Truffles, the mysterious hypogeal fungi. The ABC

Truffles – mysterious hypogeal fungi that grow spontaneously – are one of the delicacies characterizing agri-food productions in Italy and France, as well as in other countries. The ABC follows.

Premise. Forest and desert truffles

Most truffles renowned for culinary use come from temperate northern woods and forests in Europe, Australia, New Zealand, Asia and North America and belong to the genus  Tuber.

This analysis focuses on  Tuber magnatum Picco (precious white truffle),  Tuber melanosporum (precious black truffle),  Tuber borchii Vittad (bianchetto truffle) and  Tuber aestivum (summer truffle).

Other hypogeal fungi, desert truffles, grow in arid and semi-arid areas (e.g. Syria, Iraq, Kuwait, Saudi Arabia, Morocco, Egypt, Tunisia, South Africa) and belong to the genera  Terfezia e Tirmania.

From Babylon and ancient Rome to the present day

The appearance of truffles on Earth is attributed to over 100 million years ago, their first inscriptions on Babylonian clay tablets date back to 3700 BC. (1)

Ancient Rome celebrated these precious hypogeal fungi at sumptuous banquets, as a sign of ostentatious wealth and opulence of patrician families.

Over the centuries  the growing demand for truffles has stimulated the development of domestication practices and scientific research, to increase their availability.

Botany

The forest truffle is the fruiting body of edible symbiotic fungi of the genus  Tuber, class of Ascomycetes. Truffles – hypogeous fungi, since they complete their entire life cycle underground – consist of:

• a fertile part (hymenophore), immersed in
• a more or less compact structure (gleba), in turn enclosed by
• an external membranous, protective and sterile layer (peridium). (2)

The various species of truffle are distinguished by a series of morphological characteristics such as shape, size, peridium color, appearance and color of the gleba, scent.

The  terroir – type and quality (pH, moisture and nutrients) of the soil – plays an essential role in determining the development of truffles, their size and composition. (3)

Mutualistic symbiosis

The life cycle and the fruiting of truffles, therefore classified as obligate ectomycorrhizal fungi, depend on the mutualistic symbiosis with the roots of a host plant. Which are  Quercus pubescens Willd.,  Quercus ilex L.,  Quercus cerris L.,  Quercus robur L.,  Corylus avellana L.,  Tilia cordata Mill.,  Ostrya carpinifolia Scop.,  Carpinus betulus L.,  Pinus pinea L.,  Pinus halepensis L.,  Pinus nigra L..

The symbiotic bond with these plants offers organic matter necessary for the development of truffles. At the same time, thanks to their high capacity to extract nutrients from the soil, they increase accessibility also for the plant, which thus improves growth and performance productive. (4)

Nutritional properties

The nutritional properties  of truffles are appreciated for their richness in fiber, proteins and amino acids, mineral salts (potassium, magnesium, calcium and phosphorus), vitamin C. (5)

Various species – such as  T.eastivum, T.borchii, T.melanosporum  e T. magnatum – are also characterized by the presence of unsaturated fatty acids such as oleic acid and linoleic acid.

The contribution of oleic acid has been associated with the reduction of blood cholesterol levels, cardiovascular prevention, and inhibition of cancer progression. (5)

Potential health benefits

The potential benefits of truffles for health have been explored in a recent  scientific review  (Lee  et al., 2020) which highlighted its anti-tumor, antioxidant, antibacterial, anti-inflammatory and hepatoprotective capabilities. (6)

The bioactive compounds (terpenoids, phenols) contained therein, along with polysaccharides and fatty acids, have also shown the potential efficacy of truffles in enhancing the effects of various drugs such as antidepressants, immunostimulants, cholesterol reducers.

