Helen Czerski

This month, Helen grabs a spoon and mixes up some hot chocolate.

Almost everyone loves a mug of hot chocolate. Yet this silky, elegant drink gets its delicious flavour from cocoa powder, which is a bitter, gritty and unpolished substance – a classic ugly duckling.

The awkwardness of cocoa powder is most obvious when you try to stir it into liquid, such as milk or vanilla essence. It just doesn’t want to go. The powder somehow perches on top of the liquid without getting wet. You can stir for ages, but when you push a clump of powder into the liquid with a spoon, it disappears only briefly before cheerfully popping up to the surface, releasing a puff of dry cocoa just to prove its point. But get the mixing right and silky hot chocolate awaits. What’s the secret of the transformation?

We often talk about ‘dissolving’ cocoa powder into milk, but cocoa doesn’t ever dissolve. It’s mostly made up of dietary fibre and starches, with only 10 to 12 per cent cocoa butter and a few other components. All of that means that it’s insoluble in water. But the solid lumps of cocoa powder are around 0.01mm in diameter, which is slightly too small for our tongues to detect any graininess. So as long as the cocoa particles are dispersed evenly in a liquid to form a suspension, it feels smooth to us. One teaspoon of cocoa powder has about two billion particles in it, and they all need to be separated and coated in liquid. This is where the fun starts.

Cocoa is hydrophobic, which means that it resists getting wet. But the bigger problem is that cocoa particles tend to stick to each other quite easily, making loose clumps. Cocoa itself is more dense than water and so you might expect it to sink, but there’s so much air in the clumps that they float. Each clump might also develop a thin waterproof shell, because the starch in the cocoa on the outside of the clumps will absorb a tiny bit of water and form a thin gloopy layer which stops water from penetrating any further inwards. The only way to disperse the cocoa evenly is to break up these clumps, and there are two weapons in your arsenal. The first is to pick your liquid well. Hot water/ milk will penetrate the clumps more easily than cold, because hot liquid is slightly more runny and doesn’t repel the cocoa quite as much. And full-fat milk is better than water or skimmed milk, because the fat droplets will surround the cocoa particles and that helps split them off from the rest of the clump. But the real trick is to break up the clumps by stirring in a particular way. You need ‘shear’, which is a sideways force: like pushing your flat hand down hard on a table and then moving it along the surface. This is why people add a small amount of liquid and stir the cocoa into that first – this mixture is thicker and it’s harder for clumps to slide out of the way and avoid being broken up. Vigorous stirring works too, but it won’t help much if the mixture is too wet. And once the particles are all in direct contact with the liquid, your problem is solved because they won’t clump together again. So here’s the scientific method to get it right: squish your cocoa powder sideways into a small amount of warm full-fat milk to break up the lumps, and then gradually add the rest of the liquid. And then the lovely chocolatey potential of the cocoa powder will be all yours, f ree of lumps and frustration. Just thinking about it brings on the urge to experiment. Happy stirring!