Valorisation of keratinous wastes: A sustainable approach towards a circular economy.

The valorisation of keratinous wastes involves biorefining and recovering the bioresource materials from the keratinous wastes to produce value-added keratin-based bioproducts with a broad application, distribution, and marketability potential. Valorisation of keratinous wastes increases the value of the wastes and enables more sustainable waste management towards a circular bioeconomy. The abundance of keratinous wastes as feedstock from agro-industrial processing, wool processing, and grooming industry benefits biorefinery and extraction of keratins, which could be the optimal solution for developing an ecologically and economically sustainable keratin-based economy. The transition from the current traditional linear models that are deleterious to the environment, which end energy and resources recovery through disposal by incineration and landfilling, to a more sustainable and closed-loop recycling and recovery approach that minimises pollution, disposal challenges, loss of valuable bioresources and potential revenues are required. The paper provides an overview of keratinous wastes and the compositional keratin proteins with the descriptions of the various keratin extraction methods in biorefinery and functional material synthesis, including enzymatic and microbial hydrolysis, chemical hydrolysis (acid/alkaline hydrolysis, dissolution in ionic liquids, oxidative and sulphitolysis) and chemical-free hydrolysis (steam explosion and ultrasonic). The study describes various uses and applications of keratinases and keratin-based composites fabricated through various manufacturing processes such as lyophilisation, compression moulding, solvent casting, hydrogel fabrication, sponge formation, electrospinning, and 3D printing for value-added applications.