The difference of opinion when it comes to whether chocolate tastes better out of the fridge or not has caused many a discussion between me and my peers, and has almost torn friendships apart (not really); but after one of my lecture courses from last semester, I can finally definitively say that chocolate DOES in fact taste better from the fridge.
First, a little background. This is all to do with a phenomenon known as polymorphism, which is the ability for a solid to exist in more than one crystal structure. Each crystal structure is called a polymorph, and each polymorph has its own set of distinct properties.
The main ingredient in chocolate, cocoa butter, has six polymorphs which can be distinguished between each other by measuring their melting points:
Polymorph I, 16-18 degrees
Polymorph II, 22-24 degrees
Polymorph III, 24-26 degrees
Polymorph IV, 26-28 degrees
Polymorph V, 32-34 degrees
Polymorph VI, 34-36 degrees
Polymorphs I – IV are not suitable for making chocolate since they are too sticky and unstable at room temperature. Polymorph VI is the most stable, but tastes bland and is too brittle. Polymorph V is the ideal form for eating (but of course, sod’s law dictates that it’s the hardest to manufacture).
This is great, I hear you saying, but why does this mean I should keep my chocolate in the fridge? Well… at room temperature, the fatty molecules in polymorph V have enough energy to slowly (days/weeks scale) convert to polymorph VI. This transformation in in the crystal structure is facilitated by the vibrational energy stored in the molecules which allow the molecules to wriggle about and realign with each other. This can be stopped by keeping your chocolate in a cool, dark place (i.e., the fridge!!) to make sure the molecules don’t have enough vibrational energy to convert to polymorph VI.
Fat blooms in chocolate
You then might ask how you can tell this has happened? The change in crystal structure is usually accompanied by something called ‘fat bloom,’ which is where the chocolate begins to look dusty, and pale spots appear on the surface as shown in the attached image. We’ve all been there (you’re incredibly lucky if you haven’t). It’s off putting, but still safe to eat. It happens because of partial melting in the solid which cases the fats within it to rise to the surface. It’s this strange occurrence that leads me to believe that keeping my chocolate in the fridge is in fact the correct way to keep it, and also why all the chocolate I bought on my exchange year in Australia just didn’t taste as good as the stuff at home in the UK due to their hotter climate!
Polymorphism doesn’t only have implications on your chocolate, but on almost anything that can crystallise. There are many patent issues with regards to the different polymorphs of drug molecules in the pharmaceutical industry, and a range of other industries. Feel free to comment and ask questions for more detail 🙂