Why Does Chocolate Seize?

Chocolate seizes because a small amount of water dissolves a little of its sugar into sticky syrup, and that syrup glues the cocoa and sugar particles together into a stiff, grainy clump. Chocolate is mostly fat with dry particles floating in it, so it has almost no water to begin with. A drop of steam or a wet spoon is enough to tip it over, while, oddly, a lot of liquid keeps it perfectly smooth.

What's actually inside a bar of chocolate?

Melted chocolate looks like a single smooth liquid, but it's really a crowd of tiny solid particles floating in fat. A typical dark chocolate is roughly:

• Cocoa butter, the fat that melts around 34 to 36°C (93 to 97°F), just below body temperature. That's why chocolate melts on your tongue.
• Cocoa solids, finely ground bits of roasted cocoa bean, which carry the flavour and colour.
• Sugar, ground into particles so fine, around 20 to 30 micrometres, that your tongue can't feel them.
• A dash of lecithin, usually from soy or sunflower, an emulsifier that helps the particles slide past each other.
• Milk powder in milk and white chocolate.

What's missing is water. Good chocolate contains less than about 1% moisture. Those billions of sugar and cocoa particles are each coated in cocoa butter, and they move past each other freely because nothing sticky is holding them together. That's the secret of chocolate's glossy flow.

What happens when water gets in?

Sugar loves water. When a few drops get into melted chocolate, the sugar particles near them instantly start to dissolve, forming tiny beads of thick syrup. Those beads are sticky, and they stick to the cocoa particles and other sugar grains around them.

Picture dry sand flowing through your fingers, then a splash of water turning it into clumps you can mould. Same idea. The syrup forms bridges between particles, and once enough bridges form, the whole mass locks up. The cocoa butter gets squeezed out from between the clumps. You're left with a dull, thick, gritty paste that won't flow, sometimes with a slick of fat around it.

Common culprits:

• Steam rising from a double boiler and condensing into the bowl.
• A spoon, spatula or bowl that wasn't fully dry.
• A lid on the pan, dripping condensation back in.
• Adding a small splash of liqueur, juice or extract to melted chocolate.

Chef Fizz says: with chocolate and water, it's all or nothing. A drop is trouble; a good pour of cream is ganache.

Why doesn't chocolate seize when you add lots of cream?

This is the part that surprises people. If water ruins chocolate, how does ganache, which is chocolate plus a good amount of hot cream, come out silky?

It comes down to amount. With only a few drops, there's enough water to make sticky syrup but not enough to dissolve all the sugar and carry the particles. Add plenty of liquid and the situation flips. Now there's enough water to dissolve the sugar completely and form a smooth, continuous liquid. The cocoa butter breaks into tiny droplets spread through it, and the cocoa particles float freely again.

Ganache is really an emulsion: fat droplets dispersed in a sweet water-based liquid, held stable partly by the lecithin and milk proteins. It works on the same principles as mayonnaise and vinaigrette, which you can read about in how emulsions work.

A commonly quoted rule of thumb is to use at least 1 tablespoon (15 ml) of liquid per 60 g (2 oz) of chocolate, and to add it all at once rather than drip by drip. Below that, you're in the seizing danger zone.

How do you fix seized chocolate?

Since too little water is the problem, the fix is more liquid.

1. Add 1 teaspoon of boiling water (or hot cream, milk or coffee) to the seized chocolate.
2. Stir firmly. It'll look worse for a moment, then start to loosen.
3. Keep adding a teaspoon at a time and stirring until it turns smooth and glossy.

You'll end up with something closer to a thick chocolate sauce than plain melted chocolate. It's perfect for brownies, cakes, hot chocolate, drizzling over ice cream, or stirring into frosting. What it can't do anymore is set hard and snappy, so it won't work for dipping strawberries or coating truffles. For those, start a fresh batch.

What if it turned thick from overheating?

That's a different problem, and it looks similar: chocolate that goes thick, dull and grainy, sometimes with a burnt smell. This time it's heat, not water.

Cocoa butter only needs to be a little above body temperature to melt. Push chocolate much hotter and the sugar and milk solids start to scorch, and milk proteins clump. Milk and especially white chocolate are most at risk because they're full of milk solids and sugar, with no dark cocoa to hide behind.

