Thursday, May 22, 2014

Chocolate Chemistry: Tempering

Untempered (left) vs. Tempered Chocolate (right)

This was one of my favorite days because I learned so much! I had always read in recipes, things like "melt the chocolate slowly and at 50-percent power, stirring frequently!" and wondered why on earth you had to be so careful. I always warmed my chocolate at full power and it was totally fine (I thought). You do have to be a little careful not to burn chocolate (or it will seize), but that seemed like such a rare occurrence that it was hardly worth mentioning.

After learning about tempering, I finally understand! As I understand it, cocoa butter crystallizes as it cools, and at different temperatures, it forms different types of crystals. (Just like in igneous rocks!) The crystals are named alpha, beta, gamma, etc. based on the temperature at which they form. The tempering process is basically this: you heat the chocolate high enough that all the crystals dissolve. Then you cool it slowly to a certain temperature (84 F) and then slowly and carefully heat it again, not letting it get above 90 degrees for dark chocolate, or 88 degrees for milk and white chocolate. At that temperature, only one type of crystal can form, and when all the crystals are the same type and shape, they form a really tight, interlocking lattice. That makes the chocolate shiny, it makes it have a snap to it when it breaks, and it holds the fat molecules tightly within the lattice so they can't get out.

Untempered chocolate, as you can imagine, has the opposite characteristics: the varying crystal sizes mean that there is no tight lattice of crystals, so the chocolate is dull, doesn't snap, and the fats can escape and cause that white film called "fat bloom."
Sometimes fat bloom just looks like a white film, as in the picture at the top, but sometimes it actually makes these ugly dots on your chocolate, as above.

Here is an experiment you can do with fat bloom, or you can do as we did, and simply get some of your chocolate out of temper so you can compare it with the tempered stuff.

Storing your chocolate at too high a temperature, or melting and re-hardening, can also cause fat bloom.

When you're melting chocolate chips, as I so often do, there's another factor: chocolate chips actually have additives that make them resist melting. This is so they will keep their shape and look nice in cookies. (You can see the difference in the cookies above, which were made with a mix of chocolate chips and chopped melting chocolate. The chips in the left cookie are mostly intact, while the chocolate piece at the bottom of the right cookie has melted and lost its shape.) 

So, if you're trying to melt chocolate chips, you have to do it at a higher temperature (meaning they will probably get out of temper) and they will not be as smooth and runny as chocolate intended for melting. If you're going to add liquid anyway (say, to make the chocolate frosting for Sebby Cake), it doesn't matter whether you use chocolate chips or not. But for dipping or coating things in chocolate, it matters quite a lot.

When chocolate is made at the factory, it's always tempered, so when you buy it the tempering is already done, and the easiest thing to do is just keep the chocolate in temper. That means never heating it above 90 F (or 88 for milk and white chocolate), which is why all the caution and half-power recommendations that used to so puzzle me in recipes. If your chocolate gets too hot and gets out of temper, you can re-temper it, using a couple different methods. There is a ton of great information on chocolate tempering here and here. It seems intimidating at first, but it's really fairly straightforward once you have the general idea.

We melted some chocolate, some hastily and too high so it would get out of temper, and some slowly and carefully to keep it in temper. Then we dipped lots of things in it.
Tempered (left) and Untempered (Right)
Boys dipping various things in chocolate
Again, the contrast between tempered chocolate on the left (shiny and glossy) and untempered on the right (filmy and dull)

I should point out that tempering your chocolate (or keeping it in temper) is unnecessary if you're adding other ingredients to it, and even if you're planning to eat all of it right away. The fat bloom can be a bit unsightly, but the untempered chocolate tastes fine and sometimes even looks fine until the next day. But if you're trying to make chocolates that look really beautiful and fancy, and keep their lovely shiny appearance for several days, you should make sure you're using tempered chocolate. This was a revelation to us and we're really glad we learned about it!

Oh---and also of interest, speaking of chocolate chemistry, is this page on the difference between theobromine and caffeine (scroll down a bit). My neighbor when I was growing up was a chemist, and I can still remember talking to him at the ward campout one time, about how and why caffeine in chocolate was different than that in coffee. Very interesting.

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