Collagen, as a thermal-sensitive protein, is the most abundant structural protein in animals. Native collagen has been widely applied in various fields due to its specific physicochemical and biological properties. The beneficial properties would disappear with the collapse of the unique triple helical structure during heating. Understanding thermal stability of collagen is of great significance for practical applications. Previous studies have shown the thermal stability would be affected by the different sources, extraction methods, solvent systems in vitro and modified methods. Accordingly, the factors affecting thermal stability of collagen are discussed in detail in this review.
The economic and sustainable use of fish-processing by-products has received considerable attention. Large amounts of fish skin discarded by industry processes can be a source of collagen extraction. This study developed a method to extract collagen with high purity and yield from Southern catfish skins through improved pretreatment methods. Multiple degreasing methods effectively removed fat from fish skins and obtained a maximum degreasing rate (90.24%). The results of electrophoretic, hydroxyproline-content, and extraction-rate analyses revealed that the collagen extracted from defatted skin, which decoloured in 0.5% hydrogen peroxide solution (pH 10), exhibited higher purity and the highest yield (23.14% wet weight and 78.57% dry weight, respectively) as compared with other decolouration conditions. The histological appearance of pretreated fish skin indicated that the non-collagenous substances were removed effectively and the fibres loosened. Amino acid analysis, ultraviolet spectra, Fourier transform infrared (FTIR) spectroscopy, and thermal-stability analysis indicated that the collagen isolated from fish skin under optimal pretreated conditions was classified as type I. Our method for the optimal pretreatment of collagen extracted from fish skin following degreasing and decolouring procedures resulted in improved collagen purity and yield.
Intrafibrillar mineralization is crucial in the synthesis of collagen-hydroxyapatite (HAP) composites with hierarchical structure resembling natural bone. The polymer-induced liquid precursor method, which involves immersing the matrix into polymer-stabilized amorphous calcium phosphate (ACP) solutions, has been widely used to regulate collagen mineralization; however, heterogeneous infiltration and crystallization of ACP precursors from the surface to the inner matrix remain a concern. In this study, the polymer-mediated co-organization strategy in which poly (acrylic acid) (PAA)-stabilized ACP solutions were initially introduced to collagen solutions before sol-gel transformation was employed to achieve homogeneous mineralization. Moreover, the effects of PAA with various molecular weights (2, 10, and 450 kDa) and concentrations (40, 80, and 120 mg/L) on collagen mineralization were evaluated. Increasing concentration and decreasing molecular weight both improved the stability of ACP precursors, which affected collagen mineralization. A lower concentration (40 mg/L) of 2 kDa PAA and higher concentration (80 and 120 mg/L) of 100 and 450 kDa PAA effectively retarded ACP crystallization, thereby inducing intrafibrillar mineralization.Extremely unstable ACP precursors failed to infiltrate the interior of fibrils using 100 and 450 kDa of PAA at 40 mg/L, resulting in extrafibrillar mineralization.Inversely, collagen fibrils were unmineralized until 7 days owing to excessive stability of ACP precursors using 2 kDa of PAA at higher concentrations. The present results further explain the mechanisms of intrafibrillar mineralization, and may provide a facile method for producing biomimetic materials.
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