Reasons for Cooking & Heat Transfer

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Why Do We Cook Food?

Cooking food in different ways alters the texture, smell, flavour, appearance and nutritional value. Here are some explanations for why we cook:

Cooking involves chemical reactions. The chemical reactions that take place alter the flavour of the food.
When cooking different foods together, their flavours combine.
Roasting intensifies flavours. When we roast meats and vegetables, they brown and become crispier (because of added fat). This happens because the water within the foods evaporates.

Cooking food at high temperatures helps to kill any bacteria and mould present.
Pasteurising milk involves heating milk to a high temperature before cooling so that it stays fresher for longer.

Lots of foods contain dangerous toxins (e.g. rhubarb) or bacteria (e.g. sprouts and poultry) that are destroyed during the cooking process.
- For example, it's important to cook chicken properly (for long enough at a high enough temperature) to kill any Salmonella bacteria present.

Many foods are harder to swallow, chew or digest pre-cooking.
Cooking softens some foods (e.g. cauliflower becomes less rigid and couscous expands as it's starch molecules become softer).
Cooking makes meats more tender because the solid fats in the meats melt and the proteins become softer. But, overcooking meats can make them tough.
Some people may prefer the textures of foods cooked in certain ways (e.g. brownies can have a soft interior and crusty exterior).

We use heat energy to cook. Heat energy is transferred in three ways:

Heat energy is transferred through vibrating particles. Particles are tightly packed in solids, so vibrations pass quickly from one particle to another when they collide.
When you put a pan on a stove (hob), heat energy transfers to the pan, causing the particles in the pan to start vibrating more.
Pans are often made of metal because metal conducts heat well.
The particles pass on heat energy by colliding with other particles until heat energy has transferred throughout the pan.
When food is added to the hot pan, the particles in the pan transfer heat energy to those in the food. This process happens until the food in the pan is cooked all the way through.

Heat energy is transferred through liquids and/or gases.
When liquids are heated, the liquid closest to the heat source heats up first.
The warmer liquid rises and the cooler liquid flows beneath it.
The cooler liquid then starts to heat up as it is closer to the heat source. It rises and even cooler liquid falls beneath it.
The movement of water in this way creates convection currents. Eventually, all of the fluid is heated.
This process also takes place in ovens - where hot air rises and colder air replaces it below.

Heat energy is transferred through radiation waves.
The heat source and food don't come into direct contact.
Microwaves warm up the water molecules, fat and sugar in food via radiation.
Toasters and grills also heat up food using radiation.
- They send out radiation waves. Food absorbs these waves and heats up.