Fudge should melt on your tongue. When it is grainy, it feels sandy, like there is sugar you can crunch, and that is exactly what it is: sugar that has set in big crystals instead of millions of tiny ones.
The same chemistry decides whether my condensed milk brownies come out fudgy or sugary, so here is how it works.
The short answer
Fudge goes grainy when the sugar crystals grow too big. That happens when the syrup starts crystallising while it is still hot, when a stray crystal gets in and sets off the rest, or when you stop beating too soon. A spoonful of glucose or corn syrup in the pan helps prevent all three.
What is going on in the pan
Fudge is not meant to be crystal free. It is meant to be full of sugar crystals so small that your tongue cannot feel them. The whole method is about making lots of tiny crystals instead of a few big ones.
In a hot syrup the sugar molecules move fast. They find an existing crystal and pile on to it quickly, and new crystals struggle to start, so you end up with fewer, bigger crystals: grainy. In a cooler syrup the crystals grow more slowly and far more of them start, so each one stays small: smooth.
The things that make it grainy
- Stirring it while it is hot. Any agitation while the syrup is still very hot sets crystals off early, and early crystals grow big. Cook it to temperature, then leave it alone to cool before you beat it.
- A stray crystal. Sugar dried on the side of the pan, or on the spoon, is a ready made seed. Drop it into the cooling syrup and the crystals grow off it. Wipe the sides of the pan down before the syrup cools.
- Stopping beating too soon. Once it is cool enough to beat, keep going until it loses its shine and starts to set. The moment you stop, the crystals you have made carry on growing.
- Cooking it too far, or not far enough. Cook it too far and it sets dry and crumbly. Not far enough and it stays soft, even runny. A degree or two either side makes a difference, so a thermometer is worth having.
Why a spoon of syrup helps
Glucose, invert sugar and corn syrup all get in the way of ordinary sugar crystals growing. Their molecules sit on the crystal surface and block it, so the crystals stay small. That is why so many fudge recipes add some. Honey does the same job and brings its own flavour.
Condensed milk fudge
A lot of easy fudge recipes are built on a tin of sweetened condensed milk. They need the same care as any other: stirring while it is hot or cooking it too far can still make it grainy, and the lactose in condensed milk can form sandy crystals of its own.
If it has already gone grainy
It is still sweet and still edible, just sandy. The usual fix is to put it back in the pan with a splash of milk or water, melt it down gently until it is a syrup again, and take it through the cooling and beating more carefully.
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