Wen-Song Hwang

Effect of dilute acid pretreatment of rice straw on structural properties and enzymatic hydrolysis.

This study aim is to propose operational conditions for the dilute acid pretreatment of rice straw and to explore the effect of the structural properties of the solid residues on the enzymatic hydrolysis. A maximal sugar yield of 83% was achieved when the rice straw was pretreated with 1% (w/w) sulfuric acid with a reaction time of 1-5 min at 160 degrees C or 180 degrees C, followed by enzymatic hydrolysis. The completely release of sugar (xylose and glucose) increased the pore volume of the pretreated solid residues resulted in an efficiency of 70% for the enzymatic hydrolysis. The extra pore volume was generated by the release of acid-soluble lignin and this resulted in the enzymatic hydrolysis being enhanced by nearly 10%. The increase in the crystallinity index of the pretreated rice straw was limited. These results were consistent with those from the Fourier transformer infrared (FTIR) analysis.

Characterization of dilute acid pretreatment of silvergrass for ethanol production.

Pretreatment with dilute sulfuric acid of silvergrass was compared with the pretreatments effect on other commonly used lignocellulosic materials, namely rice straw and bagasse, in order to evaluate the potential of this feedstock for ethanol production. The highest yield of xylose from silvergrass was between 70% and 75%, which was similar to bagasse. However, silvergrass gave a higher level of fermentability than bagasse using the hydrolysate because less acetic acid was formed. The release of sugars resulted in an about 2.0-fold increase in specific surface area of the pretreated silvergrass. Increasing the specific surface area did not obviously enhance enzymatic digestibility. The hydrophilicity of the acid pretreated silvergrass was characterized using its Fourier transform infrared (FTIR) spectra. The increase in hydrophilicity may enhance enzymatic adsorption onto lignin and increase the accumulation of cellobiose for enzymatic hydrolysis as pretreatment severity increases.

Enhanced ethanol production by fermentation of rice straw hydrolysate without detoxification using a newly adapted strain of Pichia stipitis.

An enhanced inhibitor-tolerant strain of Pichia stipitis was successfully developed through adaptation to acid-treated rice straw hydrolysate. The ethanol production obtained by fermentation of NaOH-neutralized hydrolysate without detoxification using the adapted P. stipitis was comparable to fermentation of overliming-detoxified hydrolysate. The ethanol yield using the adapted P. stipitis with both types of hydrolysate at pH 5.0 achieved 0.45 g(p) g(s)(-1), which is equivalent to 87% of the maximum possible ethanol conversion. Furthermore, the newly adapted P. stipitis demonstrated significantly enhanced tolerance to sulfate and furfural despite the fact that both inhibitors had not been removed from the hydrolysate by NaOH neutralization. Finally, the ethanol conversion could be maintained at 60% and above when the neutralized hydrolysate contained 3.0% sulfate and 1.3gL(-1) furfural.

Development of a yeast strain for xylitol production without hydrolysate detoxification as part of the integration of co-product generation within the lignocellulosic ethanol process.

The present study verified an applicable technology of xylitol bioconversion as part of the integration of co-product generation within second-generation bioethanol processes. A newly isolated yeast strain, Candida tropicalis JH030, was shown to have a capacity for xylitol production from hemicellulosic hydrolysate without detoxification. The yeast gives a promising xylitol yield of 0.71 g(p) g(s)(-1) from non-detoxified rice straw hydrolysate that had been prepared by the dilute acid pretreatment under severe conditions. The yeasts capacity was also found to be practicable with various other raw materials, such as sugarcane bagasse, silvergrass, napiergrass and pineapple peel. The lack of a need to hydrolysate detoxification enhances the potential of this newly isolated yeast for xylitol production and this, in turn, has the capacity to improve economics of lignocellulosic ethanol production.

Pretreatment of rice straw using an extrusion/extraction process at bench-scale for producing cellulosic ethanol.

A combination of a twin-screw extrusion and an acid-catalyzed hot water extraction process performed at a bench-scale was used to prepare high monomeric xylose hydrolysate for cellulosic production. The influences of the screw speed (30-150 rpm), barrel temperature (80-160 °C) and corresponding specific mechanical energy of the extruder on the structural properties of the pretreated rice straw, sugar concentration and conversion were investigated. The optimal condition for the extrusion step was determined to be 40 rpm with 3% H2SO4 at 120 °C; the optimal condition for the extraction step was determined to be 130 °C for 20 min. After the pretreatment at the optimal condition, 83.7% of the xylan was converted to monomeric xylose, and the concentration reached levels of 53.7 g/L. Finally, after the subsequent enzymatic hydrolysis, an 80% yield of the total saccharification was obtained.

Pilot-scale ethanol production from rice straw hydrolysates using xylose-fermenting Pichia stipitis.

