Why Does Some Sugar Turn Grainy? Controlling When Crystals Form

Dissolving sugar in water gives a solution, and there is a limit to how much will dissolve at a given temperature, with more dissolving as the temperature rises. Boiling a syrup drives off water and raises the boiling point, so a hotter syrup is a more concentrated one, and the temperature reached is therefore a direct measure of concentration, which is why confectionery recipes specify temperatures rather than times. Cooling that syrup takes it past the point where the sugar can stay dissolved, leaving it supersaturated and unstable, and from there it will crystallise given the opportunity. Everything the cook does next either encourages crystals to form in a controlled way or prevents them forming at all.

A few factors control whether crystals appear and how large they grow:

• •Temperature at which stirring begins, with cooler syrups producing many small crystals and warmer ones few large ones
• •Agitation, which starts crystallisation and distributes the crystals that form
• •Seed crystals, whether deliberate or from a stray grain on the pan wall
• •Interfering agents including glucose syrup, honey, acid and fat, which obstruct crystal growth
• •Acid, which splits some sucrose into two different sugars that do not fit the crystal
• •Cooling rate, since slow cooling gives crystals time to grow large

Sweets divide into those where crystals are wanted and those where they are not, and the techniques are opposite. Crystalline confections including fudge, fondant and some caramels are cooled undisturbed to a specific temperature, well below the point of saturation, and only then agitated, which causes an enormous number of tiny crystals to form at once, producing a smooth texture where no individual crystal is large enough to feel. Stirring while still hot produces fewer and larger crystals and a gritty result, which is the standard way to ruin fudge. Non-crystalline confections including hard sweets, toffee and caramel are prevented from crystallising at all, by including interfering agents and by avoiding agitation entirely, and the result is a glass rather than a solid with any ordered structure.

Sugars other than ordinary table sugar behave differently and the differences explain several familiar foods. Table sugar is sucrose, a single compound that crystallises readily into a well-defined form. Invert sugar is what results when sucrose is split into its two components, which crystallise far less willingly and which hold moisture, so a syrup containing it stays soft. Honey is largely invert sugar and does crystallise eventually, which is why a jar granulates on the shelf, and gentle warming redissolves it. Glucose syrup contains a mixture of chain lengths that obstruct each other, which is why it is the standard anti-crystallising ingredient. Sugar alcohols used as substitutes crystallise in their own ways and absorb heat as they dissolve, which is why some of them make the mouth feel cold.

The traditional instructions each address a specific mechanism. Washing down the sides of the pan with a wet brush removes stray crystals that would seed premature crystallisation throughout the syrup. Not stirring after the sugar has dissolved prevents both seeding and the splashing that deposits crystals on the pan wall. Adding a spoonful of glucose syrup or a little acid supplies molecules that do not fit the growing crystal and physically obstruct it. Cooling undisturbed and then beating is the fudge procedure. Covering the pan briefly early in boiling lets condensing steam wash the walls. And in humid conditions hard sweets absorb moisture from the air and turn sticky, which is why they are wrapped and why confectionery is traditionally difficult in damp weather.

The temperature a syrup reaches measures its concentration, and cooling leaves it supersaturated and ready to crystallise. Agitating while hot gives few large gritty crystals, while cooling undisturbed and then beating gives many tiny ones and a smooth texture. Glucose syrup, acid and fat obstruct crystal growth, which is how hard sweets stay glassy.