To keep chocolate from turning white, keep it chilled.
WHY DOES CHOCOLATE BECOME COVERED with an ugly white film after a few days? Michel Ollivon and Gérard Keller of the Centre National de la Recherche Scientifique (CRNS), in collaboration with Christophe Loisel and Guy Lecq of the Danone Group, have recently explained how certain constituents of this partially liquid mixture migrate and crystallize on its surface, causing it to change color.
Chocolate is a dispersion of sugar crystals and cocoa powder in cocoa butter. Once crystallized, the cocoa butter serves as a binding agent for the solid particles, just as cement binds the sand and gravel in concrete. Nonetheless, chocolate does not easily cohere: The sugar crystals are hydrophilic, whereas the cocoa butter is hydrophobic. Master chocolate makers add lecithin molecules as part of a process known as conching in order to promote the coating of the sugar by the cocoa butter.
Since the 1960s it had been known that the whitening of chocolate resulted from the thermal behavior of cocoa butter. The mixture of semisolid and semiliquid fats that make up cocoa butter can crystallize in six different ways (called forms 1–6), and only a precise sequence of reheatings and recoolings, collectively known as tempering, yields crystals that are sufficiently stable to prevent chocolate from whitening. The recooling phases are crucial; failure in any one of them will ensure a disappointing result.
Chocolate X-Rayed
The CRNS and Danone researchers investigated the crystallization process by measuring variations in viscosity during tempering. X-rays showed that well-tempered chocolate is crystalized in form 5, and further observation by means of polarizing microscopy disclosed that the surface of the chocolate immediately after fabrication is free of crystals; only a few holes, no doubt created by the bursting of air bubbles on the surface, and a few minute cracks are visible. By contrast, when the chocolate is subjected to substantial variations in temperature, the whitening appears after a few days. Both the surface and the interior crystalize in form 6.
How are we to interpret these observations? It was long thought that the whitening resulted from a migration of a part of the cocoa butter toward the surface, where it recrystallized, probably in form 6. Today we know that the composition of fatty matter in the body of the chocolate differs from that of the whitened surface. Hervé Adenier and Henri Chaveron at the University of Compiègne have confirmed that form 6 crystals are more stable than those of form 5; the difference in their fusion temperatures is 1.5°C (2.7°F). Moreover, form 6 is more compact than form 5. During the transition between the two forms, a part of the cocoa butter is pushed out toward the surface of the chocolate.
The difference in fusion temperatures corresponds to a difference in composition. The molecules of cocoa butter, triglycerides, are like a comb with three teeth: The base of the comb is a glycerol molecule, and the three teeth are fatty acid molecules. Carbon atoms in these acids are held together by means of single or double bonds. The fusion temperature of a triglyceride diminishes with the number of double bonds.
Pushed by Fusion
The proportion of the liquid phase naturally increases with temperature while becoming enriched by triglycerides having one double bond, which melt at a lower temperature. Whitening corresponds to the recrystallization, during cooling, of these enriched liquid parts.
Whitening threatens mainly chocolates whose interior, or lining, is rich in fats. When two blocks of fatty matter are combined, their fats become mixed together, especially at high temperatures (when the proportion of liquid and molecular motion both increase). The surface crystals of the whitened chocolate are quite different from those of the interior mass, being composed for the most part of triglycerides having a single double bond.
The composition of the whitened crystals is virtually invariable, no matter what the fatty content of the lining, and similar to those of whitened chocolate without a lining. The migration of fats in the lining toward the covering layer of chocolate leads to an enlargement of the liquid phase of the covering layer. Consistent with the findings of the CNRS and Danone researchers, this enlargement accelerates whitening without changing the composition of the outer crystals.
The moral of the tale is that if you want to preserve the aroma and appearance of chocolate, store it at a low temperature, say 14°C (57°F). In this way you will limit the migrations that accompany storage. Then warm up the chocolate before eating it.