AQA GCSE Food Preparation and Nutrition: Student Book

Why is it important in cooking? • Convection also happens when you heat an oven, only this time it is the molecules of air that move upwards as they receive heat energy, then fall back down as they cool. • This sets up convection currents of hot air. • In gas ovens, the temperature is hottest at the top of the oven, and coolest at the bottom of the oven. • This results in ‘zones of heat’ in the oven so, for example, a cake that is put into a gas oven on the top shelf will cook faster than a cake put on the middle or lower shelf of the oven. • There is also a risk that the outside of the cake may burn on the top shelf, and the middle might not be fully cooked. • If put on the lowest shelf, the cake may not rise properly because it is too cool and may take quite a bit longer to set. • The usual advice with a gas oven is to cook foods, such as cakes, on the middle shelf, where the temperature will be what you have set the oven to. • Some electric ovens are fan assisted. • This means that they have a fan positioned at the back that blows the hot air around the oven so that the temperature on each shelf is the same, and there are no zones of heat. • Because of this, foods tend to cook quite quickly in fan ovens, so in recipes you will often see two separate temperatures given for electric ovens, with the fan oven temperature being between 10°C and 20°C lower than an ordinary electric oven, which will have zones of heat like a gas oven. • If you are cooking a sauce in a pan on the hob, you need to stir it all the time. • This is because the sauce liquid is thicker than water, so the convection currents move more slowly than they do in water. • Stirring the sauce prevents the starch granules in the sauce from staying at the bottom of the pan for too long and makes sure that they are evenly spread throughout the sauce to thicken it evenly. • It will also prevent the sauce from becoming lumpy and possibly burning on the base of the pan. To help you to learn about heat transfer, produce a mind map or a set of revision cards to explain: • Conduction • Convection • Radiation. Give examples of foods cooked by these methods. Activity Section 2: Food Science Chapter 3 Cooking of food and heat transfer 88