Mushrooms do not soak up water like sponges, and a scale proves it

These are listed in the order worth acting on, which with washing mushrooms is not the order they are usually presented in.

The rule and its authority

Generations of cooks have been told never to wash mushrooms, but to brush or wipe them individually with a cloth. The instruction appears in highly influential cookbooks and was repeated in cookery schools and television programmes for decades.

The stated reason is that mushrooms absorb water like sponges and become soggy and impossible to brown. It is an oddly labour-intensive instruction, and the labour is part of why it was believed, since nobody suffers for nothing. The claim is testable with equipment every kitchen has, which is what eventually undid it.

What the weighing shows

Rinsing mushrooms briefly under running water and weighing them before and after produces a small increase. The gain is a modest percentage of their weight, and much of that is water clinging to the outside rather than inside the tissue.

Even soaking them for several minutes produces a limited increase, because the tissue is already close to saturated. Mushrooms are largely water by weight to begin with, which sharply limits how much more they can take on. Anyone can repeat this test, which is the useful thing about a myth that makes a quantitative claim.

The real obstacle to browning

Water on the surface keeps the temperature at boiling point until it has evaporated, and browning requires considerably more heat than that. So wet mushrooms genuinely do resist browning, and the observation behind the myth is not imaginary.

The primary source says otherwise: the fix is to dry them rather than to avoid washing them, which takes a clean cloth and very little time. Mushrooms also release a great deal of their own water when heated, which is a far larger obstacle than anything a rinse adds. Cooking them in a hot pan without crowding, and waiting through the released liquid, is what actually produces colour.

Why brushing is worse

Mushrooms are grown in compost, and the material clinging to them is not something a dry cloth removes reliably. Brushing pushes grit into the gills rather than lifting it away, particularly on varieties with exposed undersides. It is also slow enough that people skip it, which means the alternative in practice is not brushing but nothing.

Checked against the record, running water is more thorough, faster and does not require handling each item individually. This is a case where the recommended technique is both more work and less effective than the one it replaced.

Where the rule came from

The instruction seems to have entered wide circulation through mid-century cookbooks that were copied extensively. It has the shape of a plausible generalisation from the appearance of mushrooms, which are visibly porous and spongy. Nobody appears to have measured it before publishing it, and once printed it was reproduced rather than rechecked.

Somewhere in the retelling, culinary writing has historically transmitted technique by repetition rather than by testing, which is how such rules survive. The correction came from a generation of food writers who treated kitchen claims as things you could weigh.

Varieties that behave differently

Morels and other highly convoluted varieties trap grit and often need proper washing, sometimes more than once. Very fresh, dry-cultivated mushrooms may genuinely need nothing more than a wipe, so the instruction is not always wrong. Sliced mushrooms expose far more cut surface and will take on more water than whole ones, so wash before slicing.

Trace it back and dried mushrooms are a separate matter entirely, since rehydrating them is the point and the soaking liquid is valuable. The general rule that survives is simple, namely wash if they need it, dry them properly, and give them a hot uncrowded pan.

Everything above, in order of what to do first

1. The rule and its authority. Generations of cooks have been told never to wash mushrooms, but to brush or wipe them individually with a cloth.
2. What the weighing shows. Rinsing mushrooms briefly under running water and weighing them before and after produces a small increase.
3. The real obstacle to browning. Water on the surface keeps the temperature at boiling point until it has evaporated, and browning requires considerably more heat than that.
4. Why brushing is worse. Mushrooms are grown in compost, and the material clinging to them is not something a dry cloth removes reliably.
5. Where the rule came from. The instruction seems to have entered wide circulation through mid-century cookbooks that were copied extensively.
6. Varieties that behave differently. Morels and other highly convoluted varieties trap grit and often need proper washing, sometimes more than once.

The takeaway

The instruction asked for more work and delivered less cleanliness, on the strength of a claim nobody weighed.

Believing it was ordinary. Continuing to is the avoidable part.