Effect of mild alkaline pretreatment on cellulose and hemicellulose hydrolysis yields from rice straw

This work aimed to study mild alkaline pretreatment conditions on the yields of hemicellulosic dilute acid and cellulosic enzymatic hydrolysis of rice straw (RS). Alkaline treatments were performed by 2³ central composite full-factorial design, varying temperature (50 – 70°C), NaOH concentration (0.2 – 0.8% m/v) and RS consistency (6.0 – 10.0% m/v). Alkaline liquor showed acetate extraction ranging from 20.19 to 93.37%, and low solubilizing sugars. Nevertheless, in solid residue the largest variations were observed in lignin content (3.5 - 16.9% w/w), whereas cellulose and hemicellulose presented values of about 40 and 26% w/w, respectively. To evaluate the effects of alkaline treatment on polysaccharides hydrolysis, assays of enzymatic hydrolysis (4% w/v consistency and enzyme loading of 25 FPU and 25 IU β-glucosidase/g) and dilute acid hydrolysis (1% w/v H₂SO₄, 10% w/v consistency, 121°C and 30 minutes) were carried out. The results showed wide variations on cellulose digestibility (22.45 - 66.67%) and hemicellulose yield (39.25 - 73.17%). From statistical analysis was assigned an alkaline treatment condition (69 °C, 0.8% NaOH and 10% RS consistency) that simultaneously maximize the responses acetate removal, cellulose digestibility and hemicellulose yield, being attained values of 82.5%, 52.2% and 68.3%, respectively. Under these conditions, acetic acid (0.06 g/L) and phenolic compounds (0.17 g/L) in hemicellulosic hydrolyzate were reduced by 94 and 61%, respectively, in relation to those obtained from untreated-RS. Therefore, this milder alkaline pretreatment is an interesting strategy for utilization of rice straw carbohydrates in bioprocess, mainly ethanol production.

Supported by FAPESP, CNPq and CAPES (BRAZIL)