If the recipe is for a stuffed mushroom, you remove the gills just to make more room for the stuffing. More often, though, the gills are removed because they will color whatever you’re cooking with quite a strong dark gray color.

Generally, if you’re making a cream of mushroom soup or other light or clear sauce containing mushrooms, you choose mushrooms with closed caps. The caps begin to open and the gills darken substantially as the mushrooms become more mature, and those open gills spill thousands of dark spores into whatever you’re cooking – tending to turn light dishes dark gray, which is not a universally palatable color.