Especially after the Christmas season, the phenomenon is well known: The leftover cookies do not taste anymore. Hard varieties become soft, soft pastry hard. But why is it like that?
The reason is the moisture difference between the pastry and the air. Soft biscuits contain more moisture than their environment, with hard ones it is exactly the other way around. If you leave the soft pastry too long, the moisture evaporates. Unwanted result: It is hard as stone. Many people know the same phenomenon about bread. It consists of up to 40 percent water. The moisture migrates from the inside to the outside, so it gets harder and harder the longer it's outside.
What about hard biscuits?
Very hard pastries, such as butter biscuits, on the other hand absorb the humidity. Like a small sponge, they suck the water out of the air - and thus become softer. This effect can be used in a failed baking attempt. For example, if you put biscuits that have been baked too long with a piece of apple together in a tin and let them stand for a day or two, they absorb the moisture of the apple and become softer again.