Perspectives on Converting Keratin-Containing Wastes Into Biofertilizers for Sustainable Agriculture

Recovery of pure PET from wool/PET/elastane textile waste through step-wise enzymatic and chemical processing

Textile waste is mostly incinerated because few recycling processes are available to recover valuable materials. In this work, a feasible chemo-enzymatic recycling process of wool/polyethylene terephthalate (PET)/elastane blends to recover pure PET is for the first time successfully demonstrated. Two novel enzyme formulations were selected for wool hydrolysis, whereas the recovered amino acids were quantified using high-performance liquid chromatography and two assays (Ninhydrin and Folin-Ciocalteu). Kinetic studies on the amino acid formation alongside reaction observations by scanning electron microscopy proved sufficient removal of wool within 8 hours with the new enzyme formulation, marking an acceleration compared to previous studies. Finally, elastane was separated with a non-hazardous solvent to obtain pure PET. Tensile tests on the recovered PET fibres reveal only slight changes through the enzymatic treatment and no changes induced by the applied solvent. The enzyme formulation was successfully tested on five different post-consumer wool/PET textile waste samples. This valorization approach enhances the circular economy concept for textile waste recycling.

Keratinase and its diverse applications

Keratinase is a proteolytic enzyme specialized in the degradation of keratin-rich materials and has garnered significant attention for its potential in various biotechnological applications. This review provides an overview of keratinase, focusing on its structure, classification, function, biochemical properties, mechanisms of action and diverse applications. Keratinase plays an important role in bioremediation and stands out prominently, as it facilitates the eco-friendly degradation of keratinaceous waste materials addressing environmental concerns by reducing pollution and waste accumulation. Moreover, in the textile industry, keratinase plays a pivotal role in bio-pretreatment processes, enhancing the dyeing and finishing properties of animal fibers such as wool and silk. Beyond textiles, this enzyme contributes significantly to animal feed production by hydrolyzing keratin-rich byproducts into digestible components, thereby fostering the creation of high-protein feeds. Its impact extends to the cosmetic and pharmaceutical realms, where keratinase finds use in skincare formulations and in treating certain dermatological conditions owing to its ability to modify and break down keratin structures. By assisting in the removal of dead tissue, it demonstrates potential in biological applications for wound healing. Additionally, the challenges and future perspectives on the commercial scalability of keratinase production and its integration into various sectors are discussed.

Impact of Steam-Exploded Feather Incorporation on the Biodegradation Performance of Renewable Biocomposites

The increasing environmental concerns regarding plastic waste, especially in agriculture, have driven the search for sustainable alternatives. Agricultural plastics, such as mulching films and greenhouse covers, are heavily reliant on petrochemical-derived materials, which persist in the environment and contribute to long-term pollution. This study explores the use of biodegradable biocomposites made from steam explosion-treated chicken feathers and various polymer matrices to address these issues. Chicken feathers, a waste by-product of the poultry industry, present an excellent biodegradability as a result of the steam explosion treatment and contain nitrogen, potentially enhancing soil fertility. The biocomposites were characterized by thermal stability, mechanical properties, and biodegradability, and ecotoxicity assessments were carried out studying the incorporation of feathers into the soil. Results showed that the incorporation of treated chicken feathers increased the water absorption capacity of the composites, promoting faster disintegration and biodegradation. In particular, biocomposites made with polyhydroxyalkanoates and Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) exhibited a significant increase in degradation rates, from 3-10% in the first month for pure matrices to 40-50% when reinforced with treated feathers. Meanwhile, those made from polylactic acid showed slower degradation. Furthermore, the addition of feathers positively influenced crop growth at low concentrations, acting as a slow-release fertilizer. However, high concentrations of feathers negatively affect plant growth due to excess nitrogen. These findings highlight the potential of poultry feathers as a valuable, sustainable filler for agricultural bioplastics, contributing to waste valorization and environmentally friendly farming practices.

