Both refrigeration and a good-quality container can extend the life of the oil, but nothing lasts forever. Though expiration dates for cooking oils vary, most begin to go rancid within six months to a year. Rancid oil lacks the flavor and health benefits the oil had in its prime, and it’s darker in color. You’ll also notice a change in odor. And, like oils that are cooked past their smoke points, rancid oils can produce free radicals. All in all, it’s better to cut your losses and get rid of rancid oil.
But disposing of used or rancid cooking oil isn’t as easy as you’d think. Rinsing it down the drain isn’t a good idea, since it can build up in your pipes and lead to slow or clogged drains. It can even result in problems for city sewer systems. Public works departments recommend putting cooled oil into a sealable, non-recyclable container and disposing of it.