Dissociation And Extraction Of Feather Keratin By Steam Flash Explosion

With the development of large scale livestock farming, the deficiency of protein feed resources is a basic question that restricts sustainable development of poultry farming. Thus, it has to import considerably more fish meal or soybean meal annually in China. Moreover, feathers, containing about 90% keratin, are obtainable in large quantities in the poultry industry. It is estimated that 3–4 billion pounds of feathers are produced annually as a byproduct in the United States and more than 1.5 billion pounds in China. Internationally, feathers are considered as a new protein feed resource with great potentiality. However, there is limited use of feathers for industrial applications, and most of them are currently disposed as solid wastes by incineration or land?lling. With extensive disul?de bonds and cross-linkages, feather keratin forms ?lamentous structures made of polypeptide chains folded into β-sheets, which renders it insoluble and resistant to most proteolytic enzymes. Thus, without appropriate processing, feather meal has poor digestibility and very low economic value. In this research, steam flash explosion(SFE) was used to treat feathers. The effect of SFE on the structure of feather keratin was examined, and a qualitative model describing the degradation process of feather keratin during SFE treatment was developed. Then, the e?ect of protein structure modi?cation on pepsin digestibility and protein solubility was conducted. The feasibility of extracting keratin from feathers using a SFE assisted alkaline method was also studied. The main research contents and results are as follows.SFE was employed to treat feather waste. The physicochemical properties of feather meal treated by SFE process were studied. Results showed that SFE could effectively improve pepsin digestibility and protein solubility in 0.2% KOH solution of feather meal. Under the steam pressure of 1.6 MPa for 1min,the pepsin digestibility and protein solubility of the exploded feather meal achieved approximately 85% and 35%, respectively, which was about 8 and 10 times higher than that of the original feathers. The crude protein of feathers was not affected by the SFE treatments. The amino acid compositions of the exploded feather meal was similar to that of the raw feather except for cysteine, which has a drastic reduction in concentration. Results showed that the available cystine of feather meal increased firstly, and then decreased with the increase of steam pressure. Thus, a proper range of pepsin digestibility 75%~85% and protein solubility 25%~35% was obtained. An approach known as “Amino Acid Requirements” was used to evaluate dietary protein for monogastric animals. Results indicated that the exploded feather meal was a potential protein source for monogastric animals.Effect of SFE on the structure of feather keratin was further examined by Scanning electron micrograph(SEM), Wide-angle X-ray diffraction(XRD), Attenuated total reflectanceFourier transform infrared spectroscopy(ATR-FTIR) and differential scanning calorimetry(DSC). SEM showed that as the pressure increased, the central axis of the feather disappeared and the original structure could not be identi?ed. XRD analysis presented that increasing spacing between two adjacent lattice planes within the feather keratins. ATR-FTIR analysis indicated that the conformation changed from intermolecular β-sheets to intramolecular β-sheets, and less hydrogen-bonded peptide groups were contained in the feather keratin. Moreover, β-sheets in the keratin decreased and the random coil increased after SFE treatment. Thermal properties analysis indicated that a stronger interaction of bound water and feather powder due to the increase in amorphous domain. The relationship of protein molecular structure changes to protein solubility and pepsin digestibility were further investigated. A high negative linear correlation coe?cient(R2= 0.98) was found between disul?de bond contents of feather keratin and pepsin digestibility, over the full investigated steam pressure range, while a negative linear correlation was found with protein solubility only after being processed. At a lower steam pressure, a linear relationship was observed between the relative spectral weights of the β-sheet structures and pepsin digestibility(R2= 0.93). Hence, there was a synergistic e?ect between hydrogen and disul?de bonds on the stability of feather keratin.The effect of SFE on the changes of chemical crosslinking and molecular weight distribution of feather proteins was also studied through changes of molecular interactions. Results showed that the interactions between disulfide bonds and noncovalent interactions decreased with the increasing steam pressure, and noncovalent bonds gradually played a key role in holding the protein structure. The analysis of gel permeation chromatography of the urea, urea plus β-mercaptoethanol, and dilute NaOH(pH 12) extracts indicated that feather keratin would depolymerized with aggregation. The free sulfhydryl content of exploded samples decreased at low pressures and then increased with increasing steam pressures, concomitant with decreases in disul?de bonds. The disproportion indicated that some new crosslinking would be formed during SFE process. Urea-SDS-PAGE and the aggregated protein showed that when steam pressure reached 1.8 MPa, the peptide chains would be cleaved. A mechanism of dissociation and degradation of feather keratin by SFE was then proposed. Due to the relatively low energy requirement, disulfide bonds would be cleaved preferentially under high temperature. At lower steam pressure, the mechanical shearing force generated in the explosion phase is not large enough to disrupt the ambient matrix. Once beyond a certain steam pressure(around 1.4 MPa), the disulfide bonds in feather keratin would decrease to a certain extent. As the protective role of the matrix disappears, the embedded filaments would denature, involving a molecular transformation from an ordered structure to a disordered and unfolded polypeptide chain. It indicated that the scission of disulfide bonds was a critical step for the degradation of feather keratin.Finally, a SFE process followed by alkali extraction for extracting keratin from feathers was developed. The results indicated that the SFE process could significantly decrease the concentration of NaOH, and improve the extraction efficiency and yield of feather keratin. An extraction rate of feathers of 65% and a yield of keratin of 42% was obtained when the steam exploded feather(1.6 MPa, 1 min) was treated under the optimal condition. Urea-SDS-PAGE, ATR-FTIR, XRD, DSC and TG analysis showed that the extracted feather keratin retained most of the protein backbone, with the breakage of disul?de cross-links and hydrogen bonds. It was also found that the dissolution involved the fragmentation of macromolecular chains and a loss of some ordered structure. The functional properties of feather keratin was improved compared with the feather protein prepared by traditional alkali process. SFE assisted dissolution of keratin from feathers, without the use of harmful reagents, o?ers a possibility of large-scale exploitation.