Keratinous and corneous-based products towards circular bioeconomy: A research review

Keratins and corneous proteins are key components of biomaterials used in a wide range of applications and are potential substitutes for petrochemical-based products. Horns, hooves, feathers, claws, and similar animal tissues are abundant sources of α-keratin and corneous β-proteins, which are by-products of the food industry. Their close association with the meat industry raises environmental and ethical concerns regarding their disposal. To promote an eco-friendly and circular use of these materials in novel applications, efforts have focused on recovering these residues to develop sustainable, non-animal-related, affordable, and scalable procedures. Here, we review and examine biotechnological methods for extracting and expressing α-keratins and corneous β-proteins in microorganisms. This review highlights consolidated research trends in biomaterials, medical devices, food supplements, and packaging, demonstrating the keratin industry's potential to create innovative value-added products. Additionally, it analyzes the state of the art of related intellectual property and market size to underscore the potential within a circular bioeconomic model.

Harnessing the potential of microbial keratinases for bioconversion of keratin waste

The increasing global consumption of poultry meat has led to the generation of a vast quantity of feather keratin waste daily, posing significant environmental challenges due to improper disposal methods. A growing focus is on utilizing keratinous polymeric waste, amounting to millions of tons annually. Keratins are biochemically rigid, fibrous, recalcitrant, physiologically insoluble, and resistant to most common proteolytic enzymes. Microbial biodegradation of feather keratin provides a viable solution for augmenting feather waste's nutritional value while mitigating environmental contamination. This approach offers an alternative to traditional physical and chemical treatments. This review focuses on the recent findings and work trends in the field of keratin degradation by microorganisms (bacteria, actinomycetes, and fungi) via keratinolytic and proteolytic enzymes, as well as the limitations and challenges encountered due to the low thermal stability of keratinase, and degradation in the complex environmental conditions. Therefore, recent biotechnological interventions such as designing novel keratinase with high keratinolytic activity, thermostability, and binding affinity have been elaborated here. Enhancing protein structural rigidity through critical engineering approaches, such as rational design, has shown promise in improving the thermal stability of proteins. Concurrently, metagenomic annotation offers insights into the genetic foundations of keratin breakdown, primarily predicting metabolic potential and identifying probable keratinases. This may extend the understanding of microbial keratinolytic mechanisms in a complex community, recognizing the significance of synergistic interactions, which could be further utilized in optimizing industrial keratin degradation processes.

Construction of artificial microbial consortia for efficient degradation of chicken feathers and optimization of degradation conditions

Microbes within a consortium exhibit a synergistic interaction, enhancing their collective capacity to perform functions more effectively than a single species, especially in the degradation of keratin-rich substrates. To achieve a more stable and efficient breakdown of chicken feathers, a comprehensive screening of over 9,000 microbial strains was undertaken. This meticulous selection process identified strains with the capability to degrade keratin effectively. Subsequently, antagonistic tests were conducted to isolate strains of fungi and bacteria that were non-antagonistic, which were then used to form the artificial microbial consortia. The optimal fermentation conditions for the keratinophilic microbial consortia were determined through the optimization of response surface methodology. The results revealed that 11 microbial strains-comprising of 4 fungi and 7 bacteria-were particularly proficient in degrading chicken feathers. The artificially constructed microbial consortia (AMC) comprised two bacterial strains and one fungal strain. The optimal conditions for feathers degradation were identified as a 10 g/L concentration of chicken feathers, a 2.6% microbial inoculation volume and a fermentation fluid pH of 9. Under these conditions, the degradation rate for chicken feathers reached a significant 74.02%, representing an 11.45% increase over the pre-optimization rate. The AMC developed in this study demonstrates the potential for efficient and economical process of livestock and poultry feathers. It provides innovative insights and a theoretical foundation for tackling the challenging degradation of keratin-rich materials. Furthermore, this research lays the groundwork for the separation and purification of keratins, as well as the development of novel proteases, which could have profound implications for a range of applications.

Keratin from Animal By-Products: Structure, Characterization, Extraction and Application-A Review

Keratin is a structural fibrous protein and the core constituent of animal by-products from livestock such as wool, feathers, hooves, horns, and pig bristles. This natural polymer is also the main component of human hair and is present at an important percentage in human and animal skin. Significant amounts of keratin-rich animal tissues are discarded worldwide each year, ca. 12 M tons, and the share used for keratin extraction and added-value applications is still very low. An important stream of new potential raw materials, represented by animal by-products and human hair, is thus being lost, while a large-scale valorization could contribute to a circular bioeconomy and to the reduction in the environmental fingerprint of those tissues. Fortunately, scientific research has made much important progress in the last 10-15 years in the better understanding of the complex keratin architecture and its variability among different animal tissues, in the development of tailored extraction processes, and in the screening of new potential applications. Hence, this review aims at a discussion of the recent findings in the characterization of keratin and keratin-rich animal by-product structures, as well as in keratin recovery by conventional and emerging techniques and advances in valorization in several fields.

