Caramel that crystallises turns cloudy, pale and gritty instead of smooth and amber. The sugar has gone back to being sugar crystals. It happens to everyone at least once, and it is easy to stop once you know what sets it off.
I make caramel in the pan for my apple tarte tatin, with butter, caster sugar and a squeeze of lemon, so that is the method I am going by here.
The short answer
Hot syrup holds more sugar than it can keep dissolved, so it is always ready to crystallise. One small crystal is enough to start it, and then the whole pan follows. Stirring, a dried crumb of syrup falling back in from the side of the pan, or a cold spoon can all give it that first crystal. Swirl instead of stirring, brush the sides down, and add a little lemon juice.
What is going on
Hot water can hold a lot more dissolved sugar than cold water. As syrup cooks and cools, it ends up holding more sugar than it really should for its temperature. Sugar in that state only needs a small push to start coming back out as crystals.
Once one tiny crystal forms, more sugar sticks to it and it grows, and new crystals start off it. That is why caramel rarely goes grainy in one corner. It goes all at once.
What starts the crystals
- Stirring. Moving the syrup around knocks the sugar molecules into each other, and that is how the first crystals form. I swirl the pan by the handle instead, which moves the syrup without stirring it.
- Splashes on the side of the pan. Syrup that splashes up the side dries out into sugar crystals. If one falls back in, it starts the whole pan off. Brush the sides down with a wet pastry brush.
- A metal spoon. Metal pulls heat away from the syrup it touches, and that cooler patch is where crystals start. Dried syrup on a spoon does the same thing. If you have to touch the caramel, use a wooden spoon.
Why the lemon juice helps
Table sugar is made of two smaller sugars, glucose and fructose, joined together. Heated with a little acid, some of it splits back into those two. They get in the way of the table sugar when it tries to pack into crystals, so the syrup stays smooth for longer. Honey is mostly those two sugars already, and glucose syrup also gets in the way of the crystals, which is why some caramel recipes use them.
That is why my tarte tatin has lemon juice in the caramel and not only in the water the apples wait in.
Watch the heat at the end
The water in the pan, which in my tarte tatin comes from the butter and the lemon juice, is what keeps the sugar from burning early on. As it boils off, the syrup gets more concentrated and its temperature keeps rising. The longer it cooks after that, the darker and more bitter it gets, so I turn the heat down as the bubbling slows.
If it has already crystallised
In my experience the easiest fix is to start again with fresh sugar. Sugar is cheap, and a gritty caramel will not turn smooth in the oven.
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