Flesh color change

For a mycologist, cutting or breaking a mushroom isn't just a way to check it for worms. It's a genuine chemical test. How quickly the flesh changes color when exposed to air, and what color it turns, is often the deciding clue for identifying a species.

Why does the flesh change color?

M.Ya. Zerova, in her “Key to the Fungi of Ukraine,” explains that flesh color change (oxidation) is an enzymatic process. Mushroom cells contain specific phenolic compounds and enzymes (tyrosinase, laccase). As long as the mushroom is intact, these substances are kept apart. But the moment you break the cap or cut the stem, oxygen from the air instantly sets off an oxidation reaction.

David Arora, in “Mushrooms Demystified,” stresses that the speed and intensity of this reaction are unique to different species:

- Some mushrooms react instantly (for example, the lurid bolete turns blue within seconds).

- Others change color slowly and in stages (for example, the red-capped scaber stalk first turns pinkish, then purple, and later black).

- Still others lack the necessary enzymes altogether, and their flesh stays unchanged.

How to test for color change properly

Roger Phillips recommends using several methods for an accurate check:

1. A deep lengthwise cut: Slice the mushroom in half from the cap right down to the base of the stem. This lets you see whether the color changes evenly or only in certain zones (for example, only at the stem base).

2. Pressing (bruising): Press your thumb onto the tubular hymenophore (the sponge) on the underside of the cap. If the hymenophore turns blue or darkens when touched, that's an important clue for identifying boletes.

3. Scratching the stem: Lightly scrape the stem with your fingernail. This often helps reveal yellowing or reddening in Agaricus mushrooms.

Questions and answers

If a mushroom turns blue or black when cut, does that mean it's poisonous?

This is one of the most widespread folk myths! A change in flesh color (especially bluing) is NOT a sign of toxicity. Plenty of first-class edible mushrooms (lurid boletes, red-capped scaber stalks, cornflower boletes) change color dramatically. Toxicity is determined only by the species, not by its ability to oxidize.

Why do some mushrooms turn black when cooked?

It's a continuation of the same enzymatic oxidation process, sped up by hot water. Brown birch boletes and hazel boletes always turn the broth black. This affects only how the dish looks, not its taste or safety.

Does the intensity of the color change depend on the weather?

Yes. In dry, hot weather, when the mushroom is dehydrated, the oxidation reaction may be very slow or absent altogether. In a moist young mushroom, the color changes in a flash.

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