Process optimization for ethanol production from photoperiod-sensitive sorghum: Focus on cellulose conversion

https://doi.org/10.1016/j.indcrop.2011.04.012Get rights and content

Abstract

Photoperiod-sensitive sorghum, as a competitive biomass for ethanol production, was investigated to develop an integrated process for improving ethanol yield. Response surface methodology was employed to study the relationship between pretreatment variables (including temperature, sulfuric acid concentration, and reaction time) and cellulose recovery, as well as efficiency of enzymatic hydrolysis (EEH) in the solid part. Recovery yield decreased and EEH increased as the pretreatment temperature, acidic concentration, and reaction time increased. A model was successfully developed to predict total glucose yield with a maximum value of 82.2%. Conditions of co-fermentation were also optimized, and the optimal ethanol yield was obtained with constant-temperature simultaneous saccharification and fermentation at 38 °C. Acetate buffer at a concentration of 50 mM was found helpful for increasing efficiency of enzymatic hydrolysis, as well as ethanol yield. The maximum ethanol yield was 0.21 g ethanol per dry mass at the conditions of 38 °C, 0.05 g yeast/L, and 50 mM acetate buffer. A complete cellulose balance was provided for the whole process.

Highlights

Photoperiod sensitive sorghum is a new feedstock for ethanol production. ► Response surface methodology was employed to optimize the process. ► About 0.21 g ethanol was obtained from 1 g PS sorghum.

Introduction

Production of ethanol from lignocellulosic biomass can effectively reduce the risk of food price increase because of sugar- or starch-based bioethanol production (Fargione et al., 2008, Tilman et al., 2006). Although corn stover has been studied for ethanol production (Zhu et al., 2009), the available amount of corn stover is not sufficient to support biorefineries with an annual production of 36 billion gallons of ethanol in 2022 (Graham et al., 2007). Photoperiod-sensitive (PS) sorghum, which can grow in semiarid parts of the world and especially in areas too dry for corn, contains a significant amount of soluble sugar and a low ratio of lignin to cellulose. These advantages make PS sorghum an excellent source for bioethanol production.

Many studies about the effects of diluted sulfuric acid pretreatment on enzymatic hydrolysis of cellulose have been reported in which the acid pretreatment was proven effectively in removing most of the hemicellulose and some lignin component (Schell et al., 2003, Zhu et al., 2004). According to previous study on PS sorghum (Xu et al., 2011), a harsher condition of pretreatment should be applied to make cellulose easily degradable, thus achieving a high efficiency (more than 90%) of enzymatic hydrolysis (EEH). Cellulose degradation during high temperature and diluted sulfuric acid pretreatment comprises the following steps. First, hydrogen bonds between glucan units are broken and cellulose is degraded to cellobiose and glucose, and cellobiose is not stable at acid condition and will be degraded to glucose. Glucose will further be degraded to form degradation products (Mosier et al., 2002), some of which are considered inhibitors to yeast fermentation (Klinke et al., 2004). Thus, it is more important to study the relationship between cellulose (glucose) recovery and EEH than to focus only on a high EEH because cellulose loss is not negligible even for mild pretreatment conditions (e.g., 143.2 °C, 0.4% sulfuric acid concentration and 30 min) (Bienkowski et al., 1987).

A previous study on diluted sulfuric acid pretreatment of PS sorghum has shown promising results: about 74% of the sugar source, including cellulose (structural glucose) and soluble sugar (sucrose, glucose, and fructose), could be converted to ethanol (Xu et al., 2011). Thus, an in-depth investigation of how to maximize glucose yield is necessary to reveal the relationship between pretreatment conditions and total glucose yield from enzymatic hydrolysis. Response surface methodology (RSM) with the advantage of efficient design was employed in this study. Since PS sorghum contains a significant amount (about 17.5%) of soluble sugar, which can be easily degraded at high temperature and in acid conditions, a direct pretreatment will result in significant sugar loss. An integrated method was designed in this research to solve this problem: this method separates soluble sugar by washing the ground sample before pretreatment. The washing juice containing soluble sugar was then added to the pretreated sample for co-fermentation (Fig. 1).

Although sugar- and starch-based ethanol fermentation has been studied for a long time (Inlow et al., 1988), these processes are different from cellulose-based ethanol fermentation. Simultaneous saccharification and fermentation (SSF) has been accepted in most biomass processing, since it saves energy and time and reduces end-product inhibition of enzyme hydrolysis (Wright et al., 1988). The low biomass loading, which varies depending on material characteristics and pretreatment method, makes it difficult for cellulose-based SSF to achieve a high ethanol concentration (more than 10% (v/v)). Thus, for yeast fermentation of low sugar concentration and low ethanol concentration, substrate inhibition or end-product inhibition is not critical, because it was reported that ethanol begins to inhibit yeast fermentation at 25 g ethanol/L (Maiorella et al., 1983). Meanwhile, one of the current barriers preventing commercialization of lignocellulosic ethanol is its high processing cost. The optimization of cellulose-based fermentation is consequently supposed to include a mechanism for lowering processing cost. In this research, the optimization of SSF was balanced between ethanol yield and processing cost.

The objective of this study was to optimize the processing method, including both pretreatment and co-fermentation (SSF), for maximizing the ethanol yield from PS sorghum.

Section snippets

Material preparation and chemicals

The PS sorghum (1990/AC791) used in this research was harvested from the Kansas State University Agronomy Research Farm in Manhattan, Kansas, from 2007 to 2009. After being ground with a cutting mill (SM 2000, Retsch Inc., Newtown, PA) to less than 1 mm particle size, the sample with moisture content of 7% was stored at room temperature. The chemical composition of PS sorghum, analyzed by using the NREL procedure (Sluiter et al., 2004), is 32.5%, 19.8%, 11.7% and 17.5% for cellulose,

Optimization of pretreatment and hydrolysis by RSM

Through analysis and optimization in Design Expert, three different models were obtained corresponding to the three response values. ANOVA was then conducted to remove insignificant terms. Finally, the three reduced models for EEH, YREC, and Yp were obtained as shown (Eqs. (7), (8), (9)). The ANOVA for EEH showed that the model is highly significant (p-value < 0.0001), and the value of R-squared was 0.9877, indicating the regression model could provide good prediction. Lack-of-fit test was used

Conclusions

The sulfuric acid pretreatment and enzymatic hydrolysis of PS sorghum were optimized together using RSM, and three models were obtained successfully for analysis. The maximum glucose yield from the solid part is 82.2%. Cellulose recovery from the solid part decreased and EEH increased as the pretreatment temperature, resident time, and acidic concentration increased. The optimization on SSF suggested that the conditions of 0.05 g/L inoculation and 38 °C CT-SSF were effective for ethanol

Acknowledgement

The authors were grateful for cellulase from Genencor (A Danisco division, Rochester, NY).

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