My kitchen became a laboratory. I plonked 150g (5 oz) of whole button mushrooms in cold water and waited 5 minutes. With another lot I waited 10 minutes. After being submerged in water for the designated time I drained off the water and dried the mushrooms with a clean tea towel. Then I re-weighed the mushrooms. After 5 minutes in water there was a gain of 4 grams (2.6% gain) and 6 grams after 10 minutes (4% gain).
Now, I imagine if you did wash your mushrooms to clean off any residual ‘dirt’ you can do it in less than 5 minutes with 0% gain. The mushrooms were then cooked and their brief swim didn’t affect the eating quality.
Then, I read that Robert Wolke, Professor of Chemistry at the University of Pittsburgh had done the very same experiment about a decade ago and found a 2.7% weight gain after soaking mushrooms for 5 minutes. How’s that for brilliant, reproducible kitchen science?
I should warn you that I did the very same experiment with sliced mushrooms. Oh dear! They gained 27% of their weight in water within 5 minutes. Still edible after cooking, but not so good. Why the difference? The sliced mushroom have their inside flesh exposed and this is the part that absorbs water quickly. The “skin” of the whole mushroom is essentially impervious to water as you would expect from a food designed to grow in rain, fog and dew. In fact, the extra weight gained with the whole mushroom might be because I didn’t dry off all the external water.
Of course, you could just brush any visible dirt from a mushroom. I wasn’t suggesting that you need to wash them. Anyway, now you know the truth too. You can give your mushrooms a quick rinse before cooking. Trust us. Quote us. Robert and I are scientists.
Wolke RL. What Einstein Told His Cook. WW Norton & Co. 2002 (p268)