geopora, Peziza and Tuber are vital in establishing oak communities.

  The Hartig net is where most of the trading happens. It is a web of fungal threads running between the cells of a root. It starts with a dormant spore. The spore waits until chemicals leaking from a host root trigger germination. Fungal threads, called hyphae, then grow toward the roots. At a root tip they wrap the outside in a sheath called a mantle, a feature unique to these fungi. Next they push between the cells of the root cortex and build the Hartig net. From there the threads spread to other root tips until the whole root system is colonised. Only after gathering enough resources does the fungus form a fruiting body. Its growth tracks the rising photosynthesis of spring as trees come into leaf. In return for sugars, the tree receives potassium, phosphorus, nitrogen and sulfur, along with iron, copper, zinc and chloride.

  Phosphorus, nitrate, ammonium, iron and magnesium all reach a tree through its fungal partners. Mycorrhizal fungi are far smaller than fine roots, so their greater surface area lets them search the soil more thoroughly. Many form mats in the upper soil layers. These mats hold more carbon and fixed nitrogen than the surrounding ground. Because they act as nitrogen sinks, less of that nutrient washes away. They also hold organic matter in place and help prevent erosion. Bacteria feed on exudates from the mycelium and spread through the soil around it. Mites and other microscopic arthropods eat the mycelium directly and release nutrients that other organisms can take up. Truffles have also been suggested to grow near the orchids Epipactis helleborine and Cephalanthera damasonium, though not always.

  In the American Southwest, the pinyon pine Pinus edulis shows how much a fungal partner can matter. It is a foundation species of the region's arid lands, and it associates with Geopora and Rhizopogon. In dry country these fungi help their host withstand drought. As global temperatures rise, severe droughts become more common, and mortality of the pinyon has increased. Whether a seedling meets compatible fungus can decide whether it establishes at all. The fungi are therefore likely to matter more for the pinyon's survival as the climate keeps changing.

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• 3. Spores and Animals

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  In North America, the flying squirrel Glaucomys sabrinus lives largely on truffles. It is a clear case of an animal that depends on a fungus as its main food. The squirrel is well suited to the hunt. It relies on a refined sense of smell, on visual clues and on a long memory of places where truffles were plentiful. Its habits form a three-way partnership among animal, fungus and plant. By spreading truffles, the squirrel indirectly shapes how well mycorrhizal plants do.

  Most truffle fungi make two kinds of spores. Asexual spores, called conidia, are cheap and quick to produce and can travel when soil is disturbed. Sexual spores, called ascospores, cost far more energy because the fungus must build a large fruiting body to hold them. The ascospores sit inside sac-like structures called asci within that fruiting body.

  Because a truffle fruits underground, wind and water cannot carry its sexual spores anywhere. Nearly all truffles therefore depend on animals that eat fungi. The situation resembles the way flowering plants use fruit to spread seeds. When the ascospores are fully developed, the truffle releases volatile compounds that draw animals in. The spores must then survive a trip through a digestive tract. Their thick walls of chitin help them endure the gut. Birds, deer and rodents such as voles, squirrels and chipmunks all serve as carriers.

  The Oregon white oak, Quercus garryana, depends on this service. Many trees rely on animals to inoculate isolated individuals with the right fungus. The acorns of Q. garryana may be carried to new territory that lacks the fungi the seedlings need to establish. A squirrel that buries an acorn may therefore also be planting a partnership.

  Flies of the genus Suillia can also detect the scent of buried truffles. They lay their eggs above the truffles to give their young something to eat. At ground level, these flies can sometimes be seen hovering over the spot where a truffle lies hidden.

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• 4. Following the Scent

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  In 1481, the papal historian Bartolomeo Platina recorded that the sows of Notza had no equal at hunting truffles. He added that the animals should be muzzled so they would not eat the prize. Because truffles grow underground, people often find them with the help of an animal that has a refined sense of smell. Such an animal is sometimes called a truffler. Pigs were the traditional choice.

  A compound resembling androstenol, the sex pheromone in boar saliva, was once thought to explain the sow's behaviour. Her natural search for truffles and her wish to eat them were both put down to it. The sweaty and musky smell of truffles is similar to androstenol, which also occurs in humans. Studies in 1990 found something simpler. The compound that both truffle pigs and dogs actually recognise is dimethyl sulfide.

  Sulfur compounds are the backbone of the truffle's smell, and every species has them. Dimethyl sulfide is joined by its disulfide and trisulfide relatives. The white truffle Tuber borchii is marked by 2-methyl-4,5-dihydrothiophene. The compound 2,4-dithiapentane turns up in all species but is most typical of the white truffle Tuber magnatum. Some very aromatic white truffles are so pungent that cutting them can irritate the eye.

  Other ingredients add further layers. The compounds 1-octen-3-ol and 2-octenal carry a typical fungal odour, and mature Tuber borchii makes the former from linoleic acid. The compound 2-phenylethanol is also common in baker's yeast. White truffle mycelia release ethylene, which affects the root architecture of the host tree. Bacteria living inside the truffle body make thiophene compounds, and 3-methyl-4-5 dihydrothiophene is the most abundant of these. Species and varieties are told apart by their relative amounts of sulfides, ethers and alcohols. Researchers have studied the volatile profiles of seven black and six white species.

  Italy banned the use of pigs for truffle hunting in 1985. Pigs dug so hard that they damaged the truffle mycelia, and output in the affected area dropped for some years. Dogs offered a way out. They have no strong urge to eat truffles, so they can be trained to find the fruiting body without digging it up. Pigs have the skill built in, but they tend to eat what they find.

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• 5. Kinds of Truffle

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  The black Périgord truffle, Tuber melanosporum, takes its name from the Périgord region of France. It is the second most commercially valuable species. It associates with oaks, hazelnut, cherry and other deciduous trees, and it is harvested in late autumn and winter. Its genome sequence was published in March 2010.

  The black summer truffle, Tuber aestivum, is found across Europe and prized in cooking. The burgundy truffle, labelled Tuber uncinatum, is the same species. It is harvested in autumn until December and has darker, aromatic flesh.

  Tuber magnatum, the white truffle or tartufo bianco, is the high-value star of northern Italy. It is found mainly in the Langhe and Montferrat areas of Piedmont, and most famously in the countryside around the cities of Alba and Asti. A large share of Italy's white truffles also comes from Molise. The whitish truffle, Tuber borchii, is a similar species native to Tuscany, Abruzzo, Romagna, Umbria, the Marche and Molise. It is reportedly less aromatic than the Piedmont kind, though those from Città di Castello are said to come quite close. A less common species is the garlic truffle, Tuber macrosporum.

  In the Pacific Northwest of the United States, several truffles are harvested for fun and for sale. The best known are Leucangium carthusianum, the Oregon black truffle, Tuber gibbosum, the Oregon spring white truffle, and Tuber oregonense, the Oregon winter white truffle. Kalapuya brunnea, the Oregon brown truffle, has also been harvested commercially. The Oregon white truffle is increasingly exported because of its high quality. Oregon marks its truffle tradition with a festival that combines culinary shows and wine tastings.

  Farther south, the pecan truffle, Tuber lyonii, grows with pecan trees across the Southern United States. Pecan farmers once found these truffles among their nuts and threw them away as a nuisance. Chefs who experimented with them agree that "they are very good and have potential as a food commodity". They now sell for about $160 a pound and have appeared in some gourmet restaurants.

  More than one hundred other genera of fungi also count as truffles, among them Geopora, Peziza, Choiromyces and Leucangium. They belong to the class Pezizomycetes and the order Pezizales. Terfezia and Tirmania, of the family Terfeziaceae, are known as the desert truffles of Africa and the Middle East. Pisolithus tinctorius, once eaten in parts of Germany, is sometimes called the Bohemian truffle. Geopora cooperi, also called the pine truffle or fuzzy truffle, is edible.

  Some truffle-like fungi sit outside Pezizales altogether. Rhizopogon belongs to the Basidiomycota and the order Boletales, a group that normally makes mushrooms. It matters in coniferous forests, where it associates with pines, firs and Douglas fir, and nurseries use it to inoculate conifer seedlings and to support reforestation. Hysterangium, from the order Hysterangiales, forms mycelial mats that may cover 25-40% of the forest floor in Douglas fir forests. These mats make up a significant share of soil biomass and act as nitrogen sinks. Glomus is an arbuscular mycorrhizal fungus of the phylum Glomeromycota. It has low host specificity and partners with hardwoods, forbs, shrubs and grasses across the Northern Hemisphere. The genus Elaphomyces is often mistaken for a truffle.

