The other night I had a whole pound of mushrooms to clean, and not a lot of motivation. I began to wonder whether the conventional wisdom-- which had only reached me in perhaps my late twenties or early thirties, before which I had washed and enjoyed mushrooms without an awareness of my sins-- was really true, or was perhaps just a snobby affectation separating the "real cooks" from the amateurs. A quick internet search shows many, many people quietly disputing the necessity of keeping mushrooms uncontaminated by water. Cooksinfo.com calls the idea that you shouldn't wash mushrooms "a persistent myth," and points us to a discussion on cheftalk.com in which food writer Harold McGee says:
I was skeptical about the mushrooms-absorb-water idea and so did the soaking experiments with standard white mushrooms for “The Curious Cook” back in 1990. I’ve since tried a number of others, and if you make sure to shake the water out of the nooks, fresh mushrooms absorb little if any water. I’d also say that since they’re already around 90% water, a little more or less isn’t going to make much of a practical difference in the subsequent cooking.
Also note that the NEW conventional wisdom, for those who have accepted that washing mushrooms is OK, is that "a quick rinse" is the way to go, but definitely not a long soak, as then the mushrooms really would absorb excessive water. Since a quick rinse is pretty much the way I wash all vegetables except greens (or things that need to be scrubbed, like potatoes), I don't care much one way or the other. However, the last of the three links above gives experimental support to the hypothesis that this new conventional, quick-rinse-only wisdom is also bunk.
Ten minutes is up. So now we extract the mushrooms from the first bowl of water, and allow them to drain very, very thoroughly, for at least 30 seconds. In the meantime, I'm going to reset for another 10 minutes, and man the scales.
Now, if you remember, we started with 4 ounces of mushrooms, and now we have 4.2 ounces of mushrooms. That means that after a 10-minute soak in cold water, these mushrooms only sucked up ... well, right around a teaspoon of water. [...] I'll be very interested to see what another 10 minutes brings.
So another 10 minutes has elapsed. That means this next set of mushrooms has been in the drink now for 20 minutes. We're going to let that drain thoroughly before hitting the scales. And of course, we will set our clock one more time for 10 minutes.
Now we see that what once was 4 ounces of mushrooms has now ballooned to 4.25 ounces of mushrooms. Very curious. Now if we can use the last batch as any kind of indicator, that means that the extra 10 minutes of soak only brought in another 0.05 ounces of water. [...] We now, of course, extract the third batch of mushrooms, drain and weigh.
After a 30-minute soak, these 4 ounces of mushrooms now weigh 4.15 ounces. Now I am willing to accept that differences in the individual mushrooms may have resulted in this batch soaking up less than the 20-minute batch. But what's important is that basically, after 5 minutes, these mushrooms stopped soaking up water. All of them soaked up what is essentially a teaspoon of water. Very, very interesting.
Before we hand down the verdict on this whole mushroom-washing thing, I think that we should see if there's a difference between soaking in cold water and giving them a good spray.
After all, this is how most of us who would wash mushrooms, would wash them at home. Great. Drain, then we weigh.
After a cold blast of water, our 4 ounces of mushrooms weigh 4.2 ounces. Exactly the same as the mushrooms that soaked for 10 minutes. Curious.
So as it turns out, mushrooms do soak up a little, teeny-weentsie bit of water, but it doesn't matter whether they get a quick spray or a 10-minute soak, or really, even a 30-minute soak. They just don't take in much water. So I don't know about you, I'm going to wash my mushrooms, and I'm going to consider this myth smashed to bits.