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In this study, a non-sterile (open) continuous fermentation (OCF) process with no-carbon loss was developed to improve lactic acid (LA) productivity and operational stability from the co-utilization of lignocellulose-derived sugars by thermophilic Enterococcus faecium QU 50. The effects of different sugar mixtures on LA production were firstly investigated in conventional OCF at 50°C, pH 6.5 and a dilution rate of 0.20 hr-1 . The xylose consumption ratio was greatly lower than that of glucose in fermentations with glucose/xylose mixtures, indicating apparent carbon catabolite repression (CCR). However, CCR could be efficiently eliminated by feeding solutions containing the cellobiose/xylose mixture. In OCF at a dilution rate ca. 0.10 hr-1 , strain QU 50 produced 42.6 g L-1 of l-LA with a yield of 0.912 g g-1 -consumed sugars, LA yield of 0.655 g g-1 based on mixed sugar-loaded, and a productivity of 4.31 g L-1 hr-1 from simulated energy cane hydrolyzate. In OCF with high cell density by cell recycling, simultaneous and complete co-utilization of sugars was achieved with stable LA production at 60.1 ± 3.25 g L-1 with LA yield of 0.944 g g-1 -consumed sugar and LA productivity of 6.49 ± 0.357 g L-1 hr-1 . Besides this, a dramatic increase in LA yield of 0.927 g g-1 based on mixed sugar-loaded with prolonged operational stability for at least 500 hr (>20 days) was established. This robust system demonstrates an initial green step with a no-carbon loss under energy-saving toward the feasibility of sustainable LA production from lignocellulosic sugars.

There is a renewed interest in acetone-butanol-ethanol (ABE) fermentation from renewable substrates for the sustainable and environment-friendly production of biofuel and platform chemicals. However, the ABE fermentation is associated with several challenges due to the presence of heterogeneous components in the renewable substrates and the intrinsic characteristics of ABE fermentation process. Hence, there is a need to select optimal substrates and modify their characteristics suitable for the ABE fermentation process or microbial strain. This "designed biomass" can be used to establish the consolidated bioprocessing systems. As there are very few reports on designed biomass, the main objectives of this review are to summarize the main challenges associated with ABE fermentation from renewable substrates and to introduce feasible strategies for designing the substrates through pretreatment and hydrolysis technologies as well as through the establishment of consolidated bioprocessing systems. This review offers new insights on improving the efficiency of ABE fermentation from designed renewable substrates.

Despite efforts to address the composition of the microbial community during the anaerobic treatment of palm oil mill effluent (POME), its composition in relation to biodegradation in the full-scale treatment system has not yet been extensively examined. Therefore, a thorough analysis of bacterial and archaeal communities was performed in the present study using MiSeq sequencing at the different stages of the POME treatment, which comprised anaerobic as well as facultative anaerobic and aerobic processes, including the mixed raw effluent (MRE), mixing pond, holding tank, and final discharge phases. Based on the results obtained, the following biodegradation processes were suggested to occur at the different treatment stages: (1) Lactobacillaceae (35.9%) dominated the first stage, which contributed to high lactic acid production; (2) the higher population of Clostridiaceae in the mixing pond (47.7%) and Prevotellaceae in the holding tank (49.7%) promoted acetic acid production; (3) the aceticlastic methanogen Methanosaetaceae (0.6-0.8%) played a role in acetic acid degradation in the open digester and closed reactor for methane generation; (4) Syntrophomonas (21.5-29.2%) appeared to be involved in the degradation of fatty acids and acetic acid by syntrophic cooperation with the hydrogenotrophic methanogen, Methanobacteriaceae (0.6-1.3%); and (5) the phenols and alcohols detected in the early phases, but not in the final discharge phase, indicated the successful degradation of lignocellulosic materials. The present results contribute to a better understanding of the biodegradation mechanisms involved in the different stages of the full-scale treatment of POME.

Autothermal thermophilic aerobic digestion (ATAD) is used to treat human excreta hygienically. We previously reported a unique full-scale ATAD, showing distinctive bacterial community transitions and producing high-nitrogen-content liquid fertilizer; nevertheless, the mechanism remains unclear. One hypothesis involves using a gas-inducing (GI) agitator. We designed a lab-scale GI system and compared it with a disk-turbine (DT) agitator system by mimicking the temperature shift of full-scale ATAD. The agitation system and its agitation speed greatly affected physicochemical properties and bacterial community structure. GI system at 1000 rpm (GI1000; high total carbon removal efficiency, 88.3%), with few nitrifying and denitrifying bacteria, maintained a high ammoniacal nitrogen concentration and had more shared operational taxonomic units related to Acinetobacter sp., Arcobacter sp., and Longimicrobium sp. with the full-scale ATAD compared with the GI system at 490 rpm and DT system at 1000 rpm (DT1000). Furthermore, DT1000, with a high abundance of nitrifying and denitrifying bacteria such as Alcaligenes aquatilis and Pseudomonas caeni, removed 94.7% total nitrogen with 71.9% total carbon removal efficiency. These results suggested that shear stress and oxygen supply system would change the bacterial community structure, thus affected ATAD performances. Consequently, it is possible that ATAD can be applied for not only production of highly nitrogen-containing liquid fertilizer but also extremely nitrogen removal of wastewater.

Biobutanol is well known as a suitable substitute for gasoline, which can be applied without engine modification. Butanol toxicity to the producer strain causes difficulties to grow strain of higher than 4 g/L dry cell weight and to produce butanol higher than 20 g/L. Fermentation using high initial cell density has been reported to enhance butanol productivity. In addition, oleyl alcohol has been recognized for effective extraction of butanol because of its selectivity and biocompatibility with reduced the effect of toxicity. Butanol fermentation with high cell density and large extractant volume has not been reported and is expected to improve butanol production in a minimum medium volume setting. Clostridium saccharoperbutylacetonicum N1-4, C. beijerinckii NCIMB 8052 (8052), and C. acetobutylicum ATCC 824 (824) were used in this study. Three kinds of media, TYA, TY, and TY-CaCO3, were used in this conventional extractive fermentation. Then, in situ extractive fermentation with Ve/Vb ratios at 0.1, 0.5, 1.0, and 10 were used. Total butanol concentration was defined as the broth-based total butanol, which is the total amount of butanol produced in broth and extractant per the volume of broth. TYA medium yielded the highest total butanol concentrations at N1-4 (12 g/L), 8052 (11 g/L), and 824 (15 g/L), and the highest partition coefficient (3.7) among the three media with similar Ve/Vb ratio at 0.5. N1-4 yielded the highest increment of total butanol production (22 g/L) in the extractive fermentation with high cell density. Low butanol concentration of 0.8 g/L in the broth was maintained using the extractant at a broth volume ratio (Ve/Vb) much lower than 4.4 g/L with a ratio of 0.5. Ve/Vb ratio of 10 which provided 2-fold higher total butanol concentration (28 g/L) than that of 11 g/L obtained using a Ve/Vb ratio of 0.5. These results indicated that a larger volume of extractant to broth improved total butanol concentration by reducing butanol toxicity and led to high medium based butanol yield in fermentation using high cell density. ©2020. CBIORE-IJRED. All rights reserved

