Shirley O. Corriher
American Chemical Society
New York, October, 2007
Cocoa butter, the fat in chocolate, can crystallize in any one of 6 different forms (polymorphs, as they are called). Unfortunately, only one of these, the beta crystal (or Form V), hardens into the firm, shiny chocolate that cooks want. Form VI is also a stable hard crystal, but only small amounts of it form from the good beta (Form V) crystals upon lengthy standing. 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.
How can we get chocolate to set up in these hard, shiny beta crystals? 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 for compound chocolate or summer coating which contains fats other than cocoa butter.
You need a truly accurate thermometer. Most kitchen thermometers (even the digital instant-read type) can be off by 10° F. If you do much chocolate work you'll want a laboratory quality thermometer.
When I asked chocolate expert Dr. Paul Dimick about tempering he said the easy way is to never heat chocolate over 91° to 92° F. Beta crystals do not melt until 94° F. So, you never lose all of these prized crystals and your melted chocolate is already tempered. What a wonderful idea!
Your goal is to barely melt the chocolate. All of these crystals have a range over which they melt and chocolate melts at 89° to 90° F even though all the beta crystals do not melt until above 94° F. You can keep the chocolate over a very low heat source and, with constant stirring, melt 2/3 of it. Then, remove it from the heat and patiently continue stirring until all the chocolate is melted. 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.
You need to grate the chocolate or process it in the food processor until it is finely chopped so that it will melt evenly. Place it in a metal bowl and heat with very low heat. Constant stirring is a must. Some chocolate experts set up a heating pad on low or a small coffee warmer or hot tray. Some like to microwave the chocolate in short time intervals with frequent stirring (50% power for dark, low power for milk or white chocolate)--any way that you can keep the heat 90° to 92° F. You can use a warm water bath but you must take great care not to get even a drop of water in the chocolate which will cause it to seize (see Seizing above).
If you get the chocolate over 94° F, you lose your prized beta crystals and you must go through the full tempering process. First, you must completely melt the chocolate. The risk here is separation. Chocolate can irreversibly separate into golden cocoa butter and grainy black cocoa particles. You can use the cocoa butter as a great hand cream, but your chocolate is gone. Most cooking literature advises you not to get chocolate over 120° F. Melting curves of chocolate in the technical literature indicate that most of the fats in cocoa butter are melted by 122° F. Some processors recommend heating their chocolate slightly higher--up to 131° F.
Cocoa butter, as most natural products, is a complex mixture of fats and contains small amounts of fats that do not melt until high temperatures--over 200° F. Dr. Dimick explained that cocoa beans from different locations are very different and that the plants adapt to the climate of their surroundings. At the same temperature, cocoa butter from Malaysian beans which grow near the equator is quite firm, while cocoa butter from Brazilian mountain grown beans which grow in a much colder climate is quite soft. Dr. Dimick says one of the major factors causing separation is inadequate stirring.
Ideally, consult the chocolate processor for the best temperature for your chocolate. As an overall temperature, I would suggest melting dark chocolate no higher than 122° F (110° to 118° F for milk or white chocolate) with constant stirring.
Now, the chocolate must be rapidly cooled to about 82° F for dark (79° F for milk and white). Always use constant stirring. Cooling this low does allow undesirable beta-prime crystals to form, but it gets good crystallization of the beta crystals started. Now, the chocolate is warmed gently to raise the temperature to 86° F for dark (84° F for milk and white). It should be held at this temperature for a few minutes, then warmed up to 91° to 92° F for dark (87° to 89° F for milk or white). Bringing the chocolate up to this higher temperature melts the undesirable beta-prime crystals that were formed.
There are a number of methods to achieve this rapid cooling. Regardless of the method, the one thing that you must do is to stir constantly. Some chefs like to spoon 2/3 of the chocolate out onto a cold surface like a marble slab and scrape the chocolate back and forth with a spatula until it is about 82° F. One must work fast doing this and it is difficult to get a quick temperature reading on the slab. Then, the cooled chocolate is blended with the warm reserved chocolate to bring it back to the desired temperature.
Dr. Dimick uses a cold water bath. He places the stainless steel bowl of melted chocolate into a bowl of ice water and stirs constantly until the chocolate cools to 82° F for dark (79° for milk or white). Then, he warms it to 86° F for dark (84° F for milk or white). He lets it remain at this temperature for a couple of minutes then heats it back up to 91° to 92° F for dark (89° F for milk or white).
A fairly simple way to lower the temperature fast is to stir the chocolate with a big lump (3-inches or so) of tempered chocolate or grated chocolate. This melting chocolate both cools and seeds with the right kind of crystals. You can lift out and reuse the lump of chocolate when you reach the correct temperature. If using grated chocolate to cool, add only a tablespoon at a time to insure that you do not end up with unmelted fine particles. Chocolate expert Alice Medrich uses a submersible blender to constantly stir while cooling, taking care to keep it submerged. This is certainly an excellent way to stir large batches of chocolate.
Some chefs like to temper melted chocolate by simply stirring constantly in a cool room.
Tempering machines have the advantage of constant stirring and accurate temperature control. I notice that some of the machines recommend placing a lump of tempered chocolate in front of the machine stirrer blade to seed the chocolate and encourage the growth of beta crystals. These machines are available for under $400 and are worth it if you do a lot of quality chocolate work.
If you use a dry method of heating, you avoid the risk of seizing that you always have with water baths.
There are a number of ways to check to see if your chocolate is tempered. Spread a smear out on a piece of waxed paper, if it dries shiny and hard within 5 minutes you are fine. Dr. Dimick uses the "string" test. After he has cooled and brought the chocolate to temperature, he spoons up a little and drizzles a string of chocolate on the surface. If it disappears instantly, he knows he does not have enough crystals formed to hold it up for a few seconds and it is not tempered. Stirring constantly with a lump of tempered chocolate may fix it. But, if it is simply not tempered and you have the wrong kind of crystals, you must begin again and go through the entire melting and cooling as described.
|Form I (beta -prime 2)||61° to 67° F|
|Form II (alpha)||70° to 72° F|
|Form III (mixed)||78° F|
|Form IV (beta-prime 1)||81° to 84° F|
|Form V (beta 2)||93° to 95° F|
|Form VI (beta 1)||97° F|
© Copyright Shirley O. Corriher, 2007