Why Does Caramel Crystallise and Turn Grainy?

Caramel turns grainy when sugar that was dissolved in the syrup snaps back into solid crystals, usually set off by a single stray crystal, a splash on the pan wall or too much stirring. Dissolving the sugar fully, swirling instead of stirring, and adding a little corn syrup or acid keeps it smooth.

Why does sugar want to turn back into crystals?

You're boiling sugar and water, it's bubbling away nicely, and then suddenly it turns cloudy, thick and sandy. To see why, start with what sugar actually is.

Table sugar is sucrose, and solid sucrose is a crystal: billions of molecules stacked in a neat, repeating pattern. When you dissolve it in water, those molecules break away from the stack and drift around separately among the water molecules.

Sugar loves water, which is also why it keeps cakes moist, as covered in what sugar does in baking. Hot water can hold far more sugar than cold. At room temperature, water dissolves about twice its own weight in sugar. Near boiling, it can hold nearly five times its weight. As you boil a syrup, two things push the sugar toward its limit at once: water evaporates, and as the syrup cools later, it can hold less.

The result is a supersaturated syrup, one holding more sugar than it "should" at that temperature. It's a bit like a crowd of people standing on a slope. Everyone's fine until one person trips, and then they all go down together. Sugar molecules in a supersaturated syrup badly want to rejoin a crystal. All they need is a starting point.

That starting point is called a seed, and once a seed appears, it grows fast. Sugar molecules stack onto it, then onto each other, and the crystals spread through the pan in seconds. That cascade is sugar crystallisation, and it's the whole problem.

What sets off the chain reaction?

Almost anything that gives sucrose molecules a surface to line up on:

• Undissolved sugar. Grains that never fully dissolved at the start are ready-made seeds.
• Crystals on the pan walls. Syrup splashes up the sides, the water evaporates, and dry sugar crystals form there. When they fall back in, or when the boiling syrup touches them, they seed the batch.
• Stirring. Stirring a boiling syrup flicks drops onto the cooler walls and the spoon, where they dry into crystals. It also jostles molecules into contact.
• A dirty spoon or thermometer. A spoon with a crust of old dried sugar is basically a seed stick.
• Dust and specks. Tiny particles can give crystals a place to start.

Chef Fizz says: sugar syrup is like a sleeping cat. Leave it alone and it's lovely. Poke it and see what happens.

How do you keep caramel from crystallising?

There are two kinds of fixes: keep seeds out, and make it harder for crystals to grow even if a seed gets in.

Keep seeds out

1. Dissolve fully before boiling. Stir the sugar and water over medium heat until the liquid is completely clear and you can't feel grit on the spoon. Only then let it boil.
2. Stop stirring once it boils. From here on, swirl the pan gently by the handle if the colour is uneven.
3. Wash down the walls. Dip a pastry brush in water and brush around the inside of the pan just above the syrup. The water dissolves stray crystals and runs them back in as liquid.
4. Or put a lid on. Covering the pan for a minute or two right after it comes to a boil traps steam, which condenses on the walls and washes the crystals down for you.
5. Start with a clean pan. Old sugar residue is a seed waiting to happen.

Add interfering agents

This is the clever chemistry bit. If you mix other sugar molecules into the syrup, they get in the way of sucrose lining up neatly. Crystals have a harder time forming and growing.

• Corn syrup or glucose syrup. It's mostly glucose plus longer glucose chains, which are a different shape from sucrose and jam up the stacking. About 1 tablespoon of corn syrup per 200 g (1 cup) of sugar makes a big difference.
• Acid. A squeeze of lemon juice or a pinch of cream of tartar helps split some sucrose into its two halves, glucose and fructose. This is called inversion, and the resulting mix is called invert sugar. Those loose glucose and fructose molecules act as interferers.
• Fat and dairy. In caramel sauces, toffee and fudge, butter and cream coat sugar molecules and physically get in the way of crystals. Getting water and sugar to behave around fat is a theme in chocolate too, where the problem flips: a drop of water makes chocolate seize.

Wet caramel or dry caramel: which is safer?

There are two ways to make caramel, and they fail differently.