Enterococcus mundtii QU25, a non-dairy lactic acid bacterium of the phylum Firmicutes, is capable of simultaneously fermenting cellobiose and xylose, and is described as a promising strain for the industrial production of optically pure l-lactic acid (≥ 99.9%) via homo-fermentation of lignocellulosic hydrolysates. Generally, Firmicutes bacteria show preferential consumption of sugar (usually glucose), termed carbon catabolite repression (CCR), while hampering the catabolism of other sugars. In our previous study, QU25 exhibited apparent CCR in a glucose-xylose mixture phenotypically, and transcriptional repression of the xylose operon encoding initial xylose metabolism genes, likely occurred in a CcpA-dependent manner. QU25 did not exhibit CCR phenotypically in a cellobiose-xylose mixture. The aim of the current study is to elucidate the transcriptional change associated with the simultaneous utilization of cellobiose and xylose. To this end, we performed RNA-seq analysis in the exponential growth phase of E. mundtii QU25 cells grown in glucose, cellobiose, and/or xylose as either sole or co-carbon sources. Our transcriptomic data showed that the xylose operon was weakly repressed in cells grown in a cellobiose-xylose mixture compared with that in cells grown in a glucose-xylose mixture. Furthermore, the gene expression of talC, the sole gene encoding transaldolase, is expected to be repressed by CcpA-mediated CCR. QU25 metabolized xylose without using transaldolase, which is necessary for homolactic fermentation from pentoses using the pentose-phosphate pathway. Hence, the metabolism of xylose in the presence of cellobiose by QU25 may have been due to 1) sufficient amounts of proteins encoded by the xylose operon genes for xylose metabolism despite of the slight repression of the operon, and 2) bypassing of the pentose-phosphate pathway without the TalC activity. Accordingly, we have determined the targets of genetic modification in QU25 to metabolize cellobiose, xylose and glucose simultaneously for application of the lactic fermentation from lignocellulosic hydrolysates.

Product inhibition by butanol and acetone is a known drawback in acetone-butanol-ethanol (ABE) fermentation. Extractive fermentation improves butanol production by several ABE-producing Clostridium spp., but only low volume ratios ( 24 h, it produced a higher concentration of total butanol (i.e., butanol produced in all the phases per broth volume used) of 24.2 g/L-broth after 96 h compared with 14.4 g/L-broth at a Ve/Vb ratio of 1, resulting in a low butanol concentration in the aqueous phase. The use of cells immobilized with calcium alginate beads shortened the lag phase to

To improve butanol fermentation efficiencies, semi-hydrolysate with low enzyme loading using H2SO4 pretreated rice straw was designed, which preferably produced cellobiose with xylose (instead of glucose). Fermentation of semi-hydrolysates avoided carbon catabolite repression (CCR) and produced higher butanol yield to enzyme loading (0.0290 g U−1), a newly proposed parameter, than the conventional glucose-oriented hydrolysate (0.00197 g U−1). Further, overall butanol productivity was improved from 0.0628 g L−1 h−1 to 0.265 g L−1 h−1 during fermentation of undetoxified semi-hydrolysate by using high cell density. A novel simultaneously repeated hydrolysis and fermentation (SRHF) was constructed by recycling of enzymes and cells, which further improved butanol yield to enzyme loading by 183% and overall butanol productivity by 6.04%. Thus, semi-hydrolysate with SRHF is a smartly designed biomass for efficient butanol fermentation of lignocellulosic materials.

A novel actinobacterial strain, designated C12CA1T, was isolated from forest soil in the conservation area of Chulabhorn dam, Thailand, and its taxonomic position was determined by using a polyphasic approach. Strain C12CA1T contained meso-2,6 diaminopimelic acid in the cell-wall peptidoglycan, and arabinose and galactose as diagnostic sugars of the whole-cell hydrolysate. On the basis of morphological and chemotaxonomic characteristics, strain C12CA1T was classified in the genus Amycolatopsis . It contained MK-9(H4) as the predominant menaquinone, C16 : 0, iso-C15 : 0 and iso-C16 : 0 as the major cellular fatty acids, and several phospholipids consisting of diphosphotidylglycerol, phosphatidylglycerol, phosphatidylmethylethanolamine, phosphatidylethanolamine, phosphatidylserine, phosphatidylinositol, hydroxyphosphatidylethanolamine, phosphatidylinositol mannosides and an unidentified glucosamine-containing phospholipid. Based on 16S rRNA gene sequence and phylogenetic analyses, strain C12CA1T was closely related to Amycolatopsis vancoresmycina DSM 44592T (98.96 %) and Amycolatopsis pretoriensis JCM 12673T (98.82 %). The strain exhibited low DNA–DNA relatedness values with A. vancoresmycina DSM 44592T (6.9±0.2–11.6±1.9 %) and A. pretoriensis JCM 12673T (8.8±0.3–9.2±1.8 %). The DNA G+C content of strain C12CA1T was 69.8 mol%. Based on the results of polyphasic characterization, strain C12CA1T represents a novel species of the genus Amycolatopsis , for which the name Amycolatopsis silviterrae sp. nov. is proposed. The type strain is C12CA1T (=TBRC 1456T=NBRC 111116T).

Meta-l-lactic acid fermentation from non-treated kitchen refuse was reconstructed using a combination of isolated bacterial strains under several pH control strategies. The meta-fermentation system was successfully reconstructed using a combination of Weizmannia coagulans MN-07, Caldibacillus thermoamylovorans OM55-6, and Caldibacillus hisashii N-11 strains. Additionally, a simplified constant pH control strategy was employed, which decreased fermentation time and increased production. The optimum pH (6.5) for the reconstructed meta-fermentation was favorable for the respective pure cultures of the three selected strains. The l-lactic acid production performance of the reconstructed meta-fermentation system was as follows: concentration, 24.5 g/L; optical purity, 100%; productivity, 0.341 g/L/h; yield, 1.06 g/g. These results indicated that constant pH control was effective in the reconstructed meta-fermentation with the best performance of l-lactic acid production at pH optimal for the selected bacterial growth, while the switching from swing pH control would suppress the activities of unfavorable bacterial species in un-isolated meta-fermentation. This article is protected by copyright. All rights reserved.

