Solid State Fermentation of Soybean Hulls for Cellulolytic Enzymes Production

Abstract

Soybean hulls are the byproduct of soybean processing and accounts for 5-8% of the 96 million metric tons soybean crop of 2006 in USA (Mielenz et al., 2009). It is mechanically removed from the soybeans during the process of dehulling and commercially sold as it is or in form of compressed pellets mostly for the feed industry (Marshall and Wartelle, 2003). Soybean hulls have rich cellulosic composition, notably half of the composition is cellulosic i.e. cellulose and hemicellulose combined (Brijwani et al., 2010). Fungal solid state fermentation (SSF) of soybean hulls for cellulolytic enzyme production is a useful way of value addition of this under-utilized byproduct. A cellulolytic enzyme system is group of enzymes that work synergistically to hydrolyze lignocellulosic biomass. It is composed of endoglucanase (endo-1, 4-┚-D--glucanase, EC 3.2.1.4), exoglucanase (1,4-┚-D-glucancellobiohydrolase, EC 3.2.1.91), and ┚-glucosidase (┚-D-glucoside glucanohydrolase, cellobiase, EC 3.2.1.21) (Holker et al., 2004; Esterbauer et al., 1991). Xylanase (EC 3.2.1.8), though not part of the group, complements the cellulolytic enzyme system as it is needed to elicit complete and efficient hydrolysis of the lignocellulosic biomass, which has an appreciable amount of hemicellulose or xylan (Brijwani et al., 2010; Brijwani, 2011). It has been widely accepted that Solid State Fermentation (SSF) is an attractive means to produce cellulase economically because of its lower capital investment and lower operating cost (Cen and Xia 1999). Further, the ability of SSF to minimize catabolite repression has been already described for several enzymes (Aguilar and Huitron 1986; Archana and Satayanaryana 1997; Siqueira et al. 1997; Solis-Pereyra 1996). SSF is defined as a fermentation process in which microorganisms grow on solid materials without the presence of free liquid and the moisture necessary for microbial growth exists in adsorbed state or complexed with solid matrix (Krishna, 2005). Both bacteria and fungi are known to produce cellulases using complex cellulosic substrates, however, fungal enzymes are generally complete comprising of all the cellulosic activities (Stockton et al., 1991). The operating conditions like temperature, pH and moisture content are very important for microbial growth and efficient cellulase production in SSF. Optimization of parameters in multifactor experimental designs fall short in locating true optimum especially when there are interactions among independent variables, besides being time consuming (Giovanni 1983; Theodore and Panda 1995). One of the worthwhile techniques to identify the explanatory variable in the system is Response Surface Methodology (RSM) (Maddox and Richer 1977;