Eduqas GCSE Food Preparation and Nutrition: Student Book

Cooking food Cooking can be defined as the transfer of heat energy ( heat transfer ) from one item to another, such as boiling water to carrots. The heat energy changes the molecular structure of the proteins, fat, starch, sugar and water and these transformations alter the texture, flavour, aroma and appearance of the food. To cook foods successfully, youmust first understand the ways in which heat energy is transferred: these are conduction , convection and radiation ( radiant heat ). Many dishes are cooked using twomethods, for instance, roasting vegetables uses both convection and conduction. If you choose the wrong method of cooking, then the food will not have a good texture, colour or flavour. A general rule of thumb is that hard or tough pieces of food need to be cooked slowly, inmoisture, for a relatively long time. Take beef as an example: fillet or sirloin steaks are tender and so can be cooked very quickly on a hot griddle pan. By contrast, braising and stewing steak are much tougher and benefit from long, slow cooking in a casserole. If you pan-fried the stewing steak it would become very hard and inedible. Heat transfer: the transfer of heat energy between objects State four reasons why we cook our food. Check it COOLER OBJECT HEAT TRANSFER WARMER OBJECT CONDUCTION DESCRIPTION: The transfer of heat by direct contact from a hot surface This is a relatively slow method of heat transfer because there must be physical contact between the surfaces to transfer energy between molecules. Surfaces need to be good conductors of heat. That is why saucepans and frying pans are made of metal, but with plastic handles. EXAMPLES: Lamb chops, bacon, sausages or Welsh cakes touching the hot surface of a frying pan USES: Dry frying, griddling, searing, sautéing CONVECTION DESCRIPTION: The transfer of heat by the mass movement of heated particles into a cooler mass or area. Natural convection uses the tendency of warm liquids or gases to rise and cooler ones to sink, leading to a constant circulation of heat. Mechanical convection uses the fan in an oven to move heat around quickly and evenly. Food cooks evenly because the oven does not have hot or cold zones in it. Ovens without a fan rely on natural convection and food is heated from the outside – the inside heats up slowly. EXAMPLES: Hot water, air or oil surrounding the food such as boiled potatoes, chicken stew, roast beef, poached eggs, deep-fried fish Natural convection – making cheese or white sauce, gravy USES: Dry heat methods: Baking, roasting and deep frying Wet heat methods: Boiling, braising, simmering, poaching, steaming, pressure cooking RADIATION (INFRA-RED RADIANT HEAT) DESCRIPTION: The heat is transferred using electromagnetic radiation: waves of heat or light strike the food. There is no physical contact between the heat source and the food being cooked. EXAMPLES: Waves of heat are directed at the food e.g. grilling sausages, bacon, Welsh rarebit, kebabs USES: Toast, grilling and barbequed foods RADIATION (MICROWAVE) DESCRIPTION: The magnetron in the microwave oven converts electricity to radio waves called microwaves which penetrate the food EXAMPLES: Ready meals, microwave-only meals or foods, scrambled eggs, burgers, bacon USES: Heating up leftovers, quick defrosting of frozen food, ready meals such as lasagne HEAT PROCESS SHOWING CONVECTION RADIANT HEAT FROM GRILL HEAT SENSITIVE VIEW OF TOASTER HEAT PROCESS SHOWING CONVECTION IN FAN ASSISTED OVEN Water cools Hot water rises Cooler water sinks Cooler water sinks HEAT SENSITIVE VIEW OF PAN Mode Stirrer Wave Guide Magnetron Tube Oven Cavity Filter Step-up Transformer HEAT CONDUCTION IN A PAN 69 SECTION 1 THE SCIENCE OF COOKING FOOD