I made several batches of caramels over the holidays, using a recipe that combin

Question

I made several batches of caramels over the holidays, using a recipe that combines corn syrup, brown sugar, condensed sweetened milk and butter in a sauce pan. The mixture is heated over a medium heat for about 40 minutes until it reaches a final temperature of 244 degrees Fahrenheit. This year I was using a new digital thermometer and was surprised to observe that rather than rising at a constant rate throughout the entire cooking time, the temperature would rise steadily for a few minutes, then remain constant for several minutes and then rise again, repeating several times while making each batch of caramels. I expected a slow, constant-rate rise in temperature from beginning to end. Why would the temperature rise so inconsistently?

Explanation / Answer

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

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