Eucalyptus has been previously suggested as a potential substrate for acetone-butanol-ethanol (ABE) production without nutrient supplementation; however, incomplete sugar consumption has prevented improvement of ABE production. Cellulase loading with 35 FPU g−1 was first optimized in terms of high hydrolysis efficiency (95%). However, only 0.43 g L−1 ABE production and 3.44% glucose consumption rate were achieved. To improve ABE production from eucalyptus hydrolysate, supplementation of 6 nutrients in common tryptone-yeast extract (TY) medium were investigated by statistical approaches. Three nutrients including yeast extract, tryptone, and FeSO4·7H2O were screened as significant nutrients for ABE production. Subsequently, use of a modified TY medium (MTY medium: yeast extract 3.04 g L−1, tryptone 7.64 g L−1, FeSO4·7H2O 15.3 mg L−1), which was subsequently predicted by Plackett-Burman and Box-Behnken designs to stimulate ABE production, resulted in ca. 40-fold increase in ABE concentration (16.9 g L−1) and a glucose consumption rate of 100%. We first examined previously uninvestigated nutrition combinations using Plackett-Burman and Box-Behnken designs for high ABE production from eucalyptus hydrolysate. This study shows that statistical method would be a powerful tool for the optimization and enhancement of ABE production from eucalyptus hydrolysate.

In this study, Sophora flavescens residues (SFR) were used for l-lactic acid production and were mixed with food waste (FW) to assess the effects of different compositions of SFR and FW. Positive synergistic effects of mixed substrates were achieved with co-fermentation. Co-fermentation increased the proportion of l-lactic acid by decreasing the co-products of ethanol and other organic acids. A maximum l-lactic acid concentration of 48.4g/L and l-lactic acid conversion rate of 0.904g/g total sugar were obtained through co-fermentation of SFR and FW at the optimal ratio of 1:1.5. These results were approximately 6-fold those obtained during mono-fermentation of SFR. Co-fermentation of SFR and FW provides a suitable C/N ratio and pH for effective open fermentative production of l-lactic acid.

Enterococcus faecium QU 50, a novel thermophilic L-lactic acid (LA) producing strain, was used in this study to ferment sugar mixtures into LA. Under the optimal fermentation conditions (50 °C, pH 6.5), strain QU 50 could ferment both mixed glucose/xylose sugars with relaxed CCR and mixed cellobiose/xylose sugars simultaneously without CCR to produce homo L-LA. The activity of enzymes related to xylose metabolism was also investigated. In the cells grown in a medium containing cellobiose/xylose, the activity of xylose isomerase and xylulose kinase, was 3.22 and 1.91 times higher, respectively, as compared to that of cells grown in a glucose/xylose medium. Strain QU 50 produced 70.8 g L−1 of L-LA with a yield of 1.04 g g−1 and a productivity of 2.95 g L−1 h−1 from simulated energy cane hydrolysate in batch fermentation. Immobilisation of strain QU 50 improved the operational stability of open repeated fermentation (three cycles), resulting in 61.1–64.3 g L−1 of L-LA with a yield of 1.01–1.02 g g−1 and a productivity of 3.22–3.82 g L−1 h−1. Thus, an efficient and cost-effective fermentation system was successfully established for L-LA production from sugar mixtures.

To investigate the methanogenic pathway and microbial community in a mesophilic anaerobic digestion (AD) system with food waste (FW) ethanol pre-fermentation (EP), two semi-continuous AD systems were operated by feeding FW with (PSR) and without EP (control). In this study, δ13C-ethanol was supplemented as solo substrate for AD sludge when the reactors operation stabilized to analyze the methanogenic pathways. The results suggested that approximately 59.3% of methane was produced from acetotrophic methanogens, while 40.7% was formed by hydrogenotrophic methanogens in the PSR group. On the other hand, compared with control, methane produced via CO2 reduction pathway was increased by 4.70%. Meanwhile, the composition variations of the microbial community in AD supported the above conclusion, since the relative abundances of Clostridium and Methanobacterium were enhanced by 7.6% and 10.2%, respectively in PSR reactor. These results provided a theoretical basis for AD applications and biogas yield improvements with EP process.

We previously reported the metabolic analysis of butanol and acetone production from exogenous acetate by (13)C tracer experiments (Gao et al., RSC Adv., 5, 8486-8495, 2015). To clarify the influence of acetate on acetone-butanol-ethanol (ABE) production, we first performed an enzyme assay in Clostridium saccharoperbutylacetonicum N1-4. Acetate addition was found to drastically increase the activities of key enzymes involved in the acetate uptake (phosphate acetyltransferase and CoA transferase), acetone formation (acetoacetate decarboxylase), and butanol formation (butanol dehydrogenase) pathways. Subsequently, supplementation of acetate during acidogenesis and early solventogenesis resulted in a significant increase in ABE production. To establish an efficient ABE production system using acetate as a co-substrate, several shot strategies were investigated in batch culture. Batch cultures with two substrate shots without pH control produced 14.20 g/L butanol and 23.27 g/L ABE with a maximum specific butanol production rate of 0.26 g/(g h). Furthermore, pH-controlled (at pH 5.5) batch cultures with two substrate shots resulted in not only improved acetate consumption but also a further increase in ABE production. Finally, we obtained 15.13 g/L butanol and 24.37 g/L ABE at the high specific butanol production rate of 0.34 g/(g h) using pH-stat co-feeding method. Thus, in this study, we established a high ABE production system using glucose and acetate as co-substrates in a pH-stat co-feeding system with C. saccharoperbutylacetonicum N1-4.

A unique method to produce highly optically-active l-lactic acid and soil amendments that promote plant growth from food waste was proposed. Three Bacillus strains Bacillus subtilis KBKU21, B. subtilis N3-9 and Bacillus coagulans T27, were used. Strain KBKU21 accumulated 36.9 g/L l-lactic acid with 95.7% optical activity and 98.2% l-lactic acid selectivity when fermented at 43°C for 84 h in a model kitchen refuse (MKR) medium. Residual precipitate fraction (anaerobically-fermented MKR (AFM) compost) analysis revealed 4.60%, 0.70% and 0.75% of nitrogen (as N), phosphorous (as P2O5), and potassium (as K2O), respectively. Additionally, the carbon to nitrogen ratio decreased from 13.3 to 10.6. AFM compost with KBKU21 promoted plant growth parameters, including leaf length, plant height and fresh weight of Brassica rapa (Komatsuna), than that by chemical fertilizers or commercial compost. The concept provides an incentive for the complete recycling of food waste, contributing towards a sustainable production system.

Raw materials and pollution control are key issues for the ethanol fermentation industry. To address these concerns, food waste was selected as fermentation substrate, and stillage reflux was carried out in this study. Reflux was used seven times during fermentation. Corresponding ethanol and reducing sugar were detected. Accumulation of by-products, such as organic acid, sodium chloride, and glycerol, was investigated. Lactic acid was observed to accumulate up to 120g/L, and sodium chloride reached 0.14mol/L. Other by-products did not accumulate. The first five cycles of reflux increased ethanol concentration, which prolonged fermentation time. Further increases in reflux time negatively influenced ethanol fermentation. Single-factor analysis with lactic acid and sodium chloride demonstrated that both factors affected ethanol fermentation, but lactic acid induced more effects.

