What makes crackers, cereal, chips and similar foods feel soft/strange textured after they’re exposed to air for a few days?

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What makes crackers, cereal, chips and similar foods feel soft/strange textured after they’re exposed to air for a few days?

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Crackers, cereal, chips, etc are very dry. By comparison the air is very wet, do they absorb water as they sit. As they absorb water they swell and become soft. It’s a lot like those little toys that you put in water and they get bigger as they absorb water.

Moisture migration. Water will always try and equilibrate.
Atmosphere vs Water content of product. Water activity plays a part as well.

The equalise with the atmosphere.

If you have a cracker, it is typically very dry – much drier than the humidity in an average room. This means that when you leave it sitting out in the open air, it will slowly absorb some of this moisture until it reaches equilibrium with the surrounding air. In the case of a cracker this means being a bit more moist than intended and it goes soft.

This also works in reverse too – take a moist slice of cake or bread and leave it out, and because it contains more moisture than the surrounding air it will slowly dry out and this moisture will be absorbed into the atmosphere – ending with a dry and stale cake or bread.