Hygroscopic materials are those that absorb and retain moisture. This is particularly important in baked goods that need to remain tender over a longer period, as the loss of moisture is a major component in staling. When an ingredient is added to a food product explicitly to retain moisture, it is called a humectant.
Common ingredients that are hygroscopic include:
- Honey
- Glucose syrup
- White sugar (sucrose)
- Molasses
- Egg yolk and egg white
- Sugar alcohols such as glycerol, sorbitol, xylitol, and maltitol
Baked goods containing hygroscopic ingredients will have a tendency to become softer if left in an environment with ambient humidity.
Note that brown sugar is hygroscopic but still has a tendency to become hard and dry. Brown sugar contains a thin layer of molasses around many grains of sucrose (white sugar), which, when it dries out, because hard and sticky. The introduction of slowly-absorbable water, as from a piece of bread, will soften the sugar by allowing the hygroscopic molasses to become rehydrated and soft.