The nightmare of chocolate manufacturers is a process called “chocolate bloom”. This is when chocolate appears to be covered with a white layer making it unappealing and losing its right texture.
There are actually two kinds of chocolate bloom: sugar bloom and fat bloom. Sugar bloom has to do with the bad relationship between chocolate and water. We saw in a previous post that a few drops of water in melted chocolate dissolve the sugars in the chocolate but not the fats and the lecithin molecules trap this sugar lumps making the chocolate less fluid.
What happens here is similar: when a chocolate bar is stored in humid conditions the water dissolves the sugar in the surface of the chocolate forming sugar lumps in the surface.Then the water evaporates and what remains is a sugar layer covering the chocolate. In this kind of bloom, you can remove the white layer by rubbing the chocolate surface with your finger and you can also taste the sweetness when putting it in contact with your tongue.
Fat bloom is much more complicated and also interesting. The other day we were saying that diamond and graphite are actually made of the same material: carbon atoms. The only different is that the atoms are arranged in a different way in each of them: they are two phases of carbon (only that they are both solid phases). The same thing happens with chocolate. Solid chocolate is a a crystal whose building blocks are basically the fats in the chocolate. These fats can arrange themselves in six different ways resulting in six solid phases of chocolate. Like with carbon, each of these phases has different properties although the differences are not as big as between diamond and graphite.
They are classified according to their melting temperature. The first one (type I) melts at 17ºC (63F) which makes it too soft and crumbly and the last one (type VI) melts at 36ºC (97F), which is a bit too hard. The perfect phase is number V. This one has a glossy appearance and a nice snap when you bite it. It’s melting point is at 34ºC (94F), just a bit below body temperature so that it melts nicely in your mouth when you eat it.
Something to note here is that this classification is for cocoa butter and not other fats. Cocoa butter is however quite expensive, so some chocolate manufacturers use other vegetable fats instead in their chocolate. Such fats usually melt at temperatures higher than the body temperature. The effect of this is that when you eat bad quality chocolate (i.e. made with vegetable fats other than cocoa butter) you feel that it stays in your throat, rather than going down smoothly, leaving you with a nasty feeling in your mouth.
The goal for chocolate manufacturers is to force chocolate to crystallize in structure number V as much as possible. This is quite complicated and is achieved via a process of heating and cooling the chocolate carefully several times. This process is called tempering.
However, after the chocolate is crystallized in form V, there are several factors that can change the internal structure of the chocolate making it go back to a lower quality crystal, provoking fat bloom. One of these factors is sudden changes in temperature which is why when you put chocolate in the fridge on a hot day you will find it covered in a white layer after a while.
They way this happens seems to be not yet properly understood. One theory is that the lower quality crystals present in the chocolate (it is never number V 100%) melt, then the fats migrate to the surface and crystallize there creating the white layer. If the chocolate is not properly tempered, the percentage of such crystals in the chocolate will be higher and the formation of fat bloom easier. You can distinguish fat bloom from sugar bloom because it does not go away when you rub the chocolate with your finger, it also repels a drop of water rather than absorbing it and it does not taste sweet when putting it in contact with your tongue.
It seems that in any case bloom is just re-ordering of the chocolate ingredients so there is nothing wrong with it and the taste should be the same. However the texture is different and you might want to use bloom chocolate for cooking rather than just eating it. I also think that when you buy chocolate and find that it has bloomed it indicates bad storage conditions (in humid places, exposed to sudden temperature changes, etc). Since chocolate absorbs very easily odours and flavors (especially in humid places) the blooming might come together with changes in the taste. For example, if you storage chocolate in the fridge on a hot day, not only will you ruin it’s snap and colour, but also it might absorb flavours from other food in the fridge changing the taste of it.
As you can see, chocolate is actually quite tricky to manipulate. When you melt it at home and then use it for cake couverture, after cooling it won’t go back to crystal V, which is why is very difficult to achieve at home the glossy finish of couvertures. As I was telling you the other day, I know that here in Edinburgh, the Edinburgh School of Food and Wine is having a tempering lesson on August 8th where they will teach you how to do this at home. They also do such courses at Coco Chocolate (also here in Edinburgh) the last Thursday and Sunday of every month in 2009.
I read in some places that another way to do this is to nest melted chocolate with little pieces of solid chocolate which you know is in the right crystal. this way you will induce the chocolate next to it to crystallize in the same form. I tried it but I found that one has to be careful with the size and timing of the pieces added because of several reasons:
- If the melted chocolate is too hot the solid pieces will melt as well losing their crystal structure.
- If the pieces are too big you won’t obtain a flat chocolate layer (although you will on the other side, all you have to do is flip it).
- Finally, the big difference of temperature between the melted chocolate and the solid bits can also force chocolate bloom in the interface.
Here you can see the chocolate bloom I obtained:
And here small areas where the induced crystallization actually worked (the area in the right crystal has a darker and more glossy colour than the rest of the chocolate):
I apologize for the small pictures this time, the sample was quite small so it was difficult to take proper pictures. In summary, I wasn’t very successful with this technique, I will have to give a try to chocolate tempering and see what happens.