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Bad Chocolate?

I mentioned that I had a lot of fun experimenting with carbonated soda (and drinking it) and that I should do an experiment with chocolate. I got quite a few e-mails suggesting experiments with chocolate, but this one was the most common. It has to do with the white discoloration that you sometimes find on old chocolate.

For this experiment, you will need:

  • a plastic food storage bag
  • chocolate - I used chocolate chips and bar chocolate, using milk chocolate and - semi-sweet chocolate.

First, eat some of the chocolate. Notice the taste and the texture. Since we are going to change the chocolate, we need a control to compare the results with. Keep plenty of chocolate handy, as I found it necessary to refresh my memory frequently. I ate almost an entire bag of semi-sweet chocolate chips just to be sure I was familiar with their flavor.

Next, we are going to cause the chocolate to "bloom." That is the term that is used in the chocolate industry to describe the changes that happen in old chocolate. To do this, put some chocolate into the plastic bag. We want to heat the chocolate enough to partially melt it, without turning it into a puddle. I placed the bag on the dashboard of my truck and left it there for about 5 minutes. This worked much better than using the microwave, which tended to melt them too quickly. The chocolate should be soft, but should still be in a lump, not a runny liquid. If the chocolate melts completely, then get some fresh chocolate and try again. So as not to waste the over-melted chocolate, I put it on some sliced bananas for a snack.

Once you have some partially melted chocolate, place the bag someplace where no one will eat it. By the following day, you should notice quite a change. The surface of the chocolate probably is lighter color and has very light colored blotches. Break off a piece and taste it. Don't worry, the light splotches are not mold. While the taste has not changed much, you will notice a big difference in the texture. Instead of being smooth, the chocolate now feels grainy. Be sure to eat some more fresh chocolate to compare the two.

What has happened? The light colored patches are cocoa butter, one of the main ingredients in chocolate. When cool enough to be solid, cocoa butter forms crystals. It is polymorphic, which means that at different temperatures, it forms different kinds of crystals. For chocolate to be smooth and creamy, the crystals must be very small. When we melted the chocolate and let it cool again, it formed larger crystals, giving it a grainy texture.

Cocoa butter is monotropic, which means that even if it does not melt, over time the small crystals will slowly change to the larger form. To prevent this, chocolate should be kept in a cool, dry place, but not in the freezer. If frozen, condensation can leach out the sugar, again causing white blotches. I feel that by far the best strategy for chocolate is to eat it all quickly, before it has a chance to go bad.