How Can You Tell When Cooking Oil Has Gone Rancid?

Rancidity is deterioration of oil through oxidation. Air, light, and heat can accelerate quality loss. Oil may look normal while developing an unpleasant paint-like or stale-oil smell; do not taste suspicious oil to test it.

Used frying oil. Foaming, marked darkening, an unusual off odor, or excessive smoke can indicate that oil should be discarded. Straining does not reverse this chemical deterioration. No single number of reuses applies to every oil and frying process.

Storage. Keep new oil closed and away from light and heat according to its label. For frying oil you plan to reuse, let it cool completely, strain, and refrigerate in a covered container; discard if it deteriorates. Adding fresh oil does not restore spoiled oil.

Disposal. Do not pour oil down the sink. Let it cool and dispose of it in a suitable container according to local used-oil collection instructions. Do not add water to hot oil.