A few strains of lactic acid bacteria metabolise xylose into optically pure L-lactic acid (LA). This study achieved an effective homofermentative cell recycle continuous fermentation of xylose to L-LA with high concentration, productivity, and yield using Enterococcus mundtii QU 25. In conventional continuous fermentation, the optimal xylose concentration in the feeding solution is 50 g L−1, and the optimal dilution rate is 0.15 h−1. Continuous fermentation with cell recycling using a microfiltration membrane module produced an L-LA concentration of 32.3 g L−1 with a yield of 0.789 g g−1 and a productivity of 5.33 g L−1 h−1. Controlling pH and optimising the feeding medium were important for achieving a high L-LA yield with strain QU 25. Using corn steep liquor-containing medium at pH 6.2, a maximum L-LA concentration, yield and productivity were achieved at 41.0 g L−1, 1.01 g g−1, and 6.15 g L−1 h−1, respectively. This study is the first to report on continuous fermentation with cell recycling for lactic acid production from xylose.

Utilization of lignocellulosic biomasses for biobutanol fermentation usually requires costly processes of pretreatment and enzymatic hydrolysis. In this study, paper pulp (93.2% glucan) was used as a starting biomass material to produce biobutanol. We conducted enzymatic semi-hydrolysis of paper pulp without pretreatment and with low enzyme loading, which produced high concentrations of cellobiose (13.9 g L−1) and glucose (21.3 g L−1). In addition, efficient fermentation of the semi-hydrolysate was achieved similar to that with the use of commercial sugars without inhibitors. Finally, we designed a novel non-isothermal simultaneous saccharification and fermentation with in situ butanol recovery, which was composed of a repeated semi-hydrolysis process and successive butanol-extractive fermentation process under the respective optimal conditions. The consolidated system improved butanol production, butanol yields, and butanol productivities and enabled repeated use of medium when compared with other integrated hydrolysis and fermentation processes.

A unique autothermal thermophilic aerobic digestion (ATAD) process has been used to convert human excreta to liquid fertilizer in Japan. This study investigated the changes in physicochemical and bacterial community characteristics during the full-scale ATAD process operated for approximately 3 weeks in 2 different years. After initiating simultaneous aeration and mixing using an air-inducing circulator (aerator), the temperature autothermally increased rapidly in the first 1 to 2 days with exhaustive oxygen consumption, leading to a drastic decrease and gradual increase in oxidation-reduction potential in the first 2 days, reached >50°C in the middle 4 to 6 days, and remained steady in the final phase. Volatile fatty acids were rapidly consumed and diminished in the first 2 days, whereas the ammonia nitrogen concentration was relatively stable during the process, despite a gradual pH increase to 9.3. Principal-coordinate analysis of 16S rRNA gene amplicons using next-generation sequencing divided the bacterial community structures into distinct clusters corresponding to three phases, and they were similar in the final phase in both years despite different transitions in the middle phase. The predominant phyla (closest species, dominancy) in the initial, middle, and final phases were Proteobacteria ( Arcobacter trophiarum , 19 to 43%; Acinetobacter towneri , 6.3 to 30%), Bacteroidetes ( Moheibacter sediminis , 43 to 54%), and Firmicutes ( Thermaerobacter composti , 11 to 28%; Heliorestis baculata , 2.1 to 16%), respectively. Two predominant operational taxonomic units (OTUs) in the final phase showed very low similarities to the closest species, indicating that the process is unique compared with previously published ones. This unique process with three distinctive phases would be caused by the aerator with complete aeration. IMPORTANCE Although the autothermal thermophilic aerobic digestion (ATAD) process has several advantages, such as a high degradation capacity, a short treatment period, and inactivation of pathogens, one of the factors limiting its broad application is the high electric power consumption for aerators with a full-scale bioreactor. We elucidated the dynamics of the bacterial community structures, as well as the physicochemical characteristics, in the ATAD process with a full-scale bioreactor from human excreta for 3 weeks. Our results indicated that this unique process can be divided into three distinguishable phases by an aerator with complete aeration and showed a possibility of shortening the digestion period to approximately 10 days. This research not only helps to identify which bacteria play significant roles and how the process can be improved and controlled but also demonstrates an efficient ATAD process with less electric power consumption for worldwide application.

The economic feasibility of acetone–butanol–ethanol (ABE) fermentation is greatly affected by the type of raw material used. The easy availability of eucalyptus from marginal environments is an alternative feedstock for use as raw material to reduce the production cost. In this study, hydrolyzed eucalyptus was used for ABE production without any nutrients supplementation. Increasing the solid concentration in the eucalyptus slurry from 6.7% (w-dry matter/v) to 25% led to an increase in the initial glucose concentration from 33.7 g/L to 86.7 g/L after enzymatic hydrolysis. Dosed cellulases not only hydrolyzed cellulose but also supplied nitrogen source for ABE producing strain. However, ABE production from the obtained hydrolysate decreased when the solid concentration was increased to more than 10%. The maximum ABE of 12.3 g/L was obtained at 10% solid concentration, with an initial glucose concentration of approximately 40 g/L. In addition, the fermentation capability of eucalyptus hydrolysate was found to be improved by diluting the hydrolysate, which prevented inhibition by substrate and fermentation inhibitors. Finally, ABE concentration was improved to 13.1 g/L by diluting the hydrolysate from the initial solid concentration of 25% to an initial glucose concentration of 45 g/L, which resulted in ABE productivity of 0.109 g/L/h and ABE yield of 0.413 g/g. Thus, the high ABE production from eucalyptus makes it a potential feedstock for biofuel production.

The development and implementation of renewable materials for the production of versatile chemical resources have gained considerable attention recently, as this offers an alternative to the environmental problems caused by the petroleum industry and the limited supply of fossil resources. Therefore, the concept of utilizing biomass or wastes from agricultural and industrial residues to produce useful chemical products has been widely accepted. Lactic acid plays an important role due to its versatile application in the food, medical, and cosmetics industries and as a potential raw material for the manufacture of biodegradable plastics. Currently, the fermentative production of optically pure lactic acid has increased because of the prospects of environmental friendliness and cost-effectiveness. In order to produce lactic acid with high yield and optical purity, many studies focus on wild microorganisms and metabolically engineered strains. This article reviews the most recent advances in the biotechnological production of lactic acid mainly by lactic acid bacteria, and discusses the feasibility and potential of various processes.