Some extracts  of methanol from  T.aestivum e T.magnatum have then revealed significant cytotoxic effects, in two in vitro experimental studies, against different tumor cell lines. (6)

Organoleptic properties

The aromas unmistakable of truffle are linked to volatile organic compounds (volatile organic compounds, VOCs), which play an active role in regulating symbiosis and interactions with other organisms and are released throughout the entire life cycle:

• only two VOCs (2-methylbutanal, 3-methylbutanal) are present in all truffle species,
• over 200 others (e.g. alcohols, aldehydes, ketones, organic acids, and sulfur compounds) are species-specific,
• The chemical synthesis of natural-identical aromas is the basis of food frauds on truffles. (7)

Scents and aromas

The scents  are influenced by the maturation stage of the fruiting body, the  terroir, the microbial flora inhabiting the tissues of the ascocarp, the intraspecific genetic variability:

• the summer truffle (T. aestivum Vittad.) presents notes of sulfur, leather, metal, cooked potato,
• T. magnatum Picco – the white truffle, the rarest and most expensive species, highly appreciated in Italian and French cuisine – also recalls hints of garlic and cheese,
• The  T. melanosporum Vittad., black truffle, has a scent similar to that of a damp forest with hints of radish, hazelnut, and chicory,
• T. brumale Vittad instead has a characteristic musky odor, accompanied by earthy notes.

Preservation

The research  on the most suitable preservation methods to maintain the organoleptic properties of truffles has considered advantages and disadvantages of freezing, refrigeration, hot air drying, modified atmosphere, freeze-drying, irradiation.

Freeze-drying  – although definitely more expensive – has been found more effective than other techniques in preserving many VOCs. Freezing alone, as well as pasteurization and other heating methods, drastically alter the organoleptic properties of truffles.

Market

Italy and France are the leading countries in truffle production, processing and marketing, while Europe remains the top market globally. (8)

The market is characterized by relatively stable demand, typical of luxury goods, and conversely volatile prices. With significant peaks for the most prized species in years of scarce production. (9)

In Italy coexist a local market, linked to the food and wine traditions of the regions of origin, and an international market focused onexport. The largest quantities of truffles in Italy are harvested in Umbria, followed by Abruzzo and Marche. (10)

Irene Giunta and Dario Dongo

Cover image from scientific study cited in note 6 (Lee et al., 2020). Fig. 2 – Symbiotic life cycle of truffles

Notes

(1) National Truffle Study Center (2002). The main species of Tuber. tuber.it

(2) Signorini D., Valli O. The truffle, habitat, research and cultivation techniques of the white and black. Giunti Demetra, Milan, 1990. ISBN: 8871221400

(3) Streiblová E., Gryndlerová H., Gryndler M. (2011). Truffle brûlé: an efficient fungal life strategy. FEMS Microbiology Ecology. Volume 80, pp 1-8. doi: 10.1111/j.1574-6941.2011.01283.x

(4) Alessandro Deanesi. Truffle, biology and characteristics.  https://www.biopills.net/truffle/  Biopills. 25.3.20

(5) Nina Šiškovič, Lidija Strojnik, Tine Grebenc, Rajko Vidrih, Nives Ogrinc (2021). Differentiation between species and regional origin of fresh and freeze-dried truffles according to their volatile profiles.  Food Control, Volume 123  https://doi.org/10.1016/j.foodcont.2020.107698

(6) Heayyean Lee, Kyungmin Nam, Zahra Zahra, Muhammad Qudrat Ullah Farooqi (2020). Potentials of truffles in nutritional and medicinal applications: a review.  Fungal Biol Biotechnol 7, 9  https://doi.org/10.1186/s40694-020-00097-x

(7) Dario Dongo. Truffle aroma and Asian 'truffle', watch out for the scam. GIFT (Great Italian Food Trade). 27.11.21

(8) Laureti M. The black truffle and its legal relations in the Spoleto and Nursia areas. Proceedings of the first international truffle congress in Spoleto, 24-25.5.68 (Amoretti Printing, Parma, 1968)

(9) Urbani G. (1995). Rational truffle cultivation: analysis of an investment. The Agrarian Informer. Volume 31, pp. 29-32

(10) Boutahir S. (2013). New biotechnologies for the production of mycorrhizal plants with trufflesPhD thesis at the University of Bologna