Sugar is a structural ingredient and reducing it changes the bake

Sweetness is one job out of five

It is tempting to think of sugar as a flavouring that could be dialled down like salt. In a batter it is closer to a building material. It holds water, it delays the point at which proteins and starches set, it feeds the browning reactions that make a crust, it carries air into fat during creaming, and it keeps a finished cake soft for days after it was baked. Sweetness is real, but it is the property you notice rather than the property doing the work.

That is why a recipe with the sugar halved so seldom behaves like the original. You have not removed a flavour. You have removed a structural component and left everything else in place, and the other ingredients then behave as though a quarter of the recipe had gone missing — which, by weight, it often has.

Sugar holds water, and water is texture

Sucrose is hygroscopic: it attracts and binds water molecules and does not readily give them up. In a batter this means a portion of the liquid is committed to the sugar rather than free to hydrate flour proteins or to evaporate in the oven. The immediate consequence is a more tender crumb, because gluten development is slowed by the competition for water.

The longer consequence is shelf life. A cake goes stale mainly through starch retrogradation, in which gelatinised starch molecules gradually reassociate into a more ordered, firmer arrangement, squeezing out water as they do. This is a physical change and not simply drying out — a cake sealed in a bag still stales. Sugar interferes with that reordering and holds moisture in the crumb, which is why a high-sugar cake is still pleasant on the third day while a lean one is not.

It moves the temperature at which the structure sets

Dissolved sugar raises the temperature at which egg proteins coagulate and at which starch granules gelatinise. That sounds like a technicality and it is one of the most visible effects in the oven. Setting later means the batter stays fluid longer, so gas has more time to expand before the walls harden. You get more rise and, in a cookie, more spread.

Take the sugar out and the structure locks up earlier. The bake sets while the gas is still expanding, which produces a tighter, drier, often domed result. Anyone who has made a low-sugar sponge and found it small and slightly tough has watched this happen. In custards the same principle runs the other way and is useful: sugar makes a custard more forgiving, since the proteins need more heat before they curdle.

The surface of a bake is mostly a sugar reaction

Colour and crust flavour come from two distinct browning processes. Caramelisation is sugar breaking down under heat on its own, which for sucrose needs temperatures well above the boiling point of water — in the region of 160°C. The Maillard reaction is different: it is a reaction between reducing sugars and amino acids, it runs at lower temperatures, and it produces the far broader range of aromatic compounds responsible for the smell of a cake baking.

Not all sugars behave alike here. Glucose and fructose are reducing sugars and brown readily; sucrose is not, though it inverts into its component sugars in the presence of heat and acid and then browns perfectly well. Honey and golden syrup contain invert sugar already, which is why a bake made with them colours faster and often needs a lower oven than the same bake made with white sugar.

Crystals do work that syrups cannot

When butter and sugar are creamed, the aeration happens because the hard edges of sugar crystals tear air pockets into the plastic fat as they are dragged through it. A liquid sweetener cannot do that, and neither can sugar that has already dissolved. This is the reason a recipe specifies granulated rather than caster in some places and the reverse in others: crystal size changes how many air cells are created and how large they are.

Sugar also stabilises foams. In a meringue it dissolves into the water film between air bubbles, raises the viscosity of that film and slows drainage, which is why an under-sugared meringue weeps and collapses. So when a recipe adds sugar slowly to whipping whites, that is not ceremony. Dumping it in at once weighs the foam down before it has structure to lose.

All of which suggests a better way to think about the whole question. Sugar in a bake is doing the work that flour, egg and fat cannot do alone, and it is doing several jobs that have no obvious substitute. If you want less sweetness, the more productive route is usually to add something that pushes against it — salt, acidity, a darker chocolate, a bitter caramel — rather than to take away the material the crumb was built from.