Bluing

Why mushrooms turn blue: the chemistry

In "Mushrooms Demystified," David Arora explains this phenomenon in detail: bluing results from the rapid oxidation of specific organic acids (most often variegatic or xerocomic acid) by oxygen from the air. The reaction is catalyzed by the mushroom's enzymes. As soon as you break the cells open (by cutting or pressing the pores), oxygen gets inside and a blue pigment (a quinone) forms.

In the "Key to the Fungi of Ukraine," M.Ya. Zerova describes a whole host of fine edible mushrooms (mostly from the family Boletaceae) for which bluing is completely normal:

1. Lurid boletes (Suillellus luridus, Suillellus queletii): Large pored mushrooms with red or orange pores and yellow flesh that turns blue so quickly and so darkly when cut that it becomes almost black. Lurid boletes are tasty edible mushrooms (after boiling).

2. Bay bolete (Imleria badia): A popular autumn mushroom. Its yellowish pored "sponge" under the cap quickly turns blue-green if you press it with a finger. The flesh itself blues more weakly when cut.

3. Cornflower bolete (Gyroporus cyanescens): A mushroom with a felty, sand-colored cap whose white flesh instantly turns a bright cornflower blue. It is listed in the Red Data Book.

4. Xerocomus (Xerocomus): Most Xerocomus species turn slightly blue when cut.

The bluing devil's bolete

Roger Phillips warns that although bluing doesn't mean a mushroom is poisonous, there is one dangerous species among the bluing boletes — the devil's bolete (Rubroboletus satanas). It's a poisonous mushroom that looks like a stout porcini with a red stem and red pores. Its flesh also blues when cut, but rather weakly and slowly (unlike the edible lurid boletes, which blue instantly and very darkly).

Besides boletes, a weak bluing of the stem (in its lower part) occurs in some species of the genus Psilocybe, due to oxidation of the alkaloid psilocin.