What happens when the fat and oil containing food materials are left outside for a long time?

When fat and oil containing food materials are left outside for a long time, the fats and oils get oxidized. This process is known as rancidity. Atmospheric oxygen reacts with the unsaturated fatty acids in fats and oils, causing them to break down into smaller molecules including aldehydes, ketones, and short-chain fatty acids. This oxidation leads to the development of unpleasant smells and tastes, making the food unpalatable. To prevent rancidity, food manufacturers add antioxidants, use nitrogen flushing in packaging, or store oils in airtight containers away from light and heat.