Produção de bioetanol por kluyveromices marxianus a partir de subprodutos do beneficiamento de arroz

Abstract

The indiscriminate exploitation of fossil fuels, as well as a possible depletion of traditional sources of raw materials, have been pointing to a possible exhaustion of energy supply. Similar to this problem, challenges with environmental preservation bring to discussion issues involving biofuels and their means of production. Alternative sources have been explored, aiming to produce biofuels with the same energy potential, within the context of sustainable development, also avoiding dispute over lands destined for food production. Brazil is an agribusiness power, with its wide variety of agricultural products, and a highly expressive rice production, which generates rice bran and the rice husk as by-products. Studies that create or improve methodologies to enable the exploring of these sources, while economically advantageous and feasible, to obtain second-generation ethanol are being developed, using fermenting yeasts. Therefore, the objective of this work was to study rice bran as a nutritional supplement of the culture medium and the utilization of the sugars from the cellulose fraction of the rice husk for the production of second-generation ethanol by Kluyveromyces marxianus. Seven different sources for the culture’s media supplementation for ethanol production were evaluated: T1 - Control (glucose 90 g/L, without supplementation); T2 - Medium supplemented with water soluble rice bran extract (FA) (glucose 90 g/L, rice bran extract 20 g/L); T3 - Medium supplemented with water soluble rice bran extract (FA) + inorganic components (glucose 90 g/L, rice bran extract 20 g/L, CaCl2 0.1 g/L, ammonium sulfate 2 g/L) ; T4 - Medium supplemented with malt extract (ME) (glucose 90 g/L, malt extract 3 g/L); T5 - Medium supplemented with yeast extract (EL) (glucose 90 g/L, yeast extract 3 g/L); T6 - Medium supplemented with peptone (PEP) (glucose 90 g/L, peptone 5 g/L); T7 - YMP medium (with glucose concentration modified to 90 g/L, 3 g/L malt extract, 3 g/L yeast extract and 5 g/L peptone). The media were prepared with sodium citrate buffer (50 mmol/L), pH 5.5. The assays were performed in triplicates, in 250 mL Erlenmeyer flasks containing 100 mL medium and 1 g/L inoculum concentration. The treatments were incubated on a shaker at 200 rpm, at 40 °C for 96 hours. For the production from rice husks by the simultaneous saccharification to fermentation (SFS) and separate saccharification and fermentation (FSS) techniques, several pre-treatments were pre-evaluated to obtain cellulose. In the SSF process, the inoculum (1 g/L) was added after 4 hours of saccharification; in the SFS process, the inoculum (1 g/L) was added after 72 hours of saccharification, both using 40 FPU/g cellulose of the enzyme CellicCTec2, rice husk in the ratio of 10% (m/v), pH 5.0, at 50 °C, 150 rpm. For both SSF and SFS methods, the media were supplemented with the following nutrients: YMP (T1), rice bran (T2), and rice bran + CaCl2 + ammonium sulfate (T3) selected in the evaluation of the nutritional supplementation. The highest ethanol production (25.59 g/L, YP/S 0.50 g/g and QP 1.16 g/L.h) was verified in the medium supplemented with rice bran, followed by the medium supplemented with rice bran + CaCl2 + ammonium sulfate (25.50 g/L ethanol, YP/S 0.49 g/g, and QP of 2.13 g/L.h), and YMP medium (24.88 g/L ethanol, YP/S, 50 g/g, and QP 2.06 g/L.h). The rice husk pretreated with 2% NaOH + 5% H2O2 showed the following composition: 47.51% cellulose, 9.57% hemicellulose, and 20.54% lignin. In the SSF, 10.21, 10.05, and 9.78 g/L of ethanol was produced, YP/S produced 0.51, 0.52, and 0.51 g/g, and ethanol volumetric productivity (QP) of 0.58, 0.55, and 0.61 g/L.h in T1, T2, and T3 treatments, respectively. For the SFS, the maximum ethanol concentrations were 11.36, 12.96, and 13.08 g/L, corresponding to YP/S 0.18, 0.21, and 0.21 g/g, and QP of 0.80, 0.96, and 0.88 g/L.h for T1, T2 and T3, respectively. With these results, it is possible to consider that rice bran and rusks are promising raw materials for the production of second-generation ethanol.