Ethanol was produced at pilot scale from rice straw hydrolysates using a Pichia stipitis strain previously adapted to NaOH-neutralized hydrolysates. The highest ethanol yield was 0.44 ± 0.02 g(p)/g(s) at an aeration rate of 0.05 vvm using overliming-detoxified hydrolysates. The yield with hydrolysates conditioned by ammonia and NaOH was 0.39 ± 0.01 and 0.34 ± 0.01 g(p)/g(s), respectively, were achieved at the same aeration rate. The actual ethanol yield from hydrolysate fermentation with ammonia neutralization was similar to that with overliming hydrolysate after taking into account the xylose loss resulting from these conditioning processes. Moreover, the ethanol yield from ammonia-neutralized hydrolysates could be further enhanced by increasing the initial cell density by two-fold or reducing the combined concentration of furfural and 5-hydroxymethyl furfural to 0.6g/L by reducing the severity of operational conditions in pretreatment. This study demonstrated the potential for commercial ethanol production from rice straw via xylose fermentation.

Production of optically pure l-lactic acid from lignocellulosic hydrolysate by using a newly isolated and d-lactate dehydrogenase gene-deficient Lactobacillus paracasei strain

The use of lignocellulosic feedstock for lactic acid production with a difficulty is that the release of inhibitory compounds during the pretreatment process which inhibit the growth of microorganism. Thus we report a novel lactic acid bacterium, Lactobacillus paracasei 7 BL, that has a high tolerance to inhibitors and produced optically pure l-lactic acid after the interruption of ldhD gene. The strain 7 BL fermented glucose efficiently and showed high titer of l-lactic acid (215 g/l) by fed-batch strategy. In addition, 99 g/l of l-lactic acid with high yield (0.96 g/g) and productivity (2.25-3.23 g/l/h) was obtained by using non-detoxified wood hydrolysate. Rice straw hydrolysate without detoxification was also tested and yielded a productivity rate as high as 5.27 g/l/h. Therefore, L. paracasei 7 BL represents a potential method of l-lactic acid production from lignocellulosic biomass and has attractive application for industries.

Method of 2,3-butanediol production from glycerol and acid-pretreated rice straw hydrolysate by newly isolated strains: pre-evaluation as an integrated biorefinery process.

The present study validated a bioconversion technology for the production of 2,3-butanediol (2,3-BD) using sugars, glycerol and lignocellulosic material by three newly isolated strains-two Klebsiella sp. and one Serratia sp. One Klebsiella sp. afforded a high diol production yield (0.45 g/g) using the less common sugar arabinose and Serratia sp. was used for the first time to convert glycerol to 2,3-BD and afforded a yield of 0.43 g/g. Furthermore, acid-pretreated rice straw hydrolysate was used to determine the feasibility of its conversion to 2,3-BD. Both cellulose and hemicellulose hydrolysate were successfully fermented to 2,3-BD and acetoin by the isolates with yields for the diol between 0.39 and 0.44 g/g (equivalent to 78-88% of the maximum yield). These results demonstrate that 2,3-butanediol can be considered as the main product or a value-added byproduct of biofuel production and then potentially improve the economy of lignocellulosic biorefinery.

Ethanol production from dilute-acid steam exploded lignocellulosic feedstocks using an isolated multistress-tolerant Pichia kudriavzevii strain

Renewable and low‐cost lignocellulosic wastes have attractive applications in bioethanol production. The yeast Saccharomyces cerevisiae is the most widely used ethanol‐producing microbe; however, its fermentation temperature (30–35°C) is not optimum (40–50°C) for enzymatic hydrolysis in the simultaneous saccharification and fermentation (SSF) process. In this study, we successfully performed an SSF process at 42°C from a high solid loading of 20% (w/v) acid‐impregnated steam explosion (AISE)‐treated rice straw with low inhibitor concentrations (furfural 0.19 g l−1 and acetic acid 0.95 g l−1) using an isolate Pichia kudriavzevii SI, where the ethanol titre obtained (33.4 gp l−1) was nearly 39% greater than that produced by conventional S. cerevisiae BCRC20270 at 30°C (24.1 gp l−1). In addition, P. kudriavzevii SI exhibited a high conversion efficiency of > 91% from enzyme‐saccharified hydrolysates of AISE‐treated plywood chips and sugarcane bagasse, although high concentrations of furaldehydes, such as furfural 1.07–1.21 g l−1, 5‐hydroxymethyl furfural 0.20−0.72 g l−1 and acetic acid 4.80–7.65 g l−1, were present. This is the first report of ethanol fermentation by P. kudriavzevii using various acid‐treated lignocellulosic feedstocks without detoxification or added nutrients. The multistress‐tolerant strain SI has greater potential than the conventional S. cerevisiae for use in the cellulosic ethanol industry.