Hidrólise enzimática de resíduos lignocelulósicos utilizando celulases produzidas pelo fungo Aspergillus niger

Abstract

Lignocellulosic materials are the most abundant residues in the world and there is a worldwide concern to use them as raw material for bioethanol production. This is possible because these materials are rich in cellulose. Cellulose is a biopolymer composed of glucose molecules linked by ß-1-4 glycosidic bonds. Glucose can be converted into ethanol by fermentation and can be obtained from cellulose by enzymatic hydrolysis using cellulases. The cellulases can be produced by several microorganisms under appropriate environmental conditions. Amongst these microorganisms is the fungus Aspergillus niger. In this work, cellulases were obtained by fermentation cultivating A. niger in broth containing pretreated lignocellulosic materials such as sugarcane bagasse, corn straw or wheat straw as the only carbon source. The fermentation kinetic was observed when the pretreated sugarcane bagasse was used as the carbon source. Several variables that affect the enzymatic hydrolysis were analyzed using the three pretreated lignocellulosic materials as hydrolysis substrate. The variables analyzed were: pH, temperature, time of the hydrolysis, mass fraction of the substrate and dilution of the enzymatic broth. The pretreatment of the lignocellulosic materials is paramount for exposing the cellulose chain. Pretreatment consisted of using 4%w/w NaOH solution or 1%w/w H2O2 and their efficiency for removing the lignin from the residues were evaluated. The enzymatic activity also was evaluated by submeting the lignocellulosic materials to successive enzymatic hydrolysis. The enzyme deactivation was evaluated by cooling or freezing the enzymatic broth. It was concluded that Aspergillus niger produces cellulases when grown on medium with pretreated lignocellulosic materials as carbon source. Considering the fermentation kinetic, the ideal time to collect the enzymatic broth with maximum productivity was about 7 days. The cellulase complex does not suffer considerable deactivation when stored at -18°C (freezer) for 43 days, however, the broth activity drops by 43% after 48 hours when stored at 4°C (fridge). The corn straw showed better results as carbon source in fermentation and as substrate hydrolysis, compared with the other materials, with enzymatic activity of 0.895 U/ml. The ideal pH to conduct the enzymatic hydrolysis was 4.8 at 50°C for 50 minutes. The mass fraction of the substrate and enzyme concentration affects the enzymatic activity by a linear dependence. The pretreated materials provided higher enzymatic activity results than the untreated materials. The highest activity enzymatic results were obtained with H2O2 treated substrates, with enzymatic activity of 0.655 U/ml for the sugarcane bagasse, 0.892 U/ml for the corn straw and 0.801 U/ml for the wheat straw. Also, the results show that the H2O2 pretreated materials can be submitted up to, at least, four successive hydrolysis with the second one yielding the highest enzymatic activity for all pretreated residues.