what is the best way to store portobello mushrooms?
In a brown paper bag. Plastic makes mushrooms go bad and smelly faster.
A zillion years ago, I took a mushroom class. We were taught to use waxed paper, and that or parchment is still my method. The brown paper bag that nutcakes recommends makes sense as well, but I like a little bit more of a barrier. The important thing is that they need some air flow. Plastic is definitely not the way to go.
Yes, plastic will induce moisture and deterioration and fresh mushrooms are fragile enough already. Also keep in mind that portabella and cremini are just fancy marketing names for the common brown mushoom that has been cultivated in this country since the early 20th Century. By the way I'm seeing lots of chantarelles in the markets right now. Grab 'em while you can.