Chocolate + Science = HOT

20Jan07

January at MIT goes by the name of IAP, for Independent Activities Period. That means we can do WHATEVER WE WANT. There are a few classes offered for credit, but most of the Activities are purely for fun & excitement & maybe some nerd cred. So yesterday I went to a chocolate lecture. It was not, alas, a talk in a chocolate hall by a chocolate professor, but a lecture on the science behind fine chocolate, sponsored by the MIT Lab for Chocolate Science. I can see you salivating. Here are the cool parts- from memory, so no hotshot details; I didn’t take notes.

In some warm tropical land, a cacao tree produces a seed pod. The beans inside are surrounded by squishy white nutrient stuff; people pile the beans in a big sticky heap and let them ferment. Some chemistry happens. Squishy white turns into squishy brown, and squishy brown is dried in the sun, and dried brown is given a good roasting. More chemistry happens! My apologies for the elision – I remember enough basic chem to accept as plausible such explanations as ‘roasting releases aromatics’, but not nearly enough to discuss them.

All of that chemistry business is to a single end: taste. BUT! Equally important, to the enlightened chocolatier, is the physical experience of chocolate eating: the smoothness, snappiness, melt-in-your-mouth-not-in-your-handiness. And that has to do with triglyceride packing. Recall: a triglyceride is made of three fatty acids attached to a glycerol spine. When they pack and crystalize, triglycerides look a little like skewed tuning forks. Anyway, there are six different crystalline forms for chocolate, each with a separate melting point:

I melts at 17 C
II 21 C
III 26 C
IV 28 C
V 34 C
VI 36 C

The temperature inside a human mouth is 37 C. The ideal chocolate melting point would be just slightly below that – too far below, and the stuff will melt at the drop of a barely lukewarm hat. Variety VI looks like a good possibility, but it takes WEEKS and WEEKS to form. The other five varieties will crystallize from melted chocolate, but VI will only develop from solid version V. And it can’t be hurried. Plus, with a melting point so close to body temperature, it comes across as hard, unforgiving stuff.

Version IV isn’t too bad either. The problem there, though, is that its triglycerides pack into a highly plastic structure, which means that the chocolate becomes rubbery. Chocolate connoisseurs frown upon this bendy stuff. Less-finicky chocolate makers aren’t so picky, which is why the cheap stuff I tend to buy can be oddly flexible.

Version V (said Goldilocks) is just right. It snaps nicely, stands up to a reasonable temperature, and melts enticingly on the tongue. Its surface also has a nice gloss – look at some fancypants chocolate sometime. It’s shiny!

Welllll okay. You only want the fifth crystalline form. So you take some chocolate and heat it to 45 C or so. It melts completely. Then you cool it to a temperature of 27 C and keep it there for a while. At 27 C, crystals of types IV and V will start to form. Eventually you heat it up to 31 C, melting the IV crystals but allowing the V crystals to continue growing until the entire thing has solidified. That’s tempering. Alternatively, if you’re not into hanging around at a constant temperature, you can seed the 27 C batch with some bits of solid version V chocolate. Crystals will form around those bits before many version IV nuclei can form. And you have a nice batch of fine chocolate, which will impress people and get you ALL THE GIRLS.

I dunno about ALL THE BOYS though. I’ll collect some data and get back to you.

Filed under: IAP   |  3 Comments