What are the causes of rancidity in oils and fats?

Answer 1

Rancidity is caused by oxidation, hyrdolysis, or both.

Anti-oxidants help slow down the process of rancidity. Some common antioxidants found in or used with food include Vitamin E, Vitamin C, BHA, BHT (both of which are unhealthy for other reasons in contrast to vitamins e and c) and others.

Unsaturated fats, (such as oils) are touted as healthier than saturated fats, but it is important to note that unsaturated fats are much more easily oxidized (go rancid) than saturated fats, and rancid fats are extremely harmful to health.

Fats can be kept fresh and unrancid longest by keeping at very cool temperatures and in the dark, and away from exposure to oxygen. Ideally this would mean in refrigeration without light or in a light-blocking container, and either vacuum-sealed or else with airspace filled with nitrogen or other nontoxic gas without oxygen.

Note that in the supermarket, unsaturated oils such as vegetable oil, olive oil, corn oil, etc are all kept at room temperature and in fluorescent light in containers that do not block out light. Solid (or saturated) fats such as lard and shortening are kept in the same conditions, but are treated with antioxidants such as BHT and Vitamin E to help retard rancidity, and are, by their nature as saturated fats, more stable against oxidation than the unsaturated fats.

But even the more stable saturated fats are harmed by high temperatures, which may be one reason other than simple caloric value, for why eating fried foods is so harmful to health: frying temperatures cause breakdown in even saturated fats and accelerate rancidity, yet frying fats are rarely discarded after just one use, and are routinely strained and refrigerated before being used over and over, each time breaking down and rancidifying more.

Coconut oil, once reviled as the unhealthiest of fats, is now being discovered as one of the healthiest. High in lauric acid (which is also a natural component of human breast milk) and medium-chain fatty acids, coconut oil is a highly saturated, highly stable fat with many possible health benefits aside from its resistance to rancidity.
when fats and oils are oxidised they become rancid.