From feathers to leather: development of biotechnological processes for the valorisation of waste and by-products rich in keratinous materials

Every year, millions of tons of keratinous waste are generated worldwide. The improper management of keratin-rich industrial residues leads to environmental problems. The main approaches to recover these byproducts are physical-chemical treatment, using toxic compounds that cause harm to both human health and environment. This work aims to develop innovative, sustainable biotechnological approaches to valorize industrial residues, like feathers and tannery solid waste. At first, we selected keratinolytic bacteria from the Mediterranean Sea for hydrolyzing chicken feathers (72 hours). The feather protein hydrolysate was rich in keratinase activity and bioactivities, like antioxidants and antimicrobials (against S. aureus, S. epidermidis, F. psychrophilum). Then, ultrafiltration (10 kDa cut-off) separated the bioactive peptides (antioxidants in permeate) from the keratinase (retentate). Recovered keratinase enabled direct feather hydrolysis, releasing antioxidants. The best-performing isolates, Bacillus velezensis MT9, and the feather hydrolysate were characterized. MT9 genome were analyzed via bioinformatics, identifying a promising S8-peptidase keratinase. Proteomic techniques were used to analyse the secretome, abundant in peptidases, especially the predicted keratinase, and to characterize the bioactive compounds, of low MW (<1400 Da). The diluted hydrolysate resulted safe for cosmetics, showing no cytotoxicity on skin cells. The other study focused on the sustainable management of tannery leather waste (TSW), associated with chromium toxicity, using proteolytic bacteria from Sahara Desert. Bacillus safensis L81 was selected for degrading different TSW, chromium- and naturally-tanned. The wet blue (WB, 20 g/L) undergoes fewer tannings; thus, it was easier to degrade (24 h) than finished leather (15 days). WB hydrolysate resulted rich in proteins, consistent with the FTIR analysis, and peptides of low MW. WB hydrolysate has a high safe chromium content, but not in the carcinogenic form. Overall, new unconventional bacterial isolates were selected for the degradation of different industrial residues in a wide range of conditions, producing a multifunctional protein hydrolysate.