Production of protein solution from silk waste by subcritical water hydrolysis for preparation of sericin and fibroin microparticles

Abstract

This study examines non-catalytic hydrothermal decomposition of silk waste into protein and amino acids in subcritical water and the characteristics of the particles formed from the solution products. The reaction was carried out in a closed batch reactor at various temperatures between 120-160 °C for sericin, and between 160-220 °C for fibroin. The reaction time was varied between 10-30 minutes and different silk to water ratios of 1:20, 1:50, and 1:100 were examined. The reaction products were separated into solid residue, whose dried weight was measured, and aqueous product, which was analyzed for protein and amino acid content. The results demonstrated that for the hydrolysis of silk for the removal of sericin, the amount of silk residue decreased with increasing hydrolysis temperature and reaction time, and as protein and amino acids were produced. The protein yield in the sericin solution was not affected greatly by temperature and time of reaction, and the highest amount was found to be 0.466 mg protein/mg raw silk (1:100, 120°C, 10 min). On the other hand the amino acids yield increased when temperature and reaction time increased, and the highest amount of amino acids was 0.203 mg AA/mg raw silk, which was found at the highest temperature and time of extraction tested (1:20, 160 °C, 60 min). Like sericin, the amount of silk fibroin residue decreased with temperature and reaction time. Both protein and amino acids yields in the fibroin solutions increased when temperature and reaction time increased. The highest amount of protein yield was 0.455 mg protein/mg silk fibroin (1:100, 220 °C, 10 min) and that amino acids was 0.755 mg AA/mg silk fibroin (1:50, 220 °C, 60 min). The results of silk fibroin decomposition in this study could be described by a surface reaction kinetics. In addition to determining the appropriate hydrolysis conditions, the aqueous solutions of silk sericin and fibroin were formed into particles by means of freeze drying at -40 °C and mechanical disintegration of the dried product. The particle morphology and characteristics such as conformation, crystal structure, and degradation temperature were examined. The analysis showed that the conformation and structure of the final product were changed, particularly in case of fibroin, where change from B-sheet conformation to a-helix/random coil was observed. This is a result of the cleavage of hydrogen linkages in the silk fibre. Although further study is needed to examine the effects of drying method and drying condition on the characterisitcs of particles, the results of this study demonstrated that subcritical water hydrolysis is a promising means for the decomposition of silk waste into useful products.