Jeffery Brewer asked:
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?
Love the show,
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 itís 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 itís 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.
I hypothesize that there is an ongoing endothermic chemical reaction that stalls the continual rise of temperature. The question is which of the ingredients are reacting and transform the sugars to caramel.
Maybe a phase change. Consider what happens when you melt ice. blind Pete, Wed, 11th Jan 2012
Combination of phase changes in ingredients plus energy absorbed during breakdown of starch and sugars will absorb heat energy during the process, so the temperature increase will not be steady during these transitions it will appear as a series of steps. Sprool, Fri, 13th Jan 2012
Do you know how they set "medium heat" on an electric hob?
Interesting. I'm sure my mum's cooker was slower than that (mine is gas so I can't check).
does it state he used an electric hob? Sprool, Mon, 6th Feb 2012