Valorization of Waste Wool to Keratin with a Green Solvent Based on a Deep Eutectic Mixture of Choline Chloride and Lactic Acid

Abstract Waste wool, a by-product of sheep husbandry, is primarily composed of keratin, which has potential use in cosmetic products, biotechnology and medical applications. Traditional chemical methods for keratin extraction face limitations due to health and environmental concerns, and thus, green alternatives such as deep eutectic solvents (DESs) have been developed. The aim of the present study was to determine whether a natural DES composed of choline chloride and lactic acid can be used to extract keratin from sheep wool. The dissolution and keratin yield were investigated under different reaction times, temperatures, and wool-to-DES ratios. Fractions of water-soluble keratin and water-insoluble keratin aggregates were obtained by dialysis, centrifugation, and drying. The results showed that both the dissolution of wool and the yield of recovered keratin increased as the reaction time and temperature increased from 1 to 8 h and from 80 to 110 $$^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> C, respectively. With the longest reaction time of 8 hours and the highest temperature of 110 $$^{\circ }$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mmultiscripts> <mml:mrow/> <mml:mrow/> <mml:mo>∘</mml:mo> </mml:mmultiscripts> </mml:math> C, the yield of water-soluble keratin increased to 23 %. Characterizations revealed that DES detached first the cuticular scales from the surface of wool fibres and subsequently degraded wool cortex layer. The extracted water-soluble keratin retained the characteristic amide structure of wool, but the secondary structure of keratin was converted to cross- $$\beta$$ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"> <mml:mi>β</mml:mi> </mml:math> sheet form. Moreover, it had a narrow size distribution and a molecular weight of 10 kDa. The results of this study indicate that the proposed DES can be used as a green solvent for recovering keratin from sheep wool. Graphical Abstract