Separation of sericin from fatty acids towards its recovery from silk degumming wastewaters

Sericin is a valuable protein, which is currently discarded as a waste in silk industry. Silk degumming wastewaters (SDW) are abundant sources of sericin, however, they contain impurities such as salts of fatty acids (soap) in conventional degumming processes. In this study, a process consisting of centrifugation (CFG), low temperature crystallization (LTC) and ultrafiltration (UF) was developed to separate sericin from fatty acids towards its recovery from conventional SDW. Complete removal of fatty acids was achieved by CFG + LTC, where CFG possibly accelerated crystal formation and separation processes. In CFG + UF (20 kDa), sericin rejection increased as pH was reduced from alkaline to acidic levels, possibly due to decreased solubility of sericin and increased amounts of undissociated fatty acids. Fatty acids were partly retained, leading to inadequate separation. Despite very high flux declines, cakes formed by the combined foulants were removable at both pH conditions. In CFG + LTC + UF (5 kDa), sericin was rejected by 92%. Although solution pH did not affect rejection performance, flux decline decreased from 85% to 61% at alkaline pH. Different fouling mechanisms were observed for different foulant types. Only cake formation occured with combined foulant (sericin + fatty acids). The cake surfaces were irregular, with denser and more heterogeneous structure at acidic pH. However, smooth cake formation plus pore narrowing occured with sericin alone at alkaline pH. Additional pore clogging occured at acidic pH, possibly due to decreased solubility of sericin. Chemical cleaning was less effective at acidic pH due to adsorption and pore clogging. Sericin recovery from conventional SDW was achieved in a three-stage process; CFG + LTC for complete separation of sericin from fatty acids and UF for recovering sericin.