During acetone–butanol–ethanol (ABE) fermentation by clostridia, acetate is reutilised for butanol production. In this study, we investigated the characteristics of ABE production from acetate and analysed the metabolism of exogenously added acetate by Clostridium saccharoperbutylacetonicum N1-4. Supplementation of 4 g L−1 exogenous acetate, to media containing glucose, increased not only concentrations of butanol (48.3%) and acetone (90.5%), but also the ratio of acetone to butanol (27.1%), which suggested that acetate addition altered the metabolic flux. Acetate could not be metabolised in the absence of glucose, thus glycolysis appeared to be necessary for acetate utilisation. In order to clarify the metabolism of exogenous acetate, 13C tracer experiments were performed by supplementing [1,2-13C2] acetate in a culture broth. Based on the results of gas chromatography-mass spectroscopy analysis, we first confirmed both butanol and acetone formation from acetate. Further, the acetate-to-butanol efficiency will significantly decrease when more acetate than 2–4 g L−1 is added to the fermentation, while acetate-to-acetone efficiency may remain high (up to a ratio of 2 mol acetate per 1 mol glucose fed). Moreover, the culture supplemented with acetate exhibited an increase in conversion efficiency of glucose to butanol and acetone, from 0.196% to 19.5% and from 0 to 7.64%, respectively, even during acidogenesis. Thus, we first revealed quantitatively that acetate addition induced solvent production during the early growth phase, and increased metabolic flux to acetone and butanol production from both acetate and glucose.

Butanol has recently attracted attention as an alternative biofuel because of its various advantages over other biofuels. Many researchers have focused on butanol fermentation with renewable and sustainable resources, especially lignocellulosic materials, which has provided significant progress in butanol fermentation. However, there are still some drawbacks in butanol fermentation in terms of low butanol concentration and productivity, high cost of feedstock and product inhibition, which makes butanol fermentation less competitive than the production of other biofuels. These hurdles are being resolved in several ways. Genetic engineering is now available for improving butanol yield and butanol ratio through overexpression, knock out/down, and insertion of genes encoding key enzymes in the metabolic pathway of butanol fermentation. In addition, there are also many strategies to improve fermentation technology, such as multi-stage continuous fermentation, continuous fermentation integrated with immobilization and cell recycling, and the inclusion of additional organic acids or electron carriers to change metabolic flux. This review focuses on the most recent advances in butanol fermentation especially from the perspectives of genetic engineering and fermentation technology.

Autothermal thermophilic aerobic digestion (ATAD) is conducted for stabilization of sludge waste and is driven by the action of various microorganisms under aerobic conditions. However, the mechanism controlling bacterial community changes during ATAD via three (initial, middle and final) phases is currently unclear. To investigate this mechanism, activity analysis and a microcosm assay with shaking were performed on a bacterial community during the initial, middle, and final phases of incubation. Cell lysis activities toward gram-negative bacteria, but not gram-positive bacteria, were detected in the ATAD samples in the middle and final phases. During shaking incubation in initial-phase samples at 30 °C, major operational taxonomic units (OTUs) related to Acinetobacter indicus and Arcobacter cibarius dramatically increased along with decreases in several major OTUs. In middle-phase samples at 45 °C, we observed a major alteration of OTUs related to Caldicellulosiruptor bescii and Aciditerrimonas ferrireducens, together with distinct decreases in several other OTUs. Final-phase samples maintained a stable bacterial community with major OTUs showing limited similarities to Heliorestis baculata, Caldicellulosiruptor bescii, and Ornatilinea apprima. In conclusion, the changes in the bacterial community observed during ATAD could be partially attributed to the cell lysis activity toward gram-negative bacteria in the middle and final phases. The microcosm assay suggested that certain physical factors, such as a high oxygen supply and shearing forces, also might contribute to bacterial community changes in the initial and middle phases, and to the stable bacterial community in the final phase of ATAD.

Production of optically pure lactic acid from lignocellulosic material for commercial purposes is hampered by several difficulties, including heterofermentation of pentose sugars and high energy consumption by mesophilic lactic acid bacteria. Here, we report a novel lactic acid bacterium, strain QU 50, that has the potential to produce optically pure l-lactic acid (≥99.2%) in a homofermentative manner from xylose under thermophilic conditions. Strain QU 50 was isolated from Egyptian fertile soil and identified as Enterococcus faecium QU 50 by analyzing its sugar fermentation pattern and 16S rRNA gene sequence. Enterococcus faecium QU 50 fermented xylose efficiently to produce lactic acid over wide pH (6.0-10.0) and temperature ranges (30-52°C), with a pH of 6.5 and temperature of 50°C being optimal. To our knowledge, this is the first report of homofermentative lactic acid production from xylose by a thermophilic lactic acid bacterium.

DNA typing from forensic evidence is commonly used to identify individuals. However, when the quantity of the forensic evidence is insufficient, successful identification using DNA typing is impossible. Such evidence may also contain DNA from bacteria that occur naturally on the skin. In this study, we aimed to establish a profiling method using terminal restriction fragment length polymorphisms (T-RFLPs) of the amplified bacterial 16S ribosomal RNA (rRNA) gene. First, the extraction and digestion processes were investigated, and the T-RFLP profiling method using the 16S rRNA gene amplicon was optimized. We then used this method to compare the profiles of bacterial flora from the hands of 12 different individuals. We found that the T-RFLP profiles from one person on different days displayed higher similarity than those between individuals. In a principal component analysis (PCA), T-RFLPs from each individual were closely clustered in 11 out of 12 cases. The clusters could be distinguished from each other, even when the samples were collected from different conditions. No major change of the profile was observed after six months except in two cases. When handprints on glass plates were compared, 11 of 12 individuals were assigned to a few clusters including the cluster corresponding to the correct individual. In conclusion, a method for reproducible T-RFLP profiling of bacteria from trace amounts of handprints was established. The profiles were obtained for particular individuals clustered in PCA and were experimentally separable from other individuals in most cases. This technique could provide useful information for narrowing down a suspect in a criminal investigation.

A novel Gram-staining-positive, endospore-forming, rod-shaped, facultatively anaerobic, thermotolerant bacterium, designated strain MO-04T, was isolated from a marine animal resources (MAR) compost. The 16S rRNA gene sequence of strain MO-04T showed 99.4 % similarity with Bacillus thermolactis R-6488T, 94.1 % similarity with Bacillus thermoamylovorans CNCM I-1378T, 93.3 % similarity with Bacillus humi LMG 22167T, 93.2 % similarity with Bacillus niacini IFO 15566T and the similarities with other species were less than 93 %. DNA–DNA relatedness between strain MO-04T and B. thermolactis DSM 23332T was 45 %. The DNA G+C content of strain MO-04T was 33.4 mol%, comparatively lower than that of B. thermolactis R-6488T (35.0 mol%). Strain MO-04T grew at 35–61 °C (optimum 50 °C), pH 4.5–9.0 (optimum pH 7.2) and tolerated up to 8.0 % (w/v) NaCl (optimum 2 %). The MO-04T cell wall peptidoglycan type was meso-2,6-diaminopimelic acid, and the major fatty acids were C16 : 1, C14 : 1, C17 : 0 and C17 : 1. The major polar lipids were represented by diphosphatidylglycerol and phosphatidylglycerol and two unidentified phospholipids. The analysed polyphasic data presented here clearly indicate that the isolate MO-04T is considered to represent a novel species within the genus Bacillus for which the name Bacillus kokeshiiformis sp. nov. is proposed. The type strain of B. kokeshiiformis is MO-04T ( = JCM 19325T = KCTC 33163T).

