Material balance calculations for sugars and ethanol yield from rice straw pretreated by sequential alkaline and dilute acid processes

The major drawback of dilute acid pretreatment is the solubilization and formation of different types of inhibitors, such as carboxylic acids, furans and phenolic compounds, which can strongly affect the fermentability of hemicellulosic hydrolysates. The use of a deacetylation step prior to dilute acid pretreatment has been proposed as a strategy to overcome this problem. By applying this strategy, the formation of inhibitor compounds is reduced and the enzymatic hydrolysis of cellulose is improved. In the present work, rice straw was firstly deacetylated by pretreatment with NaOH (loading of 80 mg NaOH/g of biomass, 70 °C for 45 minutes). Then, the washed fibers were treated with dilute acid under different conditions of acid concentration and time, and the changes on major components and individual sugars recovery from biomass were assessed. In order to evaluate the impact of the pretreatment on ethanol production in separate streams, the hemicellulosic hydrolysate was fermented by Scheffersomyces stipitis NRRL Y-7124, whereas the cellulosic fraction was submitted to simultaneous saccharification and fermentation employing commercial cellulases and Kluyveromyces marxianus NRRL Y-6860. Material balance calculations were then developed and the results showed that performing the deacetylation step prior dilute acid pretreatment considerably improved the overall glucose recovery (from 66 to 86%) and decreased hemicellulosic hydrolysate toxicity. Consequently, ethanol yield from both glucose and xylose increased by 34 and 24%, respectively, when compared to one-step dilute acid pretreatment.

Financed by FAPESP (Project n° 2013/13953-6) and CNPq.