Wet caramel

Sugar is dissolved in water first, then boiled until the water's gone and the sugar starts to colour. It's slower and more even, so it's easier to control colour, but it's the method most likely to crystallise, because it passes through that supersaturated syrup stage. All the tricks above apply.

Dry caramel

Sugar goes straight into a dry pan over medium heat with no water. It's faster. There's no syrup stage, so classic crystallisation is less of a worry, but you can still get clumps if you stir too early: partly melted sugar coats unmelted grains and forms hard lumps. Let the sugar liquefy around the edges first, then gently nudge the dry sugar from the middle into the melted parts with a heatproof spatula. Lumps usually melt if you keep the heat moderate and are patient.

Interesting fact: sucrose doesn't really melt the way ice does. Research has shown it starts breaking down chemically as it liquefies, somewhere around 160 to 185°C (320 to 365°F) depending on how fast it's heated. So "melting" and "caramelising" in a dry pan happen almost together.

What temperatures matter?

As water boils off, the syrup's temperature climbs, and the temperature tells you how concentrated it is. Confectioners name the stages:

Above about 185°C (365°F) caramel turns burnt and bitter, and the jump from deep amber to scorched can take under 30 seconds. Once the colour is right, take it off the heat or stop it with cream right away.

The browning itself is caramelisation, sugar breaking down into hundreds of new flavour and colour compounds. It's often confused with Maillard browning, but there's a key difference you can read about in Maillard reaction vs caramelisation.

Why does caramel sauce seize when you add cream?

Making a caramel sauce means pouring cream into sugar at 170°C (338°F) or so. The cream is much cooler, so the caramel violently bubbles and steams, and some of it may harden into glassy lumps. That isn't crystallisation; it's just sugar cooling suddenly and solidifying.

To avoid it:

• Warm the cream first so the temperature drop is smaller.
• Pour slowly while whisking, and stand back from the steam.
• If lumps form, keep going. Set the pan back over low heat and stir. The hardened caramel redissolves into the cream within a minute or two.

A caramel sauce that turns grainy days later in the fridge is true crystallisation, though. Sauces made with a spoonful of corn syrup stay smooth much longer.

Want a feel for caramel colour without burning a pan? The Cooking Scientist app has a short lesson on caramelising sugar and making caramel sauce, with a mini game for pulling it at the perfect amber.

Try it at home

See supersaturation and seeding in action with a mini sugar-crystal test. Hot sugar syrup can cause serious burns, so do this with care and keep kids at a distance.

1. In a small clean pan, combine 100 g (1/2 cup) sugar with 50 ml (3 1/2 tbsp) water. Stir over medium heat until the syrup is completely clear, then stop stirring and let it boil for about 2 minutes, to roughly 110°C (230°F).
2. Pour half into a clean, heatproof glass or jar. To the other half in the pan, add 1 teaspoon of corn syrup and swirl, then pour it into a second heatproof glass.
3. Let both cool, untouched, for about 30 minutes.
4. Drop a few grains of dry sugar into each glass and stir each a couple of times with a clean spoon.
5. Watch for the next hour or so.

What you'll see: the plain syrup usually starts to cloud and grow crystals around the seed grains, sometimes turning into a sandy slush. The corn syrup version stays much clearer for longer.

Why: the cooled syrup is supersaturated. The sugar grains give sucrose molecules a template to stack onto. In the second glass, glucose molecules from the corn syrup get in the way, so crystals struggle to grow.

Common mistakes

• Boiling before the sugar fully dissolves. Leftover grains seed crystals. Stir until the syrup is completely clear, then stop stirring.
• Stirring a boiling syrup. It splashes sugar onto the walls and jostles crystals into forming. Swirl the pan instead.
• Ignoring the pan walls. Dried crystals up the sides fall back in and seed the batch. Brush them down with a wet pastry brush or cover the pan briefly.
• Using a sugary old spoon or thermometer. Dried crust acts as a seed. Use clean, dry tools.
• Skipping an interfering agent. Plain sugar and water crystallise easily. Add about 1 tablespoon of corn syrup per 200 g sugar, or a squeeze of lemon juice.
• Throwing out crystallised caramel. It's fixable. Add a few tablespoons of water, heat gently until clear, then cook it back up without stirring.
• Walking away at the amber stage. Caramel can go from perfect to burnt in seconds. Stay with it and have your cream ready.