Stiffness

Stiffness is a rheological property describing how stiff an item is — that is, how firm and rigid. Technically, applying stress to a material produces strain along the stress direction, and stiffness is stress divided by strain. In foods it characterizes products from firm cheeses and set gels to crisp biscuits, and it is measured instrumentally with compression, bending, or penetration tests. Changes in stiffness during processing and storage reveal structural shifts such as starch retrogradation, protein gelation, or moisture migration, making it a useful quality-tracking parameter.