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• 6. Deep Time

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  The fungal subphylum Pezizomycotina, home to the order Pezizales, is about 400 million years old. Within Pezizales, the habit of fruiting underground evolved independently at least fifteen times. The families Tuberaceae, Pezizaceae, Pyronemataceae and Morchellaceae all contain lineages of truffle fungi. Analysis of their family trees also shows that the ectomycorrhizal way of life arose repeatedly in unrelated fungi.

  The oldest ectomycorrhizal fossils date to the Eocene, about 50 million years ago. They come from the Princeton chert site in the Okanagan Highlands, where the specimens are preserved in place by permineralization. Their rarity suggests that the soft bodies of these fungi seldom fossilize. Molecular clock estimates point much earlier, to roughly 130 million years ago. The ancestors of Geopora, Tuber and Leucangium originated in Laurasia during the Paleozoic era.

  Most underground fruiting bodies seem to have descended from mushrooms that grew above ground. Over time the stalks and caps shrank, and the caps came to enclose the reproductive tissue. The truffle habit arose in the Ascomycota, the Basidiomycota and the Glomeromycota, many times over. Rhizopogon and Hysterangium, both basidiomycetes, play roles similar to those of truffle-forming ascomycetes. Rhizopogon likely arose from an ancestor shared with Suillus, a genus that makes ordinary mushrooms. Studies suggest that selection for underground fruiting bodies, in both groups, took place in water-limited environments.

  In 2008, researchers used internal transcribed spacers in nuclear DNA to study the genus Tuber. They found five major clades, named Aestivum, Excavatum, Rufum, Melanosporum and Puberulum. A 2010 study used 28S large-subunit rRNA and expanded the count to nine. The Magnatum and Macrosporum clades were separated from Aestivum, and Spinoreticulatum was split from Rufum. The Gibbosum clade stood apart from every other clade, and it contains the Oregon spring white truffle.

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• 7. Taming the Truffle

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  Jean-Anthelme Brillat-Savarin wrote in 1825 that "the most learned men have sought to ascertain the secret and fancied they discovered the seed. Their promises, however, were vain, and no planting was ever followed by a harvest." He suggested that this failure might be for the best, since truffles are valued partly for their dearness and might be less esteemed if cheaper. His claim was already out of date. Attempts at trufficulture, as the French call truffle growing, had succeeded as early as 1808.

  In 1808, Joseph Talon of Apt, in the Vaucluse in southern France, collected seedlings at the foot of oaks known to host truffles and transplanted them. Some sources now give priority to Pierre II Mauléon, who lived from 1744 to 1831 in Loudun, in western France. Around 1790 he began sowing acorns from truffle-bearing trees in chalky soil. He had seen an "obvious symbiosis" among the oak, the rocky soil and the truffle, and he tried to recreate it. Truffles turned up around the new oaks years later.

  In 1847, Auguste Rousseau of Carpentras planted 7 hectares of oaks grown from acorns gathered under truffle-producing trees. He obtained large harvests and won a prize at the 1855 World's Fair in Paris. Others copied him in France and Italy. In the late 19th century, an epidemic of phylloxera destroyed many vineyards in southern France, and another epidemic killed most of the silkworms, leaving mulberry fields useless. Truffle growing became an important income for people hit by both disasters. The calcareous, exposed vineyard soils suited the crop well.

  From the 19th century to today, truffle production fell by 97-99%, to just 20-50 tonnes a year. The Industrial Revolution, rural flight and the wars of the 20th century all thinned the countryside. World War I alone mobilised 65% of the agricultural workers of the Lot region. Knowledge of truffles, soil and seasons left with them. Sheep and shepherds disappeared too, along with the pruning of trees for feed and fuelwood. Former plantations closed over into forest and stopped producing. Markets once sold two to six tonnes in a good week, with a twice-weekly market at Martel in the Lot. Today only Lalbenque and Limogne keep weekly truffle markets. Prices climbed until a food of the middle class became a luxury.

  The situation changed in the late 1960s and early 1970s, when researchers in France and Italy established mycorrhizas with truffle spores. From the 1980s, plantations began to make up for part of the loss in wild truffles. They now exist in France, Italy, Spain and Australia, among other countries. About 80% of the truffles produced in France come from specially planted groves. Investors in many places are using controlled irrigation to get regular and resilient harvests.

