Evaluation of acid hydrolysis conditions to obtain fermentable sugars from alkali pretreated rice straw

In this work, we evaluate dilute acid hydrolysis of alkali-pretreated rice straw to determine the conditions to produce high yields of fermentable sugars, considering both hemicellulosic and cellulosic fractions. With this purpose, rice straw was previously pretreated by an alkaline step (0.8% w/v NaOH, 10% w/v solids, 70°C during 45 min). This pretreated solid was then washed and submitted to acid hydrolysis, in 500 mL stainless steel batch cylindrical reactors. The reactions were performed varying H₂SO₄ concentration from 0 to 1% w/v and treatment time from 20 to 60 min at fixed temperature (150°C) and 10% w/v solid loading. The solid residues were then subjected to enzymatic hydrolysis (4% w/v consistency and enzyme loading of 12.5 FPU and 25 IU β-glucosidase/g substrate for 12 hours). At each run, xylan conversion yield and cellulose enzymatic digestion were assessed. In addition, concentrations of soluble components in the hemicellulosic liquors (acetic acid, furans and phenolic compounds) were measured. The results showed that the maximum hemicellulose recovery (67%) and glucan yield (65%) were reached using 1.0% w/v H₂SO₄ for 60 min. These results were close to those obtained at 0.5% H₂SO₄ for 40 min, but reducing three times the furans formation in hemicellulosic liquor. Considering these results, was shown that reducing the acid concentration was possible to obtain similar results of sugar with lower toxicity, which can improve further fermentation results. However, additional experiments are still needed to optimize the conditions of acid hydrolysis from alkali-pretreated rice straw.

Supported by FAPESP, CNPq and CAPES.