Process design of SSCF for ethanol production from steam-pretreated, acetic-acid-impregnated wheat straw

Abstract

Background: Pretreatment is an important step in the production of ethanol from lignocellulosic material. Using acetic acid together with steam pretreatment allows the positive effects of an acid catalyst to be retained, while avoiding the negative environmental effects associated with sulphuric acid. Acetic acid is also formed during the pretreatment and hydrolysis of hemicellulose, and is a known inhibitor that may impair fermentation at high concentrations. The purpose of this study was to improve ethanol production from glucose and xylose in steam-pretreated, acetic-acid-impregnated wheat straw by process design of simultaneous saccharification and co-fermentation (SSCF), using a genetically modified pentose fermenting yeast strain Saccharomyces cerevisiae.

Results: Ethanol was produced from glucose and xylose using both the liquid fraction and the whole slurry from pretreated materials. The highest ethanol concentration achieved was 37.5 g/L, corresponding to an overall ethanol yield of 0.32 g/g based on the glucose and xylose available in the pretreated material. To obtain this concentration, a slurry with a water-insoluble solids (WIS) content of 11.7 % was used, using a fed-batch SSCF strategy. A higher overall ethanol yield (0.36 g/g) was obtained at 10 % WIS.

Conclusions: Ethanol production from steam-pretreated, acetic-acid-impregnated wheat straw through SSCF with a pentose fermenting S. cerevisiae strain was successfully demonstrated. However, the ethanol concentration was too low and the residence time too long to be suitable for large-scale applications. It is hoped that further process design focusing on the enzymatic conversion of cellulose to glucose will allow the combination of acetic acid pretreatment and co-fermentation of glucose and xylose.

Keywords: Acetic acid; Ethanol; Fed-batch; Saccharomyces cerevisiae; Simultaneous saccharification and co-fermentation (SSCF); Steam pretreatment; Wheat straw; Xylose fermentation.

Fig. 1

Schematic description of the liquid fermentation process for experiments with and without pre-hydrolysis

Fig. 2

a Concentrations of ethanol, xylose, and glucose during liquid fermentation with no pre-hydrolysis (black) and with pre-hydrolysis for 2 (grey) and 4 (white) h. b Overall and metabolic ethanol yield (g/g) based on available sugars (pale blue) and consumed sugars (dark blue), respectively, after liquid fermentation with no pre-hydrolysis (0 h) and with pre-hydrolysis for 2 and 4 h. The error bars represent the highest and lowest results of duplicate experiments

Fig. 3

Overview of the four SSCF configurations investigated. A Batch SSCF (base case). B Fed-batch SSCF of the liquid, followed by the addition of solids at 48 and 50 h. C As in B, but with 4-h pre-hydrolysis of the liquid fraction. D As in C, but with 8-h pre-hydrolysis of the solid fraction

Fig. 4

Ethanol and sugar concentrations during SSCF using the Configurations A–D as defined in Fig. 3

Fig. 5

Overall and metabolic ethanol yield (g/g) based on available sugars and consumed sugars, respectively, for the four SSCF configurations defined in Fig. 3. The error bars represent the highest and lowest results of duplicate experiments

Fig. 6

Concentration of ethanol, xylose, and glucose during fed-batch SSCF (Configuration C) with pre-hydrolysis of the liquid fraction at a total WIS of 11.7 %. The error bars represent the highest and lowest results of duplicate experiments

Fig. 7

Concentration of ethanol, xylose, and glucose during fed-batch SSCF with pre-hydrolysis of the liquid fraction and 4.3 g yeast/L at a total WIS content of a 10 % and b 11.7 %. The error bars represent the highest and lowest results of duplicate (10 % WIS) and triplicate (11.7 % WIS) experiments