Reader Rainey writes in to ask me to expound a little on the phenomenon of crystallization, specifically in sugar (marshmallows) but also in chocolate. Those who know me know that if there’s one thing I like to do, it’s expound. So let’s take the first part first.
Sugar crystals can be a blessing or a curse depending on the outcome you’re after. They’re great for, say, rock candy, but not so great for marshmallows. Marshmallows are made from table sugar (sucrose), which is especially crystal-prone for the simple reason that its molecules are so similar to one another. When you have a lot of similar molecules in very close proximity to one another you get a sort of LEGO effect: the molecules stack up on each other and whammo: crystals. To prevent that from happening you need to bring some random flotsam and jetsam to the party: molecular junk that will get in between those very small, similar molecules so they won’t be able to lock together. This is the service that corn syrup performs. It’s made up of a hodgepodge of longer-chain sugars which get in between those little sucroses, preventing them from arranging themselves in orderly stacks.
It’s always tempting to think that whenever you melt table sugar down (break up the crystals with heat) you defeat their crystal-forming impulse. But any victory on that front is only temporary as they’ll just re-form again as they cool down (water doesn’t much interfere with the process). This is why you generally get crystallization in marshmallows when you don’t use corn syrup. The crystallization begins almost immediately upon cooling, but isn’t terribly noticeable to the eater for many hours, or up to a couple of days. (Individual results may vary).
Chocolate tempering is all about crystal formation as well, however in that case the crystals aren’t made of sugar but fat (cocoa butter). What, you didn’t know fat can crystallize? Oh sure it can. In fact any fat that’s solid at room temperature is a crystallized fat (butter, lard, cocoa butter). Any fat that’s not solid, well…isn’t. Those fats we call oils. The reason some fats crystallize and others don’t is because the substance we call “fat” is made up of many different kinds of fat molecules. That being the case, fat in general doesn’t form crystals all that well. However if there are enough similarly-shaped molecules in the mix, you’ll get crystallization. The degree to which you get crystallization dictates the relative firmness of the fat.
Following so far? Groovy. Cocoa butter is unusual stuff in that only a few different types of fat molecules dominate its structure. The result is lots and lots of crystallization, and a very hard fat. One very interesting thing about cocoa butter is that once it’s melted, it’ll crystallize in an orderly or disorderly way depending on how the temperature is manipulated. That manipulation process is what’s known as tempering. There’s lots more on fat crystallization if you care to check the posts that occur around mid-October of 2009.
I should add that while many people associate chocolate “seizing” with bad tempering, it’s actually a different phenomenon. More on that here. Thanks for the great questions, Rainey!