Boletus aereus

Description

Boletus aereus is a highly prized and much sought-after edible mushroom in the family Boletaceae. The fruit body has a large dark brown cap, which can reach 30 cm (12 in) in diameter. Like other boletes, B. aereus has tubes extending downward from the underside of the cap, rather than gills. The stem is tall and thick with partially covered a raised network pattern, or reticulation.

Mushrooms are mostly found during hot spells in summer and autumn, growing in mycorrhizal association with various broad-leaved trees and sclerophyllous shrubs, especially oak (Quercus), beech (Fagus), chestnut (Castanea), strawberry trees (Arbutus), treeheath (Erica), and rockrose (Cistus), showing a preference for acid soils. Roadsides and parks are common habitats. The cork oak in particular is an important symbiont, and the distribution of B. aereus aligns with the tree across Europe and North Africa. It is widely consumed in Spain, France, Italy, Greece, and generally throughout the Mediterranean.

It was included in the Red List of the Czech Republic as a vulnerable species.

Common names: Queen Bolete, Bronze Bolete, Black Piglet (Italian), Cèpe bronzé (French), Vasilikό (Greek), Bronskleurig eekhoorntjesbrood (Netherlands), Bronzeröhrling; Schwarzhütiger Steinpilz (German), Hřib bronzový (Czech Republic).

Mushroom Identification

1. Cap

   Medium and big size, from 5-30 cm (up to 12 in), fleshy, initially hemispheric then convex, finally flat, slightly overhanging margin, dry cuticle, finely velvety in the young specimen, without decorations, shows a bistre-brown coloring to assume a blackish “bronzy” tone, but also paler colorings can be found.

2. Hymenophore

   Very small tubules, long, adnate, thick, from white to yellow to olive-green with the ripening of the spores, easily loosening from the flesh of the cap, non-staining at the cut; small pores concolorous to the tubules, non-staining when touched.

3. Stem

   5-18 (20) x 5-12 cm, firm and sturdy, from obese to barrel-shaped, rounded, bulbous, of brown color, always paler than the cap, shows a concolorous to the cap small net, which extends mostly on the upper part.

4. Flesh

   White, firm, compact, and non-staining. Fungous smell and taste.

5. Chemical reactions

   The flesh, when in contact with strong bases such as the KOH (potassium hydroxide) and NaOH (sodium hydroxide), assumes a more or less brown coloring.

6. Spores

   Spindle-shaped, smooth, 13-16 x 4-5 µm.

7. Spore Print

   Olive-brown.

8. Habitat

   In woods, especially oaks, but also chestnut, purely xerophytic species, loving the warm and dry locations, much more infrequent in the north whilst it is more common in the Mediterranean and Apennine zones.

9. Season

   May to October.

10. Edibility

    Excellent; even raw, if consumed in small quantities.

Look-Alikes

1. Boletus reticulatus

   Is very similar to B. aereus, also occurring during the summer months under broad-leaved trees. It has a paler, often cracked cap and a usually paler stipe covered in a more elaborate and pronounced whitish reticulation, often extending to the stipe base.

2. Boletus pinophilus

   Occurs under conifers, mostly Pinus sylvestris, and has a reddish-brown cap. Microscopically, it can be separated by the more inflated, club- to spindle-shaped hyphal ends of the pileipellis.

3. Boletus edulis

   Occurs later in the season during lower temperatures, mostly under Picea. It has a paler viscid cap and a paler stipe with an acute white reticulation. Microscopically, it has gelatinized hyphal ends in the pileipellis.

4. Tylopilus felleus

   Poisonous, the stem is darker with a bitter taste.

5. Boletus betulicola

   Has a gray-nut-brown surface of the cap and grows with birches.

Fun Facts

Based on analyses of fruit bodies collected in Portugal, there are 367 kilocalories per 100 grams of bolete (as dry weight).

The macronutrient composition of 100 grams of dried bolete includes 17.9 grams of protein, 72.8 grams of carbohydrates, and 0.4 grams of fat.

By weight, fresh fruit bodies are about 92% water. The predominant sugar is trehalose (4.7 grams/100 grams dry weight; all following values assume this mass), with lesser amounts of mannitol (1.3 grams).

There are 6 grams of tocopherols, the majority of which is gamma-tocopherol (vitamin E), and 3.7 grams of ascorbic acid.

Boletus aereus Drying

Boletus aereus is especially suited for drying, a process that enhances its flavor and aroma. Like other boletes, the mushrooms can be dried by being sliced and strung separately on twine, then hung close to the ceiling of a kitchen. Alternatively, the mushrooms can be dried by cleaning with a brush (washing is not recommended), and then placed in a wicker basket or bamboo steamer on top of a boiler or hot water tank.

Once dry, they are kept in an airtight jar. They are easily reconstituted by soaking in hot, but not boiling, water for about twenty minutes; the water is infused with the mushroom aroma and can be used as stock in subsequent cooking. When dried, a small amount of the mushroom can improve the taste of less flavorsome fungi-based dishes.

History

In 1789 French mycologist Pierre Bulliard described this species and named it Boletus aereus. The epithet aerěus is the Latin adjective meaning "made with bronze or copper".

In 1886 Lucien Quélet transferred the species to the now-obsolete genus Dictyopus, which resulted in the synonym Dictyopus aereus, while René Maire reclassified it as a subspecies of B. edulis in 1937.

American mycologist Harry Thiers reported Boletus aereus from California in 1975; a taxonomic revision of western North American porcini boletes in 2008 formally established them as a separate species, Boletus regineus. These differ from B. aereus by nature of their more gelatinous cap skin (pileipellis), and belong in a different porcini lineage.

Boletus aereus is classified in Boletus section Boletus, alongside close relatives such as B. reticulatus, B. edulis, and B. pinophilus. A genetic study of the four European species found that B. aereus was sister to B. reticulatus. More extensive testing of worldwide taxa revealed that B. aereus was sister to a lineage that had split into B. reticulatus and two lineages that had been classified as B. edulis from southern China and Korea/northern China respectively. Molecular analysis suggests that the B. aereus/mamorensis and B. reticulatus/Chinese B. "edulis" lineages diverged around 6 to 7 million years ago.

Synonyms

1. Dictyopus aereus (Bull.) Quél. 1886

2. Boletus mamorensis Redeuilh 1978

3. Suillus aereus (Bull.) Kuntze 1898

4. Tubiporus edulis subsp. aereus (Bull.) Maire 1937

5. Boletus aereus var. aereus Bull.

6. Boletus aereus var. carne-dilute-sulfurea Bull.

7. Boletus aereus var. carne-nivea Bull.

8. Boletus aereus var. cepa (Thore) DC.

9. Boletus aereus var. cravetta (Bellardi) DC.

10. Boletus aereus var. leucoporus Pers.

11. Boletus aereus var. squarrosus De Rezende Pinto (1940)

12. Boletus edulis f. aereus (Bull.) Vassilkov (1966)

13. Boletus edulis f. aereus (Bull.) Vassilkov 1955

Photo sources:

Photo 1 - Author: GLJIVARSKO DRUSTVO NIS from Serbia (CC BY-SA 2.0)

Photo 2 - Author: Nacionalni park Una (CC BY-SA 4.0)

Photo 3 - Author: Enrico Tomschke (CC BY-SA 4.0)

Photo 4 - Author: OlexandrBohdanets (CC BY-SA 4.0)