Sugar holds on to moisture
Also searched as: moist cake, sugar binds water, hygroscopic, cake stays fresh, honey in dough
In short: Sugar attracts water and holds on to it. Cakes with enough sugar stay moist longer; biscuits with honey turn soft.
Like a storeroom for water: sugar lays in moisture and only gives it back slowly.
Simplified model
What happens
Sugar binds water (it is hygroscopic). In a batter it holds the water, so less evaporates during baking and the cake dries out more slowly afterwards. It also slows the starch from going stale. Honey and syrup even draw moisture from the air: biscuits made with them turn soft in the tin, while pure sugar biscuits stay crisp.
Why it matters
Explains why reduced-sugar cakes are drier and why honey gingerbread turns soft over time.
Rules of thumb
- Do not reduce sugar by more than a quarter, otherwise the cake turns dry and pale.
- For soft biscuits use a little honey or brown sugar; for crisp ones, white sugar.
- Store crisp and soft biscuits separately.
Levers
In practice
Cakes and baking
Honey gingerbread turns soft and better after a few days in the tin.
- Harold McGee: On Food and Cooking (2004)