Huge amount of keratinous waste, especially birds' feathers, demands more value-added application instead of dumping. The present work reports the results of experiments aimed at preparing soluble keratin useful for novel bioproduct formation. The effect of thermo-chemical treatments with various reducing agents, i.e. 2-mercaptoethanol, dithiothreitol, sodium m-bisulphite, and sodium bisulphite, as well as sodium hydroxide, on the yield of keratin extracted from chicken feathers was determined. It was shown that after 2-h reduction with 2-mercaptoethanol and sodium bisulphite, the yield of soluble keratin was about equal and amounted to 84 and 82 %, respectively. The cheaper and harmless sodium bisulphite additionally decreased the extraction time to 1 h with the same yield. Moreover, treatment of the feathers with 2.5 % NaOH further improved the extraction effectiveness by increasing the yield up to 94 %. The results of the study demonstrate the viability of hydrolytic processes to obtain soluble keratin useful for biodegradable film formation for food application, that are harmless and more effective than solubilization by reduction of the disulphide bonds. Graphical Abstract

The keratinous wastes of the textile industry and poultry slaughterhouses may be used as sources of soluble keratins or hydrolysates. This review presents methods for processing raw keratin-based materials into bioproducts with functional and bioactive properties suitable for biomedical, cosmetic, food, and agricultural applications. Soluble keratin can be obtained by thermal treatment in some organic solvents, reduction, or oxidation of the disulfide bonds. Recent studies have shown that keratins contain amino acid sequences with high biological activities such as antioxidant, angiotensin I converting enzyme inhibitory, dipeptidyl peptidase IV inhibitory, and antimicrobial.Peptides containing these sequences may find numerous applications as value-added products in the food industry. More research devoted to development of methods for conversion of animal by-products to novel products is needed. Further technological investigations to create large-scale production methods are also necessary. Practical applicationsThe keratinous wastes represent a problematic by-product to the wool textile industry and poultry slaughterhouses due to the large volumes and their high pollutant load. They are usually incinerated or used for low value purposes such as fertilizers. This review focuses on the trends of application of keratin recovered from animal by-products. Biomaterials for regenerative medicine, cosmetic formulations, and biodegradable food packaging can be obtained as a result of keratin self-assembly. Several peptide sequences released by hydrolysis as bioactive peptides should be studied further for their in vivo antihypertensive, and antidiabetic effects, as well as functional ingredients in foods. K E Y W O R D Sbioactive peptides, functional properties, keratin hydrolysates, keratin isolates

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