Rancid is what you call oils that have gone bad due to extended exposure to light, heat, air, moisture or bacteria.
Rancid oils have altered taste and odour that makes them less palatable. Consuming such oils wouldn’t cause disease in the short run but has been associated with accelerated ageing, tissue damage and development of Cancer, Diabetes and Alzheimer’s disease.

The leading reason for cooking oils to go rancid is if you heat it beyond its smoking point.  Research has demonstrated that all oils can withstand varying levels of heat. A heated cooking oil is considered unsafe when it reaches its smoking point — the temperature at which it begins to chemically break down and smoke continuously. At this point, the fat molecules break down into glycerol and free-fatty acids, and the glycerol breaks down further to produce toxic fumes and free radicals — not to mention an unpleasant flavor.

Temperatures for different cooking styles and ideal oils

Deep frying & Searing
Temperature: 160 – 180°C (320 F – 356°F).
Best oils – Coconut oil, Ghee, Avocado oil, Mustard Oil or animal fats like lard

Pan frying (sauté) + Over baking
Temperature: 120°C (248°F)
Best oils: All the above and sunflower, extra light olive oil and canola oil

Virgin oils like extra virgin olive oil, flaxseed oil, grapeseed should be used to drizzle over salads and other foods and not subjected to heat

Smoke point decreases at different pace in different oils.^[

How to check if oil has gone bad

If the oil has an unpleasant sweetness, like fermenting fruit or fruit that’s just gone completely bad or looks dark in colour, it’s rancid. If your food has bitter, metallic, or soapy aromas, or just smells “off,” you’re probably dealing with rancidity. Another easy way to tell if there may be rancidity: If your bottle of oil feels sticky. That’s oil residue undergoing polymerization, says LaBorde—an advanced stage of the rancidity process. If your oil looks darker than usual, that’s another sign.

Keeping oils from going rancid

• Light is an enemy of olive oil, so oils packaged in dark glass or metal tins should last longer than oils packaged in clear glass.
• If you’re buying oil in clear glass, chose a bottle from the back of the store shelf where it’s darker.
• Once you have the oil home, store it in a cool, dry, dark place.
• Do not store your oil above the stove because the heat and steam from the stove will make it spoil more quickly. Oil does not like humidity.