When mushrooms age, they develop mold on the outside surfaces, just as other foods do. Monitor your mushrooms by checking to see whether they feel slimy or have darkened. Once the mushrooms begin to feel just a bit slimy, usually after about two days when kept at room temperature, you can saute them in oil to extend their shelf life. Store the cooked mushrooms for a few days more in the fridge. Canned or dried mushrooms last much longer than fresh ones do.

The ideal place to store mushrooms is in your refrigerator, where it's cool and dry. Set your fridge to 40 degrees Fahrenheit or below to keep the mushrooms fresh for as long as 10 days. Sliced mushrooms last about seven days, so it's best to keep them whole and in their own packaging. Alternatively, place them in a paper bag to keep them dry in the fridge. Freeze cooked -- but not raw -- mushrooms. Saute them in a bit of olive oil or butter, and place them into a sealed, airtight container for up to eight months in the freezer at 0 F.

Slimy mushrooms with an off odor will likely ruin your gravy, stew or other yummy dish. Simply rinsing your mushrooms won't remove mold or the off flavors it causes. Mold can burrow deep into foods, including fungi such as mushrooms, which means it may not be visible on the surface. Besides tasting bad, some types of mold are harmful if ingested.

Moisture is the enemy of your mushrooms. Keep them dry to prevent the growth of mold, mildew and bacteria on surfaces. Never wash mushrooms before storing them, because they are porous and absorb the water. This leads to sliminess in short order. Wash them quickly and scrub them gently with a soft brush just before cooking them. Feel and smell mushrooms before you buy them. They should have an earthy aroma -- not an ammonia-like odor, which can indicate spoilage.