The common supermarket-variety mushroom is bright white when harvested. By the time it reaches the produce bin or salad bar table, it will have developed a light-to-medium brown discoloration. This is natural. What is not natural is for the mushroom to have a bright lily-white hue at this stage. That would indicate that the mushroom was treated with the chemical sodium bi-sulfite to retard discoloration. This type of preserving agent can cause allergic reactions.
** Asian Recipes