Oxidation in the food cupboard

Cooking oil, nuts and crisps kept too long take on a stale smell and a bitter taste. They have gone rancid: oxygen from the air has attacked their fat.

Fats and oils, as you have seen, are esters of glycerol with three long fatty acid chains. Oxygen attacks a chain next to its C=C double bonds, where a C–⁠H bond breaks more easily. Each chain that reacts makes reactive pieces that attack the next chain, so once the reaction starts, it keeps itself going. The damaged chains break into smaller molecules, among them aldehydes such as hexanal, and these give the stale smell.

The more C=C bonds in a fat's chains, the faster it goes rancid.

Coconut oil keeps for many months. Linseed oil, also sold as flaxseed oil, keeps best in the fridge once the bottle is open. Heat and light speed the reaction up (lesson 16 shows how heat does it), so all oils keep longest in a cool, dark cupboard, and crisp bags are filled with nitrogen gas to keep oxygen away from the oil in the crisps.

Antioxidants

An antioxidant reacts with oxygen, or with the reactive pieces oxygen makes, more readily than the fat does. The antioxidant is oxidised first, and the fat stays fresh until the antioxidant is used up. Two vitamins do this job in food.

Vitamin E is a phenol, like the apple molecules in the last section. The -⁠OH on its ring gives up its hydrogen to the reactive pieces and stops the reaction spreading from chain to chain. Its long tail mixes with fat, so nuts and seeds carry vitamin E in their oils. Vitamin C, with four -⁠OH groups, dissolves in water and protects the watery parts of food, such as fruit juice. Food makers add both: on ingredient lists in Europe, vitamin C is E300 and vitamin E is E306 to E309.