Sugar Crystals, Fat Crystals

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!

9 thoughts on “Sugar Crystals, Fat Crystals”

  1. Sometimes, when making caramel by boiling sugar and water, I end up with a solid layer of crystallized sugar at the bottom of the pan. You mention above that water doesn’t interfere with crystallization, and that heat breaks up the crystals. However, this happens while the sugar + water mixture is being heated.

    So, why do I sometimes end up with a solid mass of crystallized sugar, and how can I prevent that?

    1. Hey B!

      Those crystals are a phase sugar naturally passes through on its way to becoming caramel. However you can essentially leapfrog over it if you apply hight heat to the pan instead of heating it slowly, which I suspect you are doing. Try applying high heat neat time and the crystals will probably disappear.

      – Joe

      1. Thanks Joe! You’re right, I do tend to apply medium-low heat when I make caramel. I will try using high heat next time.

  2. I love that you took the time to explain this in such depth and clarity. But I’m still not getting why the marshmallows that I made with a simple syrup weren’t stable while the ones I cooked to the soft ball stage were if there isn’t some kind of crystal or something critical involved.

    Thanks so much. I really admire the depth of your understanding and your generosity in sharing what you know. Really! mwwwwahhhh!

    1. Hey Rainey!

      Hm…you mean that after all that I didn’t even answer your question? Amazing, but really no surprise given me and my ramblings. But that’s a good question. I confess I’m not completely sure about the answer. Crystals generally start because they have a seed of some sort to start from. Might there have been a few unmelted crystals on the rim of the pan that got into the whip?

      – Joe

  3. I make marshmallows using golden syrup, whenever I make it in small quantities it comes out perfect. Whenever I try to increase the quantities to pour into a large tray for some reason the marshmallow becomes rock hard after a few hours.

    I cannot figure out why it would do this, I have changed the recipe, increased the amount of syrup and still it isn’t soft however when I reduce the quantities it seems to set perfectly. Any thoughts?

    1. Hey Louisa!

      That’s an interesting problem. It seems all but certain the sugar crystallization is the problem. Do you think you might be leaving any un-melted sugar around the rim of the pan when you’re making the syrup? That could be causing it. You might try upping the amount of gelatin and/or corn syrup, since those will help stop crystals from forming.

      I’ll keep thinking, Louisa! Keep me updated on the situation!

      – Joe

    2. I also meant to ask…you’re using golden syrup in place of the corn syrup…correct?

      – Joe

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