Effect of Extraction Time on the Rheological Properties of Sericin Solutions and Gels

Int. J. Indust. Entomol. Vol. 27, No. (1), pp. 180-184 (2013) 180 181 The dry mass of sericin and cocoons was determined using a moisture analyzer (XM60, Precesa, Switzerland). Rheological measurement were performed on 1) the 0.3% (w/ v) sericin aqueous solutions, 2) sericin gels prepared from the 0.3% sericin solution, and 3) frozen and thawed sericin gels. The steady-state flow of sericin solutions and gels were measured using a rheometer (MARS III, Hakke, Germany) using a 35 mm parallel-plate geometry at 25 °C. The compression strength of sericin gels was measured using the axial test function of the rheometer. The 35 mm plate of the rheometer compressed the gels with a speed of 0.12 mm/s. Results and Discussion Production yield and amino acid composition Production yield is an important issue for the industrialization and mass production of silk sericin. Therefore, the production yield of sericin with different extraction times in hot water was determined and the results were shown in Fig. 1. With an extraction time of 30 min, the production yield was 7.7%. The yield increased with increasing extraction time and reached 11.4% at an extraction time of 120 min. Considering that the sericin content of Baekokjam cocoons is about 26% (Ko et al., 2013), these values are not high. The yield can be increased further by increasing the extraction time or strengthening the treatment conditions including treatment temperature and pressure; however, in this case, molecular degradation of sericin can become a problem. time is increased, more sericin can be extracted, but at the same time, the sericin molecules become more degraded. Considering that many studies have been conducted to improve the mechanical properties of sericin and to extract high molecular weight (MW) sericin (Oh et al., 2007, 2011; Yun et al., 2013), it is clearly desirable to avoid the severe molecular degradation of silk sericin during the extraction process. That is, it is important to properly control the extraction time of sericin in hot water to simultaneously obtain both a high yield and a high MW. Rheology has previously been used as an indicator to examine the structure of materials and to evaluate their processability for various applications including gelation (Yoo and Um, 2013), wet spinning (Cho et al., 2012) and electrospinning (Cho et al., 2012; Ko et al., 2013). In this study, sericin was obtained using different extraction times in hot water and rheological measurements were performed on the resulting sericin solutions and gels to gain a better understanding of the effect of extraction time on the solution properties and gel character of sericin. Materials and Methods