Molecular strategies to enhance the keratinase gene expression and its potential implications in poultry feed industry

The tons of keratin waste are produced by the poultry and meat industry which is an insoluble and protein-rich material found in hair, feathers, wool, and some epidermal wastes. These waste products could be degraded and recycled to recover protein, which can save our environment. One of the potential strategy to achieve this target is use of microbial biotreatment which is more convenient, cost-effective, and environment-friendly by formulating hydrolysate complexes that could be administered as protein supplements, bioactive peptides, or animal feed ingredients. Keratin degradation shows great promise for long-term protein and amino acid recycling. According to the MEROPS database, known keratinolytic enzymes currently belong to at least 14 different protease families, including S1, S8, S9, S10, S16, M3, M4, M14, M16, M28, M32, M36, M38, and M55. In addition to exogenous attack (proteases from families S9, S10, M14, M28, M38, and M55), the various keratinolytic enzymes also function via endo-attack (proteases from families S1, S8, S16, M4, M16, and M36). Biotechnological methods have shown great promise for enhancing keratinase expression in different strains of microbes and different protein engineering techniques in genetically modified microbes such as bacteria and some fungi to enhance keratinase production and activity. Some microbes produce specific keratinolytic enzymes that can effectively degrade keratin substrates. Keratinases have been successfully used in the leather, textile, and pharmaceutical industries. However, the production and efficiency of existing enzymes need to be optimized before they can be used more widely in other processes, such as the cost-effective pretreatment of chicken waste. These can be improved more effectively by using various biotechnological applications which could serve as the best and novel approach for recycling and degrading biomass. This paper provides practical insights about molecular strategies to enhance keratinase expression to effectively utilize various poultry wastes like feathers and feed ingredients like soybean pulp. Furthermore, it describes the future implications of engineered keratinases for environment friendly utilization of wastes and crop byproducts for their better use in the poultry feed industry.

Comprehensive insights into the mechanism of keratin degradation and exploitation of keratinase to enhance the bioaccessibility of soybean protein

Keratin is a recalcitrant protein and can be decomposed in nature. However, the mechanism of keratin degradation is still not well understood. In this study, Bacillus sp. 8A6 can completely degrade the feather in 20 h, which is an efficient keratin degrader reported so far. Comprehensive transcriptome analysis continuously tracks the metabolism of Bacillus sp. 8A6 throughout its growth in feather medium. It reveals for the first time how the strain can acquire nutrients and energy in an oligotrophic feather medium for proliferation in the early stage. Then, the degradation of the outer lipid layer of feather can expose the internal keratin structure for disulfide bonds reduction by sulfite from the newly identified sulfite metabolic pathway, disulfide reductases and iron uptake. The resulting weakened keratin has been further proposedly de-assembled by the S9 protease and hydrolyzed by synergistic effects of the endo, exo and oligo-proteases from S1, S8, M3, M14, M20, M24, M42, M84 and T3 families. Finally, bioaccessible peptides and amino acids are generated and transported for strain growth. The keratinase has been applied for soybean hydrolysis, which generates 2234 peptides and 559.93 mg/L17 amino acids. Therefore, the keratinases, inducing from the poultry waste, have great potential to be further applied for producing bioaccessible peptides and amino acids for feed industry.

Biodegradable Cellulose/Polycaprolactone/Keratin/Calcium Carbonate Mulch Films Prepared in Imidazolium-Based Ionic Liquid

