I have started the practice of storing Champignons (Portobella, Cremini, button mushrooms, etc.; Agaricus bisporus) unwrapped in the supermarket container on the window ledge in the pantry. The pantry is a small room that is generally at a relative humidity of 55-80% and temperature of 9-12 degrees Celsius in the winter. Every morning it is aired out by opening the window and door.
The air circulation helps to prevent fuzzy mold formation and rotting of the mushrooms. Today, I saw a pack of mushrooms in the supermarket that had condensation in it and the mushrooms were on the brink of getting mushy but were still plump. After unwrapping them and airing them for an hour by the pantry window, they are no longer wet but still a little slimy. I am hoping for sunshine tomorrow.
Exposing mushrooms to sunlight, in particular ultraviolet light, has at least two beneficial effects. The first is that intense UV light can kill mold and bacteria that might be hanging out with your ‘shrooms. The second is that UVB irradiation increases the Vitamin D content of mushrooms.
Many fungi, including mushrooms, contain the precursor for vitamin D2. The previtamin is ergosterol, and under UVB radiation (harmful to human cells), ergosterol is converted to ergocalciferol (vitamin D2). Most foods do not contain enough Vitamin D to meet the daily requirement (without eating a ridiculous amount every day). Studies have been done which confirm that eating UVB irradiated mushrooms can help prevent Vitamin D deficiency. Vitamin D3 (cholecalciferol) is also generated by UV-mediated conversion; this occurs in your skin. However, UV light is damaging to skin cells and causes radical-induced damage and cancer if antioxidants are not present or destroyed with prolonged exposure. Vitamin D2 rich foods are probably a safer way of obtaining Vitamin D (along with other nutrients) than sunbathing. Though the two types are not equal, Vitamin D2 can be converted in the body to Vitamin D3, which is the active form. A note that I want to add is from [Br J Gen Pract. 2008 Sep 1; 58(554): 644–645]: