Ultrafiltration recovery of sericin from the alkaline waste of silk floss processing and controlled enzymatic hydrolysis

Abstract

Abstract This study was designed to improve the traditional scouring procedure using a solution of sodium silicate, sodium carbonate, H2O2, surfactant etc. as a refining agent for degumming machine-drawn raw silk sheet (RSS) and raw silk from the cocoon shell of the silkworm Bombyx mori. The RSS was scoured firstly in strongly alkaline electrolyzed water (SAEW, pH ≥ 11.5) consisting mainly of hydroxyl ions by boiling for 30 min and then treated by the traditional scouring procedure (without alkaline compounds), resulting in refined silk floss. The recycled SAEW containing sericin was separated by ultrafiltration into a retentate with a range of high molecular mass values accounting for 10% (w/w) of the cocoon shell and the filtrate was separated by nanofiltration into a purified H2O filtrate and a retentate containing oligopeptides and free amino acids, which can be used directly as food additives or in biological growth media. The ultrafiltration retentate was hydrolyzed by papain under three different conditions into three groups of sericin peptides with high, middle or low molecular mass. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and gel permeation high-performance liquid chromatography (GP-HPLC) showed the molecular mass of these sericin peptides was in the range 0.2–60, 0.2–30 and 0.2–15 kDa. These clean products have a variety of applications, including coating materials for surface modification, cell culture media and food additives. The procedure described here could be applied to the manufacture of silk floss quilts and the process of refining raw silk, which results in three clean products of the silk protein fibroin (i.e. stripped of sericin) and reduces environmental pollution from scouring waste containing sericin.