Reasons for cooking food

 To kill harmful bacteria/make food safe to eat – e.g. meat.  To destroy natural toxins – e.g. red kidney beans.  To preserve – e.g. making fruit into jam.  To aid digestion: cooked starch easier to digest – begins in mouth  To aid absorption e.g. raw starch in potatoes and flour cannot be easily be absorbed.  To make food easier to eat, e.g. meat is tenderized.  To make food more attractive: meat changes from red to brown  To develop extractives/flavor: grilled steak, toasted cheese

Reasons for cooking food  Smell stimulates appetite/flow of digestive juices: e.g. curry  To provide hot food in cold weather: e.g. soup in winter  To reduce bulk/allow more to be eaten: e.g. cabbage  Create new dishes: e.g. quiche, chocolate cake etc.  Add variety to diet: e.g. eggs can be cooked in many different ways  Necessary for some cooking processes: e.g. thickening sauces, baking cakes and biscuits  Mix together different foods: cakes, sauces, casseroles etc.

Different ways of transferring heat in the preparation of dishes.

Methods of transferring heat: Conduction  Through solids by contact (also occurs within food in microwave cooking), molecules vibrate rapidly cause neighbouring molecules to vibrate, adjoining molecules vibrate etc.  Transfer, vibrations / heat energy, from one molecule to the next; requires contact with a source of heat  Heat is conducted at varying rates through different substances; metals are good conductors; wood, plastic are poor conductors;  E.g. metal spoon in hot liquid, frying bacon in pan, cake in cake tin, pan standing on a hotplate, oven shelf, etc.

Methods of transferring heat: Conduction  Advantages:  Quick. e.g. frying  Needs constant attention.  Boiling can be left: several dishes can be baked at once.  Heat from all oven shelves passes to baking tins.  Disadvantages:  Nutrients may be lost in liquids: e.g. boiling green vegetables,  Cooking tins/pans may be too hot to handle (safety), need wooden spoons to stir (metal conducts heat and would burn hands)  Oven gloves required to handle hot trays.  Some metals better conductors than others (more efficient at transferring heat) e.g. copper is good etc.

Methods of transferring heat: Convection  Through liquids and gases.  Molecules rise when heated because they are less dense, colder molecules fall and they are heated again. Thus, convection currents are created etc. e.g. boiling water in pan, heated oven etc.  Until a constant temperature is reached, heat energy is transferred by the movement of the gas or liquid.  E.g. boiling potatoes/steaming fish/baking a cake, boiling water in pan, heating an oven, roasting, deep frying;

Methods of transferring heat: Convection  Advantages:  May cook several dishes at once, tiered steamer, oven shelves filled.  No added fat if steam or water used, more healthy  Does not need constant attention.  Disadvantages:  Some methods take a long time: steaming.  Boiled and steamed dishes lack colour, have a soft texture, some water soluble nutrients lost.  Zones of heat

Methods of transferring heat: Radiation  No medium (through space or vacuum). No heated molecules.  Rays, from source of heat, travel in straight lines (fall on food in their path). Food needs to be turned. e.g. grill, barbecue  In microwave oven (electromagnetic waves given off by radiation)  Cause a rise in temperature in the object in path of waves. e.g. heat rays are infra-red rays  Heat transferred within food by conduction etc.  Space between heat source and food is not heated  Food need to be turned etc.  E.g. grilled steak/spit- roasted chicken, barbecued sausages etc.

Methods of transferring heat: Radiation  Advantages:  A quick method (grill – barbecue)  Extractives developed on surface, attractive brown surface, crisp, fat drips off, more healthy.  Disadvantages:  Needs careful attention (easy to overcook).  Dries surface, food needs to be turned and basted.  Only suitable for thin pieces of food (would be overcooked before inside was cooked)

Microwave cooking  Electromagnetic waves given off (by magnetron). Water molecules in food vibrate generate heat passes to adjoining molecules by conduction (quick method)  Oven does not need to be preheated, stays cool, so food does not burn on sides of oven.  Penetrate to depth of 5–7.5 cm, used on thin pieces of food. Suitable for small, thin pieces of food (easy to overcook) cannot judge when food is cooked.  Container does not get hot (glass, china, certain plastics can be used but no metal or metal decoration. Causes arcing and will damage the microwave oven  May have a ‘stirrer’, to spread rays, for more even heating.  Works best on foods with high water content.

Microwave cooking  Advantages:  Quick. Fuel saved. No pre-heating necessary. No mess in oven.  Saves cleaning time, same dish can be used for cooking and serving, less washing up.  Micro-organisms destroyed by heating of water molecules.  Minimum loss of water-soluble vitamins, little or no cooking liquid.  Maintains colour of vegetables (quick cooking)  Heat produced immediately, can be used for defrosting.  Safer than leaving food in a warm kitchen.  Re-heats food very quickly, less destruction of nutrients.  Easy to use, for children, elderly, disabled, etc.