Bioconversion of Keratin Wastes Using Keratinolytic Microorganisms to Generate Value-Added Products

Abstract

The management of keratinous wastes generated from different industries is becoming a major concern across the world. In each year, more than a billion tons of keratin waste is released into the environment. Despite some trials that have been performed and utilize this waste into valuable products, still a huge amount of keratin waste from different sources is a less explored biomaterial for making valuable products. This indicates that the huge amount of keratin waste is neither disposed properly nor converted into usable products rather thrown away to the environment that causes environmental pollution. Due to the introduction of this waste associated with different pathogenic organisms into soil and water bodies, human beings and other small and large animals are affected by different diseases. Therefore, there is a need for modern and ecofriendly approaches to dispose and convert this waste into usable products. Hence, the objective of this review is to give a concise overview regarding the degradation of keratin waste by biological approaches using keratinase producing microorganisms. The review also focuses on the practical use of keratinases and the economical importance of bioconverted products of keratinous wastes for different applications. Various researches have been studied about the source, disposal mechanisms, techniques of hydrolysis, potential use, and physical and chemical properties of keratin wastes. However, there is negligible information with regard to the use of keratin wastes as media supplements for the growth of keratinolytic microorganisms and silver retrieval from photographic and used X-ray films. Hence, this review differs from other similar reviews in the literature in that it discusses these neglected concerns.

Figure 1

Summarized schematic illustration of integration between sources of keratin wastes, bioconversion of keratin wastes, generation of value-added products, and their applications (source: author).

Figure 2

Structure of keratin [16]: (a) hierarchy of α-keratin showing the assembly from two polypeptide chains (i) to a fibrous structure (iv), (b) TEM micrograph of α-keratin from a sheep horn displaying the composite structure of a crystalline keratin core within an amorphous keratin matrix, and (c) β-keratin with a pleated sheet shape that consists of anti-parallel chains with R-groups that extend between sheets [17].

Figure 3

Management options of keratin wastes.

Figure 4

Approaches to convert keratin wastes into keratin hydrolysates.

Figure 5

Enzymatic or biological treatments of keratinous materials [43].

Figure 6

Possible mechanisms for microbial degradation of keratin (LPMO, lytic polysaccharide monooxygenase [6].

Figure 7

Sheep hair keratinolysis efficiency of Vibrio sp. strain R11 isolated from Lake Arenguade, one of the Ethiopian soda lakes, at room temperature: (a) control (flask Vibrio sp. strain R11) and (b) completely degraded sheep hair afterward 144 h of fermentation with R11 [62].

Figure 8

Efficiency of keratinase (58 U/ml), produced by Vibrio sp. strain R11, for dehairing activity without any addition of Na₂S and lime in the reaction mixture at 37°C for 12 h incubation: (a) control (sheepskin treated with denatured or heat-inactivated keratinolytic enzyme at 100°C for 1 h, with buffer alone) and (b) enzyme-treated sheepskin incubated with nondenatured enzyme (normal) [62].

Figure 9

Goatskin dehairing efficiency of keratinase produced by P. stutzeri strain K4: (a) skin immersed in distilled water (control), (b) skin immersed in culture filtrate or crude keratinolytic enzyme and after 20 h of incubation (experimental), and (c) goatskin dehaired with chemical (Na₂S) as a positive control [147].

Figure 10

Dehaired goatskin examination result with a stereo microscope under 10x magnification power: (a) dehaired hide after strain k4 culture filtrate (crude keratinase) was applied and (b) dehaired hide after the chemical is applied [147].

Figure 11

Dehaired goatskin examination result with stereo microscope under 15x magnification: (a) physical status of the hair after the dehairing process using crude keratinolytic enzyme obtained from strain k4 and (b) physical status of the hair after dehairing process by the chemical method [147].

Figure 12

Examination of lime sulfide and keratinase dehairing approaches. Despite they share comparable dehairing mechanism and breakdown of disulfide bonds in the polypeptides, the chemical method attacks the hair shaft outside the skin, and remaining hairs are available on the skin surface, while keratinase attacks the hair root to produce shaft-free skins: Note: a = whole intact hair on the skin without treatment, b = dehairing with chemicals and parts of hair shaft was still remaining on the surface of the hide, and c = dehairing with keratinolytic enzyme and skins are free from any hair shaft on the surface of the hide [148].

Figure 13

Stain (egg yolk) removal efficiency of commercial detergent (upper pictures) and crude keratinase produced by Vibrio sp. strain R11 (lower pictures): (a) nontreated stained cotton fabric, (b) stained cotton fabric treated with 11.6 Uml⁻¹ enzyme or 7 mg/ml commercial detergent, 30 min incubation at 37°C [62].

Figure 14

Stain (blood) removal efficiency of commercial detergent (upper pictures) and keratinase produced by Vibrio sp. strain R11 (lower pictures): (a) stained cotton fabric before reaction mix, (b) stained cotton fabric treated with glycine/NaOH buffer alone (control), and (c) stained cotton fabric treated with 11.6 Uml^–1 enzyme or 7 mg/ml commercial detergent, 30 min incubation at 37°C [62].

Figure 15

Gelatin removal efficiency of keratinase produced by Vibrio sp. strain R11: (a) before and (b) after 3 minutes of incubation at 55°C with 11.6 U/ml enzyme concentrations [62].

Figure 16

Diagrammatical illustration of biofertilizer preparation from chicken feathers [159].

Figure 17

Biomedical applications of human hair keratin-based biomaterials [179].