Lipid production through simultaneous utilization of glucose, xylose, and l-arabinose by Pseudozyma hubeiensis: a comparative screening study
© The Author(s) 2016
Received: 17 June 2016
Accepted: 23 August 2016
Published: 26 August 2016
Co-fermentation of glucose, xylose and l-arabinose from lignocellulosic biomass by an oleaginous yeast is anticipated as a method for biodiesel production. However, most yeasts ferment glucose first before consuming pentoses, due to glucose repression. This preferential utilization results in delayed fermentation time and lower productivity. Therefore, co-fermentation of lignocellulosic sugars could achieve cost-effective conversion of lignocellulosic biomass to microbial lipid. Comprehensive screening of oleaginous yeasts capable of simultaneously utilizing glucose, xylose, and l-arabinose was performed by measuring the concentration of sugars remaining in the medium and of lipids accumulated in the cells. We found that of 1189 strains tested, 12 had the ability to co-ferment the sugars. The basidiomycete yeast Pseudozyma hubeiensis IPM1-10, which had the highest sugars consumption rate of 94.1 %, was selected by culturing in a batch culture with the mixed-sugar medium. The strain showed (1) simultaneous utilization of all three sugars, and (2) high lipid-accumulating ability. This study suggests that P. hubeiensis IPM1-10 is a promising candidate for second-generation biodiesel production from hydrolysate of lignocellulosic biomass.
The lipid produced by microorganisms is considered to have powerful potential for the development of a new kind of energy, and has received significant interest from sustainable energy researchers. Lipid accumulated by oleaginous yeast is viewed as a promising alternative to second-generation biodiesel, since the composition of the fatty acids produced by yeast is suitable for biodiesel production. That is, it contains palmitic (16:0), stearic (18:0), oleic (18:1), and linoleic (18:2) acids at a high ratio, mainly in the form of triacylglycerol (TAG) (Beopoulos et al. 2011; Knothe 2009; Meng et al. 2009; Sitepu et al. 2014). Compared to other oleaginous microorganisms, oleaginous yeasts are advantageous due to their rapid growth rate (Li et al. 2008), and they are deemed to have the potential to convert various carbon sources, such as cellobiose, xylose and starch, to lipid (Gong et al. 2012; Hu et al. 2011; Huang et al. 2014; Tanimura et al. 2014a).
Second-generation biodiesel is made from non-food sources such as rice straw, wood residue, corncob, and sugarcane bagasse. Lignocellulosic hydrolysates from these feedstocks are composed mainly of glucose, xylose, and l-arabinose (hereafter referred to simply as arabinose) (Huang et al. 2009; Kumar et al. 2009; Madhavan et al. 2012; Roberto et al. 1995; Tsigie et al. 2011). The ratio of the sugars and their concentration in the hydrolysates vary depending on the feedstock used and pretreatment conditions (Behera et al. 2014; Kumar et al. 2009). A previous study investigated lipid accumulation using a medium containing 3 % glucose by Vanrija musci JCM 24512 (formally Cryptococcus musci) (Tanimura et al. 2014b). The strain showed higher lipid-producing ability from glucose compared to typical oleaginous yeasts such as Lipomyces starkeyi and Rhodosporidium toruloides. Strains like this that can convert glucose to lipid with high productivity are well suited for the production of glucose-rich hydrolysate such as the hydrolysate of starchy biomass. However, because pentoses content ranged from 20 to 40 % of the total released sugars (Sumphanwanich et al. 2008; Tanimura et al. 2012), glucose utilization alone is insufficient for the conversion of lignocellulosic biomass. In other words, sequential utilization of the sugars extends fermentation times. Therefore, economically feasible production of lipid will require a yeast strain with the ability to co-ferment the lignocellulosic sugars.
