The second question we tackled was a question we get a lot at the shop, “I heard that I wasn’t supposed to wash mushrooms because they soak up water”. This of course is usually followed by an explanation that they are not interested in eating “mushroom growing material” or poop. We gotcha.
The plan was to have specifically weighed mushrooms, wash them three different ways, weigh the result and then cook the mushrooms exactly the same and that’s right weigh them again.
First we used button mushrooms and weighed them to 8 ounces precisely. Then the mushrooms were cleaned. 
The first bowl of mushrooms were soaked in water and then brushed off with a paper towel.
The second bowl of mushrooms were rinsed under running water and then brushed off with a paper towel.
And the last bowl was just wiped off with a paper towel.
Let’s be clear about a couple things, we can be as precise as we want to but there are variables that need to be taken into consideration. If we wanted to be really picky about it, this should be done with a number of different mushrooms since they are all going to react in a different way. I can tell you with confidence that, yes mushrooms (or at least buttons) do absorb some water when they are washed.
After soaking: 8.5 ounces
After rinsing: 8.4 ounces
After dry brush: 8 ounces (no change even with removal of the dirt)
Then comes the cooking part and here the variables become clear because of subjectivity. We can tell you that we used the same kind of pan for all three groups, the same amount of hight temperature cooking oil and we can only tell you that we used the same heat on all three pans. Since we’re not in a lab and even our induction burners don’t have all the same temperatures we got the heat the same on all three pans as closely as we could. BUT should that make a difference? We didn’t think so because we planned on cooking each group of mushroom until they were DONE and not by time. Any culinary student can tell you, and we know more than one student at PCW as well, that their most hated answer to the question”How long do I cook it”, is “Until it’s done”. It is the truth, we go by degrees on a thermometer, color, doneness and only use time as a very vague guideline.
Our hypothesis was that it didn’t matter if the mushrooms gained any water in the cleaning process because if you were going to cook mushrooms properly, you would be cooking the water out anyway. The majority of people cook mushroom too hot, they need to be at a lower temperature so that the water does come out and they develop a nice brown on the outside.
We don’t have any kind of equipment that’s going to tell us that all of the mushrooms have been cooked properly. We have nothing but our eye and our experience and that is us looking into the pan and saying ,” Yup they’re done.” There you go that is the biggest variable. Well we can can at least tell you that we didn’t skew the outcome as we were completely surprised by the results. After the cooking process:
Soaked mushrooms: 5.4 ounces
Rinsed mushrooms:5.2 ounces
Dry brush mushrooms: 5 ounces
Without doing this experiment multiple times our conclusion would be that yes mushrooms are going to soak up water when you wash them, yes there might be residual water after cooking but, it is negligible and will be even less so if you cook them properly. So unless you care about less than a half an ounce of water feel free to wash the manure off your shrooms.
Thank you for your analyses of this question. I have always rinsed mushrooms because it just feels right to me, but there was a nagging in my head that it was WRONG. Now I will happily wash my mushrooms with no remorse. Thank you!
This is silly. It should be obvious that button mushrooms don’t soak up much water but others such as
maitake soak up enormous amounts of water. Just squeeze one after soaking it.
After washing, I make sure to dry the mushrooms by blotting them on paper towels. And with varieties like portobellos, I try to keep them right-side-up when they’re under the water–I’ve found that the gills on the undersides do seem to absorb water. All in all, I’ve never noticed that mushrooms added too much additional liquid to my cooking.
and when cooking mushrooms, the water will come out of the mushroom and be cooked off at which point the mushroom will take on caramelization.
the ONLY time you wash mushrooms is just before cooking them. Mushrooms should never have any moisture added to them while you are storing them. wait to wash until they will be cooked. Alton Brown had a 5 or 10 minute section in one of his shows about a year ago. I hunt wild mushrooms as well as buy lots of mushrooms and have known the facts he states on his show for a long time but, AB did a great job of explaining the why’s, etc.