Chocolate Polymorphs

With Easter upcoming, here’s another graphic on chocolate to go with the previous food chemistry graphic which looked at some of the chemicals it contains. This one examines the different structures of chocolate, why the tempering process is important to produce the highest quality chocolate – and why you shouldn’t leave your Easter chocolate lying around for months before eating it.

Cocoa butter is the primary ingredient in chocolate; it is composed primarily of fat molecules, and how these fat molecules are arranged determines the structure of chocolate. The molecules themselves don’t change at all in these different structures; what does change is how they are arranged or stacked together. The more efficiently they are stacked together, the higher the density of the structure formed, and the more stable.

Cocoa butter has at least six different structures, or ‘crystal forms’; the ability of a substance to display different structures is known as ‘polymorphism’. These crystal structures differ in how the molecules are arranged, which in turn influences their properties, such as appearance, taste, and texture. These, additionally, can affect the taste and quality of the chocolate, so it’s an important factor to consider for chocolate makers.

The best crystal form for chocolate, in terms of it exhibiting the best appearance and taste, is form V. This form has a shiny appearance, produces an audible snap when broken, melts in the mouth, and has a smooth texture. Unfortunately, it’s not the most stable of the six, and requires a process called ‘tempering’ in order for it to be the main structure present in manufactured chocolate.

If melted cocoa butter is simply allowed to cool naturally, a mixture of forms I-V will be obtained. Obviously, this isn’t ideal, as forms I-IV have less than desirable properties that impact on the quality of the chocolate. Tempering involves allowing the melted chocolate to cool very slowly, as this increases the amount of form V that is formed in the mixture. Once cooled, the chocolate can then be heated again to just below the melting point of form V – this melts forms I-IV, which all have lower melting points, but not the form V crystals. When the chocolate is again allowed to cool, it solidifies following the pattern of the existing form V crystals, and adopts a similar structure, with the end result being chocolate that has very little of the other forms of structure present.

We’ve yet to mention form VI, because it doesn’t form as melted chocolate solidifies, like the other forms. Instead, it forms only after several months, from form V. The fat molecules in form V have enough energy over this time period to change to form VI, which is harder, and melts too slowly in the mouth due to the higher melting point. It’s also possible to see fat ‘bloom’ forming on the chocolate – due to partial melting, causing the fats to rise to the surface.

This transformation is one that impacts on the quality of the chocolate, and therefore is one you probably want to avoid. It’s not something you have to worry about if you’ve no plans to leave chocolate lying around for long periods of time, but if you prefer consuming chocolate in moderation, the transformation of form V to form VI can be arrested simply by storing the chocolate in the fridge. This is because, at a lower temperature, the molecules in the structure don’t have enough energy to convert to form VI.



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