Flesh color change

Darkens

When mycologists describe flesh as "darkening" or "browning" when cut, they usually mean a slow, not very contrasting color change. This differs from dramatic "blackening" (as in birch boletes) or instant "bluing." Darkening most often means brown, rusty or dirty gray spots forming on contact with oxygen.

24 species in the database.

The darkening process and the mushrooms that show it

In her "Key to the Fungi of Ukraine," M.Ya. Zerova often uses the phrase "flesh browns where broken" to describe reactions in mushrooms with brittle flesh, especially in the brittlegill family (Russulaceae).

David Arora and Roger Phillips describe several classic examples of mushrooms whose flesh simply darkens or browns when cut:

1. Ugly milk cap (Lactarius turpis): This mushroom (often called the "blackie") starts out with an olive-green cap, but when damaged, the flesh or the spots where it was touched slowly turn brown and later darken almost to black.

2. Crab brittlegill (Russula xerampelina): A distinctive mushroom with a fishy smell. If you cut or firmly scratch its white stem, it will gradually turn brownish (darken). This color change is an important trait that confirms the species identification.

3. Bitter bolete (Tylopilus felleus): The flesh of this inedible porcini lookalike can sometimes brown slightly when cut or take on a dirty brown tint (and the tube layer turns pink).

Darkening from age and rot

Apart from genetically built-in enzymatic reactions, darkening (browning) of a cut surface is a completely universal sign of aging in any mushroom. If a porcini or an Agaricus mushroom is too old or waterlogged, its once-white flesh quickly turns dull, rusty or dark brown when cut. This signals the start of bacterial breakdown of proteins. Eating such mushrooms is not recommended.

Questions and answers

Why do the gills of Agaricus mushrooms darken over time?

This has nothing to do with enzymatic oxidation of the flesh. The gills of Agaricus mushrooms darken from pink to chocolate brown or black solely because the spores are ripening. Agaricus spores are dark, and as they are released en masse, they color the gills. It is a sign that the mushroom is mature.

If a mushroom has darkened where cut, has it gone bad?

Not always. If you picked a fresh, springy crab brittlegill and the cut on its stem turned brownish after 15 minutes, that is its natural property. But if the flesh is limp, watery and darkens at the same time as a sour smell appears, the mushroom has gone bad.

How do you stop the flesh of picked mushrooms from darkening?

To keep freshly picked mushrooms (especially Agaricus mushrooms or brittlegills) from browning and losing their good looks in the basket or in the kitchen, don't soak them in water for long. Cooks often drop sliced mushrooms into water with added citric acid or vinegar: the acidic environment neutralizes the oxidizing enzymes and stops the darkening.

Sources

1.Зерова М.Я. та ін. Визначник грибів України. 1979
2.Roger Phillips. Mushrooms (Roger Phillips)
3.David Arora. Mushrooms Demystified
4.Index Fungorum