So how long is too long? Still Tasty notes that in order to stay safe and protect yourself from bacteria, the cooked mushrooms cannot be left at room temperature for more than two hours before going into the fridge. They also recommend storing them in an airtight container or sealed plastic bag. You can also freeze sautéed mushrooms for several months and reheat them at your convenience, according to Good Cheap Eats, but you need to freeze them immediately after you finish cooking them.
If you put the mushrooms in the refrigerator within that time frame, and your reheating simply involves last night’s leftovers, you should be fine. According to the European Food Information Council (EUFIC), if cooked mushrooms are kept in the fridge for no longer than 24 hours, they can safely be reheated. The EUFIC recommends reheating to a temperature of 158 degrees Fahrenheit or 70 degrees Celsius (via Business Insider).
If you’re sure you’ve managed to keep everything within that time window, then your mushroom pizza or pasta sauce can still be reheated. If you’re not, you may want to heed My Recipes’ advice and eat them cold, or simply cut your losses and toss the leftovers. Although not life threatening, nobody wants to deal with an upset stomach.