  Growers must check the quality of the mycorrhizal seedlings carefully. Between 7 and 10 years pass before the trees build their fungal network and begin to produce. A full soil analysis guards against contamination by other dominant fungi. An irrigated plantation fenced against wild boars can cost up to €10,000 per hectare. Farmers who neglect soil or seedling conditions face a high risk of failure.

  The first black truffles in the Southern Hemisphere were harvested at Gisborne, New Zealand, in 1993. In July 2012, a Waipara truffle farm found New Zealand's first burgundy truffle. It weighed 330 grams, and the farm owner's beagle found it. In Tasmania, the first Australian truffles were harvested in 1999 after eight years of work to inoculate trees with the fungus. That success and the value of the crop encouraged a small industry. Production has spread into the colder regions of Victoria, New South Wales and Western Australia. In 2014, Truffle Hill at Manjimup, Western Australia, harvested over 5000 kilograms. In June of that year, a grower at Robertson in the Southern Highlands of New South Wales dug up Australia's largest truffle. It was a French black Périgord weighing 1.172 kilograms and valued at over $2,000 per kilogram.

  Périgord truffles were first farmed in Tennessee in 2007. At its peak, in the 2008-2009 season, one farm produced about 200 pounds. Then Eastern filbert blight almost wiped out the farm's hazel trees by 2013, and production fell until the business essentially ended. The blight likewise destroyed the orchards of other promising farms in East Tennessee. Newer farms started in California, North Carolina, Oregon and Arkansas. Meanwhile, the Appalachian truffle, Tuber canaliculatum, was being developed as a possible new market.

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• 8. Table and Legend

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  The Latin word tūber, meaning a swelling or lump, is the root most sources give for the word truffle. It passed through the Vulgar Latin tufera and then into other languages by way of Old French dialects.

  The first mention of truffles appears in neo-Sumerian inscriptions about the eating habits of their Amorite enemies. Theophrastus wrote about them in the 4th century BCE. In classical times their origin was a puzzle. Plutarch and others believed lightning, warmth and water in the soil produced them, while Juvenal credited thunder and rain. Cicero called them children of the earth, and Dioscorides took them for tuberous roots. Rome and Thracia recognised three kinds: Tuber melanosporum, T. magnificus and T. magnatum.

  The Romans, however, relied on a different fungus called terfez, a desert truffle. It came from Lesbos, Carthage and especially Libya, where the coastal climate was less dry in ancient times. Its flesh is pale and tinged with rose. Terfez has little flavour of its own, but it absorbs the flavours around it. Cooks used it as a carrier for flavour, which suited a cuisine full of spices.

  Truffles were rarely used in the Middle Ages. During the Renaissance they regained popularity in Europe and were honoured at the court of King Francis I of France. In the 1780s they were popular in Parisian markets, shipped in season from the truffle grounds, where peasants had long enjoyed them. Brillat-Savarin noted that they were so costly they appeared only on the tables of great nobles and kept women. They were sometimes served with turkey.

  Truffles often sell for thousands of dollars per pound because they are rare and intensely savory, so cooks use them sparingly. They are sold fresh or preserved, typically in a light brine. Their aroma compounds blend well with fats such as butter, cream, cheese, avocado and coconut cream. Heat drives off the aroma quickly, so truffles are usually served raw, shaved over warm and simple foods such as buttered pasta or eggs. Thin slices can go into meats, under the skin of roast fowl, or into foie gras, pâtés and stuffings. Some specialty cheeses contain them, and they flavour truffle salt and truffle honey. Chefs once peeled truffles. Most restaurants now brush them and shave or dice them with the skin on, and some stamp out discs of flesh and use the skins for sauces.

  Truffle oil is a cheaper, convenient stand-in for the real thing. Some products sold under that name contain no truffle at all. Others hold pieces of inexpensive varieties with no culinary value, added for show. The vast majority is oil artificially flavoured with 2,4-dithiapentane. Oils infused with real truffle list the scientific name among the ingredients.

  More aromatic molecules dissolve in alcohol than in oil, so truffle vodka can carry a more complex and accurate flavour. Many commercial producers use 2,4-dithiapentane regardless. It has become the dominant flavour that consumers, who know oils but not fresh truffles, link with truffles. Most Western nations do not require ingredient labels on spirits, so buyers often cannot tell whether artificial flavouring was used.

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