We've had some sweet potatoes siting around on the counter for a while. I started to cut them up to roast but not sure if they're too old. The flesh has some yellow parts on in in addition to the usual beige/off white color, and the skin on one looks like it has like thick veins inside it. Is there a way to know when a sweet potato is too old? What's the shelf life when kept on the counter at room temp? They weren't exposed to sun but weren't in a cool dark place either. Thank you!