By applying non-sterile open fermentation of food waste, various thermotolerant l -lactic acid-producing bacteria were isolated and identified. The predominant bacterial isolates showing higher accumulation of l -lactic acid belong to 3 groups of Bacillus coagulans , according to their 16S rRNA gene sequence similarities. B. coagulans strains M21 and M36 produced high amounts of l -lactic acid of high optical purity and lactic acid selectivity in model kitchen refuse medium and glucose–yeast extract–peptone medium. Other thermotolerant isolates resembling to Bacillus humi , B. ruris , B. subtilis , B. niacini and B. soli were also identified. These bacteria produced low amounts of l -lactic acid of more than 99% optical purity. All isolated strains showed the highest growth rate at temperatures around 55–60°C. They showed unique responses to various oxygen supply conditions. The majority of isolates produced l -lactic acid at a low overall oxygen transfer coefficient ( K L a ); however, acetic acid was produced instead of l -lactic acid at a high K L a . B . coagulans M21 was the only strain that produced high, consistent, and reproducible amounts of optically pure l -lactic acid (>99% optical purity) under high and low K L a conditions in a homo-fermentative manner.

The use of lignocellulosic biomass for the production of optically pure lactic acid remains challenging because it requires efficient utilisation of mixed sugars without carbon catabolite repression (CCR). Enterococcus mundtii QU 25, a novel L-lactic acid-producing strain, was used in this study to ferment mixed sugars. This strain exhibited apparent CCR in a glucose–xylose mixture; however, replacement of glucose by cellobiose (cellobiose–xylose mixture) led to simultaneous consumption of both sugars without CCR. The production of lactic acid and activity of enzymes related to xylose metabolism were also investigated. Xylose isomerase and xylulokinase specific activity in cellobiose–xylose grown cells was three times higher than that in glucose–xylose grown cells. The addition of yeast extract and ammonium hydroxide effectively improved sugar utilisation and cell growth. Under the optimal conditions with simulated lignocellulosic hydrolysates, a high L-lactic acid concentration (up to 163 g L−1) was produced with a yield of 0.870 g g−1 and maximum productivity of 7.21 g L−1 h−1 without CCR in the fed-batch fermentation. Thus, we could establish rapid and simultaneous consumption of hexose and pentose sugars by using a lactic acid bacterium strain, which significantly increased production of high-purity L-lactic acid.

Factors including the composition and concentration of mixed sugars, dilution rate, and cell density were examined to establish a highly efficient continuous acetone–butanol–ethanol (ABE) fermentation process using Clostridium saccharoperbutylacetonicum N1-4. During conventional continuous fermentation at a dilution rate of 0.15 h−1, the glucose/xylose mixture exhibited carbon catabolite repression (CCR) with an intensity (I CCR) of 4.18 and ABE productivity of 0.574 g L−1 h−1, while CCR was alleviated using a cellobiose/xylose mixture with 1.60 and 1.09 g L−1 h−1, respectively. Continuous fermentation with high cell density by cell recycling improved ABE productivity to 5.32 g L−1 h−1 at a dilution rate of 0.686 h−1 owing to high xylose consumption. Further optimization of mixed sugar concentration in the feeding medium induced the best performance of continuous ABE production from mixed sugar without CCR, relative to previous studies: ABE productivity, 6.09 g L−1 h−1; ABE yield, 0.214 g g−1; I CCR, 1.48.

Acetone-butanol-ethanol (ABE) fermentation using Clostridium saccharoperbutylacetonicum N1-4 and mixed sugars containing cellobiose and xylose was studied to establish efficient butanol production process without carbon catabolite repression (CCR). Although batch culture with glucose and xylose exhibited apparent CCR, we achieved simultaneous consumption of cellobiose and xylose. Moreover, preculture of the N1-4 strain with xylose yielded maximum butanol and solvent concentrations (16 and 23 g/L, respectively). Thus, we succeeded in ABE fermentation with mixed sugars of hexose and pentose, without CCR, by using wild-type ABE-producing clostridia. We also investigated the effect of various ratios of cellobiose and xylose on the fermentation process and yield. Increasing initial xylose concentration improved butanol and solvent concentrations and maximum xylose consumption rate. Fed-batch culture with cellobiose and xylose showed rapid and simultaneous sugar consumption and improved maximum consumption rate of both sugars.

We investigated l -lactic acid production in static batch fermentation of kitchen refuse using a bacterial consortium from marine-animal-resource (MAR) composts at temperatures ranging from 30 to 65 °C. At relatively low temperatures butyric acid accumulated, whereas at higher temperatures l -lactic acid was produced. In particular, fermentation at 50 °C produced 34.5 g L−1 l -lactic acid with 90% lactic acid selectivity and 100% optical purity. Denaturing gradient gel electrophoresis indicated that dominant bacteria present in the original MAR composts diminished rapidly and Bacillus coagulans strains became the dominant contributors to l -lactic acid production at 45, 50 and 55 °C. This is the first report of the achievement of 100% optical purity of l -lactic acid using a bacterial consortium.

Presently, several researchers are increasingly focusing on producing butanol as the next-generation fuel by acetone-butanol-ethanol (ABE) fermentation. Butanol has many superior characteristics compared to other biofuels, such as ethanol. However, its production by ABE fermentation faces the challenges of low productivity and yield because of product inhibition and heterofermentation, respectively, and thereby, high costs. Until date, molecular biological techniques and fermentation engineering methods have been applied for high butanol production. Although glucose remains the substrate of choice since traditional research, it is now necessary to substitute glucose derived from edible starch to other substrates from low-cost feedstock, such as agricultural residue. In addition, ABE-producing clostridia cannot directly produce butanol from lignocelluloses. Therefore, recent research is focusing on pretreatment and enzymatic saccharification of the complex molecules derived from agricultural residue for use as feedstock in butanol production. This article reviews traditional research, including the metabolism patterns and characteristics of ABE-producing clostridia. Furthermore, this article describes developments in ABE fermentation with respect to the establishment of highly efficient butanol production processes, such as batch, fed-batch, and continuous cultures, with the introduction of butanol removal, as well as butanol production from lignocellulosic biomasses or alternative substrates to sugars.