Research has shown that engineered yeast can be valuable in expanding the substrate range. For example, Tai engineered Yarrowia lipolytica to make it utilize xylose (Tai 2012). In that case, the xylose reductase encoding gene (XYL1) and xylitol dehydrogenase encoding gene (XYL2) were transferred from the xylose-fermenting yeast Scheffersomyces stipitis into the strain. The uptake of arabinose has not yet been reported, and therefore, research in this area is expected. In addition, to avoid the problem caused by glucose repression, the quest for novel oleaginous yeasts able to co-ferment glucose, xylose, and arabinose would seem to be an efficient strategy. To the best of our knowledge, there has not yet been a screening study of oleaginous yeasts able to ferment the three sugars. The application of the following new oleaginous yeasts to the conversion of lignocellulosic sugars to lipids has been carried out: Trichosporon fermentans (Huang et al. 2009, 2014), L. starkeyi (Anschau et al. 2014), Cryptococcus curvatus (Liang et al. 2014), R. toruloides (Wiebe et al. 2012) and Y. lipolytica (Tsigie et al. 2011). However, in these sugar-consumption profiles, sequential utilization of arabinose was not observed.
In this study, exhaustive screening of 1189 isolates was undertaken to identify an oleaginous yeast strain that was able to convert the glucose, xylose, and arabinose in artificial hydrolysate to lipid. We here report the discovery of Pseudozyma hubeiensis IPM1-10, which shows a significant utilization of a mixture of the sugars.
Materials and methods
Strains and media
Yeast strains collected and taxonomically identified by Takashima et al. (2012) were our primary resources. Yeast strains isolated by Dr. Ando, Kyoto University, from the Kushiro and Kyoto area (Japan) were also assessed. YM agar medium (Difco, Detroit, MI, USA) was used for pre-culture and maintenance of yeast strains.
The artificial hydrolysate of lignocellulosic biomass (mixed-sugar medium) was based on the medium used by Gong et al. (2012), which contained ammonium sulfate 1 g/L, yeast extract 0.5 g/L, potassium dihydrogenphosphate 1 g/L, magnesium sulfate 1 g/L, glucose 20 g/L, xylose 10 g/L and arabinose 5 g/L. The single sugar medium contained ammonium sulfate 1 g/L, yeast extract 0.5 g/L, potassium dihydrogenphosphate 1 g/L, magnesium sulfate 1 g/L, and glucose 35 g/L or xylose 35 g/L or arabinose 35 g/L.
In the secondary screening, the yeast strains selected by the first screening were used. These strains have been deposited in the Japan Collection of Microorganisms (JCM). One loop of 3-day-old yeast culture was suspended in 25 mL of mixed-sugar medium in an Erlenmeyer flask and incubated at 28 °C, with rotary shaking at 150 rpm. The process was performed in batch culture. Culture broth was withdrawn after 4 days. Sugars concentrations of the supernatants were determined by HPLC. Cells from culture broth were harvested by centrifugation (15,000 rpm for 10 min), and washed with distilled water. Cell mass was determined by dry weight after lyophilization. Intracellular total lipids were determined by gas chromatography, as described below.
Kinetic analysis of selected strains
The yeast strains screened by the secondary screening were used. One loop of 3-day-old yeast culture was suspended in 100 mL of mixed-sugar medium and single sugar medium in Erlenmeyer flasks and incubated at 28 °C, with rotary shaking at 150 rpm for 10 days. All of the experiments were performed in batch culture. Fermentation broth was withdrawn at specific time intervals, and intracellular total lipids and sugar concentrations were determined. All experiments were performed in triplicate.
Measurement of fatty acids
Total intracellular lipid was estimated as total fatty acids. The accumulated lipid of the yeast strain was extracted from the lyophilized cells by a hydrochloric acid-catalyzed direct methylation method (Ichihara and Fukubayashi 2010). In brief, after cultivation, the centrifuged cells were lyophilized and weighed. The cells were suspended in toluene and methanol, then directly transmethylated with 8 % methanolic HCl at 100 °C for 1 h. The resultant fatty acid methyl esters were extracted with n-hexane and analyzed using a gas chromatograph (GC-2010 Plus; Shimadzu, Kyoto, Japan) equipped with a flame ionization detector (FID) and an autosampler (AOC20; Shimadzu). A TC-17 capillary column (GL Science, Tokyo, Japan) was used. Heptadecanoic acid (C17: 0) was used as an internal standard for the determination of fatty acid concentrations.
Measurements of sugars
Glucose, xylose, and arabinose concentrations were determined using an HPLC (Shimadzu, Kyoto, Japan) equipped with an Aminex Fermentation Monitoring Column (Bio-Rad Laboratories, Hercules, CA, USA) and Micro-Guard Cation H Refill Cartridges with a Standard Cartridge Holder (Bio-Rad Laboratories). The detector was an RID 10A refractive index detector (Shimadzu). The column was kept at 60 °C using a CTO 20A column oven (Shimadzu). Sulfuric acid solution (5 mM) was used as the mobile phase at a constant flow rate of 0.6 mL/min.
