|Industrial processing of bovine hides into leather results in a large number of unusable hide off-cuttings, shavings and trimmings. This waste raw material is under-utilised and presents a waste valorisation opportunity to derive a high-value product such as collagen. Collagen is a highly sought after protein which consists of three polypeptide chains, comprising 30% of the mammalian body’s protein, being the main component of skin, connective tissue and cartilage. The purpose of this study was to extract and characterize collagen from bovine hide off-cuttings in order to establish the potential of using these waste raw materials as sources of collagen. Secondly, to prepare and characterize biopolymer films from the extracted collagen.
Methods of acid-solubilisation (AS), acid-enzyme solubilisation (AES1) and a modified method of acid-enzyme solubilisation (AES2) were applied to extract collagen from bovine hide off-cuttings of bull, calf, cow, ox-hide and bovine face-pieces. The hide sources differed with respect to the animal’s age, sex, diet and environment and influenced collagen yield, and therefore the economic benefit of extraction. The highest dry collagen content was obtained from cowhides using the AES2 method (75.13%), followed closely by bull hides at 74.45%. On the other hand, the lowest collagen content was also from cowhides (3.80%) but with the AS extraction method. The AS method proved to be inefficient for collagen extraction from bull, cow, face-piece and ox-hide sources. The analysis concluded that all the samples were of Type I collagen with α, β, and γ components.
Waste bovine hide off-cuttings proved to be a potential source of high-value product collagen. AES2 proved to be the most preferable method of extraction out of the three methods applied and considering leather to collagen revenue, these waste bovine hide off-cuttings could potentially result in substantial revenue.
Further work was carried out on BH and CH1 collagen to be investigated as potential raw materials for film preparation. Scanning electron microscopy (SEM) showed collagen sheets with a combination of threads and collagen strands, that were bundled together to form a fibril network and hence, a dense sheet-like structure was observed for both BH and CH1 collagen. Differential scanning calorimetry (DSC) and Fourier transform spectroscopy (FTIR) indicated some age-related changes between calf hide collagen and bull hide collagen. DSC analysis resulted in thermal denaturation temperature of BH and CH1 collagen to be 51.90 ℃ and 45.36 ℃, respectively. FTIR spectra indicated BH collagen to possess a higher molecular order than CH1 collagen as indicated by a shift in the wavenumber of characteristic bands of amide A, amide I, amide II and amide III. Transmission electron microscopy (TEM) showed uniform, defined and highly ordered collagen fibrils for BH collagen.
BH and CH1 collagen-based films with varying glycerol concentration were successfully prepared. Films were characterized for their physical properties, secondary structure, thermal and mechanical properties. The films appeared to be uniform in thickness, transparent and showed good flexibility. Surface properties and thickness of both collagen sources were very similar. Film solubility (%) decreased with decreasing content of glycerol, which is a feature of glycerol. DSC and DTA curves of films indicated higher thermal stability for BH collagen-based films in comparison to CH1 collagen-based films. The FTIR spectra of films helped to understand the structural changes and interactions occurring with the collagen sources and glycerol. Films prepared with 40% glycerol were found to be optimum in terms of resulting in the most preferable film properties, especially resulting in superior mechanical properties for both BH and CH1 collagen sources. . Both BH and CH1 collagen-based films resulted in similar film properties. This is a positive finding as CH1 collagen is extractable without the use of enzymes, hence presenting a cheaper alternative.
Collagen-reinforced gelatine-based films were prepared in order to decrease the amount of collagen used with film preparation. The concentration of collagen reinforced had an influence on film moisture content, solubility (%), mechanical properties, thermal stability and the morphology of the gelatine-based films. Reinforcement of gelatine-based films with collagen improved both physical, thermal and mechanical properties of films. Increased addition of collagen resulted in a reduction of film moisture content, lower water solubility, high glass transition temperature (Tg) values and an increase in tensile strength of films. Tg values of pure gelatine films (0C-100G) to reinforced films at 30% collagen increased from 46.80 ℃ to 52.76 ℃, respectively. The only downfall of adding increased amounts of collagen as a reinforcement was the reduction in elongation (%).
In general, the results showed that bovine waste hide off-cuttings have potential as sources of high-quality collagen. The modified acid-enzyme solubilisation proved to be an efficient method of extraction. Films were successfully prepared from extracted collagen and proved to have favourable properties. Further, collagen-reinforced gelatine-based films showed to have excellent film-forming properties and exhibited good thermal stability and mechanical strength. Blend films containing 30 wt % collagen displayed the best mechanical and physical properties. The blend film properties investigated by scanning electron microscopy, differential scanning calorimetry, and film surface properties showed a clear interaction between extracted collagen and gelatine - forming a new material with enhanced mechanical properties.