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Guest Blog: The Chemically Perfect Hot Chocolate

Chocolate has been enjoyed in sumptuous liquid form since the long bygone days of the Mayan civilisation and Aztec empire. In fact, the word chocolate itself derives from the Aztec/Nahuatl word, xocolatl, translating roughly as ‘bitter water’. A ceremonial beverage, consumed during sacrificial rituals, was made from the fermented nibs of the cocoa or cacao plant. It is widely thought that the resemblance of the heart-shaped cocoa pod to the vital human organ was critical to the establishment of this sacred link.

The scientific name for the genus of the cacao plant, Theobroma, which derives from ‘food of the gods’ in Ancient Greek, further alludes to its historical religious significance. The drink was believed to empower one with the strength of the gods and, as such, was included as basic rations for the Aztec army. Those wishing to increase their libido also turned to the drink; the mooted aphrodisiacal properties of the divine beans have, however, since been attributed to the chemical compound, theobromine. As a chemical cousin of caffeine, it shares its, albeit weaker, stimulating effect.

The drink has evolved over the centuries into one which we now most readily associate, somewhat surprisingly given its history, with a cosy night in or a wander through the warming embrace of a festive market. Sugar arrived relatively late to the cocoa scene, it may well be this excess of energy provided what we now associate with times of abundance, contentment and celebration. Here, I would like to share with you a recipe for this winter warmer and take the opportunity to talk about the secret science that goes into the perfect mug of hot chocolate.

First things first: find yourself a state-of-the-art Chemistry lab


  • 50g dark chocolate
  • 250ml whole milk
  • ~25g sugar (to taste)
  • 1 stick of cinnamon
  • 1tbsp cocao powder
  • 1/4tsp chilli powder
  • Cream (to finish)

Let’s begin

  1. First, use a bain-marie to warm the milk to a temperature of 45oC and add a stick of cinnamon. The flavour compound cinnamaldehyde is miscible with the fat droplets suspended in the milk and will diffuse out of the dried bark.
  2. Once, the milk has reached temperature, allow some time for infusion of the cinnamon flavour. In the lab I used a magnetic stirrer bar to continuously stir the milk, allowing for more effective heating and dispersion of the cinnamaldehyde. These devices are readily available in a chemistry lab and indispensable when needing to stir for long periods of time and when dealing with air-sensitive mixtures, where thrusting a spatula in to stir it simply won’t do.
  3. Now, the mixture should be ready for the chocolate. Break up the chocolate and add to the warm milk. The smaller the pieces of chocolate, the higher their surface area to volume ratio, the faster the heat transfer, and the quicker a given amount of chocolate will melt. A bain-marie is the least error-prone method of melting chocolate. The device maintains the temperature of the inner vessel walls at the boiling temperature of the liquid used, commonly 100oC for water.
    As the water begins to boil, excess heat is spent in vaporising the H2O molecules instead of raising their temperature. This prevents the sugar in the chocolate from burning on the sides of the container which, particularly if metallic, can transfer heat at a much faster rate than through hot air alone. In the lab, I used a thermostat to maintain the temperature far below the boiling point of the water, in an effort to retain the multitude of delicate aroma compounds present in the chocolate.
  4. Add the cocoa powder to provide further colour and impart a rich, earthy flavour without the excess sugar and milk of the chocolate. Dried powders often show the tendency to lump together when added to a liquid (think ready-made hot chocolate powder or adding cornflour to gravy or custard). These powders are starch-based, starch being the energy storage molecules of plants in which many sugar molecules are packed into ‘supply’ chains. These molecular chains are branched along their length.
    When starch powders are added to water, the water molecules are attracted to the sugars along the chain, wriggling their way between the branches and fanning out the dendritic polymer. The starch chains exposed to the water then mesh together, shielding the dry interior starch from the water to form the persistent characteristic lumps.
    To overcome this problem, I use a simple solution. It’s more brute physics than chemistry but effective, nonetheless. Using as small a container as possible, add a small amount of milk to the powder and stir, pressing the lumps into the walls of the container. This handy hack enables exposure and hydration of the desiccated interior without chasing the floating lumps like herding cats. Once a smooth paste is achieved, add to the mixture.
  5. Add vanilla essence to finish with the associated sweet, creamy flavour and, finally, chilli powder. Capsaicin, the active compound in the chilli powder will mix with the fat droplets from the cocoa butter and milk, and diffuse throughout the drink. Once in contact with the tongue, it binds to receptors and tricks them into relaying the same signal to the brain as would be released in response to excessive heat or physical abrasion. Originally a defence mechanism for Capsicum or chilli plant, it is used here as a nod to the original xocolatl and to impart a warming tingling sensation, characteristic of the festive season.

And there you have it! A winter-warming, chemically perfect hot chocolate.


  • Russell

    How much vanilla should we add? It’s not listed. Also when should we add the cream and how much? At the end I assume and to taste. Any help appreciated, thanks!

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