In the early parts of the 1700’s, there weren’t that many ways to investigate the identity or purity of a material. Sophisticated electronics and fancy scanning equipment had not yet been invented. One of the first analytical techniques developed for testing an unknown material was the melting point, or “m.p.”, as it is usually abbreviated in science journals.
The principle was simple. You would load a few crumbles or crystals of the unknown compound into a tiny glass tube that was closed on one end. The glass was essentially a mini test tube. You would then put the tube into a pile of sand that was being heated by flames or a heater of some sort, and you would put a thermometer in the sand as well. You would slowly heat up the sand until the material in your test tube melted. The temperature of the thermometer (which is also in the hot sand, and which shoudl equal the temperature of the material inside the tube at any given moment in time) could be read, and that number became the melting point of the material. Ice, for example, melts at 0 degrees Celsius.
The reason melting points gained so much popularity as an analytical technique is that the precise mp is very characteristic of a given material. If you have ice under normal, every day conditions, you can be sure that it will always melt at 0 degrees. It will never melt at 123 degrees, or -17 degrees. The exception to this rule is if the ice is somehow impure, as impurities can throw off the melting point of a material. You could take water, freeze it, melt it at 0 degrees, and it would still be water. It’s just a physical change, not a chemical change. So scientists would test their new materials being made in the lab and if the melting point was sharp and well defined and repeatable over many tests, they could be assured that they had one pure compound in the tube and they would then report that mp value in their journals and communications. That value became characteristic of that particular compound.
Enter sucrose, aka table sugar. Scientists have obviously been looking at sugar for hundreds of years. If you look at published reports on sucrose, you see a wide range of melting points. The authors would blame this range of values on a variety of factors – maybe user error, or maybe the thermometer wasn’t calibrated correctly, or the sugar had an impurity in it, and so forth. However, that isn’t a very satisfying conclusion. If every scientist in the world has to come up with an explanation for why his result doesn’t agree with every other scientist, it’s time to take a step back and look at what’s going on here.
We can make very pure sugar. We know the chemical identity of sugar; we can draw the molecule. We have very precise thermometers and very carefully controlled heating blocks and heating elements. We can perform melting point analysis. Why, then, are there so many different varying values for the melting point of sugar? A new paper published by chemists from the University of Illinois has solved the mystery.
When sucrose is heated up to 186 degrees Celsius, it appears to melt. However, if you then immediately cool it back down and analyze the resulting material with all the battery of tests that science can throw at it, you find that it’s no longer sugar. It’s something else. The material has decomposed into a mixture of products; the sucrose has disappeared. The scientists also noted that the rate at which the sugar was heated had an effect on the “melting point”; faster rates of heating led to a lower apparent melting point. In all cases, once the sugar became molten and was then cooled, it was no longer sugar. It had undergone not only a physical change, but also a chemical change.
That’s the crucial difference with sugar, and it explains the wide range of melting points in the literature. This discovery is important because food scientists will now be able to do a lot more with sugar. Instead of searching for different ingredients for their products, they can experiment with these (delicious) decomposition products of sugar like caramels. As is usually the case, a deeper understanding of the chemistry behind a process should lead to new applications for that material. Expect future foods to have slightly different ranges of tastes and degrees of sweetness as this peculiar behavior of sucrose is put to good use.
The source of this article can be found at:
Joo Won Lee, Leonard C. Thomas, Shelly J. Schmidt. “Can the Thermodynamic Melting Temperature of Sucrose, Glucose, and Fructose Be Measured Using Rapid-Scanning Differential Scanning Calorimetry (DSC)”. Journal of Agricultural and Food Chemistry, 2011; 59 (7): 3306.