stiffen

Stiffen is a verb meaning to make or become stiff or rigid. It can describe physical changes in materials and tissues, as well as metaphorical or procedural changes in behavior, appearance, or design. The noun form stiffener and the gerund stiffening are commonly used in crafts, engineering, biology, and cooking contexts.

In materials science and engineering, stiffness refers to a body's resistance to deformation under load. Stiffening can be achieved by altering geometry, adding reinforcing elements such as ribs or stiffeners, or by changing material properties (for example, increasing the modulus of elasticity). Designers often stiffen panels, beams, or skins to improve load-bearing performance or shape retention.

In textiles and paper products, stiffening agents or resins (often starch, gum, or polyvinyl acetate-based formulations) are applied to fabrics to enhance body and shape. Stiffened fabrics hold crisp silhouettes for collars, hats, or crafts, but may lose drape and breathability as a trade-off.

In cooking, to stiffen means to set a mixture into a firmer, structured form—for example, beating egg whites or cream to stiff peaks, or gently cooking gelatin until it firms. Stiffening can also refer to thickening a sauce with starch or reducing moisture to create a firmer texture.

In biology and medicine, stiffness describes resistance to movement or deformation in tissues such as muscles and joints. Increased stiffness, or rigidity, can arise from injury, arthritis, inflammatory conditions, or neurological disorders such as spasticity or Parkinson's disease.