As mentioned above, the experimental flow scheme is shown in Fig. 1. A total of 1189 yeast strains were tested in test tubes containing 3 mL of mixed-sugar medium during the first screening step. The consumed glucose, xylose, and arabinose concentration ranged from 0–20 g/L (0–100 %), 0–5.8 g/L (0–58 %) and 0–5 g/L (0–100 %), respectively. Twelve oleaginous yeast strains with relatively high sugar-consuming ability were obtained through the process (Table 2). Among the 12 yeast strains selected, seven strains belonged to P. hubeiensis.
Sugar consumption and lipid production by P. hubeiensis IPM1-10
The fatty acid compositions of P. hubeiensis IPM1-10 after the 10-day culture are shown in Table 3. Although slight differences can be seen among the fatty acid compositions, the predominant fatty acids found in all cultures were palmitic (C16:0), stearic (C18:0), oleic (C18:1) and linoleic (C18:2) acids.
Mixed sugar consumption and lipid production by P. hubeiensis IPM1-10
Sugar composition of lignocellulosic hydrolysates
Huang et al. (2009)
Roberto et al. (1995)
Oberoi et al. (2012)
Tsigie et al. (2012)
Tsigie et al. (2011)
Zhang et al. (2014)
Huang et al. (2012)
Yeast species, source, and JCM number of 12 selected oleaginous yeasts
Plant, Iriomote Island
Plant, Iriomote Island
Plant, Iriomote Island
Soil, Rishiri Island
Plant, Iriomote Island
Plant, Iriomote Island
Plant, Iriomote Island
Plant, Iriomote Island
Plant, Iriomote Island
Plant, Iriomote Island
Unidentified Ustilaginales species
Plant, Iriomote Island
Plant, Iriomote Island
As shown in Fig. 2, all 12 candidates showed favorable results in terms of the assimilation of pentoses. The lipid concentration of M. aphidis RS041, U. siamensis IP037, M. antarctica IP040, and A. elionuri IPM46-16 were relatively higher from the viewpoint of sugar yield (g of lipid produced per g of sugar consumed). However, their sugar consumption was not comparable to that of P. hubeiensis IPM1-10, which led to the lower lipid productivity (duration of time needed for lipid concentration), because the slow sugar uptake increased cultivation time. Lipid productivity is considered to be the most important parameter. Higher lipid productivity decreases production cost. In the selected strain, P. hubeiensis IPM1-10, the highest lipid concentration and cell mass were achieved with almost complete utilization of the sugars.
Similar to the other Ustilaginales species, P. hubeiensis has been recognized as a biosurfactant producer (Konishi et al. 2008). P. hubeiensis produces lipases, assimilates oil (soy oil or bovine fat), and secrets biosurfactant (Bussamara et al. 2010, 2012). Since P. hubeiensis can also convert lignocellulosic sugars to lipid, it has great potential for utilization of unused biomass and low-cost raw materials.
Fatty acid composition of P. hubeiensis IPM1-10 after a 10-day culture
Glucose, xylose and arabinose
When grown in the mixed-sugar medium, P. hubeiensis IPM1-10 required a 10-day culture. There have been several previous reports on lipid production by oleaginous yeast from mixtures of glucose, xylose, and arabinose. Sugar exhaustion was achieved at 11 days from rice straw hydrolysate by T. fermentans (Huang et al. 2009), 10 days from a semi-defined medium by T. fermentans (Huang et al. 2014), and 7 days from sugarcane bagasse hydrolysate by Y. lipolytica (Tsigie et al. 2011). Further consideration is needed to determine how best to improve fermentation conditions. On the other hand, to increase lipid accumulation, continuous or fed-batch culture might be effective (Gong et al. 2012; Zhao et al. 2008).
