Tempering cools the chocolate to particular temperatures to produce shiny, smooth bars.
It does this by creating the desired crystalline structure in solid chocolate. Crystal formation is a very complex procedure, and it differs depending on the type of chocolate being tempered.
In a nutshell, cocoa butter, the fat in chocolate, can crystallize in several ways, but only beta crystals harden into firm, shiny chocolate that breaks with a snap. These are the characteristics of chocolate that professional chefs and bakers strive to produce. When you buy commercial chocolate, it is in the form of beta crystals.
When you melt chocolate and get it above 94° F, you melt these much desired beta crystals, and other types of crystals can set up. If you simply let melted chocolate cool, it will set up in a dull, soft, splotchy, disgusting-looking form. Even the taste is different. Fine chocolate has a snap when you break it and a totally different mouthfeel from the other cocoa butter forms.
The process of melting and then cooling the melted chocolate so that it will form beta crystals is called tempering. Tempering is necessary only for real chocolate which contains cocoa butter (not coating chocolate). Basically, chopped or grated chocolate is added to the same type of melted chocolate, to slowly cool it to the desired temperature. It is stirred the entire time. For dark chocolate, ideally you want to end up with a temperature of 89° to 91° F (87° to 89° F for milk or white chocolate). If you have kept the chocolate below 92° F during all of this, it is still tempered and ready for use.
The above information was provided by renowned American food scientist and chef Shirley Corriher, in the following article about proper tempering of chocolate. There are also links to articles about chocolate chemistry in general. She really knows her stiff, and it’s a very informative read.
You may also wish to check out Shirley’s acclaimed books:
And, in case you’re game to try tempering at home, here is a great how-to video for dark chocolate courtesy of Brad Kintzer, a chocolate maker/product developer at Scharffen Berger.
Here are some other instructive websites with great photos:
Accompanying article and recipe for chocolate-dipped strawberries with mint whipped cream:
And, to finish off, here are some beautiful examples of professionally-tempered chocolate products. Let your imagination be your guide.