Ke-Ke Cheng

Organosolv pretreatment of lignocellulosic biomass for enzymatic hydrolysis

Production of ethanol by bioconversion of lignocellulosic biomass has attracted much interest in recent years. However, the pretreatment process for increasing the enzymatic digestibility of cellulose has become a key step in commercialized production of cellulosic ethanol. During the last decades, many pretreatment processes have been developed for decreasing the biomass recalcitrance, but only a few of them seem to be promising. From the point of view for integrated utilization of lignocellulosic biomass, organosolv pretreatment provides a pathway for biorefining of biomass. This review presents the progress of organosolv pretreatment of lignocellulosic biomass in recent decades, especially on alcohol, organic acid, organic peracid and acetone pretreatments, and corresponding action mechanisms. Evaluation and prospect of organosolv pretreatment were performed. Finally, some recommendations for future investigation of this pretreatment method were given.

Multiple growth inhibition of Klebsiella pneumoniae in 1,3-propanediol fermentation

The inhibition of substrate and product on the growth of Klebsiella pneumoniae in anaerobic and aerobic batch fermentation for the production of 1,3-propanediol was studied. The cells under anaerobic conditions had a higher maximum specific growth rate of 0.19 h−1 and lower tolerance to 110 g glycerol l−1, compared to the maximum specific growth rate of 0.17 h−1 and tolerance to 133 g glycerol l−1 under aerobic conditions. Acetate was the main inhibitory metabolite during the fermentation under anaerobic conditions, with lactate and ethanol the next most inhibitory. The critical concentrations of acetate, lactate and ethanol were assessed to be 15, 19, 26 g l−1, respectively. However, cells grown under aerobic conditions were more resistant to acetate and lactate but less resistant to ethanol. The critical concentrations of acetate, lactate and ethanol were assessed to be 24, 26, and 17 g l−1, respectively

Improved Succinate Production by Metabolic Engineering

Succinate is a promising chemical which has wide applications and can be produced by biological route. The history of the biosuccinate production shows that the joint effort of different metabolic engineering approaches brings successful results. In order to enhance the succinate production, multiple metabolical strategies have been sought. In this review, different overproducers for succinate production, including natural succinate overproducers and metabolic engineered overproducers, are examined and the metabolic engineering strategies and performances are discussed. Modification of the mechanism of substrate transportation, knocking-out genes responsible for by-products accumulation, overexpression of the genes directly involved in the pathway, and improvement of internal NADH and ATP formation are some of the strategies applied. Combination of the appropriate genes from homologous and heterologous hosts, extension of substrate, integrated production of succinate, and other high-value-added products are expected to bring a desired objective of producing succinate from renewable resources economically and efficiently.

Integrated production of xylitol and ethanol using corncob

Xylitol production from corncob hemicellulose is a popular process in China. Microbial conversion of xylose to xylitol, as a biological process with many advantages, has drawn increasing attention. As a by-product from the manufacturing of xylitol, corncob cellulosic residues are produced in very large amounts and represent an environmental problem. As a result, considering the large amount of xylitol production in China, the conversion of corncob cellulosic residues has become a widespread issue having to be tackled. After the hemicellulose in corncob has been hydrolyzed for xylitol production, the corncob cellulosic residue is porous and can easily be hydrolyzed by cellulases into glucose and further converted to ethanol, another high-added-value chemical. Based on the latest technology advancements in xylitol, cellulase, and ethanol production, the integrated production of ethanol from corncob cellulosic residues appears as a promising way to improve the profit of the whole xylitol production process.

Aerobic and sequential anaerobic fermentation to produce xylitol and ethanol using non-detoxified acid pretreated corncob

BackgroundFor economical bioethanol production from lignocellulosic materials, the major technical challenges to lower the production cost are as follows: (1) The microorganism should use efficiently all glucose and xylose in the lignocellulose hydrolysate. (2) The microorganism should have high tolerance to the inhibitors present in the lignocellulose hydrolysate. The aim of the present work was to combine inhibitor degradation, xylitol fermentation, and ethanol production using a single yeast strain.ResultsA new process of integrated aerobic xylitol production and anaerobic ethanol fermentation using non-detoxified acid pretreated corncob by Candida tropicalis W103 was proposed. C. tropicalis W103 is able to degrade acetate, furfural, and 5-hydromethylfurfural and metabolite xylose to xylitol under aerobic conditions, and the aerobic fermentation residue was used as the substrate for ethanol production by anaerobic simultaneous saccharification and fermentation. With 20% substrate loading, furfural and 5-hydroxymethylfurfural were degraded totally after 60 h aerobic incubation. A maximal xylitol concentration of 17.1 g l-1 was obtained with a yield of 0.32 g g-1 xylose. Then under anaerobic conditions with the addition of cellulase, 25.3 g l-1 ethanol was produced after 72 h anaerobic fermentation, corresponding to 82% of the theoretical yield.ConclusionsXylitol and ethanol were produced in Candida tropicalis W103 using dual-phase fermentations, which comprise a changing from aerobic conditions (inhibitor degradation and xylitol production) to anaerobic simultaneous saccharification and ethanol fermentation. This is the first report of integrated xylitol and ethanol production from non-detoxified acid pretreated corncob using a single microorganism.