When the sugar mixtures were used as the carbon source, the lipid concentration was higher than with glucose alone. Increasing the proportion of pentoses in the carbon source increased lipid accumulation. Papanikolaou and Aggelis indicated that xylose affected lipid yield rather than glucose, because oleaginous microorganisms exclusively utilize the phosphoketolase pathway for xylose (Papanikolaou and Aggelis 2011). Therefore, P. hubeiensis IPM1-10 provides an efficient process for converting lignocellulosic biomass, such as the glucose, xylose, and arabinose present in hydrolysates, into lipid.
Comprehensive screening of oleaginous yeasts capable of simultaneously utilizing glucose, xylose, and l-arabinose was performed. Among the strains tested here, P. hubeiensis IPM1-10 had the best lipid productivity grown on lignocellulosic sugars. The strain may also be useful as a genetic resource for engineering pentoses metabolism in oleaginous microorganisms in order to improve their ability to convert sugar mixtures to lipid. More importantly, the absence of glucose repression could facilitate further study to unravel the unique sugar-assimilation mechanism.
AT performed experiments and drafted the manuscript. MT, TS, RE and MO isolated and identified the tested yeast strains and revised the manuscript. SK and JO assisted with the data analysis. JS managed the overall project and revised the manuscript. All authors read and approved the final manuscript.
This work was supported partly by the Research Institute for Food and Agriculture of Ryukoku University, and partly by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST). The authors would like to thank Dr. Akinori Ando, Kyoto University, for supplying a portion of the yeast strains used in this study.
The authors declare that they have no competing interests.
Ethics approval and consent to participate
This article does not contain any studies with human participants or animals performed by any of the authors.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
- Adsul MG, Bastawde KB, Gokhale DV. Biochemical characterization of two xylanases from yeast Pseudozyma hubeiensis producing only xylooligosaccharides. Bioresour Technol. 2009;100:6488–95.View ArticlePubMedGoogle Scholar
- Anschau A, Xavier MCA, Hernalsteens S, Franco TT. Effect of feeding strategies on lipid production by Lipomyces starkeyi. Bioresour Technol. 2014;157:214–22.View ArticlePubMedGoogle Scholar
- Behera S, Arora R, Nandhagopal N, Kumar S. Importance of chemical pretreatment for bioconversion of lignocellulosic biomass. Renew Sust Energ Rev. 2014;36:91–106.View ArticleGoogle Scholar
- Beopoulos A, Nicaud JM, Gaillardin C. An overview of lipid metabolism in yeasts and its impact on biotechnological processes. Appl Microbiol Biotechnol. 2011;90:1193–206.View ArticlePubMedGoogle Scholar
- Bussamara R, Dall’agnol L, Schrank A, Fernandes KF, Vainstein MH. Optimal conditions for continuous immobilization of Pseudozyma hubeiensis (strain HB85A) lipase by adsorption in a packed-bed reactor by response surface methodology. Enzyme Res. 2012;2012:329178.View ArticlePubMedPubMed CentralGoogle Scholar
- Bussamara R, Fuentefria AM, Oliveira ESd, Broetto L, Simcikova M, Valente P, Schrank A, Vainstein MH. Isolation of a lipase-secreting yeast for enzyme production in a pilot-plant scale batch fermentation. Bioresour Technol. 2010;101:268–75.View ArticlePubMedGoogle Scholar
- Gong Z, Wang Q, Shen H, Hu C, Jin G, Zhao ZK. Co-fermentation of cellobiose and xylose by Lipomyces starkeyi for lipid production. Bioresour Technol. 2012;117:20–4.View ArticlePubMedGoogle Scholar
