Optimization of phosphoric acid pretreatment conditions to produce lactic acid from eucalyptus residues.

Abstract

Forestry lignocellulose biomass can be used as a potential substrate in an integrated biorefinery, given the wide products variability that can arise from the chemical structure of its components (cellulose, hemicellulose, lignin and extractives). The production of lactic acid has been studied using different lignocellulosic biomass types: sugarcane bagasse, wheat straw, elephant grass, among others. Studies demonstrate that an efficient bioconversion of lignocellulosic biomass into lactic acid can be achieved through the selection of an optimal combination of pretreatment, saccharification and fermentation process configuration [1]. In this study, the phosphoric acid pretreatment of eucalyptus residues was evaluated to separate and recover the hemicellulosic fraction (mainly xylan) for further conversion to lactic acid by fermentation. Even though phosphoric acid is usually more expensive than sulfuric acid, it is less corrosive, which could contribute to reducing the plant costs, and it generates smaller amounts of toxic inhibitory compounds, which negatively affects hemicellulosic hydrolysate fermentation [2]. The pretreatment conditions were optimized using central composite design to achieve maximum xylose recovery yield and concentration in the pretreatment liquor with minimum amount of phosphoric acid. The independent variables were acid concentration (0.5-1.0%), temperature (140-160ºC) and reaction time (10-40 min). After pretreatment, the liquor and solid fractions were separated by centrifugation. The liquid fraction (pretreatment liquor) was used as substrate for lactic acid fermentations, while the solid fraction (pretreated eucalyptus) was washed with distilled water for enzymatic hydrolyzed assay. Both pretreatment liquor and pretreated eucalyptus were characterized following NREL protocols. In the solid fractions, between 41-63% glucan, 4-12% xylan and 31-37% lignin was obtained. The maximum xylose concentration and recovery yield obtained at optimal pretreatment conditions (160ºC, 0.52% H3PO4, 40 min) were 15.8 g/L and 11.9 g/100 g, respectively. The pretreatment liquor containing xylose was converted into lactic acid using Bacillus coagulans DSM 2314 resulting in 7.9 g/L lactic acid and 100% consumption. Fermentations were performed under anaerobic conditions at 55ºC and initial pH 7 with orbital agitation (150 rpm). The pretreatment liquor was supplemented with peptone (10 g/L), yeast extract (5 g/L), meat extract (10 g/L), Tween 80 (1 g/L), sodium acetate (5 g/L), ammonium citrate (2 g/L), MgSO4 (0.2 g/L) and MnSO4 (0.05 g/L). The pretreated eucalyptus was hydrolyzed using 16% solids, enzyme complex Cellic Ctec2 25 FPU/g glucan with sodium citrate buffer (pH 4.8) 0.05 N, 48°C and 150 rpm. After 72 h, the hydrolysis efficiency reached a value near to 30%.