Hydrothermal production and characterization of protein and amino acids from silk waste

Non-catalytic hydrothermal decomposition of sericin and fibroin from silk waste into useful protein and amino acids was examined in a closed batch reactor at various temperatures, reaction times, and silk to water ratios to examine their effects on protein and amino acid yields. For the decomposition of sericin, the highest protein yield was found to be 0.466 mg protein/mg raw silk, obtained after 10 min hydrothermal reaction of silk waste at 1:100 silk to water ratio at 120 °C. The highest amino acid yield was found to be 0.203 mg amino acids/mg raw silk, obtained after 60 min of hydrothermal reaction of silk waste at 1:20 silk to water ratio at 160 °C. For the hydrothermal decomposition of fibroin, the highest protein yield was 0.455 mg protein/mg silk fibroin (1:100, 220 °C, 10 min) and that of amino acids was 0.755 mg amino acids/mg silk fibroin (1:50, 220 °C, 60 min). The rate of silk fibroin decomposition could be described by surface reaction kinetics. The soluble reaction products were freeze-dried to obtain sericin and fibroin particles, whose conformation and crystal structure of the particles were shown to differ from the original silk materials, particularly in the case of fibroin, in which the change from β-sheet conformation to α-helix/random coil was observed.