Statistical optimization of xylitol production from corncob hemicellulose hydrolysate by Candida tropicalis HDY-02.

The statistical experimental designs were adopted to optimize the culture medium in xylitol production by Candida tropicalis HDY-02 with corncob hemicellulose hydrolysate as substrate. In the first step, Plackett-Burman design was used for screening the important variables. KH(2)PO(4), yeast extract, (NH(4))(2)SO(4) and MgSO(4)·7H(2)O were found to significantly affect xylitol yield. In the second step, central composite design (CCD) was used to determine the optimum level of each of the significant variables. A second-order polynomial was determined by the multiple regression analysis of the experimental data. The interactive effects of yeast extract and MgSO(4)·7H(2)O on xylitol yield of C. tropicalis HDY-02 were determined to be significant. The validation experimental was consistent with the prediction model. The optimum combinations for xylitol yield were 5 gl(-1) (NH(4))(2)SO(4), 1.3 gl(-1) KH(2)PO(4), 4.6 gl(-1) yeast extract and 0.6 gl(-1) MgSO(4)·7H(2)O. Under these optimal conditions, the continuous fed-batch experiments could produce xylitol of 58 gl(-1) with a yield of 0.73 g g(-1) xylose.

Statistical optimization of sulfite pretreatment of corncob residues for high concentration ethanol production.

In this study, a central composite design of response surface method was used to optimize sulfite pretreatment of corncob residues, in respect to sulfite charge (5-10%), treatment time (1-2h), liquid/solid (l/s) ratio (6:1-10:1) and temperature (150-180°C) for maximizing glucose production in enzymatic hydrolysis process. The relative optimum condition was obtained as follows: sulfite charge 7.1%, l/s ratio 7.6:1, temperature 156°C for 1.4h, corresponding to 79.3% total glucan converted to glucose+cellobiose. In the subsequent simultaneous saccharification and fermentation (SSF) experiments using 15% glucan substrates pretreated under this kind of conditions, 60.8 g ethanol l(-1) with 72.2% theoretical yield was obtained.

Effects of pH and dissolved CO2 level on simultaneous production of 2,3-butanediol and succinic acid using Klebsiella pneumoniae

The influences of pH and dissolved CO2 level on the regulation of growth and formation of catabolic end products have been investigated in Klebsiella pneumoniae. With increasing CO2 levels, there were no apparent changes in 2,3-butanediol production but succinic acid productions were enhanced significantly. A novel strategy for co-production of 2,3-butanediol and succinic acid using K. pneumoniae was developed by controlling pH and dissolved CO2 concentration in fermentation medium. Under the optimum condition, maximal 77.1 g l(-1) 2,3-butanediol and 28.7 g l(-1) succinic acid were obtained after 60 h of fed-batch fermentation, giving a 2,3-butanediol+succinic acid yield of 1.03 mol mol(-1) glucose. This type of fermentation producing two commercial interests at the same fermentation process might be considered for a promising biological production process which will decrease the production cost by sharing the operation and recovery cost.

Strain Isolation and Study on Process Parameters for Xylose-to-Xylitol Bioconversion

ABSTRACT A xylitol producing yeast was isolated from soil and identified as Candida tropicalis W103 by 18S r DNA gene sequence analysis and physiological characteristics. The optimal fermentation conditions for Candida tropicalis W103 were: 35°C, pH 4.5 and 120 g l−1 initial xylose concentration. However, the maximum yield and productivity of xylitol were obtained at KLa 16.5 h−1 and 18.3 h−1, respectively. A two-stage aeration strategy (0–24 h, KLa 18.3 h−1; after 24 h, shift KLa to 16.5 min−1) was applied in the fermentation to get higher xylitol yield and productivity. After 60 h in batch fermentations, both the xylitol concentration and xylose consumption reached the maximum, obtaining 87.1 g l−1 of xylitol with 1.45 g l−1 h−1 productivity and 0.72 g g−1 xylose yield. Fed-batch fermentation with 120 g l−1 initial xylose and regulating xylose concentration to 40–55 g l−1 during 24–96 h was performed to reach a higher productivity of 1.82 g l−1 h−1, and xylitol concentration of 218.7 g l−1.

Kinetic Modeling of Fermentative Production of 1, 3-Propanediol by Klebsiella pneumoniae HR526 with Consideration of Multiple Product Inhibitions

During the fermentative production of 1, 3-propanediol (1,3-PD), the multiple product inhibitions cannot be negligible to accurately describe the kinetics of fermentation process. A kinetic model for fermentative production of 1,3-PD by Klebsiella pneumoniae HR526 with glycerol as carbon source under aerobic condition was proposed. The inhibitions of multiple products including 1,3-PD, 2, 3-butanediol (2,3-BD), acetate, and succinate were considered in the model. It was found that 1,3-PD, 2,3-BD, and acetate showed strong inhibitions to cell growth depending on their concentrations. The kinetic model was relatively accurate to predict the experimental data of batch, fed-batch, and continuous fermentations. The model thus can serve as a tool for further controlling and optimizing the fermentation process.