Why does caramel cooking temperature rise in steps?

We posed this question to Amy Chesterton from the University of Cambridge and retired science teacher John Wenham...

Amy - When you add heat energy to a simple substance, it leads to an increase in the kinetic energy of the molecules so the temperature increases. If we drew a graph of this, the temperature would increase along a straight line as we added more heat energy. Sugar syrup isn't simple though because there are chemical bonds between the atoms that form the molecules and even between the different molecules. These hold the water molecules and sugar molecules together, preventing them from evaporating when the mixture is cool. When the mixture is heated beyond a certain temperature, it begins to boil. This causes the molecules of water to break apart from one another, allowing the water to escape as steam. This uses up energy so the temperature increases more slowly while this is happening. At other temperatures, sugar molecules themselves begin to break apart which also consumes energy again, slowing the increase in temperature when this happens. At other temperatures, the sugar molecules begin to bond together to form long chains which is what is happening when you actually make caramel, forming new bonds like this releases energy which causes the temperature of the mixture to increase more quickly. This is why it's hard to only slightly caramelise sugar without burning it. So returning to the graph analogy from earlier, if you plot a graph of temperature against time for heating sugar syrup, there would flatter areas where bonds are breaking and steeper ones where they're being formed. Hannah - So, the inconsistent rise in temperature that Jeff observed whilst cooking his caramels was due to bonds, between water molecules and inside sugar molecules, being broken and new bond is being made as the caramel cooked. This caramel cooking conundrum caused quite a stir amongst all of you. According to retired science teacher John (Winnam) from Sussex... John - In the case of the caramel, the solid sugar will reach its melting point. Now when a solid melts, the forces of attraction that hold the particles of the solid together in a lump start to loosen and break and it's this process that absorbs the energy supplied instead of it being used to make the particles move faster. And consequently, the substance will stay at about the same temperature until melting is completed. Hence, the temperature of the mixture will not constantly rise.