Chocolate Crystals!

Kids love Chocolates. I like it too! What makes chocolates so special is their taste, the way they melt on our tongues and how they become a mess if they are out of the refrigerator for a while! These characteristics depend on the way chocolates are made. Chocolate is a common confectionery material throughout the world which has seen increasing production trends over the last ten years. The preference of chocolates varies from country to country and the different tastes and uses for chocolate reflect the history of the industry in these places.

The taste of chocolate is partially determined by the chemistry of the product. The type and taste of the chocolate depends on the micrometer-scale structure of the chocolate consisting of crystals and particles ranging from 10 µm to 120 µm in diameter depending on the final product. Taste depends on the release of flavor compounds to the mouth and nose, while perceived texture is a function of the way in which the material melts and breaks up in the mouth. Chocolate is made primarily of cocoa and milk. Cocoa butter is chemically a multicomponent mixture of triglycerides and trace compounds. Palmitic, Oleic and Stearic acids are the most prevalent fatty acids attached to the glycerol base. The exact composition depends on factors such as growing conditions and therefore can vary between batches, especially from different geographic regions. Milk fats typically consist of one long chain and two short chains. Milk fat is one of the few fats that are compatible with cocoa butter, with which it builds a continuous phase. Addition of milk fats gives ‘milk chocolate’, the sweeter and cheaper cousin of dark chocolate.

Making chocolates requires an understanding of how the consumer perceives it – no one would like a chocolate which melts too quickly, or too slow, and which tastes crunchy or smooth. The actual chocolate manufacturing process is thus a material science problem, and takes into consideration all the requirements of the product. Industrial chocolate processing is well developed and includes several complex operations for the development of flavor and texture. Chocolate is first mixed and ground to give a mixture of the correctly sized particles. The process of ‘conching’ then involves the mixing and shear of the chocolate under controlled conditions and results in the removal of volatile components and adjustments in moisture content and viscosity. This process results in chocolate with the correct flavor profile. The production and structuring of the solid material prior to molding involves a further complex step, that of tempering, in which the chocolate is heated and sheared prior to its final solidification.

Processing looks easy, but the complexity of chocolate arises from the polymorphic nature of its constituent fats, which can come in at least five crystal forms. Forms I to IV form on varying intensities of heating, and have different crystal structures. The lower-melting polymorphs generally form by cooling to a lower temperature at a faster rate, and transform to higher-melting forms with time. Form VI is generally associated with fat ‘bloom’, an unpleasant-looking appearance of the chocolate surface. It is widely believed that good temper prevents fat bloom to some extent, as does the presence of free milk fat. The microstructure of the sucrose in the chocolate is also a critical factor: chocolate made with amorphous sugar does not bloom, while that made with crystalline sugar does. The key practical problem for the chocolate maker is that the thermodynamically stable Form VI is not the one that we find attractive, but the more transient Form V. The complexity of the chemistry is such that it is crucial to get the right crystal form. Recently, ultrasound, high shear/low temperature crystallizer, and cold extrusion have been used to improve tempering of plain chocolates.

So when we eat chocolates, it’s not just a piece of sweet wrapped in a golden foil, but also a souvenir of the science involved!

*This post is not sponsored by Dairy Milk!