When it comes to cleaning mushrooms you can do one of two things. First, place them in a colander, rinse under cold water for 30 seconds, and pat dry them with paper towels.
Second, fill a small bowl with water, soak a clean sponge (or a kitchen towel), and squeeze out as much water as you can. Then, gently wipe the mushroom caps. Be sure to get the underside of the caps and the stems until no dirt or debris remains.
Turn the portobello mushroom over. Holding the mushroom in the palm of your hand, use a paring knife to slice off the inner edges while slowly rotating the mushroom. This paring will expose the gills located on the underside of the mushroom.
Then, using a dessert spoon, gently scrape the gills away. Be careful not to go too deep into the mushroom's flesh during this step. Continue working around the mushroom until you remove all of the gills.
Remove or Trim the Stems:
To remove the stem, use your fingers to grip it from the bottom of the mushroom, twist it, and pop it right out. Alternatively, you can also trim the stem using a sharp knife.
Slice or Use as A Whole
Depending on your recipe, you can prepare portobello mushrooms using the whole cap or slice them into long thin strips.
The best way to store portobello mushrooms is to place them in a partially open plastic bag, such as a Ziploc bag. It's essential to leave the bag partly open to maximize air circulation and allow the ethylene gas emitted from the mushrooms to release. Additionally, you can also store fresh mushrooms in their original packaging. Those containers are designed to breathe, which helps release ethylene gas and balance the amount of moisture in the mushrooms.If you've heard that storing portobello mushrooms in a brown paper bag is a good option, I don't recommend it. According to Cook's Illustrated, paper bags "turn the fungi spongy and wrinkly."