The Polymorphism of Chocolate

Have you ever wondered why a chocolate bar that has melted and resolidified doesn’t have the same shine or snap that it did before? We are going to take a quick detour from talking about how atoms can group together to talk about some of the messy, interesting, and delicious chemistry of chocolate.

Chocolate is actually a fermented food, like sourdough bread or beer. To make chocolate, you start by taking the fruit pods of the cacao tree and letting them sit for several days, so that the wild yeasts which are on the surfaces of most fruit can start digesting the sugar in the fruit. After the fermentation, the pods are dried, and the seeds are removed from the dried and fermented fruit flesh; these seeds are called cocoa beans. The beans are roasted, then removed from their shells, and finally ground into a thick paste from which the dissolved fat, cocoa butter, can be removed (or more can be added!). Vanilla, sugar, and other flavors can also be added at this point, but much of the character of the chocolate comes from the initial fermentation and the roasting. Once the flavors are as desired, the chocolate is ready to be tempered!

Tempering is the process of heating a material to a specific temperature and then letting it cool into a solid, where the temperature is chosen so that the solid will have certain properties. It’s important in chocolate because the fats in chocolate have six different crystal phases, that is, different configurations in which they can solidify. This is called polymorphism, and it’s not unique to chocolate! The orientation of the fat molecules relative to each other, and their bonding, determine how easy it is to break apart the solid. This means that the melting temperature for each crystal phase is different, with some phases being easier to break apart than others. The six phases of chocolate are generally numbered with Roman numerals in ascending order of melting temperature, and it’s phase V that has the glossy sheen and satisfying snap that we are used to in commercial chocolate. And as an added virtue, its melting temperature is very close to human body temperature, so that it can melt in your mouth!

So how can chocolate be preferentially solidified in phase V? It has to be heated above the melting temperature of the unwanted phases I-IV, so that none of those crystal structures can form. But if it’s heated above the melting temperature of phase VI, it may resolidify in that phase, which is exactly what happens when you leave a bar of chocolate in the sun and it melts and resolidifies. Thus, the usual tempering procedure in chocolate manufacture is the following: first, heat the chocolate above all the melting points, to get rid of existing crystal phases. Then cool it to around the phase V crystallization temperature, and keep it there for awhile so that that regions of phase V begin to form. If you stir during this process, you will get many small crystals which can act as seeds for the desired crystal phase. Finally, cool the chocolate, causing the phase V seeds to spread so that the entire chocolate bar has the desirable crystal phase.

Chocolate is a great example of how different material properties result from the arrangement of matter at a small scale. And if you’re interested, you can always learn more about the history, manufacture, and usage of chocolate at its wikipedia page.


17 responses to “The Polymorphism of Chocolate

  1. Is that why the surface of chocolate turns white after it’s melted in the car? Is that something about phase VI, or an additive in the chocolate coming to the surface?

    • Jessamyn Fairfield

      In most cases, the white stuff on the surface is cocoa butter in the chocolate that has slightly melted and recrystallized in phase VI (which is the most stable solid phase). This is appealingly labeled ‘fat bloom’. You can also get sugar bloom, when condensation on the surface draws out sugar that’s left behind when the condensed water evaporates. If you touch the white bits on the chocolate and they feel dry, that’s sugar bloom, and if they feel oily, it’s fat bloom.

      One way to reduce fat bloom is by adding a bit of milk fat to the chocolate, which seems to slow crystallization and thus improve the purity of the phase V structure. Even a tiny bit of milk fat in dark chocolate slows fat bloom. But since the phase VI structure is more energetically favorable, it’s hard to permanently prevent chocolate from trying to get there. Thus it’s important to eat chocolate promptly!

  2. Two related physics and phases bits.
    In many alloys, the tempering that works for chocolate to attain a particular phase does not work. Steel avoids a lot of phases that would seem to be preferred.
    As to energetically favorable, graphite is favored over diamond. The transition time, however, is millions of years.

  3. Hi Jessie! What about cooking with chocolate? For example, chocolate chips in cookies retain their original shape. The oven is more than melting temperature so….? If you melt choc chips on the stove, they retain their outer shape until you stir and they are instantly creamy. Does it have to do with the surface of the chip? That is, do the insides melt but the surface tension holds the outer form together as long as it is undisturbed by touch?
    Thanks for the article on chocolate, all my knowledge of chocolate before this has come from my senses!

    • Jessamyn Fairfield

      Chocolate chips are different in a few ways! They often have either a lower fat content, or vegetable oil instead of cocoa butter, in order to raise the melting temperature since they are so often used in baking. In cookies, the chocolate does melt to some degree but the dough holds it together.

      I’m guessing that the chips retaining their shape as you melt them on the stovetop depends on how much above their melting temperature they get. At a high enough temperature, they should be completely liquid, but during the phase transition they may smear out when stirred before they’ve lost their shape (like warm butter does).

  4. And a single 3 oz. bar of dark chocolate (85 g to you foreigners) provides 30 – 40% of the dietary fiber anybody sane would want to consume in a day. Seems odd, since chocolate is so smooth.

    • Jessamyn Fairfield

      You know, I recently found out that coffee has more fiber than orange juice and was pretty surprised by that too. Cacao and coffee beans are very high in fiber, and it seems that quite a bit of that fiber makes it through all the processing steps.

  5. Bring on the mocha!

  6. Very apropos for the holiday! Congrats on the blog and you tricked me into learning a new thing for the day.

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  9. terrence chaplin

    I recently bought a chocolate bar , melted it , added some nuts and coconut , then did not get it back to the phase v I wanted. I did it by eye, no thermometer. but afterwards , it was very hard to break off a piece, it was messy. had to leave in the frig. to eat it that way. how do you know when to stop heating and pour it out to cool it back to the proper temper? would love to learn that.

    • Jessamyn Fairfield

      The most reliable method would be to use a thermometer! If you overheat chocolate it takes a lot of work to temper it again. But you can find a lot of recipes for tempering chocolate around, and you’ll want to adjust the temperature you heat to depending on the milk content of the chocolate. One thing that helps a lot is to add some unmelted chocolate bits just before you cool things down, to seed the proper crystal phase.

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  11. excellent article. I am glad you wrote this as we are having an food chemistry midterm tommorow and this subject is on it and I did not understand the phases or tempering before reading this. thanks.

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