Experiments have been carried out on the tempering of chocolate using a temperature-controlled shearing rig with a concentric cylinder geometry. This design maximizes uniformity of shear rate during tempering in contrast to most tempering devices where shear is often concentrated in a small part of the chocolate mass. Samples were subsequently cooled in a differential scanning calorimeter (DSC) to monitor how the sample crystallized, and then reheated to gain further information on the melting points of the polymorphs formed. The results can be interpreted using established theories on the crystallization mechanism. It was found that at least two polymorphic forms could be generated. The higher melting form predominated provided the shear rate was high enough, the temper time long enough, the rewarm temperature low enough, and the DSC scan rate slow enough. For parameters affecting the nucleation stage (temper time and shear rate), it was found that the transition was a sharp one, reflecting the notion that seed crystals need to grow past a threshold point in order to be stably formed. Raising the rewarm temperature had the effect of destroying seed nuclei, but this was a more gradual process. The bimodal nature observed of the melting points compared to the more spread-out behavior of crystallization temperatures reflects the kinetic constraints found in crystallization which are not found on melting.Paper no. J9013 in JAOCS 76, 677-685 (June 1999).In many processed foods the product food structure depends not only on the ingredient formulation but also on the processing history of the material. This is particularly true for chocolate molding during which the fat component (cocoa butter for dark chocolate and cocoa butter and milk fat for milk chocolate) solidifies. The cocoa butter forms crystals but these can be in a number of different "polymorphic" forms (1). At least five (and possibly six) independent polymorphic forms are known to exist. Those with the lower melting points are formed most readily but are thermodynamically the least stable (2). The polymorph known as Form V has long been accepted as the optimal form for commercial chocolate manufacture, in that its melting range (which for milk chocolate lies between 20 and 30°C) is high enough for chocolate to be hard at room temperature, but also low enough that the product can melt in the mouth. The form also exhibits good demolding properties (3). Unfortunately, Form V does not form in bulk by simple cooling of chocolate. A small quantity of seed crystals of the polymorph form needs to be present so the remaining fats can solidify around it. This can be achieved by adding pre-made seed crystals to the mixture, but most current large-scale chocolate production involves subjecting the chocolate to tempering. During tempering the chocolate is subject to a well-controlled temperature program under the action of shear. Proper tempering is a vital part of successful chocolate manufacture as it determines the quantity and size distribution of seed crystals of...

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