The Materials Science of Chocolate
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The Materials Science of Chocolate
Peter Fryer and Kerstin Pinschower Introduction Chocolate is a common confectionery material throughout the world that has seen generally increasing production trends over the last 10 years.1 Making chocolate requires an understanding of how the consumer perceives it. The preferred type of chocolate varies from country to country; for example, common U.S. and U.K. chocolate tastes are mutually incomprehensible, while the rest of Europe hates both of them! The different tastes and uses for chocolate reflect the histories of the industry in different places. The taste of chocolate is partially determined by the chemistry of the product; typical formulations of chocolate are shown in Table I. Whatever type or taste of chocolate, however, the taste experienced by the consumer also depends critically on the micrometer-scale structure of the chocolate, which can consist of crystals and particles ranging from 10 m to 120 m in diameter, depending on the product.2,3 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. This is a materials-science problem; making chocolate involves solving problems that are familiar in other areas of science. 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
MRS BULLETIN/DECEMBER 2000
which the chocolate is heated and sheared prior to its final solidification. The complexity of chocolate arises from the polymorphic nature of its constituent fats, which can come in at least five crystal forms. Cocoa butter is chemically a multicomponent mixture of triglycerides and trace compounds.4 Approximately 85% of the composition consists of just three triglycerides: POP (20%), POS (40%), and SOS (25%), where palmitic (P), oleic (O), and stearic (S) acids are the 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.5 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,” which is sweeter and cheaper than dark chocolate. Examples of the conditions under which the various polymorphs arise are given
in Table II. The melting points are those given by Wille and Lutton,6 who als
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