Immobilization of papain on chitin for natural rubber latex deproteinization

Abstract

Deproteinized natural rubber (DPNR) produced by papain treatment has improved the dynamic properties but the cost of using free papain is too high with only one use. Therefore, the purpose of this study is to immobilize papain on chitin by physical adsorption and covalent-binding methods and to select the best method for production of immobilized papain for latex deproteinization. The results show strong evidences that immobilization of papain on chitin by covalent-binding method is more suitable because covalently-immobilized papain (CIP) shows 7-time higher activity (450-470 CDU/g chitin), 4-time higher specific activity (1,200 CDU/mg protein) and 10-time higher yield (23% yield) than physically-adsorbed immobilized papain (PIP). Moreover, CIP is more stable in wider range of temperature and pH (50-80°C and pH 5-9). From kinetics studies, CIP also has higher affinity for casein, ovalbumin and proteins in rubber latex and higher efficiency than PIP as evident by lower K(m) and higher V(max). Hence, CIP was chosen for natural rubber latex deproteinization. The results indicate that at the optimal conditions, the constant viscosity-DPNR (CV-DPNR Mooney viscosity 70) obtained showed significant reduction in nitrogen content to 0.077±0.003 g%. CIP treatment was found to improve the physical properties of CV-DPNR due to less water adsorption (0.16 g% volatile matter), more resistance to storage hardening and low color index (2.5) with weight average molecular weight 7.6x10(5). However, dirt (0.018 g%) and ash content (0.186 g%) are rather high due to steam coagulation. Comparative study on the cure characteristics between CV-DPNR produced by CIP, free papain and control, high protein rubber showed that deproteinization resulted in faster cure rate. The CV-DPNR produced by CIP also shows better stress-strain properties by increasing tensile strength and % elongation at break but decreasing hardness comparing to the control. Test for ageing resistance at 70°C for 7 days showed that tensile strength and 300 % modules of the CV-DPNR increased by 3.14% and 133.43%, respectively with a small decrease (8.13%) in elongation at break, which evident for better quality of CIP produced CV-DPNR