Anyone who has baked in more than one oven knows that the number on the temperature dial often has little to do with reality. When you are baking –er, _harnessing temperature-specific biochemical reactions_ — an untried oven can be a nightmare. So it goes without saying, I need to test the calibration on the kwartzlab oven, before the bread baking begins.
Thermometers are easy to use, but not particularly interesting. Plus, you are relying upon your thermometer being accurate, something many of us take for granted but have no idea how to confirm.
In Cooking for Geeks, Jeff Potter offers a more entertaining alternative. On the grounds that “sucrose (table sugar) melts at 367°F / 186°C”, he suggests using a bowl of granular sugar to test your oven’s calibration. Specifically, Potter suggests putting a bowl of sugar in an oven set for 350°F. Based on a melting temperature of 367°F, that sugar should not melt if the oven is either true, or running cold. Increase the oven temperature to 375°F, and if the oven is true, the sugar should melt.
That sounds straight forward enough, but 367°F? Candy makers the world around have long held that plain old sucrose melts at around 320°F, and then caramelizes at around 335°F. So who’s right?
To the internets!
I found lots of received wisdom proclaiming 320°F, but the only source I could find for 367°F was the product listing for Domino’s Di-Pac® Direct Compacting & Tableting Sugar. But received wisdom is not science, and “a unique dry fondant sugar manufactured by a patented technique solely for use in the pharmaceutical industry” does not sound like normal sucrose to me.
Enter chemists Lee, Thomas, and Schmidt, authors of “Investigation of the Heating Rate Dependency Associated with the Loss of Crystalline Structure in Sucrose, Glucose, and Fructose Using a Thermal Analysis Approach”. Enter Harold McGee, who summarized their research thusly:
“…sucrose doesn’t have a true melting point. Instead it has a range of temperatures in which its molecules are energetic enough to shake loose from their neighbors, and a range in which the molecules jitter themselves apart and form new ones. And these two ranges overlap. Whenever sugar gets hot enough to liquefy, it’s also breaking down and turning into caramel. But it starts to break down even before it starts to liquefy. And the more that sugar breaks down while it’s still solid, the lower the temperature at which it will liquefy.”
Schmidt et al found that sucrose could start liquifying at temperatures as low as 290°F, and as high as 380°F, depending upon how quickly the sugar was brought up to temperature. Which means that determining the temperature of your oven based on whether or not sugar melts depends on knowing a lot more about both sugar and ovens than you or I know.
Time to test some thermometers!