In products such as biscuits and hard caramel, which contain small amounts of moisture and large amounts of sugar, the relative moisture level is lower than the ambient humidity. Without protective packaging, these products will absorb moisture from the air.
Sugar’s ability to bind water in foods is based on the relationship between crystallised and loose sugar. Any change in this relationship during production or storage affects the amount of water that the food can bind.
Because sugar binds water, reactions that need water are destroyed if sugar is present. The shelf life of bread is extended because sugar causes water to be retained for longer in the bread.
Starch in bread gels more slowly if the dough contains sugar or syrup. This effect is primarily caused by fructose, which is left behind when the yeast reacts with glucose. This causes the bread to rise and makes it more airy and porous than unsugared bread, thus ensuring that it stays fresh for longer.