Vol. VII · Starches and Sugars Plate 9 of 11invert sugar, inversion, honey, glucose, fructose, acid, hygroscopic, soft cookies, golden syrupMT

Invert Sugar and Moisture.

A spoon of honey in a cookie dough keeps the cookies soft for days. The reason is invert sugar, which pulls water from the air.

Heated with a little acid, sucrose splits into its two halves, glucose and fructose: invert sugar. The mixture behaves quite differently from sucrose. It does not crystallise easily, so syrups and jams made with some of it stay smooth (VII.7). It is hygroscopic — it draws water from the air — so cakes and cookies made with honey, golden syrup or invert syrup stay soft and moist. It lowers water activity more per gram (III.9), browns sooner (V.2) and tastes a little sweeter.

Honey is mostly invert sugar made by bees' enzymes; golden syrup and Chinese 转化糖浆 (used in mooncake pastry) are made by heating sugar syrup with acid. A cook makes some every time a syrup is boiled with lemon juice or cream of tartar. The workshop boils a syrup at the pH you choose.

Below: the share of sucrose inverted against minutes of boiling at three pH values and yours (a teaching rate, faster tenfold per pH unit lower, as acid catalysis implies).

What you will learn
Workshop
Teaching model: first-order inversion at 100 °C with a rate proportional to the hydrogen-ion concentration (tenfold per pH unit), set so that a pH 3 syrup inverts about half its sucrose in an hour.

Split sugar, soft cakes

Boiled thirty minutes at pH 3, as with a generous squeeze of lemon, a syrup has about 29% of its sucrose inverted in this model; with cream of tartar (pH about 3.5), 10%; in plain water, only 0.03%. Even a modest share is enough to stop a syrup crystallising, which is why many candy and jam recipes add a little acid or glucose syrup.

Use invert sugar for what it does. A tablespoon of honey or golden syrup in cookies keeps them chewy; mooncake pastry uses invert syrup for a soft, glossy crust that browns well; sorbets and ice creams with some invert sugar or glucose stay scoopable. Too much makes doughs sticky and baked goods brown too fast and too dark; and honey's fructose browns at lower temperatures, so honey cakes are baked a little cooler.

At the stove

Bake two small batches of sugar cookies, replacing a quarter of the sugar with honey in one. Store both in open air for two days. The honey cookies stay soft; the others go crisp.

The word "invert" comes from optics: sucrose solution rotates polarised light to the right, and after inversion the mixture rotates it to the left — the direction of rotation is inverted. Arrhenius studied this very reaction (V.10).

Terms

Inversion
Splitting sucrose into glucose and fructose with acid and heat or with the enzyme invertase.
Invert sugar
The resulting mixture of glucose and fructose.
Hygroscopic
Drawing water from the air.
Golden syrup
A partly inverted sugar syrup used in baking.
Acid and heat split sucrose into invert sugar, which resists crystallising, holds water, browns sooner and keeps bakes soft.

Sources

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