Glucose and invert sugar are there to stop something happening

Sucrose is orderly, which is the problem

Table sugar is a single compound with a regular molecular shape, and regular shapes stack neatly into crystals. That tidiness is exactly what a confectioner is fighting most of the time: a syrup of pure sucrose is always looking for an excuse to arrange itself into a solid lattice, and a fudge, a caramel or a jam that does so has failed. The commonest defence is to introduce molecules that do not fit the lattice and get in the way of the ones that do.

Glucose syrup, invert sugar and honey all do this, and none of them are simply sweeter or cheaper alternatives to sugar. Understanding what they contribute makes it obvious why a recipe specifying glucose syrup usually cannot be made with sugar and water. The substitution changes the physics of the finished sweet, and the resulting failure — grainy, dry, hard, short-lived — tends to be blamed on technique instead.

Invert sugar is sucrose taken apart

Sucrose is a compound of glucose and fructose joined together. Heat it in the presence of acid, or treat it with the enzyme invertase, and that bond breaks, leaving a mixture of the two simpler sugars. This is what happens when a recipe calls for a squeeze of lemon or a pinch of cream of tartar in a syrup, and it is why that small addition has an effect out of all proportion to its quantity. Some of the sucrose has stopped being sucrose.

The mixture behaves differently in three useful ways. It resists crystallisation, because glucose and fructose disrupt the sucrose lattice. It tastes somewhat sweeter, since fructose registers as sweeter than sucrose does at the same concentration. And it holds water tenaciously, which keeps a finished sweet soft. Honey is largely a naturally occurring invert sugar, made by bees using an enzyme of their own, which is why it behaves so similarly in a recipe.

Glucose syrup is a mixture, and the mixture is the point

Glucose syrup is produced by breaking starch down into shorter chains, and the result is not one sugar but a distribution of them: some free glucose, some two-unit and three-unit sugars, and a long tail of larger fragments. That distribution is described by a dextrose equivalent number, which indicates how far the breakdown was taken. A low number means more long chains, more viscosity and less sweetness; a high number means more small sugars and behaviour closer to invert.

The long chains do work that no simple sugar can. They raise viscosity, which physically slows the movement of molecules towards a growing crystal face, and they contribute body and chew to a sweet without contributing much sweetness. That is why a chewy caramel or a soft nougat lists glucose syrup high in its ingredients. It is providing texture, and the sucrose alongside it is providing most of the sweetness.

Water-holding is the second job and it cuts both ways

These sugars are hygroscopic, meaning they bind water and pull it out of the air. In a ganache or a soft caramel this is entirely desirable, since it keeps the texture yielding and slows the staling that dries a sweet out. It also lowers the amount of water available to microbes, which is why heavily sugared preserves keep so long — the water is present but chemically tied up rather than free to support spoilage.

The same property becomes a nuisance elsewhere. A brittle, a hard-crack sweet or a dry biscuit made with invert sugar will go sticky and soften as it draws in moisture, and no packaging entirely prevents it in a humid climate. This is a real trade rather than an oversight. You choose crystallisation resistance and softness, and you accept a shorter crisp life, or you choose crispness and accept the risk of graininess.

Where the choice shows up on a recipe card

A ganache with glucose syrup stays smooth and glossy for longer and resists the grain that pure sucrose can develop. A caramel sauce made with a little corn syrup is far less likely to seize on cooling. Ice cream formulations often include glucose syrup partly for crystal control and partly because it depresses the freezing point less per unit weight than an equivalent quantity of sucrose would, which gives the maker a second control over hardness.

Substituting between them is possible with care. Honey brings a strong flavour and slightly more acidity; golden syrup is a partially inverted product with its own character; corn syrup in its ordinary retail form is mild and mostly glucose. Treacle and molasses are not interchangeable with any of them, since they carry acidity and mineral content that will react with leavening. When a recipe names one specifically, it is generally naming it for a reason.