Corn stover, the residue left in the field following the harvest of cereal grain, is a very common agricultural product in areas with high levels of corn production. The current annual production of corn stover is about 250
M tons in the U.S. [1
] and 220
M tons in China [2
]. Corn stover is in many ways an ideal feedstock for biomass ethanol production. Although biomass ethanol can be made from a wide range of biomass materials, stover from existing corn production is by far the most abundant crop residue readily available today.
Biomass ethanol is ethanol made from non-grain plant materials known as biomass. The bulk of most plants is fibrous material consisting of cellulose, hemicellulose and lignin. The bioconversion of biomass to ethanol requires hydrolysis of the carbohydrate polymers, cellulose changes to its constituent monomeric sugars prior to microbial fermentation. Due to the complicated structure of the cell wall in biomass, additional pretreatment is needed for biomass ethanol production compared to grain ethanol.
Biomass pretreatment is an essential step in biomass ethanol production with high yield [3
]. Many studies have been published about different pretreatment methods for enhancing the digestibility of biomass [5
]. Biological pretreatment that utilizes the metabolite of microorganism in nature to break up the cell wall of biomass for ethanol production is a promising technology due to its advantages of having a low energy requirement and being friendly to the environment [7
]. Compared to chemical pretreatment, it is no need to recycle the chemical and does not bring exotic materials to environment. These reductions in the severity of pretreatment conditions could result in less biomass degradation and consequently lower inhibitor concentrations compared to conventional thermochemical pretreatment [5
]. Fungal pretreatment using wood-rot fungus is one of the most effective methods for enhancing the efficiency of enzymatic saccharification [9
]. [Fissore et al 13
] evaluated a process of combined brown-rot decay –chemical delignification as a pretreatment for bioethanol production. The combination of brown-rot fungi and organosolv processes result in 210
ml ethanol/ kg wood. Some thermochemical pretreatment methods have been performed for biomass ethanol production [4
]. However, biological pretreatment of corn stover with wood-rot fungi has been neglected. As is widely known, due to the ability to degrade lignin extensively, white-rot fungi (WRF) have received considerable attention for their potential to remove lignin for bio-ethanol pretreatment [8
]. In contrast, brown-rot fungi (BRF) such as Gloeophyllum trabeum
, have different mechanisms for the degradation of wood that rapidly depolymerize the cellulose and hemicellulose in wood with modified lignin in the brown residue. BRF degrade lignocellulose via a theorized two-part mechanism, with modification of the plant cell wall induced non-enzymatically and secretion of cellulases and hemicellulases likely occurring after modifications [18
]. The initial stages of decay are thought to involve Fenton chemistry (Fe2+
) for the production of hydroxyl anions and radicals [20
]. The low molecular weight reactants, unlike enzymes, are small enough to penetrate the wood lignocellulose fabric, and have been shown in immunolabeling studies to be present throughout the S2 layer of the brown rot-degraded cell wall [21
]. Cellulase production by brown rot fungi is different in that it is typically constitutive, not influenced by free glucose concentrations, and most often lacks exo-acting cellobiohydrolase [22
]. G. trabeum
has the ability of fermenting sugar to ethanol [24
]. The enzymatic and non-enzymatic mechanisms used by G. trabeum
to degrade wood could potentially be employed for the bioconversion of other biomass, such as corn stover. The complicated structural modification of the cell wall plays a role in the initial degradation of BRF in the pretreatment for the purpose of bio-ethanol production.
In this study, 40 strains of wood-rot fungi (33 strains of WRF and 7 strains of BRF) were screened for corn stover pretreatment. A strain of brown-rot fungus KU-41 was selected for corn stover pretreatment due to its having the highest conversion of cellulose to glucose (CCG). A molecular biological identification showed that KU-41 was most closely related to Gloeophyllum trabeum. To gain a deeper understanding of the mechanisms of brown-rot fungus pretreatment, the fungal-pretreated corn stover was evaluated in detail, including the lignin and structural carbohydrate contents, cellulose crystallinity, initial adsorption capacity of cellulase, SEM and specific surface area.