Safe maximums when melting:

Truly scorched chocolate tastes burnt and can't be rescued. Mildly overheated chocolate sometimes loosens if you take it off the heat right away and stir in a few chunks of unmelted chocolate to cool it down.

What's the best way to melt chocolate?

Over a double boiler

Chop the chocolate into small, even pieces. Put it in a dry heatproof bowl set over a pan with 2 to 3 cm (about an inch) of barely simmering water. The bottom of the bowl shouldn't touch the water. Stir often and take it off as soon as only a few lumps remain; they'll melt as you stir. Keep the steam away by making sure the bowl fits snugly over the pan.

In the microwave

Use a dry microwave-safe bowl and 50% power. Heat in 20 to 30 second bursts, stirring well between each one. Chocolate often holds its shape even when melted, so stir rather than waiting for it to slump, or the bottom will scorch.

A word on tempering

Melting chocolate destroys its "temper". Cocoa butter can solidify into six different crystal forms, and only one, known as form V, gives that glossy surface and clean snap. Melted chocolate that's simply left to cool sets into the wrong crystals and turns soft, streaky and dull. Tempering means melting, cooling and gently rewarming the chocolate (for dark: about 45 to 50°C, down to 27 to 28°C, back up to 31 to 32°C) so only form V crystals survive. For baking or sauces, you can skip it.

Why does ganache split, and how do you fix it?

Ganache sometimes turns oily and grainy, with fat pooling on top. That's a broken emulsion, the same thing that happens to a split sauce. It's usually because:

• It got too hot, so the cocoa butter separated out.
• The ratio is fat-heavy, with too little liquid to hold all the fat droplets.
• It was stirred too hard while it was cooling.

To fix it, warm 1 to 2 tablespoons of milk or cream to about body temperature, add it to the split ganache and blend with an immersion blender, keeping the head under the surface so you don't whip in air. The extra liquid gives the fat droplets somewhere to sit again.

Rough ganache ratios by weight, for dark chocolate:

• 2 parts chocolate to 1 part cream: firm, for rolling truffles.
• 1 to 1: soft, for glazes and tart fillings.
• 1 part chocolate to 2 parts cream: loose, for sauces or whipping.

Milk and white chocolate have less cocoa solid, so they need more chocolate for the same firmness, about 2.5 to 3 parts white chocolate to 1 part cream for truffles.

Want to practise without wasting good chocolate? The Cooking Scientist app has short lessons on melting chocolate gently and getting ganache ratios right, with a quick mini game for each.

Try it at home

Prove the "little water versus lots of water" paradox with 60 g (2 oz) of dark chocolate.

1. Melt the chocolate gently and divide it between two small dry bowls.
2. To bowl one, add 1/4 teaspoon of water. Stir.
3. To bowl two, add 2 tablespoons of hot water all at once. Stir.
4. Now take bowl one and keep adding hot water, a teaspoon at a time, stirring after each.

What you'll see: bowl one tightens within seconds into a thick, dull, grainy lump. Bowl two turns into a smooth, glossy, pourable sauce. As you keep adding water to bowl one, it loosens and ends up looking just like bowl two.

Why: a tiny bit of water makes sticky syrup that glues the particles together. A lot of water dissolves the sugar completely and gives the fat and cocoa particles a smooth liquid to float in.

Common mistakes

• Using a damp bowl or spatula. Even a few drops can seize the whole batch. Dry everything completely before you start.
• Letting steam reach the chocolate. Condensation from a double boiler drips straight in. Use barely simmering water and a bowl that fits snugly over the pan.
• Adding a small splash of liquid to melted chocolate. Too little water is the worst amount. Add flavourings into the cream or other liquid first, then combine.
• Microwaving on full power. The chocolate scorches before it looks melted. Use 50% power in short bursts and stir.
• Melting white chocolate like dark. Its milk solids scorch at lower temperatures. Keep it under 45°C (113°F) and stir constantly.
• Throwing out seized chocolate. It makes great sauce. Stir in hot liquid a teaspoon at a time until smooth, and use it in baking.