Why Do Mushrooms Release Water When Cooked?

Mushrooms release a lot of water while cooking in a pan because they are about 90% water. As heat causes the cell walls to collapse, the water is released and flows into the pan. Although mushrooms may look dry at first, they are made up largely of water. When too many mushrooms are added to the pan at once, the released liquid quickly covers the bottom, and the mushrooms cook more like they are boiling than browning.

Why Do Mushroom Cells Hold So Much Water?
Mushroom cell walls are made of chitin, a different structure from plant cell walls. This structure holds water inside the cells like a sponge. When heated, the cell walls quickly soften and collapse, releasing the water all at once. That is why raw mushrooms do not release water even when squeezed firmly by hand, yet a noticeable pool can form in the pan within minutes.

Why Do Pan Crowding and Heat Matter So Much?
If too many mushrooms are added at once, steam from them cannot escape quickly enough, and they essentially boil in their own liquid. Spreading them in a single layer in a wide pan so they do not touch can largely prevent this. Cooking over high heat also makes the water evaporate faster, helping the mushrooms start browning sooner.

Why Is Salting Before the Water Evaporates a Mistake?
If mushrooms are salted at the beginning of cooking, the salt starts drawing water out of their cells more quickly through osmosis, causing more liquid than usual to collect in the pan. Add salt after the water has fully evaporated and the mushrooms have started to brown to avoid this problem. That is why recipes generally add salt during the last few minutes of cooking, not at the beginning.

How to Brown Mushrooms After They Release Water
Once all the water in the pan has evaporated, the mushrooms begin to contact the pan's surface. At this point, turn the heat up a little and add a few tablespoons of oil; the mushrooms will start to turn golden. Adding oil before this stage makes it ineffective in the watery pan. Browning becomes noticeable within 5–7 minutes, depending on how crowded the pan is and how strong the heat is.

Can You Cook Mushrooms Without Any Water Release?
It is not possible to cook mushrooms without them releasing any water because their water content is a natural characteristic. However, high heat, a wide pan, and a single layer help the water evaporate much faster and shorten the process. Some cooks first cook mushrooms in a dry pan, without oil, until all their water has evaporated, then add the oil. Since the oil is used only during browning, this method uses less of it and helps the mushrooms brown more distinctly. Knowing that wet mushrooms will not brown properly can keep you from adding oil too early out of impatience. Different mushroom varieties also behave differently: high-moisture varieties such as cultivated mushrooms release more water, while more fibrous varieties such as beech mushrooms release relatively less and start browning a little sooner. Wiping mushrooms with a damp cloth instead of washing them before cutting can also reduce the amount of water they release during cooking by preventing the pores from absorbing extra water.