Non-mulberry Silk Fibers

Abstract

Wild or non-mulberry silks are produced from various species of insects. Most popular non-mulberry silks that are commercially available are tasar (Antheraea mylitta), eri (Samia cynthia ricini), and muga (Antheraea assamensis). Properties of these three common types of wild silks are compared to Bombyx mori silk in Table 36.1. A typical life cycle of a wild silkworm (Antheraea mylitta) is shown in Fig. 36.1 [12Kun]. During production of the wild silk fibers, in addition to the cocoons, some sericin proteins are extruded external to the cocoons and are called peduncles. These peduncles (Fig. 36.2) act as reservoir for sericin and are seen only in the non-mulberry silks. Silk produced in these peduncles was found to be similar to the sericin in the cocoons [06Das] with proteins having molecular weight of 200 kDa and mainly composed of glycine and serine with 36.7 % β-sheets, 52.7 % random coils, and 10.6 % turns with no helices. Other researchers have suggested that Antheraea mylitta contains polyalanine repeat sequences, and fibroin extracted from the silk gland of Antheraea mylitta had a molecular mass of 395 kDa with monomers of approximately 197 kDa [09Ach]. To determine the structural differences using nuclear magnetic resonance (NMR), 13C and 15N labeling, select amino acids were orally fed to the fifth instar larvae. Silk obtained contained 75 % alanine and 65 % glycine residues, the alanine content being much greater than that found in B. mori silk [99Asa, 04Asa]. In the solid state, the glycine-rich regions stretched up to 10 times indicating that β-sheets were predominant.