A Thermostable Crude Endoglucanase Produced by Aspergillus fumigatus in a Novel Solid State Fermentation Process Using Isolated Free Water

Aspergillus fumigatus was grown on chopped wheat straw in a solid state fermentation (SSF) process carried out in constant presence of isolated free water inside the fermentation chamber. The system allowed maintaining a constant vapor pressure inside the fermentor throughout the fermentation process. Crude endoglucanase produced by A. fumigatus under such conditions was more thermostable than previously reported enzymes of the same fungal strain which were produced under different conditions and was also more thermostable than a number of other previously reported endoglucanases as well. Various thermostability parameters were calculated for the crude endoglucanase. Half lives (T1/2) of the enzyme were 6930, 866, and 36？min at 60°C, 70°C, and 80°C, respectively. Enthalpies of activation of denaturation ( ) were 254.04, 253.96, and 253.88？K？J？mole？1, at 60°C, 70°C and 80°C, respectively, whereas entropies of activation of denaturation ( ) and free energy changes of activation of denaturation ( ) were 406.45, 401.01, and 406.07？J？mole？1？K？1 and 118.69, 116.41, and 110.53？K？J？mole？1 at 60°C, 70°C and 80°C, respectively. 1. Introduction Endoglucanases (EC 3.2.1.4) constitute a large proportion of the group of enzymes collectively known as cellulases which are the 3rd largest enzymes sold worldwide and have applications in a number of industries [1]. Their demand is increasing fast especially because of the emergence of second-generation-advanced biofuel industries which require tremendous amounts of various enzymes in their processes [2, 3]. In order to decrease process costs and increase the efficiencies, it is desirable to use thermostable enzymes in the industrial processes [3]. However, most cellulases are not stable at high temperatures [4], and a number of efforts are being made in order to obtain thermostable cellulases [3]. Solid state fermentation (SSF) has long been used for the production of cellulases and other enzymes or bioproducts [5]. It was recently shown that A. fumigatus produced a more thermostable endoglucanase using SSF than that produced through a submerged process [6]. SSF is carried out in the absence or nearly absence of free water in the fermentation medium [5, 7]. In many of the reported experiments, moisture level of the substrate is neither monitored nor controlled after the onset of the SSF process. Even when monitored, it is often estimated “off-line” thus creating technical problems regarding determining the actual water activity ( ) of the substrate medium [8]. The problems can be overcome by designing a system which would