Concerning the Existence of Fractions of the Sericin in Raw Silk

Raw silk fibres consist of at least two proteins, silk fibroin and silk sericin. The latter forms a coating over the fibroin and constitutes 15 to 30% of the total weight of the raw fibre. In practical silk processing, the sericin is generally allowed to remain on the fibre until the fabric is ready to be dyed. In this way, it acts as a protective coating for the silk in the weaving or knitting operations. Removal of the sericin is usually accomplished with hot soap solutions. The results of numerous studies of this process led earlier investigators to a variety of con cepts concerning the state and properties of the sericin in the raw silk fibre. Some stated that the sericin is a single protein substance, whereas others claimed it to be a mix ture of proteins which they designated as sericins A, B, etc. The latter concept formed the basis for further claims concerning the desirability of retaining certain fractions of the sericin to improve some wltimate proper ties of the silk. As many of these claims were based on insufficient experimental evidence, it seemed desirable to make a more extensive investigation of the question of the possible existence of several fractions of the sericin in raw silk. The results of the present investigation show that the relative yields of the so-called sericins A and B, obtained after treatment of silk in an autoclave, were dependent on the duration of the treatment. Practically all of the sericin removed in the early stages of autoclaving was in the form generally designated as sericin B. On continu ing the treatment, this fraction was rapidly converted into the more soluble form, sericin. A. Analyses of the fractions, obtained after autoclaving the silk for differ ent lengths of time, showed that the alleged serieins A and B are not definite chemical entities, but are substances whose chemical compositions vary with the duration of the autoclave treatment. It is concluded from these stud ies that the sericin in raw silk does not exist as a mix ture of fractions, but that the fractions are artifacts which result from the hydrolytic decomposition of the naturally occurring sericin during its removal from the raw fibres.