There are several steps involved in cleaning portobello mushrooms, not all of which are necessary or sometimes desirable. The stems have a very woody texture and, if they are cut away, they can be diced finely and fried or frozen for later use in vegetable stock. The gills do not always have to be removed but can easily be scraped away with a spoon. The top of the mushroom cap, which usually is a brownish color, can be carefully removed so the portobello has a pale, clean appearance when presented. None of the cleaning methods is really necessary, with the exception of removing the gills if the mushroom has grown too old and the gills have acquired an off odor.
One area of confusion about cleaning portobello mushrooms is whether to wash them in water. While the mushrooms will absorb a small amount of water, it is not really a significant enough amount to affect the flavor unless the mushroom is sliced or soaked for a long time. The main reason to avoid submerging the mushrooms in water is that the surface of the mushroom will be damaged, ruining the appearance if they are not used very soon after being washed. When cleaning portobello mushrooms that will not be used until a few hours or days afterward, it is best just to use a damp cloth to brush away any dirt on the surface.
The gills under the cap of a portobello mushroom are often removed because they cause food to take on a dark color, although the gills are perfectly edible and can add an earthy flavor to a dish. They are easily removed by holding the mushroom upside down and scraping them away with the tip of a spoon. If the gills have opened and have an unpleasant odor, then they should definitely be removed. Care should be taken when cleaning portobello mushrooms of their gills, because they can still turn cutting boards or other foods dark if they come into contact with them, although this can usually be washed away if caught soon enough.
The surface on the top of the cap of the mushroom also can be removed. The thin skin can be pulled off in strips, usually by grabbing a small piece on the outer edge of the cap and pulling upward. A mushroom cleaning brush or vegetable brush also can work. The main reason to remove the skin when cleaning portobello mushrooms is to create a uniform presentation when they are used in salads or white sauces.