Tuber melanosporum


Tuber melanosporum, called the black truffle, Périgord truffle or French black truffle, is a species of truffle native to Southern Europe. It is one of the most expensive edible mushrooms in the world.

Taxonomy

Italian naturalist Carlo Vittadini described the black truffle in 1831.

Description

External characteristics

The round, dark brown fruiting bodies have a black-brown skin with small pyramidal cusps. They have a strong, aromatic smell and normally reach a size of up to. Some may be significantly larger, such as a black truffle found 2012 in Dordogne with a mass of.
Their flesh is initially white, then dark. It is permeated by white veins which turn brown with age. The spores are elliptical and measure about 22–55 µm by 20–35 µm. They are dark brown and covered with large spikes.

Aroma

The fruiting bodies of the black truffle exude a scent reminiscent of undergrowth, strawberries, wet earth or dried fruit with a hint of cocoa. Their taste, which fully develops after the truffles are heated, is slightly peppery and bitter. If stored at room temperature, the aromatic compounds dissipate, while storage around the freezing point leads to an increased synthesis of these compounds.
The volatile compounds that contribute to the aroma and are developed by the fruiting bodies include 2-methyl-1-butanol, isoamyl alcohol, 2-methylbutyraldehyde and 3-methylbutyraldehyde, as well as traces of sulfur compounds. One of these, dimethyl sulfide, is what attracts truffle dogs, truffle hogs and truffle flies to the fruiting bodies. Several species of yeast, which produce part of the aromatic compounds, have been isolated from Tuber melanosporum and Tuber magnatum.

Genome

The genome of the black truffle was published in 2010. It contains 125 million base pairs, 58% of the genome consists of transposable elements, and the genome contains only 7500 identified protein-encoding genes. During symbiosis, genes involved in the decomposition of plant cell walls and lipids are induced. This indicates that black truffles decompose the cell walls of their host plants at the beginning of the symbiosis.

Identification

The black truffle is morphologically very similar to the commercially less valuable Chinese truffle. To avoid fraud or misidentifications in commerce, a RFLP genetic test has been developed to distinguish the two species. Externally, they can be distinguished by their skin, which is smoother and dark red or dark brown in the Chinese truffle. Two other similar truffle species are the summer truffle and the winter truffle, whose flesh is of a lighter color.

Ecology

Development and phenology

Black truffles grow at a depth of to as ectomycorrhizae, preferably in loose calcareous soil, close to the roots of their plant symbionts. These include holm oaks, French oaks, hazel, cherry and other deciduous trees. The symbiosis of holm oak saplings and black truffles has been shown to improve photosynthesis and root growth in the plant.
Black truffles suppress the growth of plants around their symbiont, creating the impression of a burnt area around it. They do so by parasitizing the roots of other plants, which may lead to necrosis of the root bark and the death of the parasitized plant. Moreover, part of the scent emitted by the truffles may limit the growth of other plants through oxidative stress.
In the Northern Hemisphere, the fruiting bodies develop from April to June, and are harvested from November to March.

Reproduction

s and the larvae of the truffle fly, which eat the fruiting bodies, aid in the distribution of the species by excreting the indigestible spores. Their excrement likely also serves to fertilize the spores. Black truffles are sometimes found together with winter truffles, which aid the growth of black truffles in wet soils.
Until 2010, all truffle species were thought to be homothallic, that is, capable of sexual reproduction from a single organism. Subsequent research indicated that black truffles are heterothallic, that is, sexual reproduction requires contact between the mycelia of different mating types. If a tree is surrounded by mycelia of different mating types, eventually one type becomes predominant. This is relevant for the operation of truffle plantations: To improve production, planters must ensure that neighboring trees harbor mycelia of different mating types, such as by inoculating new saplings with a mycelium of a particular type.