A continuous butanol production system with high-density Clostridium saccharoperbutylacetonicum N1-4 generated by cell recycling was established to examine the characteristics of butanol fermentation from xylose. In continuous culture without cell recycling, cell washout was avoided by maintaining pH>5.6 at a dilution rate of 0.26 h(-1), indicating pH control was critical to this experiment. Subsequently, continuous culture with cell recycling increased cell concentration to 17.4 g L(-1), which increased butanol productivity to 1.20 g L(-1) h(-1) at a dilution rate of 0.26 h(-1) from 0.529 g L(-1) h(-1) without cell recycling. The effect of dilution rates on butanol production was also investigated in continuous culture with cell recycling. Maximum butanol productivity (3.32 g L(-1) h(-1)) was observed at a dilution rate of 0.78 h(-1), approximately 6-fold higher than observed in continuous culture without cell recycling (0.529 g L(-1) h(-1)).

Uninvestigated control factors of meta-fermentation, the fermentative production of pure chemicals and fuels in a mixed culture system, were examined for production of optically pure l-lactic acid (LA) from food waste. In meta-fermentations by pH swing control, l-LA production with 100% optical purity (OPl-LA) was achieved even using unsterilized model kitchen refuse medium with preferential proliferation of l-LA-producing Bacillus coagulans, a minor member in the seed, whereas agitation decreased OPl-LA drastically. pH constant control shortened the fermentation time but decreased OPl-LA and LA selectivity (SLA) by stimulating growth of heterofermentative Bacillus thermoamylovorans. Deliberately switching from pH swing control to constant control exhibited the best performance for l-LA production: maximum accumulation, 39.2gL(-1); OPl-LA, 100%; SLA, 96.6%; productivity, 1.09gL(-1)h(-1). These results present a novel pH control strategy for efficient l-LA production in meta-fermentation based on a concept different from that of pure culture systems.

We previously reported a butanol production process with pH-stat continuous feeding of dl-lactic acid and glucose as the co-substrate (Oshiro et al., Appl. Microbiol. Biotechnol., 87, 1177-1185, 2010). To accomplish butanol production from completely inedible substrates, in this study, we investigated acetone-butanol-ethanol (ABE) fermentation of Clostridium saccharoperbutylacetonicum N1-4 with lactic acid by using pentose as the co-substrate. Examination for optimum co-substrate indicated that arabinose was superior to glucose and xylose for ABE fermentation. Actually batch culture with lactic acid and arabinose without pH control exhibited higher butanol production (7.11 g/l) and lactic acid consumption (2.02 g) than those (6.62 g/l and 1.45 g, respectively) with glucose. Fed-batch culture without pH control increased these values to 12.08 g/l and 15.60 g/l butanol production, and to 3.83 g and 5.91 g lactic acid consumption by feeding the substrate once and twice, respectively. Finally, the result of gas chromatography-mass spectroscopy analysis using [1,2,3-(13)C(3)]-lactic acid indicated that lactic acid was converted to butanol with the efficiency of 51.9%. Thus, we established a novel high butanol production from lactic acid using arabinose as the co-substrate in simple fed-batch culture.

A polytetrafluoroethylene (PTFE) membrane was used in membrane-assisted extractive (MAE) fermentation of acetone–butanol–ethanol (ABE) by Clostridium saccharoperbutylacetonicum N1-4. The growth inhibition effects of 1-dodecanol, which has a high partition coefficient for butanol, can be prevented by employing 1-dodecanol as an extractant when using a PTFE membrane. Compared to conventional fermentation, MAE–ABE fermentation with 1-dodecanol decreased butanol inhibition and increased glucose consumption from 59.4 to 86.0 g/L, and total butanol production increased from 16.0 to 20.1 g/L. The maximum butanol production rate increased from 0.817 to 0.979 g/L/h. The butanol productivity per membrane area was remarkably high with this system, i.e., 78.6 g/L/h/m2. Therefore, it is expected that this MAE fermentation system can achieve footprint downsizing.

Living cells are alive and have the butanol-producing ability but not much proliferation under nitrogen source-limited condition. We investigated various butanol production systems with high density of living cells of Clostridium saccharoperbutylacetonicum N1-4 supplemented with methyl viologen (MV) as an electron carrier and nutrient dosing for activity regeneration. In continuous butanol production with high density of living cells, butanol yield was drastically increased from 0.365 C-mol/C-mol with growing cells to 0.528 C-mol/C-mol at a dilution rate of 0.85 h −1 , being increased with the butanol to total solvent ratio. This yield was increased to 0.591 C-mol/C-mol by adding 0.01 mM MV. MV addition increased not only butanol yield but also butanol concentration and productivity as compared to those without MV addition. However, living cells lost their activity with incubation time, which lowered the operational stability of the system. Therefore, to maintain constant stability, activity regeneration was carried out with high density of living cells and MV. This system produced butanol at high concentration (9.40 g l −1 ) and productivity (7.99 g l −1 h −1 ) for approximately 100 h with maintenance of considerably high yield of butanol (0.686 C-mol/C-mol). Thus, we established a high-speed and highly efficient butanol production system.

In order to achieve high butanol production by Clostridium saccharoperbutylacetonicum N1-4, the effect of lactic acid on acetone-butanol-ethanol fermentation and several fed-batch cultures in which lactic acid is fed have been investigated. When a medium containing 20 g/l glucose was supplemented with 5 g/l of closely racemic lactic acid, both the concentration and yield of butanol increased; however, supplementation with more than 10 g/l lactic acid did not increase the butanol concentration. It was found that when fed a mixture of lactic acid and glucose, the final concentration of butanol produced by a fed-batch culture was greater than that produced by a batch culture. In addition, a pH-controlled fed-batch culture resulted in not only acceleration of lactic acid consumption but also a further increase in butanol production. Finally, we obtained 15.5 g/l butanol at a production rate of 1.76 g/l/h using a fed-batch culture with a pH-stat continuous lactic acid and glucose feeding method. To confirm whether lactic acid was converted to butanol by the N1-4 strain, we performed gas chromatography-mass spectroscopy (GC-MS) analysis of butanol produced by a batch culture during fermentation in a medium containing [1,2,3-(13)C(3)] lactic acid as the initial substrate. The results of the GC-MS analysis confirmed the bioconversion of lactic acid to butanol.

A kinetic simulation model of acetone–butanol–ethanol (ABE) fermentation of xylose (ModelXYL) was proposed by substituting Embden–Meyerhof–Parnas (EMP) pathway equations in the glucose model (ModelGLC) by pentose phosphate (PP) pathway equations of xylose utilization. We estimated the equation parameters of the PP pathway and set other equation parameters to the same as those in ModelGLC, by which ModelXYL exhibited an r2 value of 0.901 between the experimental time course of metabolites with initial xylose concentrations ranging from 40.7 to 292 mM and the calculated values. The results with the developed model suggested that Clostridium saccharoperbutylacetonicum N1-4 has a robust metabolic network in acid- and solvent-producing pathways. Furthermore, sensitivity analysis revealed that slow substrate utilization would be effective for higher butanol production; this coincided with the experimental results. Therefore, we consider the proposed model to be one of the best kinetic simulation candidates describing the dynamic metabolite behavior in ABE production.

