Why food is cooked and heat transfer.

• Why is food cooked?


  To improve shelf life, to develop flavours, to make it safe to consume, to improve texture and to give variety.
• How do different cooking methods change our food?


  Changes in appearance, texture, flavour, smell and nutritive value.
• How does cooking food make it safe to eat?

  Bacteria in food like meat, poultry and eggs are killed if the food is cooked long enough at a high temperature. Some foods contain harmful toxins

  Some foods contain harmful toxins that are destroyed when the food is cooked - e.g. red kidney beans.
• How does cooking food improve shelf life?

  When food is cooked at high temperatures, bacteria and mould are destroyed. This is one of the ways that food can be preserved during manufacturing, e.g. milk is pasteurised to help it stay fresher for longer.
• How does cooking food develop flavours?

  Chemical reactions take place during cooking which change the flavour of the food (caramelisation - makes onions sweeter).

  Roasting meats and vegetables creates more intense flavours. The food becomes browner and crispier with more fat added to it and as water evaporates from inside the food.

  Cooking allows the flavours of different foods to combine (braising meat in a pot of liquid and veg).
• How does cooking food improve texture?

  Cooking usually makes it easier for us to chew, swallow and digest our food.

  Some foods become softer when cooked - vegetables like broccoli and carrots become more flexible and foods like rice and pasta swell as their starch molecules soften.

  Meats become tender - solid fats melt and proteins soften.

  Some foods become firm - proteins in egg whites coagulate.

  Meats can become tough and egg whites rubbery if they are overcooked.

  A change in texture can make it more pleasant to eat - roast potatoes (soft in the centre but crispy exterior).
• How does cooking food give variety in the diet?

  Foods can be cooked in different ways to create variety. For example, if you wanted beef it could be grilled, roasted, stewed, pan-fried or barbecued.
• What is conduction?

  The transfer of heat energy through solids by the vibration of particles.
• How does conduction work? (part 1)


  In a solid, the particles are held tightly together. So when one particle vibrates, it bumps into other particles nearby and quickly passes the vibrations on.

  When a pan is placed on a hob, heat energy from the hob causes particles in the pan to vibrate more and gain heat energy.

  When these particles collide with nearby particles, they pass some of their extra heat energy on to them.

  This process continues throughout the pan until the heat has passed all the way through.
• How does conduction work? (part 2)

  When food is placed into the hot pan, heat energy is transferred from the particles in the pan to the particles in the food in a similar way until the food is cooked through.

  Most pans are made of metal as they are good conductors of heat.
• What is convention?

  Convention is the transfer of heat energy through gases or liquids.
• How does convection work? (part 1)

  When you heat up a liquid near the heat source, it warms faster. The warmer liquid rises above its colder surroundings.

  As the warm liquid rises, colder liquid takes its place.

  The colder liquid is heater and when it starts to rise, colder liquid takes its place.
• How does convection work? (part 2)

  As this process continues, you end up with a circulation of fluid (convection currents) - after a while, this circulation of heat results in the whole fluid being heated.

  Convection also occurs in ovens - hot air rises and cooler air falls in the same way.
• What is radiation?

  The transfer of heat energy through waves of radiation.
• How does radiation work?

  There is no direct contact between the heat source and the food.

  Cookers like grills and toasters use radiation.

  They emit waves of radiation, when these waves reach the food, they are absorbed and heat the food up.

  Microwaves use radiation to heat up the fat, sugar and water molecules in our food.