Technical knowledge
A snack can lose crispness quickly as moisture rises.
Water plasticises the snack matrix and can lower its glass-transition temperature, contributing to loss of crispness. The rate and moisture threshold depend on the product, and crispness can decline before the macroscopic glass transition.
Which is why a product can be right for weeks and wrong in a fortnight, and why the seasoning on the outside is sometimes what brought the water in.
Read the technical note