Members of the genus Bacillus are considered to be both, among the best studied and most commonly used bacteria as well as the most still unexplored and the most wide-applicable potent bacteria because novel Bacillus strains are continuously being isolated and used in various areas. Production of optically pure l-lactic acid (l-LA), a feedstock for bioplastic synthesis, from renewable resources has recently attracted attention as a valuable application of Bacillus strains. l-LA fermentation by other producers, including lactic acid bacteria and Rhizopus strains (fungi) has already been addressed in several reviews. However, despite the advantages of l-LA fermentation by Bacillus strains, including its high growth rate, utilization of various carbon sources, tolerance to high temperature, and growth in simple nutritional conditions, it has not been reviewed. This review article discusses new findings on LA-producing Bacillus strains and compares them to other producers. The future prospects for LA-producing Bacillus strains are also discussed.

Glycerol is a by-product in the biodiesel production process and considered as one of the prospective carbon sources for microbial fermentation including lactic acid fermentation, which has received considerable interest due to its potential application. Enterococcus faecalis isolated in our laboratory produced optically pure L-lactic acid from glycerol in the presence of acetic acid. Gas chromatography-mass spectrometry analysis using [1, 2-(13)C2] acetic acid proved that the E. faecalis strain QU 11 was capable of converting acetic acid to ethanol during lactic acid fermentation of glycerol. This indicated that strain QU 11 restored the redox balance by oxidizing excess NADH though acetic acid metabolism, during ethanol production, which resulted in lactic acid production from glycerol. The effects of pH control and substrate concentration on lactic acid fermentation were also investigated. Glycerol and acetic acid concentrations of 30 g/L and 10 g/L, respectively, were expected to be appropriate for lactic acid fermentation of glycerol by strain QU 11 at a pH of 6.5. Furthermore, fed-batch fermentation with 30 g/L glycerol and 10 g/L acetic acid wholly exhibited the best performance including lactic acid production (55.3 g/L), lactic acid yield (0.991 mol-lactic acid/mol-glycerol), total yield [1.08 mol-(lactic acid and ethanol)]/mol-(glycerol and acetic acid)], and total carbon yield [1.06 C-mol-(lactic acid and ethanol)/C-mol-(glycerol and acetic acid)] of lactic acid and ethanol. In summary, the strain QU 11 successfully produced lactic acid from glycerol with acetic acid metabolism, and an efficient fermentation system was established without carbon loss.

A newly isolated Bacillus sp. MC-07 showed 99.2 % 16S rRNA gene sequence similarity with the Bacillus thermoamylovorans LMG 18084(T). It demonstrated optimum and maximum growth temperatures of 50 and 62 °C, respectively. The ability of MC-07 to produce optically pure L-lactic acid via direct fermentation of starch without enzymatic hydrolysis was investigated at different pH values (6.0-8.0) by intermittent adjustments every 12 h. During batch fermentation in mineral salt medium containing 0.001 % yeast extract at pH 7.0, 20 g/L of soluble starch was utilized to produce 16.6 g/L L-lactic acid at 50 °C within 24 h of fermentation, with 100 % optical purity, 92.1 % lactic acid selectivity, and an L-lactic acid yield of 0.977 g/g. Direct starch fermentation at pHs 6.0, 6.5, 7.5, and 8.0 resulted in considerably lower concentrations of lactic acid than did at pH 7.0. Compared with B. thermoamylovorans LMG 18084(T), the ability of strain MC-07 to produce L-lactic acid was superior.

There has been tremendous growth in the production of optically pure l-lactic acid from lignocellulose-derived sugars. In this study, Enterococcus mundtii QU 25 was used to ferment a glucose/xylose mixture to l-lactic acid. Maintenance of the xylose concentration at greater than 10 g/L achieved homo-lactic acid fermentation and reduced the formation of byproducts. Furthermore, carbon catabolite repression (CCR) was avoided by maintaining the glucose concentration below 25 g/L; therefore, initial concentrations of 25 g/L glucose and 50 g/L xylose were selected. Supplementation with 5 g/L yeast extract enhanced the maximum xylose consumption rate and consequently increased lactic acid production and productivity. Finally, a 129 g/L lactic acid without byproducts was obtained with a maximum lactic acid productivity of 5.60 g/(L·h) in fed-batch fermentation with feeding a glucose/xylose mixture using ammonium hydroxide as the neutralizing agent. These results indicate a potential for lactic acid production from glucose and xylose as the main components of lignocellulosic biomasses.

Fermentative production of optically pure lactic acid has roused interest among researchers in recent years due to its high potential for applications in a wide range of fields. More specifically, the sharp increase in manufacturing of biodegradable polylactic acid (PLA) materials, green alternatives to petroleum-derived plastics, has significantly increased the global interest in lactic acid production. However, higher production costs have hindered the large-scale application of PLA because of the high price of lactic acid. Therefore, reduction of lactic acid production cost through utilization of inexpensive substrates and improvement of lactic acid production and productivity has become an important goal. Various methods have been employed for enhanced lactic acid production, including several bioprocess techniques facilitated by wild-type and/or engineered microbes. In this review, we will discuss lactic acid producers with relation to their fermentation characteristics and metabolism. Inexpensive fermentative substrates, such as dairy products, food and agro-industrial wastes, glycerol, and algal biomass alternatives to costly pure sugars and food crops are introduced. The operational modes and fermentation methods that have been recently reported to improve lactic acid production in terms of concentrations, yields, and productivities are summarized and compared. High cell density fermentation through immobilization and cell-recycling techniques are also addressed. Finally, advances in recovery processes and concluding remarks on the future outlook of lactic acid production are presented.

Lactic acid is an industrially important product with a large and rapidly expanding market due to its attractive and valuable multi-function properties. The economics of lactic acid production by fermentation is dependent on many factors, of which the cost of the raw materials is very significant. It is very expensive when sugars, e.g., glucose, sucrose, starch, etc., are used as the feedstock for lactic acid production. Therefore, lignocellulosic biomass is a promising feedstock for lactic acid production considering its great availability, sustainability, and low cost compared to refined sugars. Despite these advantages, the commercial use of lignocellulose for lactic acid production is still problematic. This review describes the "conventional" processes for producing lactic acid from lignocellulosic materials with lactic acid bacteria. These processes include: pretreatment of the biomass, enzyme hydrolysis to obtain fermentable sugars, fermentation technologies, and separation and purification of lactic acid. In addition, the difficulties associated with using this biomass for lactic acid production are especially introduced and several key properties that should be targeted for low-cost and advanced fermentation processes are pointed out. We also discuss the metabolism of lignocellulose-derived sugars by lactic acid bacteria.

Enterococcus mundtii QU 25, a newly isolated lactic acid bacterium, efficiently metabolized xylose into l -lactate. In batch fermentations, the strain produced 964 mM l -(+)-lactate from 691 mM xylose, with a yield of 1.41 mol/mol xylose consumed and an extremely high optical purity of ≥99.9% without acetate production.