Tempering Chocolate

Introduction:

Do you ever wonder what makes metals strong and shiny? What makes plastics flexible? What makes ceramics really good at absorbing heat? All of these material properties are related to how the atoms are arranged in a material. In other words, the way a material behaves is a function of the material’s crystal structure. In this activity, you will learn about crystal growth and material properties through tempering chocolate. Just like the properties of materials (like hardness and electrical conductivity) are controlled through crystal growth, the properties of chocolate like texture, taste, and shine are obtained in the same manner: by controlling the type of crystals that grow in chocolate.  

There are 6 different kinds of crystals in chocolate, but not all of them look good and taste good. So how do you get the chocolate that you buy at the store? Through material science! Each of the 6 crystals in chocolate are made of the same chemicals, but they have a different shape/structure. We call crystals with the same composition but different structure polymorphs. For example, graphite and diamond are polymorphs. This means that the graphite in your school pencil and the diamonds found in jewelry are made of the same atoms (carbon atoms), but the carbon atoms are arranged in different crystal structures in graphite versus diamond.

In the case of chocolate, each of the 6 crystal polymorphs has a different melting temperature, so by controlling the heating and cooling rates of molten chocolate, we can melt the crystals we don’t want to obtain the type of chocolate crystals we do want. The process of heating chocolate to a desired temperature to grow only one crystal structure (phase V) is called tempering chocolate (Fig. 1). Phase V is the crystal phase that gives chocolate its glossy, shiny appearance, its satisfying “snap” when you break it, and its smooth texture.

However, there is another way to temper chocolate through introducing little bits of already tempered chocolate. These little bits of tempered chocolate act as seed crystals and initiate preferential crystal growth of phase V chocolate crystals (Fig. 2). Seed crystals are often used in ceramic and metal processing to initiate a desired crystal phase to grow.

Material scientists call seed crystals nucleation sites. Nucleation sites are areas in a material that make it easy for crystals to grow. Using nucleation sites to cause preferential crystal growth is a very important process in material science as it allows scientists and engineers to optimize material properties.

Although we may not always think of chocolate as a common scientific material, tempering chocolate is actually quite similar to synthesizing and processing metals and ceramics. Tempering chocolate is a great way to learn about the important role crystal structure plays in material properties. Even the desired properties of chocolate such as its smoothness and shininess are obtained through controlling the crystal structure of chocolate. By performing this activity, we can all learn to be material scientists in our kitchens!

Purpose:

To learn how crystal nucleation and growth impact material properties through tempering chocolate.

Materials:

• • Chocolate bar (We used Ghirardelli 60% Cacao chocolate) 
  • Candy thermometer (Preferably measuring from 75 degrees to 150 degrees temperature) 
  • 1-2 cups water
  • Pot
  • Pyrex or glass bowl that fits inside the pot
  • Knife
  • Spatula
  • Parchment paper
  • Stove top

Instructions :

Step 1: Place 180 g (~ 75%) of the chocolate in the bowl. Place the pot with 1-2 cups of water in it on the stovetop.

Step 2: Simmer the water in the pot and place the bowl with 75% chocolate on top of it as shown the picture below. The steam from the simmering pot will start to melt the chocolate.

Step 3: Keep stirring until all the chocolate melts. Use the candy thermometer to monitor the temperature of the melted chocolate until it reaches 120˚ F.

Step 4: Once the molten chocolate reaches 120˚ F, remove the bowl from the stove.

Caution: Use oven mitts when handling the hot bowl! Do NOT touch the bottom! Be careful!

Step 5: Add the remaining 25% chocolate and keep stirring until all the chocolate is completely melted.

Step 6: Keep stirring while the chocolate melts. Use the candy thermometer to monitor the temperature of the melted chocolate until it reaches 75˚ F – 80˚ F.

Step 7: Using the spatula, spread some melted chocolate on the parchment paper in a thin layer. Let the chocolate cool to room temperature by letting it sit on the kitchen counter for 2-5 minutes, or by placing it in the refrigerator for 1-2 minutes.

Step 8: The chocolate is now tempered, and it should look smooth and shiny. Tempered chocolate should have a nice snap to it too! Enjoy!