Habitat and cultivation

The natural habitat of the black truffle includes various regions in Spain, France, and Italy. These are presumably the areas where the host plants found refuge during the last Ice Age. In these areas, the search for black truffles and their cultivation is a tradition going back more than 200 years. Truffles are still collected manually in a traditional way in large areas of natural forests. For example, the county of Alto Maestrazgo has an ideal ground with suitable conditions for cultivating truffles. Albocàsser, Atzaneta, Culla, or Morella are just some of the villages in this region where one can find black truffles in large amounts.
Nevertheless, this form of recollection is increasingly affected by climate change and since 2010, a significant drop in productivity has occurred in naturally producing forests. In exchange, cultivated areas are increasingly popular, and in central Spain, several thousands of hectares are dedicated to truffle cultivation. Some experiments have also been conducted in burnt areas, with promising results, as legally no need to ask for a land-use change exists when planting truffles, as it can be considered as forest land.
Black truffles are now also cultivated in Australia, New Zealand, Chile, North America, South Africa, and Wales. Cultivation involves the planting of, for example, hazel trees whose roots are inoculated with truffle mycelium. The first fruiting bodies can be harvested about 4-10 years after planting the trees.

Production

France accounts for some 45% of the world production of black truffles, Spain for 35% and Italy for 20%. Smaller amounts are produced in Slovenia, Croatia, and the Australian states of Tasmania and Western Australia. In 2005, black truffles were found in Serbia. About 80% of the French production comes from southeast France: upper Provence, part of Dauphiné, and part of Languedoc. About 20% of the production comes from southwest France: Quercy and Périgord. The largest truffle market in France is at Richerenches in Vaucluse. The largest truffle market in southwest France is at Lalbenque in Quercy. These markets are busiest in January, when the black truffles have their highest perfume.
Production has considerably diminished during the 20th century, dropping to around 20 metric tonnes per year, with peaks around 46 tonnes in the best years. By comparison, in 1937, France produced around 1,000 metric tonnes of black truffles.
The following table shows the production in the EU of T. melanosporum in Spain, France, and Italy. Production data are in metric tons, and country weights in percentage, and come from the Groupe Européen Truffe et Trufficulture, an association of the main producers in Europe.
YearsSpainFranceItalyEU% of average yearSpain vs EUFrance vs EUItaly vs EU
1990/1991301755283583310
1991/1992102053556295714
1992/199323313579040545
1993/19949222335227676
1994/199541230467392665
1995/199620192564102313039
1996/199725502095151265321
1997/1998803024134213602218
1998/199971442540285616
1999/200035401085135414712
2000/20016354457113789
2001/2002201554063503813
2002/200340352095151423721
2003/20047962235324127
2004/20052227105994374617
2005/2006141583759384122
2006/20072028105892344817
2007/2008252685994424414
2008/200914582092146156322
2009/201093284978186516
2010/20111837,21267,2107275518
2011/201214,542,3864,8103226512
2012/20131538,22073,2116205227
2013/2014455030125198364024

As the data show, France has been the main producer of black truffles in the last decade and a half, but is rapidly challenged by Spain, where regions have made use of the EU-funded Rural Development Programme to subsidise cultivated plantations. This is particularly visible in the Teruel province of the Aragón region, where the black truffle represents the first and main economic activity, especially since 2010-2011 when many plantations opened under the last Rural Development Programme 2000-2006 came into production phase.

Commerce and use

With a price of about 1,000 to 2,000 euros per kilogram, black truffles are the second-most expensive truffles after white truffles, and one of the most sought-after edible mushrooms in the world.
In cooking, black truffles are used to refine the taste of meat, fish, soups, and risotto. Unlike white truffles, the aroma of black truffles does not diminish when they are heated, but becomes more intense.
In Spain, where black truffles are often used in cooking, one truffle dish is known as blanched eggs, or huevos escaldados in Spanish.

Physiology

Truffles contain the endocannabinoid anandamide and the major metabolic enzymes of the endocannabinoid system. The AEA content increases in the late stages of truffles’ development. AEA and ECS metabolic enzymes may have evolved earlier than endocannabinoid-binding receptors, and AEA might be an ancient attractant to truffle-eating animals, which are well-equipped with endocannabinoid-binding receptors.