The origins of chocolate can be traced to (modern) Mexico and neighbouring countries, where Aztec myth has it that Quetzalcoatl, the feathered serpent god, brought cacoa beans from Paradise to teach the Aztecs how to make the bitter drinkchocolatl. (A similar story can be found in Mayan mythology.)
For this deed he was punished and exiled by the other gods, who thought the drink too good for humans.Initially, only royalty and priests were allowed to consume the drink created by roasting and then grinding the beans which were added to water and whipped up to make a frothy liquid. Allegedly it was a favourite of Montezuma’s, as well as the subsequent Spanish conquistadors.
This myth is enshrined in the name of the cocoa tree Theobroma cacao, which literally means ‘food of the gods’. These days the tree grows not only in its original territory but has been introduced to many wet tropical regions around the equatorial belt for commercial production – countries such as Brazil, Ghana, Malaysia and Colombia.
However, as with any natural crop, different countries of origin and different local environments lead to substantial variations in the chemical make-up of the bean – specifically in the triglyceride composition – rendering it a challenge to process optimally in a reliable fashion.
Triglycerides are a kind of fat, and are esters of glycerol with (three) fatty acids. The glycerol means these molecules have a hydrophilic head group with three pendant hydrocarbon tails based on fatty acids.
These chains may be all identical but typically are not in this case. In the cocoa butter obtained from cocoa beans, the fatty acids involved are palmitic acid (P, with 16 carbon atoms), stearic acid (S, consisting of 18 carbon atoms strung together with all the carbon-carbon bonds saturated), and oleic acid (O, which also has 18 carbons but one of the bonds is unsaturated, meaning there is a double bond joining two of the carbons in the middle of the chain).
Cocoa butter contains a mixture of SOS, POP and POS triglycerides, where the letters identify the individual chains on the triglycerides, and the relative amounts depend on the source of the beans. That is the beginning of the complexity of chocolate.
One of the attributes that makes chocolate particularly appealing to our senses is the fact that it melts so close to room temperature;
in other words it deliciously melts on our tongue when we pop a bit into our mouths. But unlike a material like ice, which melts at a well-defined temperature, because there is a mixture of triglycerides present in chocolate, each with a slightly different melting point, it actually melts over a range of temperatures.
Some readers may remember the advertisement that referred to “chocolate that melts in your mouth not in your hand”.
This is a case of having exactly the right composition of cocoa butter, as well as exactly the right processing conditions, to make sure the chocolate melts only when in the slightly warmer environment of one’s mouth.
Furthermore, and also unlike many simple materials such as (table) salt, the different triglycerides can actually exist in a variety of crystal forms – so-called polymorphs, which literally means ‘many forms’ – and these also have different properties (including melting point).
So when I say “exactly the right processing conditions”, what I actually mean is a complicated series of thermal operations, heating to and then holding at different temperatures, to make sure the right kind of triglyceride crystals form.
This process is known as tempering. It isn’t exactly a black art by now, but it certainly started off as one in the confectioner’s recipe book.
A familiar manifestation of when things go wrong – usually after purchase – is the unattractive whitish ‘bloom’ that forms on the surface of a chocolate bar, typically after it’s got a bit too hot but before it’s actually melted.
This bloom is attributed to a particular (and very stable) polymorph forming, and may be associated with the previous thermal history being inadequate to transform all the crystals into the right polymorph in the first place.
So far I’ve leaped over a few key stages, including getting the prized cocoa butter with all its constituent fatty triglycerides out of the cocoa bean, as well as what else goes into chocolate to transform it from the bitter product the Aztecs drank to the chunks of stuff we know and love to our waistline’s detriment.
Extraction of the cocoa butter is another multi-stage process, perfected over centuries, but it is a crucial set of actions involving fermentation, drying, roasting, breaking up, grinding, alkalisation (also known as Dutching after its invention by van Houten, a Dutch chocolatier in the 19th century) and finally pressing to separate the cocoa butter from the cocoa solids.
Some of the cocoa solids are then added back in when regular chocolate is made – as opposed to white chocolate which omits this ingredient – along with sugar, plus milk solids and some butter fat in the case of milk chocolate. Typical chocolate contains around 30% fat, but for inexpensive chocolate often the proportion of cocoa butter is rather lower, with cheaper vegetable fats being added instead. There is no escaping the fact, chocolate is a high-fat food.
Various attempts have been made to make chocolate lookalikes from alternatives to the cacoa tree. The most successful of these uses the immature fruit and flowers from the linden tree (the lime family,Tiliaceae) in a method originally developed by a French chemist, Missa, in the 18th century. Although it was claimed (and you can still findrecipes on the web if you want to test this out) that this had an aroma very similar to normal chocolate, it transpired it did not keep at all well and various attempts at commercialisation failed.
So, as you bite into that last piece of chocolate, consider just how much art and science has gone into its production. Sinful luxury though it may seem, in England its production has largely been in the hands of Quaker families since the Civil War until the very recent past: Fry, Rowntree and Cadbury were all significant Quaker families.
Chocolate is no trivial piece of confectionery; much skill is required to mould Easter eggs or fill liqueur chocolates successfully. Production of the myriad forms in which we consume it has been perfected over the centuries, starting off as an art with the science coming rather later. Its production is an astonishingly complex and time-consuming series of operations. Our own consumption of it is, on the other hand, all too rapid.
Athene Donald is professor of experimental physics at the University of Cambridge, and tweets as @AtheneDonald