From dairy by-product to high-performance material: Casein as a sustainable platform for functional textile...

Casein, a major dairy protein and an abundant by-product of milk processing, is being studied as a sustainable material platform for developing high-performance functional fiber and biomaterial systems. However, the successful implementation of casein-based fibers depends on an understanding of their compositional variability, colloidal behavior, and processing response. Casein availability and functionality depend strongly on milk composition, processing conditions, and breed-related differences, while comparisons across studies remain challenging due to non-standardized reporting metrics. Acid precipitation represents a critical extraction step, as processing conditions influence yield, molecular organization, and downstream material performance. At the molecular level, casein exhibits inherent heterogeneity (αs1, αs2, β, and κ fractions), polydispersity, and a tendency to aggregate, which directly affect solution stability and fiber spinnability. Conventional wet spinning remains the primary route for casein fiber production, whereas electrospinning enables the fabrication of nanostructured fibers with enhanced functional properties, although formulation optimization is required. Emerging green crosslinking strategies are being developed to improve water resistance, mechanical stability, and durability under use conditions. This review examines current advances, processing challenges, and future opportunities for positioning casein as a dairy-derived material, with an emphasis on structure-processing-function relationships and sustainable routes toward functional applications.