Vol. VII · Starches and Sugars Plate 7 of 11crystallisation, supersaturation, fudge, fondant, seeding, interfering agents, glucose syrup, stirringMT

Crystallisation.

Every candy is a decision about crystals: none (glass), very many tiny ones (smooth fudge), or a few large ones (grainy disaster).

Hot water dissolves much more sugar than cold: about 487 g per 100 g of water at 100 °C against about 204 g at 20 °C. A syrup cooked to soft ball and cooled holds far more sugar than cool water can, so it is supersaturated and wants to crystallise. What decides the texture is how many crystals form and how large they grow. Many small crystals, all at once, give the smooth, firm texture of fudge and fondant. A few big ones, growing slowly, give a grainy, sandy candy.

Cooks control crystallisation three ways: by when they stir (stirring a hot syrup seeds a few large crystals; beating it once cooled to about 50 °C seeds many small ones at once), by keeping stray crystals out (washing down the pan sides with a wet brush, covering the pan at first), and by adding interfering agents — glucose syrup, honey, cream of tartar or lemon juice (which invert some sucrose, VII.9), butter, milk — that get in the way of crystals.

Below: the solubility of sucrose against temperature, and how supersaturated a fudge syrup becomes as it cools from the stage you choose; and the effect of an interfering agent.

What you will learn
Workshop
Sucrose solubility in water (published table values). Supersaturation = sugar held per 100 g of water in the cooked syrup ÷ what water dissolves at that temperature. Interfering agents are a teaching factor that lowers the effective drive to crystallise.

Many small, or few large

Cooked to 114 °C, a fudge syrup holds about 567 g of sugar per 100 g of water. Cooled undisturbed to 50 °C, it is 2.16 times supersaturated: beat it then and crystals form everywhere at once, too many to grow large, and the fudge sets smooth and firm. Stir it while it is still hot and the few crystals that form grow big as it cools; the fudge is grainy.

For a clear brittle, toffee or caramel the goal is no crystals at all: cook to a high stage where so little water is left that the sugar sets as glass, keep the pan sides clean, avoid stirring once the syrup boils, and add glucose syrup or a little acid. One stray crystal — from a spoon, a speck on the rim — can seed a whole pan into sugar sand, the "seizing" every confectioner has suffered.

At the stove

Make two small batches of fudge cooked to 114 °C. Beat one at once while hot; leave the other undisturbed until it cools to about 50 °C (warm to the touch), then beat until it thickens and loses its gloss. Compare the textures the next day.

Fondant, the smooth white icing of cakes and centres of chocolates, is pure crystallisation control: syrup cooked to soft ball, cooled, then worked on a marble slab until it turns white with countless microscopic crystals.

Terms

Supersaturation
A solution holding more dissolved substance than it could at equilibrium.
Seeding
Starting crystallisation with a crystal or a disturbance.
Interfering agent
An ingredient that hinders crystal growth: glucose syrup, invert sugar, fat, milk proteins.
Seizing
A syrup suddenly crystallising into a grainy mass.
Cool, then beat: many small crystals make smooth candy. Keep crystals out entirely for glass.

Sources

  1. H. McGee, On Food and Cooking: The Science and Lore of the Kitchen, 2nd ed., Scribner, 2004.
  2. A teaching value chosen for the course: the right order of magnitude and shape, not a measurement of a particular food.