Vegetable fats and oils are lipid materials derived from plants. Spoilage mainly happens if you live in a warm climate (where indoor temps tend to be over 72F over half the year), or if the oil is exposed to light. Keeping it in a dark place helps. What happens is that the oil gets oxidized and chemically changes and the products are consumed by bacteria, producing a variety of substances that are not edible.

Michael L. Bentley, EdD, Expert Panelist