Power of Polymorphism

Do you like chocolate? Who doesn’t! But do you know why chocolate from some places or brands is much more snappier and melts much better in the mouth than others? Yes sure ingredients and recipe are a reason but there is something beyond ingredients that makes the same cocoa butter taste much nicer! Cocoa butter is basically a collection of really small (0.01 -0.1 mm) crystals of cocoa. And crystals can exist in multiple crystals structures. When crystals having the same chemical composition exist in different structures they are called polymorphs and the phenomena is known as polymorphism. Many crucial properties of the substance like density, melting temperature etc. change when the polymorph changes. Some are desired and some aren’t, so it is always important to have the most desired polymorph of a substance! Achieving the best polymorph is dependent on the way the substance is crystallized. In case of chocolate, it has 6 polymorphic forms and the form that tastes the best is form 5. So one who has the best recipe for form 5 of chocolate sells the most chocolate!

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A wrong polymorph of chocolate will only affect the taste and experience of person consuming it, but imagine if the same happens in case of medicine! About 90% pharmaceutical drugs like Paracetamol, Aspirin etc. are crystalline and some do exhibit polymorphism. It is crucial to get the right polymorph in case of medicine. Undesired polymorphs can have adverse effects on the human body. Crystallizing the desired polymorph is not always easy. Usually at a given temperature and pressure there could be stable or metastable polymorphs. Depending on the compound and properties the desired form could be stable or meta stable. For example in case of Aspirin, the commercial form is the stable form – I but it was found that a form – II exists which is very difficult to crystallize but has better dissolution in blood than form – I. There are some parameters and techniques that can enable better control of polymorph during crystallization processes. Some parameters that play an important role are type of solvent, supersaturation and pH. Some techniques that have shown potential to control polymorph are the use of ultrasound, use of laser for inducing nucleation and polymer heteronucleation.
Now that I have given an idea of what polymorphs are, let me end the blog on a sweet note. Below is picture that shows the procedure to get different chocolate polymorph forms and the best one, form – 5. This should help in choosing the best chocolate next time you shop!

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Mohammed Noorul Hussain, KU Leuven




  1. The importance of structure: chocolate
  2. Polymorphism in Pharmaceutical Solids – Harry G. Brittain – 2009
  3. On the Polymorphism of Aspirin –Andrew D. Bond, Roland Boese, and Gautam R. Desiraju – Chem. Int. Ed.- 2007