Produção de etanol de segunda geração e de ácido láctico a partir da casca de arroz e do capim-elefante utilizando pré-tratamento a vapor e hidrólise enzimática

Abstract

Rice husk (RH) and elephant grass (EG) are abundant lignocellulosic raw materials in the South of Brazil and a feasible resource that can be converted into a range of value-added chemicals and biofuels. In this study, both biomasses were investigated by uncatalyzed steam explosion followed by hydrolysis, in order to obtain a satisfactory content of C6-sugar in the water insoluble solids (WIS) and C5-sugar in the liquor to subsequent fermentation aiming ethanol and lactic acid (LA), respectively. Thus, the effect of the variables temperature (181 to 229 °C for RH and 178 to 212 °C for EG) and residence time (1.6 to 11.4 min for RH and 5.9 to 10.1 min for EG) employed in the pretreatments was evaluated by means of experimental design. Altogether, among the different pretreatments performed, those involving the use of RH favoured the recovery of hemicellulose contained in the liquor, whereas those involving the use of EG were more susceptible to the recovery of cellulose contained in the WIS. In addition, it was possible to verify that intermediate temperature conditions and prolonged periods of time led to the obtaining of balanced contents of glucose in the pretreated material and xylose in the filtrate, in a sole experiment. Further pretreatments with EG, conducted with the aid of an acid catalyst, have shown an improvement in the yield of the sugars of interest in each of the target fractions. For the following hydrolysis and fermentations, the optimized condition of 205 °C for 11.5 min was employed for RH pretreatment; in turn, the condition of 190 °C for 5 min was employed for EG pretreatment, which was impregnated with 2% (w/w) of sulfuric acid. For both pretreated feedstocks, enzymatic hydrolysis assays indicated the advantages in the use of high solids loading, resulting in a more concentrated glucose and promoting the subsequent ethano production. Thus, the WIS was subjected to hydrolysis using high solids consistency arranged with distinct doses of enzymes. Assays employing 20% (w/v) solids loading and an enzyme dosage of 20 FPU g-1 substrate augmented the conversion of cellulose into glucose, releasing 33.68 g L -1(RH) and 95.57 g L-1 (EG). Different fermentation strategies were employed aiming ethanol production: prehydrolysis, separate hydrolysis and fermentations, and simultaneous saccharification and fermentation; the latter of which increased the productivity of the overall process. Ethanol production by accharomyces cerevisiae CAT-1 reached 19.17 g L-1 (RH) and 42.25 g L-1 (EG), after 72 h of cultivation, using WIS as substrate. Filtrates were detoxified and used as a substrate for the production of LA by Lactobacillus buchneri NRRL B-30929. In the specific case of steam-exploded RH, the liquor was previously subjected to hydrolysis with an enzymatic complex produced by Penicillium echinulatum S1M29 strain. Already the pretreatment catalyzed for the EG also acted as an acid hydrolysis. Therefore, the total sugars belonging to each of the liquors were consumed by LA bacteria, and a racemic blend of DL-LA was produced, totaling 12.69 g L-1 (RH) and 13.35 g L-1 (EG), after 30 h of cultivation. The findings of this study suggest that both RH and EG are potential renewable resources which can generate bioproducts from C5-sugars, while producing second generation ethanol from C6-sugars. Likewise, the possible insertion of these raw materials, with low market cost, can foment the generation of processes linked with technological innovation, also contributing to the reduction of environmental problems.