Using their melting point graphs students should be able to work out which forms or polymorphs are present in their two samples, given the melting points of the polymorphs.
The ordinary sample usually has a melting point of around 33–35˚C, showing that form V is probably present. This chocolate has a good ‘mouth feel’ and students may notice a cooling effect on eating it as melting is an endothermic process.
The melted and re-hardened sample melts at a much lower temperature and is probably form II or III. The chocolate often tastes stronger in this sample, but it does not snap so well and has a less smooth texture. The cooling effect in the mouth is less pronounced.
This is a genuine experiment as the results are difficult to predict exactly. As can be seen from the table above, the cocoa butter polymorphs will readily change from one form to another at the sort of temperatures experienced by chocolate. The plots which students generate from this may therefore be rather ragged and are rarely as clear to read as those which they may get from stearic acid, for example. In spite of this, the practical is well worth doing as genuine research is always interesting. There is almost always a clear difference between the plots and students enjoy studying a 'real life' example of chemistry.