Today is Easter Day. Traditionally a celebration of fertility and birth and more recently combined with the story of the resurrection of Jesus Christ, Easter is often symbolised by eggs, newly hatched fluffy chicks, happy hopping bunny rabbits, and of course, CHOCOLATE. I for one am a fan of both adorable animals and chocolate, but what is it about chocolate that drives us so wild, and how much science was involved in creating your chocolate bunnies? My science hero, top bloke, and one of the best materials scientists around, Prof. Mark Miodownik, has dedicated a whole chapter of his book Stuff Matters to the marvellous material that is chocolate. If you are yet to read the book, I urge you to buy it, as it is beautifully written and explores chocolate as a material. As I was a chemist before I was a Materials Scientist, I am also curious about the chemistry of this wondrous substance, and the scientific reason behind why it drives so many of us to unhealthy obsession!
Chocolate is made from cocoa solids (although there is none of this ingredient in white chocolate), cocoa butter, sugar and milk in varying proportions. The cocoa components come from the seed of the Theobroma cacao tree. The journey of chocolate from revolting-tasting seed through bitter drinks, fermented heaps in forests and hot beverages to the solid chocolate that we know and love today is a long story. I hope to cover it in a future post when I will share my experience of visiting the Thorntons Chocolate Factory in Derby, where I was lucky enough to check out 3D printing in chocolate and the science behind creating confectionery masterpieces.
Cocoa butter is one of the main components of chocolate. Unlike dairy butterfat, cocoa butter does not go rancid very quickly, thus preserving chocolate for prolonged periods of time. On a molecular level it also displays polymorphism, which means that the overall composition of the cocoa butter stays the same, but the molecules can be positioned differently in relation to one another, giving rise to different types of cocoa butter with varying physical properties. A common example of polymorphism can be seen in diamond and graphite, both of which are made of carbon, but vary greatly in terms of their hardness and physical appearance due to the arrangement of carbon atoms in each. Depending on the arrangement of molecules in cocoa butter, the various types display differences in their appearance, melting point and hardness among other things. The melting points of each of the six types can be seen in the table below.
Melting points of cocoa butter (Wille and Lutton, Journal of the American Oil Chemists Society, August 1966, Volume 43, Issue 8, page 491-496)
For chocolate to look and feel appealing to us while also being stable enough to be portable, it must have the correct melting temperature; high enough to ensure it does not melt too easily when being transported or stored, but low enough to melt quickly and smoothly on the tongue, releasing all the delicate aromas and flavours in the chocolate. The perfect cocoa butter type to achieve this is Type V. This type has an optimal hardness that gives that familiar snap or bite to good quality chocolate. It is also the glossiest type, which results in an aesthetically more appealing shiny chocolate than the duller chocolate that would result from the presence of other cocoa butter types. The cocoa butter polymorphs have varying degrees of thermodynamic stability. Each type of cocoa butter will want to change to the most stable form where the molecules are in their most comfortable, lowest energy position in relation to all others. This can once again be compared to the thermodynamic instability of many carbon polymorphs; less thermodynamically stable diamond will, eventually, turn into more thermodynamically stable graphite, although for those of us that like shiny things, this is thankfully an incredibly slow process. Again, analogous to carbon polymorphs, it is a less thermodynamically stable form of cocoa butter that we favour for its more desirable material properties; Type V instead of Type VI. Each one of the six types of cocoa butter can be encouraged to form over other types by changing the formation conditions; the rate of heating and cooling of chocolate, and the temperatures at which chocolate is heated and cooled are incredibly important and very precise. Meandering outside this very narrow range of temperatures will allow less desirable types of cocoa butter to form. Anyone that has ever tried tempering chocolate in the kitchen will be aware of how tricky this process can be.
Once the chocolate has melted on the tongue, it continues to display more fascinating physical properties. Among these, my favourite is the fact that molten chocolate is a ‘non-Newtonian’ fluid; it disobeys the normal laws of physics and mechanics. A normal Newtonian fluid that follows the classical laws of physics would move out of the way when a force is applied to it, maintaining a regular viscosity or thickness.
Non-Newtonian fluids can be grouped into two categories; those that become more viscous (thicker) when a force is applied to it, such as a classic suspension of cornflour in water which behaves as a solid when hit but otherwise behaves as a liquid (and also D3O that I mentioned in my previous post about ballet pointe shoes), and those that become less viscous (more runny) when a force is applied to it, such as ketchup that will only shift from the bottle after a lot of bottle bashing. Molten chocolate behaves like ketchup. As we press the molten chocolate against the roof of our mouth, we are making it more liquid, allowing all of the wonderful flavours to flood across our taste buds.
Once the flavours have stimulated our taste buds, the chemicals in chocolate get to work and stimulating our minds. Chocolate is packed with mood-enhancing chemicals such as theobromine, phenylethylamine, tryptophan and caffeine, as well as sugar. Theobromine is a chemical cousin of caffeine, and the two occur naturally in chocolate in higher and much lower concentrations respectively. While caffeine promotes alertness, theobromine works in a slightly different way, by inhibiting the feeling of relaxation by competing with the chemical adenosine. Adenosine promotes sleepiness, so by preventing adenosine from working, theobromine contributes to the increase in alertness of a post-chocolate pick-me-up. Adenosine also lowers feelings of arousal and excitement, so by inhibiting the effect of this, theobromine also allows the consumer to feel excited, which could be one of the reasons that chocolate could possibly be addictive. In medicine, theobromine is used to treat high blood pressure. It is a vasodilator and lowers blood pressure by widening blood vessels. Like caffeine, theobromine is also a diuretic, a chemical that makes you want to pee, and has been used to treat oedema, or water retention. Some animals cannot break down theobromine, so chocolate is poisonous to animals like dogs and cats. Accidental consumption of chocolate occurs more in dogs, who are attracted by the sweetness of the sugar in chocolate, compared to cats, who are unable to detect sweet flavours in chocolate and are not very keen on eating it.
The lowering of blood pressure is a great side effect, but eating lots of chocolate will lead to other problems owing to the high sugar content. While theobromine is getting us excitable and sugar is fuelling us, the two other chemicals further contribute to these feelings of chocolate-related elation and obsession, albeit at a lower level. Tryptophan is a precursor for serotonin, which promotes happy feelings, again contributing to that contented high achieved when you eat chocolate. Phenylethylamine is a chemical that can be produced by the body when you are in love, which from what I remember is a very lovely feeling indeed, and this is why we keep coming back for more. Chocolate is packed with mood-enhancing chemicals, and engineered to makes us want to eat it just by looking at it and hearing it break. The United Kingdom is often rated one of the top five chocolate consuming countries in the world based on mass consumed per person, with the chocolate industry here worth around £3.5 billion. When we compare this to the UK fashion industry, known to be big business and estimated to be worth £21 billion, it is easy to see why such scientific precision is used in creating the most tempting chocolate. It is an excellent investment.
On that note, I’m off to see whether the Easter Bunny has visited, and to scientifically test out the snap, smoothness of melt, and non-Newtonian properties of my chocolate treasure. For science, you understand… I suggest you do the same, as ‘n=1’ will never create a scientific study that will stand up to peer review. Happy Easter!
The science of CHoCoLaTe BUnNiEs?