- Hu C, Wu S, Wang Q, Jin G, Shen H, Zhao ZK. Simultaneous utilization of glucose and xylose for lipid production by Trichosporon cutaneum. Biotechnol Biofuels. 2011;4:25.View ArticlePubMedPubMed CentralGoogle Scholar
- Huang C, Xf Chen, Xiong L, XD Chen, LL Ma, Chen Y. Single cell oil production from low-cost substrates: the possibility and potential of its industrialization. Biotechnol Adv. 2013;31:129–39.View ArticlePubMedGoogle Scholar
- Huang C, Li YY, Liu LP, Wu H, Zong MH, Lou WY. Kinetics and Mechanism analysis on microbial oil production by Trichosporon fermentans in Rice straw hydrolysate. Ind Eng Chem Res. 2014;53:19034–43.View ArticleGoogle Scholar
- Huang C, Zong MH WuH, QP Liu. Microbial oil production from rice straw hydrolysate by Trichosporon fermentans. Bioresour Technol. 2009;100:4535–8.View ArticlePubMedGoogle Scholar
- Huang C, Wu H, Li RF, Zong MH. Improving lipid production from bagasse hydrolysate with Trichosporon fermentans by response surface methodology. N Biotechnol. 2012;29(3):372–78.View ArticlePubMedGoogle Scholar
- Ichihara K, Fukubayashi Y. Preparation of fatty acid methyl esters for gas-liquid chromatography. J Lipid Res. 2010;51:635–40.View ArticlePubMedPubMed CentralGoogle Scholar
- Jaseetha AS, Das N. Enhanced biodegradation of lindane using oil-in-water bio-microemulsion stabilized by biosurfactant produced by a new yeast strain, Pseudozyma VITJzN01. J Microbiol Biotechnol. 2013;23:1598–609.View ArticleGoogle Scholar
- Knothe G. Improving biodiesel fuel properties by modifying fatty ester composition. Energ Environ Sci. 2009;2:759–66.View ArticleGoogle Scholar
- Konishi M, Morita T, Fukuoka T, Imura T, Kakugawa K, Kitamoto D. Efficient production of mannosylerythritol lipids with high hydrophilicity by Pseudozyma hubeiensis KM-59. Appl Microbiol Biotechnol. 2008;78:37–46.View ArticlePubMedGoogle Scholar
- Kumar P, Barrett DM, Delwiche MJ, Stroeve P. Methods for pretreatment of lignocellulosic biomass for efficient hydrolysis and biofuel production. Ind Eng Chem Res. 2009;48:3713–29.View ArticleGoogle Scholar
- Li Q, Du W, Liu D. Perspectives of microbial oils for biodiesel production. Appl Microbiol Biotechnol. 2008;80:749–56.View ArticlePubMedGoogle Scholar
- Liang Y, Jarosz K, Wardlow AT, Zhang J, Cui Y. Lipid production by Cryptococcus curvatus on hydrolysates derived from corn fiber and sweet sorghum bagasse following dilute acid pretreatment. Appl Biochem Biotechnol. 2014;173:2086–98.View ArticlePubMedGoogle Scholar
- Madhavan A, Srivastava A, Kondo A, Bisaria VS. Bioconversion of lignocellulose-derived sugars to ethanol by engineered Saccharomyces cerevisiae. Crit Rev Biotechnol. 2012;32:22–48.View ArticlePubMedGoogle Scholar
- Meng X, Yang JM, Xu X, Zhang L, Nie QJ, Xian M. Biodiesel production from oleaginous microorganisms. Renew Energ. 2009;34:1–5.View ArticleGoogle Scholar
- Morita T, Takashima M, Fukuoka T, Konishi M, Imura T, Kitamoto D. Isolation of basidiomycetous yeast Pseudozyma tsukubaensis and production of glycolipid biosurfactant, a diastereomer type of mannosylerythritol lipid-B. Appl Microbiol Biotechnol. 2010;88:679–88.View ArticlePubMedGoogle Scholar
- Papanikolaou S, Aggelis G. Lipids of oleaginous yeasts. Part I: biochemistry of single cell oil production. Eur J Lipid Sci Technol. 2011;113:1031–51.View ArticleGoogle Scholar
- Oberoi HS, Babbar N, Sandhu SK, Dhaliwal SS, Kaur U, Chadha BS, Bhargav VK. Ethanol production from alkali-treated rice straw via simultaneous saccharification and fermentation using newly isolated thermotolerant Pichia kudriavzevii HOP-1. J Ind Microbiol Biotechnol. 2012;39(4):557–66.View ArticlePubMedGoogle Scholar
- Roberto IC, Sato S, Demancilha IM, Taqueda MES. Influence of media composition on xylitol fermentation by Candida guiliiermondii using response surface methodology. Biotechnol Lett. 1995;17:1223–8.View ArticleGoogle Scholar
