Blanching any vegetable, including mushrooms, is a way to stop the enzymes that cause them to ripen so that you can freeze them and preserve them for later. To blanch mushrooms, use the steam from boiling water and an ice bath to complete the process. Use a steam-blanch with mushrooms instead of the traditional water-bath blanch to preserve the flavor, texture and vitamin quality of the mushrooms.
Blanching can also be used as a way to reconstitute dried mushrooms, though it is not the best method. Blanching dried mushrooms causes them to lose flavor, but you can use the same blanching times if you're in a hurry.
Preparing the Mushrooms
Start by cleaning the mushrooms before blanching them. This is especially important if you're using home-grown or farmers market mushrooms to remove dirt and debris. You can blanch whole or sliced mushrooms, so if desired, slice the mushrooms before placing them in a steamer basket.
If you want to maintain the color of the mushrooms for aesthetic appeal, soak them for 5 minutes in a lemon juice-and-water solution. You'll need 1/2 teaspoon of lemon juice per cup of water.
Fill a large pot with 2 inches of water and bring it to a vigorous rolling boil for the first half of the blanching process. Once the water is boiling, add the whole or sliced mushrooms to a steamer basket and place it over the boiling water. Since mushrooms are steam-blanched, ensure that the basket does not touch the water. With the lid on the pot, steam-blanch for 3 minutes for sliced or button mushrooms and 5 minutes for whole mushrooms.
After the steam-blanch, immediately immerse the mushrooms in a very cold or ice water bath. Fill a bowl with ice water and put the steamer basket of mushrooms directly into the ice water. Cool the mushrooms for the same amount of time that you used to steam-blanch them.
Once you've blanched the mushrooms, drain them well before packaging them tightly in freezer-safe bags. The mushrooms will keep up to 12 months.