Sucrose -- also known as table sugar -- is composed of carbon, hydrogen and oxygen. When you heat sugar, it melts, caramelises or burns, depending on the temperature. When mixed with water, saturation occurs at different temperatures. There are many applications for sugar in baking and cooking.
Sugar comes in a crystallised state. However, when heated, sugar reaches a melting point, turning into a liquid. The melting point of pure sugar occurs at 160 degrees C (320F). Melting points may vary with impure varieties of sugar. As sugar is heated, the molecules separate, break apart and dissolve. The molecules move away from one another, also known as diffusion. The molecules cool when removed from the heat source, reverting the sugar back to its crystalline form.
Caramelising and burning
As sugar is heated above its melting point, it begins to brown, also known as caramelising. If water is added to sugar and heated, caramelisation can occur at temperatures as low as 100 degrees C (212F). Water attracts the sugar molecules to form a syrup that can be thickened if you heat the mixture long enough for the water to evaporate. As with all foods, if sugar turns black it means that it has burnt. Burnt sugar reacts and oxidises, leaving mostly carbon behind.
The saturation point of sugar differs depending on the amount of heat added. Higher temperatures increase saturation. Sugar saturation refers to the maximum amount of sugar that can be dissolved into a solution. Confectionery is a good example of saturation as it consists of sugar, water and other ingredients. In sweet making, when high temperatures are added, sugar will remain in the mixture even after most of the water has burnt away.
Heated sugar has many applications in cooking and baking, as it forms bonds and affects the development of foods. During baking, sugar helps food to brown. In the oven, sugar molecules diffuse to help items like biscuits spread out on the pan to become tender and crumbly. When baking bread, sugar affects the development of gluten to create a moist dense loaf. If lemon juice is added to heated sugar, it will prevent sugar from returning to its crystalline form when the solution cools. This reaction occurs because the acid in the lemon juice separates the fructose and glucose.