# Sericin for Tissue Engineering
## 2 Sericin as a Degumming Product
### Sericin Molecular Weight and Degumming Process  
Sericin, composed of 18 amino acids, is a water-soluble globular protein with molecular weights ranging from 10 to 400 kDa [29; 30; 32; 58]. Its molecular weight varies depending on the extraction method employed [58; 59]. Several strategies have been used for the extraction of sericin from the cocoon of B. mori, including extraction using high temperature, with or without high pressure, acids, alkalis, urea, and enzymes [32; 58; 60]. Degumming conditions such as temperature, time, chemical additives, and solution concentrations can be modified to achieve better efficacy [32]. The extraction yield and the morphology of cocoon extracts vary depending on the degumming conditions such as solvent and temperature (Figure 2). In addition, protein content and molecular weight vary depending on the degumming conditions (Figure 3). The chemical structure of cocoon extracts changes as per these conditions (Figure 4). The higher the concentration of ethanol (EtOH) and the lower the temperature, the higher is the crystallinity. The random coil and \(\beta\)-sheet conformations of cocoon extracts also change based on the degumming conditions (Figure 5 and Table 1).  
Figure 2: Effects of extraction conditions on the extraction yields and morphologies of cocoon extracts. (**a**) Extraction yields (%) calculated by dividing the solid component in the various extracts by the weight of input cocoons. (**b**) Cocoon extracts solutions after concentration by an evaporator. (**c**) Cocoon extract powders obtained by lyophilization.  
Figure 3: Effects of extraction conditions on protein content (%) and molecular weight of proteins. (**a**) Percentage of protein and non-protein fractions included in the extracts. (**b**) SDS-PAGE results for various cocoon extracts. Lane “M” and “1”–”4” indicate marker (M), silk sericin extracted by using a hot water system (1), cocoon extracts prepared by 30% ethanol (EtOH) at 40 \({}^{\circ}\)C (2), 60 \({}^{\circ}\)C (3), 80 \({}^{\circ}\)C (4), respectively. Lanes “5”–”–”7” indicate cocoon extracts prepared by 70% EtOH at 40 \({}^{\circ}\)C (5), 60 \({}^{\circ}\)C (6), 80 \({}^{\circ}\)C (7), respectively.  
Figure 4: Effects of extraction conditions on the chemical structure of cocoon extracts. (**a**) FTIR spectra and (**b**) amide I crystallinity index of various cocoon extracts. A cocoon extract prepared using the hot water extraction system was considered as a control. \({}^{*}\)\(p\)\(<\) 0.05, \({}^{**}\)\(p\)\(<\) 0.01,\({}^{***}\)\(p\)\(<\) 0.001.  
Figure 5: Effects of extraction conditions on the conformations of cocoon extracts in aqueous solution. CD spectra of various cocoon extracts prepared in 30% ethanol (**a**) and 70% ethanol (**b**) at 40 \({}^{\circ}\)C, 60 \({}^{\circ}\)C, and 80 \({}^{\circ}\)C. A cocoon extract prepared by the hot water extraction system was considered as a control. Yellow marks indicate the wavelengths corresponding to random coil and \(\beta\)-sheet conformations, respectively.  
Sercin extracted by the conventional method of boiling have a molecular weight of 10 to 220 kDa [61], while those extracted by the high-temperature and high-pressure degumming technique (autoclaving) have molecular weights ranging from 20 to 400 kDa [62]. Sercin extracted by the acid-degradation method has a molecular weight in the range of 35-150 kDa, while that extracted by the alkali-degradation method has a molecular weight in the range of 15-75 kDa [63]. Sercin extracted with a urea solution was found to have a molecular weight in the range of 10 to \(>\)225 kDa [64].