The observation of a mushroom rapidly changing color from white or yellow to a vibrant blue upon being handled or cut is a phenomenon known as mycological bruising. This transformation occurs when the mushroom’s internal tissues are damaged, exposing specific compounds to atmospheric oxygen. The resulting blue pigment is not inherent to the flesh but is the final product of a rapid chemical reaction. This process serves as a remarkable visual marker often used by mycologists for field identification.
The Chemical Process of Mycological Bruising
The color change is fundamentally an oxidation reaction, triggered when the mushroom’s cell walls rupture and their contents mix with oxygen. This mechanism relies on two distinct chemical pathways depending on the fungal genus involved. In the genus Psilocybe, the reaction involves the psychoactive compound psilocybin, which is stable in its natural state. When tissue is injured, a phosphatase enzyme removes a phosphate group from psilocybin, converting it into the less stable compound psilocin.
Psilocin then undergoes a cascade of reactions catalyzed by a laccase enzyme. This enzyme promotes the oxidation of psilocin, leading to the formation of complex, multi-ringed molecules called quinoid psilocyl oligomers. These oligomers are responsible for the intense blue pigment, which is chemically similar in structure to indigo dye.
Boletaceae Family
A separate chemical mechanism causes blue staining in many species within the Boletaceae family, commonly known as boletes. These mushrooms contain derivatives of pulvinic acid, such as variegatic acid and xerocomic acid. When the flesh is damaged, these typically yellow or orange compounds are enzymatically oxidized upon contact with oxygen. This reaction transforms the pulvinic acid derivatives into blue-colored quinone methide anions. This oxidation is pH-dependent, and the resulting blue pigment can be reversed by applying an acidic solution.
Identifying Blue-Staining Mushroom Families
The blue-staining reaction is a trait shared across several distinct groups of fungi, providing a valuable clue for identification. The most well-known group exhibiting this feature is the genus Psilocybe, where the blueing reaction is directly linked to the presence of psilocybin compounds. The intensity and speed of the blueing in these species can vary, often appearing as a dark, indigo stain on the stem or cap where the fungus has been bruised.
The largest and most diverse group demonstrating this trait is the order Boletales, which includes many species of boletes. Within this group, the reaction is highly variable; some species stain a deep, immediate blue, while others show a slower, fainter blue-green change. The rapid blueing in boletes, such as the Cornflower Bolete (Gyroporus cyanescens), is a result of pulvinic acid oxidation and is entirely unrelated to the psychoactive compounds found in Psilocybe.
The phenomenon also appears in less common groups, such as some members of the genus Lactarius, known as milk-caps. The Indigo Milkcap, Lactarius indigo, is distinctive because it exudes a deep blue, milky liquid, called latex, when its gills or flesh are broken. In this case, the blue color is present in the latex itself and may darken slightly upon exposure, representing a third, distinct type of blueing mechanism.
The Relationship Between Blue Staining and Edibility
Contrary to popular belief, the color change to blue is not a definitive signal of whether a mushroom is safe to eat or toxic. Blue staining is a simple chemical reaction that occurs in species across the spectrum of edibility. Some blue-staining mushrooms are considered choice edibles, such as Lactarius indigo and a few species of boletes, provided they are properly identified and prepared. Some blue-staining boletes are safe to consume after thorough cooking, which often causes the blue pigment to disappear.
However, many blue-staining boletes are known to cause severe gastrointestinal distress, and some are considered genuinely poisonous. A general cautionary guideline for boletes is to avoid those that stain blue rapidly and also possess red-colored pores, as these features often coincide with toxicity. The blue-staining reaction is also a key indicator of psychoactive compounds in Psilocybe species, further complicating the issue. Relying on color change alone is insufficient and potentially hazardous for determining edibility. True identification requires careful examination of multiple features, including spore print, structure, habitat, and the specific nature of the bruising reaction.