Chocolate
Why it Melts in Your Mouth
Chocolate is made from the seeds found in the pods of the cacao fruit tree. These beans are first partially fermented; then dried, roasted, and crushed; and finally pressed to extract their fats in the form of cacao butter. The remaining solid particles can be milled to make cacao powder or cocoa powder, which is very similar but is processed at a higher temperature.
Dark chocolate, which is solid at room temperature, is a complex mixture of cacao powder, cacao butter, and sugar. In most types of chocolate, the individual cacao particles are ground up so finely that they are not distinguishable on the tongue. Technically speaking, chocolate is a sol, or a solid colloidal system—that is, a suspension of solid particles of sugar and cacao powder in a solid matrix of cacao butter. The very special mouthfeel of chocolate is due to the specific melting properties of the cacao butter.
Fats from plants and animals are usually made up of a number of components that have different melting points. Consequently, a given fat melts over a wide range of temperatures. This is true, for example, for butter and lard. In exceptional cases, some mixtures of fats melt over a narrow temperature interval, almost at a well-defined temperature. Cacao butter is one of these very special cases. It is primarily made up of three different types of triglycerides, which contain both saturated and unsaturated fatty acids. But due to its large proportion of saturated fatty acids, the mixture has a relatively high melting point of 90°–97°F (32°–36°C), just below the normal temperature in the mouth. This is the crucial factor that contributes a mouthfeel that is so agreeable that most people love—even crave—chocolate. And the actual process of melting the cacao butter requires a certain amount of heat, which is drawn from the body, so the overall effect can be a sensation of coolness.
The inner structure of chocolate depends on how it is processed. Even though two different pieces of chocolate have an identical chemical composition, they may not have the same mouthfeel. For instance, if a piece of chocolate is melted and then solidifies again, it will not taste the same, because its mouthfeel has been altered. This happens because the fats in the cacao butter can form six different types of crystals with discrete structures and only one of these types will result in a glossy surface and the right degree of brittleness.
Chocolate intended for use in baking and candy making is produced using a method known as tempering. This is a way to ensure that it crystallizes into precisely those structures that will result in what is considered a good mouthfeel. Untempered chocolate is soft and does not snap, whereas tempered chocolate has a glossy surface, is brittle, and does not melt when you hold it between your fingers. The desired structure can be obtained by seeding the melted chocolate mass with small pieces of chocolate before allowing it to cool. Another way of doing this is to pour the melted chocolate out onto a marble slab and repeatedly fold it over on itself with a spatula while it cools slowly. This mechanical action encourages the growth of the right crystals. In both cases, it is vital that neither water nor steam is incorporated into the chocolate, as this would result in the wrong crystals, known as seizing, producing an undesirable graininess or lumpiness. Tempered chocolate can be made more stable by the addition of a number of emulsifiers—for example, lecithin.
Incorrect crystal formation can also result in what is called a bloom, the white or grayish spots that are seen on the surface of chocolate that has been stored at too high a temperature or in too moist an environment, has been kept too long, or that has been exposed to the sun. It is best to store chocolate in a dark, dry place with a constant temperature of around 61°F (16°C), so as to prevent recrystallization of the cacao butter. The bloom is made up of either sugar or fat that has migrated to the surface and formed a crystal with a high melting point. These spots can also be attributed to fats from nuts that have been added to the chocolate. Formation of bloom can to a certain extent be prevented by adding emulsifiers or other fats that can combine with the fats in the cacao butter—for example, milk fat, which is used in baked goods.
Unlike dark chocolate, white chocolate contains only cacao butter, milk fat, and sugar. Its color is due to the absence of cacao particles. Milk chocolate also contains milk fat, making it lighter than dark chocolate. Ganache, used as a filling in chocolates and cakes, is a mixture of chocolate with cream or butter—the more cream or butter, the softer the ganache. Because it is an even more complex mixture than chocolate, it can be tricky to work with ganache.
Different cultures tend to have preferences with regard to the mouthfeel of chocolate. For example, Mexican chocolate has a coarser-grained texture than the creamy, even texture of Swiss or Belgian chocolate, which typically has particles that are only 20 micrometers in size. For Mexican chocolate, the beans are not ground as finely; the cacao is often mixed with spices and contains sugar that has bigger crystals. The result is a more complex sandy and gritty texture. Particle size also has a major influence on how the chocolate flows, which in turn has an impact on the mouthfeel of the melted chocolate. The finer the particle size, the lower the viscosity and the greater the creaminess of the melted chocolate. Swiss chocolate is renowned for its especially soft texture, which is due to a discovery made by Rodolphe Lindt in 1897. Lindt observed that by conching (which resembles kneading) the cacao particles with cacao butter, the resulting chocolate became very smooth and flavorful. Lindt then invented a machine to mechanize this process.
It is not possible to use tempered chocolate to coat ice cream and frozen desserts because the temperature in the oral cavity, thanks to the cold food, seldom rises above 86°F (30°C). At such a relatively low temperature, tempered chocolate remains solid and firm. In contrast, untempered chocolate is captured in the correct crystalline form by flash freezing it to 0°F (−18°C), causing the cacao butter to crystallize into a series of different structures with low melting points, typically around 77°F (25°C). As a result, the untempered chocolate coating melts in the mouth along with the ice cream. As long as the product is stored in a freezer kept at 0°F (−18°C) or less, the crystal structure remains intact. Another method for achieving a low melting point is to mix in other fats—for example, coconut oil.
