EXTRACTION, PARTIAL PURIFICATION AND CHARACTERIZATION OF CELLULASE FROM ASPERGILLUS FUMIGATUS AND ASPERGILLUS FLAVUS IN SUBMERGED FERMENTATION SYSTEM USING BREADFRUIT HULLS AS CARBON SOURCE

CHAPTER ONE

INTRODUCTION

Enzymes are the catalysts of biological processes. Like any other catalyst, an enzyme brings the reaction catalyzed to its equilibrium position more quickly than would occur otherwise. An enzyme cannot bring about a reaction with an unfavourable change in free energy unless that reaction can be coupled to one whose free energy change is more favourable (Nelson and Cox, 2000). The activities of enzymes have been recognized for thousands of years. However, only recently have the properties of enzymes been understood properly (Wolfgang, 2007). Indeed, research on enzymes has now entered a new phase with the fusion of ideas from protein chemistry, molecular biophysics, and molecular biology which have given rise to applications in fields ranging from agriculture to industry (Wolfgang, 2007).

The enzyme industry as we know it today is the result of a rapid development seen primarily over the past four decades and thanks to the evolution of modern biotechnology (Ole et al., 2002). Enzymes found in nature have been used since ancient times in the production of food products, such as cheese, sourdough, beer, wine and vinegar, and in the manufacture of commodities such as leather, indigo and linen (Ole et al., 2002). All of these processes relied on either enzymes produced by spontaneously growing microorganisms or enzymes present in added preparations such as calves’ rumen or papaya fruit. The development of fermentation processes during the later part of the last century, aimed specifically at the production of enzymes by use of selected production strains, made it possible to manufacture enzymes as purified, well-characterized preparations even on a large scale (Wolfgang, 2007)  

Microbial cellulases have shown their potential application in various industries including pulp and paper, textile, laundry, biofuel production, food and feed industry, brewing and agriculture. Due to the complexity of the enzyme system and immense industrial potential, cellulases have been a potential candidate for research by both academic and industrial research groups (Shang, 2013). The growing concerns about depletion of crude oil and the emissions of greenhouse gases have motivated the production of bioethanol from lignocellulose, especially through enzymatic hydrolysis of lignocellulosic materials (Bayer et al., 2004; Himmel et al., 1999)

1. Cellulose

Cellulose is a linear polymer of β-D-glucose units linked through 1,4-β-linkages with a degree of polymerization ranging from 2,000 to 25,000 (Kuhad et al., 1997). Cellulose chains form numerous intra- and intermolecular hydrogen bonds, which account for the formation of rigid, insoluble, crystalline microfibrils (Golan, 2011). Natural cellulose compounds are structurally heterogeneous and have both amorphous and highly ordered crystalline regions (Morana et al., 2011). The degree of crystallinity depends on the source of the cellulose and the highly crystalline regions are more resistant to enzymatic hydrolysis (Morana et al., 2011). Cellulosic materials are particularly attractive because of their relatively low cost and abundant supply. As the most abundant polysaccharide in nature, cellulose decomposition plays not only a key role in the carbon cycle of nature, but also provides a great potential for a number of applications, most notably biofuel and chemical production (Lynd et al., 2012). The central technological impediment to more widespread utilization of this important resource is the general absence of low-cost technology for overcoming the recalcitrance of cellulosic biomass.

1.1.1 Structure of Cellulose