Oxidation

Oxidation is a chemical change that happens when food reacts with oxygen. In the kitchen you see it in two main ways. Cut fruit and vegetables, such as apples, avocados, potatoes and basil, turn brown. Fats, such as oils, nuts, whole-grain flour and fatty fish, slowly go rancid and taste stale or bitter. Some oxidation is wanted: it develops the flavor of black tea, aged wine and some cured foods.

Why it works

When you cut or bruise produce, you break its cells. Enzymes inside them meet oxygen and turn natural compounds into brown pigments. This is called enzymatic browning. In fats, oxygen attacks the fat molecules, especially in unsaturated oils, and breaks them into small compounds that smell like paint or old cardboard. Heat, light and metal speed this up.

How to use it

To slow browning of cut produce:

• Coat it with acid, such as lemon or lime juice. Acid slows the enzymes.
• Keep it under water: put cut potatoes or artichokes in cold water, with lemon for artichokes.
• Press plastic wrap right onto the surface of guacamole or pesto, or pour a thin layer of oil on top.
• Blanch herbs or vegetables before freezing. Heat destroys the enzymes.

To slow rancidity:

• Store oils in a cool, dark cupboard with the lid closed. Buy amounts you will use in a few months.
• Keep nuts, seeds and whole-grain flours in airtight containers in the fridge or freezer.
• Vacuum-seal or wrap food tightly before freezing.

Common mistakes

• Cutting apples or avocados long before serving without any acid or cover.
• Keeping cooking oil next to the stove, where heat speeds up rancidity.
• Assuming brown fruit is spoiled. Browned apple or avocado is safe; it just looks and tastes a little different.
• Reusing deep-frying oil many times. Each use breaks it down further.
• Cooking with rancid oil or nuts. They smell sharp, like crayons, putty or paint, and taste bitter. Throw them out; they will spoil the dish.