What is Rancidity? (Causes and Examples)

Rancidity is the complete spoilage of food in such a way that it develops a terrible, foul smell and a horrible, sour taste.

• The Chemistry: It happens specifically to foods that contain Fats and Oils. When these fats are left exposed to the air for a long time, the oxygen in the air reacts with them (Oxidation), breaking the fat molecules down into smelly, toxic acids.

• Spoiled Butter: If you leave a stick of butter out on the kitchen counter for two weeks, it will turn slightly yellow/brown and smell incredibly sour. The fat in the butter has oxidized.
• Stale Potato Chips: If you leave a packet of deep-fried potato chips completely open overnight, the oil on the chips reacts with the air. By morning, the chips will taste like wet cardboard and smell bad.
• Old Cooking Oil: A bottle of peanut or mustard oil left open for months will develop a sticky texture and a sharp, foul odor.

Since oxygen is the enemy, we must keep oxygen away from the fat:

• Nitrogen Gas Packaging: Manufacturers flush bags of potato chips with pure Nitrogen gas before sealing them. Nitrogen is unreactive and blocks oxygen, keeping the chips fresh for months.
• Antioxidants: Chemicals like BHA and BHT are added to packaged foods. These chemicals 'sacrifice' themselves, letting the oxygen attack them instead of attacking the food.
• Refrigeration & Airtight Containers: Keeping food in the fridge slows down the chemical oxidation reaction. Using airtight boxes stops new oxygen from entering.