- Sitepu IR, Garay LA, Sestric R, Levin D, Block DE, Bruce German J, Boundy-Mills KL. Oleaginous yeasts for biodiesel: current and future trends in biology and production. Biotechnol Adv. 2014;32:1336–60.View ArticlePubMedGoogle Scholar
- Sumphanwanich J, Leepipatpiboon N, Srinorakutara T, Akaracharanya A. Evaluation of dilute-acid pretreated bagasse, corn cob and rice straw for ethanol fermentation by Saccharomyces cerevisiae. Ann Microbiol. 2008;58:219–25.View ArticleGoogle Scholar
- Tai M. Metabolic engineering of oleaginous yeast for the production of biofuels. Cambridge: Massachusetts Institute of Technology; 2012.Google Scholar
- Takashima M, Sugita T, Van BH, Nakamura M, Endoh R, Ohkuma M. Taxonomic richness of yeasts in Japan within subtropical and cool temperate areas. PLoS One. 2012;7:e50784.View ArticlePubMedPubMed CentralGoogle Scholar
- Tanimura A, Nakamura T, Watanabe I, Ogawa J, Shima J. Isolation of a novel strain of Candida shehatae for ethanol production at elevated temperature. SpringerPlus. 2012;1:27.View ArticlePubMedPubMed CentralGoogle Scholar
- Tanimura A, Takashima M, Sugita T, Endoh R, Kikukawa M, Yamaguchi S, Sakuradani E, Ogawa J, Ohkuma M, Shima J. Cryptococcus terricola is a promising oleaginous yeast for biodiesel production from starch through consolidated bioprocessing. Sci Rep. 2014a;4:4776.PubMedPubMed CentralGoogle Scholar
- Tanimura A, Takashima M, Sugita T, Endoh R, Kikukawa M, Yamaguchi S, Sakuradani E, Ogawa J, Shima J. Selection of oleaginous yeasts with high lipid productivity for practical biodiesel production. Bioresour Technol. 2014b;153:230–5.View ArticlePubMedGoogle Scholar
- Tsigie YA, Wang C-Y, Chi-Thanh T, Ju Y-H. Lipid production from Yarrowia lipolytica Po1g grown in sugarcane bagasse hydrolysate. Bioresour Technol. 2011;102:9216–22.View ArticlePubMedGoogle Scholar
- Tsigie YA, Wang CY, Kasim NS, Diem QD, Huynh LH, Ho QP, Truong CT, Ju YH. Oil Production from Po1g Using Rice Bran Hydrolysate. J Biomed Biotechnol. 2012;2012:1–10.View ArticleGoogle Scholar
- Wang QM, Begerow D, Groenewald M, Liu XZ, Theelen B, Bai FY, Boekhout T. Multigene phylogeny and taxonomic revision of yeasts and related fungi in the Ustilaginomycotina. Stud Mycol. 2015;81:55–83.View ArticlePubMedPubMed CentralGoogle Scholar
- Wang QM, Jia JH, Bai FY. Pseudozyma hubeiensis sp nov and Pseudozyma shanxiensis sp nov., novel ustilaginomycetous anamorphic yeast species from plant leaves. Int J Syst Evol Microbiol. 2006;56:289–93.View ArticlePubMedGoogle Scholar
- Wiebe MG, Koivuranta K, Penttila M, Ruohonen L. Lipid production in batch and fed-batch cultures of Rhodosporidium toruloides from 5 and 6 carbon carbohydrates. BMC Biotechnol. 2012;12:26.View ArticlePubMedPubMed CentralGoogle Scholar
- Yoshida S, Morita T, Shinozaki Y, Watanabe T, Sameshima-Yamashita Y, Koitabashi M, Kitamoto D, Kitamoto H. Mannosylerythritol lipids secreted by phyllosphere yeast Pseudozyma antarctica is associated with its filamentous growth and propagation on plant surfaces. Appl Microbiol Biotechnol. 2014;98:6419–29.View ArticlePubMedGoogle Scholar
- Zhao X, Kong X, Hua Y, Feng B, Zhao Z. Medium optimization for lipid production through co-fermentation of glucose and xylose by the oleaginous yeast Lipomyces starkeyi. Eur J Lipid Sci Technol. 2008;110:405–12.View ArticleGoogle Scholar
- Zhang Y, Chen X, Qi B, Luo J, Shen F, Su Y, Khan R, Wan Y. Improving lactic acid productivity from wheat straw hydrolysates by membrane integrated repeated batch fermentation under non-sterilized conditions. Bioresour Technol. 2014;163:160–66.View ArticlePubMedGoogle Scholar