Cap surface

Dry

In mycological field guides, the term dry surface doesn't simply mean "a mushroom that hasn't been rained on." It's a structural feature of the cuticle (the skin). A cap is considered dry if it never produces slime or gelatin and never becomes sticky to the touch, even in a very damp environment (in fog or after a heavy downpour).

24 species in the database.

Why does the cap stay dry?

The skin of "dry" mushrooms lacks the specific polysaccharides that can swell and turn into slime on contact with water. In rain, the water simply runs off such a cap or is absorbed evenly into it (hygrophanous), making the mushroom darker but not sticky.

Typical species with a dry cap

David Arora and M.Ya. Zerova give plenty of examples of mushrooms with this feature:

1. Chanterelle (Cantharellus cibarius): The absolute benchmark. Its yellow, slightly matte cap is never slimy or tacky. Forest debris never sticks to it (unlike slippery jacks).

2. Xerocomus boletes (Xerocomus): The cap is greenish-brown, dry and velvety. Even in rain it gets wet but doesn't produce slime. (This is the main difference from a slippery jack, whose tube layer looks similar but whose cap is gelatinous.)

3. Porcini (Boletus edulis): Its skin is usually dry and smooth, although in very wet weather it can become ever so slightly tacky (but it's never covered with a layer of slime).

4. Most brittlegills (Russula): They have a perfectly dry, matte skin that is often very brittle.

Dangerous mushrooms with a dry cap

A dry cap is no guarantee of edibility. Many poisonous mushrooms have exactly this texture:

- Fly agaric (Amanita muscaria): Its white-spotted cap is dry (moisture can wash off the spots, but the skin itself doesn't produce slime).

- Brown rollrim (Paxillus involutus): Deadly poisonous, with a dry, matte, suede-like cap.

- Fibrecaps (Inocybe): Poisonous mushrooms whose caps are very dry and radially fibrous.

Questions and answers

If a mushroom is dry, does it still need peeling?

Exactly — it doesn't. The golden rule of cooking: you only peel mushrooms that produce bitter or laxative slime (slippery jacks or spike caps, for example). Peeling dry brittlegills, porcini or chanterelles is a pointless waste of time and throws away the most aromatic part of the mushroom.

Why do some dry mushrooms crack?

Since the skin of a "dry" mushroom (such as the red cracking bolete) has no elastic layer of slime, it is very sensitive to changes in humidity. When the mushroom grows fast in dry weather, the skin can't stretch quickly enough and simply bursts, forming cracks.

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