The sugars, and when the difference matters

CRYSTAL SIZE decides whether the sugar can do mechanical work. Creaming depends on sharp crystals dragging air into fat, so caster does it best, granulated more slowly, and icing sugar not at all. Size also decides whether the sugar dissolves: undissolved crystals in a dry mixture are detectable as grit, which is why a simple buttercream needs icing sugar.\n\nMOLASSES CONTENT decides flavour, colour, moisture and acidity together. Soft brown sugars carry molasses distributed through them, so they are moister, more acidic and more hygroscopic than white sugar — which is why brown-sugar cookies are softer and chewier and why a formulation leavened with bicarbonate of soda may depend on the acid.\n\nWHETHER IT IS A SYRUP changes the category. Golden syrup, honey and treacle are inverted or partly inverted, so they resist crystallisation, hold water and brown earlier. Substituting a syrup for a crystalline sugar adds liquid and changes texture toward chewiness.

GRANULATED FOR CASTER: safe in anything wet or dissolved; slightly coarser in anything creamed; risky in a dry, delicate mixture where the crystals may not dissolve.\n\nCASTER FOR ICING SUGAR: not safe in a buttercream or an icing, where there is no water for it to dissolve into and it stays gritty.\n\nBROWN FOR WHITE: changes moisture, colour and acidity as well as flavour. Usually pleasant; occasionally consequential where bicarbonate of soda was relying on the acid.\n\nDEMERARA FOR SOFT BROWN: usually a mistake inside a mixture. The crystals are hard and do not dissolve, which is exactly why demerara is excellent as a crunchy topping.\n\nSYRUP FOR SUGAR: never arithmetic. It changes sweetness, moisture, colour and pH at once.

The most useful thing to understand about sugar in baking is that removing it does far more than reduce sweetness.\n\nSugar competes with flour for water, which restrains gluten and tenderises. It raises the temperature at which egg proteins set, which delays the structure forming and gives a batter longer to rise. It holds moisture in the finished crumb. It browns. And in a creamed cake, its crystals are the mechanism by which air enters the batter at all.\n\nThat is why reducing the sugar in a cake produces something tougher, paler, drier-keeping and often smaller — not a less sweet version of the same cake. It is a reformulation, and it is worth doing deliberately rather than by subtraction.