E61 Fusion and Solidification - Butter and Chocolate

Topic

This butter and this chocolate are mixtures of triglycerides that—unlike a pure substance—do not have a single melting point but rather a full melting range; this is because each fatty acid present in the mixture has its own specific melting temperature, and the mixture consequently undergoes partial transitions throughout this melting range rather than a single, abrupt transition. The fact that the triglyceride mixture in this butter or chocolate contains fatty acids with widely varying melting temperatures results in a broader melting range for the mixture.

The liquid fraction of the fat, measured via differential scanning calorimetry across this melting range, indicates how much of the fat has melted at a given temperature; this liquid fraction curve ultimately defines the texture of the butter or chocolate. At human body temperature, the liquid fraction of the chocolate must be nearly complete for it to melt in the mouth when eaten. Consequently, chocolate with properly tempered cocoa butter exhibits a very narrow liquid fraction curve centered precisely around human body temperature. A narrower liquid fraction curve that is better centered on human body temperature allows the chocolate to melt more cleanly and completely in the mouth.

Tempering chocolate involves deliberately creating crystal nuclei of cocoa butter in the correct polymorphic form—the most stable of all the forms the cocoa butter can adopt. Classical nucleation theory demonstrates that the energy barrier required to form these nuclei is minimized at a specific, optimal degree of supercooling, which corresponds exactly to the chocolate's tempering temperature. If the chocolate cools too rapidly—failing to maintain the optimal degree of supercooling—the cocoa butter crystallizes into a less stable polymorphic form than the correct one; this resulting form is softer and prone to developing unsightly surface bloom over time.

Once the cocoa butter crystals have nucleated in the correct polymorphic form, their growth proceeds via a diffusion process: molecules of the still-liquid fat must diffuse to the crystallization front of each crystal to become incorporated into it. Consequently, moderately agitating the chocolate during tempering enhances the diffusion of liquid fat molecules toward the crystallization front, resulting in smaller, more uniform cocoa butter crystals and giving the tempered chocolate a smoother, glossier texture.

Thus, this model—based on the melting range and cocoa butter tempering characteristics discussed earlier—also describes untempered butter, which forms crystals in a metastable state that gives it its characteristic plasticity. Clarified butter, by contrast, has had the water and proteins found in untempered butter removed, leaving only the fat, which exhibits a much sharper melting point than the unclarified product. Margarine, on the other hand, mimics this behavior using a blend of partially hydrogenated vegetable fats, adjusting its melting range to closely approximate that of natural butter.

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