Tempering
In order to make the chocolate crystallize exclusively in form V, the crystallization process has to be controlled by a sophisticated temperature regime (tempering).
First, the chocolate is melted at 50 °C. For the optimum formation of type V nucleation sites, it is then cooled at 1 °C/min to 22 °C, where it is held for several minutes so that a sufficient number of nucleation sites can form.
Subsequently, it is heated again at 4 °C/min to 31 °C so that the thermodynamically unstable nucleation sites, particularly those of form IV, are melted [13, 14].
Here, an exact temperature control is of the utmost importance; one degree too high or too low will decide on the product quality. Another cooling process follows, with the cooling rate depending on the chocolate variety and the recipe.
Conversion
The reason for the lower stability of crystal form V lies in the relatively loose packing of the lipid molecules, leaving empty spaces [15]. In the solid state, crystal form V also tends to convert into the more stable form VI. The addition of milk fat retards the conversion so that the V→VI transition is less often observed in milk chocolate.
At room temperature, the conversion takes place only slowly; it nevertheless limits the shelf life of chocolate to several months.
Therefore, chocolate should always be stored in a refrigerated environment (15–18 °C). At higher temperature, e.g., in the sun or in a warmed up car trunk, the undesirable phase transition V→VI happens quickly, even faster than during unintentional melting and subsequent cooling. If the phase transition happens, the producer’s effort will have been in vain: The chocolate is dull, soft and melts only slowly in the mouth.