Smell

Onion

The smell of green or bulb onions is much rarer in the forest than the smell of garlic, but it always surprises those who catch it. Chemically it is very close to the garlic smell and is caused by similar sulfur-containing compounds.

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Mushrooms that carry an onion aroma

An onion smell is most often a diagnostic feature of small saprotrophic mushrooms that break down dead organic matter in the forest litter or on old stumps. David Arora in «Mushrooms Demystified» points out that these smells (onion and garlic) are often considered together, since they shade into one another.

1. Some bonnets (Mycena): The genus Mycena contains many small bell-shaped mushrooms with all kinds of chemical and plant-like smells. Some species give off a smell of green onion or fresh garlic shoots when you rub the stem.

2. Marasmius: Although the best-known member of this genus — the garlic parachute — smells distinctly of garlic, some of its relatives can have a weaker, sweetish smell more like fried or boiled onion.

3. A few webcaps (Cortinarius): As an exception, among the enormous variety of webcaps there are species whose earthy, radish-like smell sometimes takes on sharp oniony notes.

How to recognize an onion smell

To catch an onion aroma, you need to tear or crush the mushroom's tissue (especially the stem). It is the damage to the cells that triggers an enzymatic reaction releasing volatile sulfur compounds (the same thing happens when we cut an onion in the kitchen).

Questions and answers

Are mushrooms with an onion smell edible?

The small mushrooms with this smell (bonnets and Marasmius species) are usually not poisonous, but they have no culinary value because of their tiny size and toughness. The Marasmius species used as a seasoning have a pronounced smell of garlic, not onion.

Why do bonnets smell of onion?

It is an evolutionary adaptation. The volatile sulfur compounds that create the smell of onion and garlic are powerful natural antimicrobial and antifungal agents. They protect the mushroom's fruiting body from being eaten by certain insects or from mold.

Does the onion smell survive drying?

Sulfur compounds are very volatile and unstable. As with ordinary onions, after cooking or air-drying the sharp onion aroma weakens considerably or disappears completely.

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