Ionic liquid 1-butyl-3-methylimidazolium chloride [BMIM][Cl] was used to prepare cellulose (CELL), cellulose/polycaprolactone (CELL/PCL), cellulose/polycaprolactone/keratin (CELL/PCL/KER), and cellulose/polycaprolactone/keratin/ground calcium carbonate (CELL/PCL/KER/GCC) biodegradable mulch films. Attenuated Total Reflectance Fourier-Transform Infrared (ATR-FTIR) spectroscopy, optical microscopy, and Field-Emission Scanning Electron Microscopy (FE-SEM) were used to verify the films' surface chemistry and morphology. Mulch film made of only cellulose regenerated from ionic liquid solution exhibited the highest tensile strength (75.3 ± 2.1 MPa) and modulus of elasticity of 944.4 ± 2.0 MPa. Among samples containing PCL, CELL/PCL/KER/GCC is characterized by the highest tensile strength (15.8 ± 0.4 MPa) and modulus of elasticity (687.5 ± 16.6 MPa). The film's breaking strain decreased for all samples containing PCL upon the addition of KER and KER/GCC. The melting temperature of pure PCL is 62.3 °C, whereas that of CELL/PCL film has a slight tendency for melting point depression (61.0 °C), which is a characteristic of partially miscible polymer blends. Furthermore, Differential Scanning Calorimetry (DSC) analysis revealed that the addition of KER or KER/GCC to CELL/PCL films resulted in an increment in melting temperature from 61.0 to 62.6 and 68.9 °C and an improvement in sample crystallinity by 2.2 and 3.0 times, respectively. The light transmittance of all studied samples was greater than 60%. The reported method for mulch film preparation is green and recyclable ([BMIM][Cl] can be recovered), and the inclusion of KER derived by extraction from waste chicken feathers enables conversion to organic biofertilizer. The findings of this study contribute to sustainable agriculture by providing nutrients that enhance the growth rate of plants, and hence food production, while reducing environmental pressure. The addition of GCC furthermore provides a source of Ca2+ for plant micronutrition and a supplementary control of soil pH.

Preparation, Purification, and Identification of Novel Feather Keratin-Derived Peptides with Antioxidative and Xanthine Oxidase Inhibitory Activities

Feather keratin is an underappreciated protein resource of high quality, with limited bioavailability, and it urgently requires eco-friendly methods to enhance its value. Here, we report on the preparation, purification, and identification of novel peptides with antioxidant and xanthine oxidase (XOD) inhibitory activities from fermented feather broth, using Bacillus licheniformis 8-4. Two peptides, namely, DLCRPCGPTPLA (DA-12) and ANSCNEPCVR (AR-10), displayed remarkable 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical and 2,2'-azino-bis(3-ethylbenzthiazoline-6-sulfonic acid) (ABTS) radical scavenging abilities with half-maximal inhibitory concentrations (IC₅₀) values of 0.048, 0.034, and 0.95, 0.84 mg/mL, respectively. These values exceed those of the previously reported feather keratin-derived antioxidant peptides. Another peptide, GNQQVHLQSQDM (GM-12), demonstrated XOD activity inhibition, with an IC₅₀ value of 12.15 mg/mL, and it quenched the fluorescence of XOD. Furthermore, after simulating gastrointestinal digestion, DA-12, AR-10, and GM-12 retained their biological activities. Meanwhile, DA-12 and GM-12 showed an unexpected synergistic inhibition on XOD activity accompanied by fluorescence quenching. This study provides new insights into the potential applications of feather keratin, including functionalized feed with antioxidative and antigout (anti-hyperuricemia) activities.

Preparation Methods and Functional Characteristics of Regenerated Keratin-Based Biofilms

The recycling, development, and application of keratin-containing waste (e.g., hair, wool, feather, and so on) provide an important means to address related environmental pollution and energy shortage issues. The extraction of keratin and the development of keratin-based functional materials are key to solving keratin-containing waste pollution. Keratin-based biofilms are gaining substantial interest due to their excellent characteristics, such as good biocompatibility, high biodegradability, appropriate adsorption, and rich renewable sources, among others. At present, keratin-based biofilms are a good option for various applications, and the development of keratin-based biofilms from keratin-containing waste is considered crucial for sustainable development. In this paper, in order to achieve clean production while maintaining the functional characteristics of natural keratin as much as possible, four important keratin extraction methods—thermal hydrolysis, ultrasonic technology, eco-friendly solvent system, and microbial decomposition—are described, and the characteristics of these four extraction methods are analysed. Next, methods for the preparation of keratin-based biofilms are introduced, including solvent casting, electrospinning, template self-assembly, freeze-drying, and soft lithography methods. Then, the functional properties and application prospects of keratin-based biofilms are discussed. Finally, future research directions related to keratin-based biofilms are proposed. Overall, it can be concluded that the high-value conversion of keratin-containing waste into regenerated keratin-based biofilms has great importance for sustainable development and is highly suggested due to their great potential for use in biomedical materials, optoelectronic devices, and metal ion detection applications. It is hoped that this paper can provide some basic information for the development and application of keratin-based biofilms.