3. Results and discussion
3.1. Extraction yield of sericin by ethanol precipitation
Natural silk is composed of two kinds of proteins, namely the crystalline fibroin (inside the silk thread) and the amorphous sericin (as a tube outside the thread) (Fabiani et al., 1996). The degumming process is used to eliminate the external sericin prior to dyeing in the silk industry. In this study, cocoon shells were swelled with water and exploded under high temperature and pressure, thus external sericin was easily removed from fibroin. Because fibroin is a water-insoluble fibrous protein, while sericin is a water-soluble globular protein, we determined the nitrogen solubility index (NSI) to character the protein solubility of spray-dried flour. The NSI of the spray-dried flour was found to be 95.3 ± 1.28% (n = 3). This data denoted that the protein composition of the spray-dried flour recovered from the silk industry wastewater was sericin, accounting for about 95%, with little or no contamination by fibroin. Next the spray-dried flour was used as materials to recovering pure sericin by removing the non-protein components.