Cyclic L-lactide synthesis from lignocellulose biomass by biorefining with complete inhibitor removal and highly simultaneous sugars assimilation

pH shifting adaptive evolution stimulates the low pH tolerance of Pediococcus acidilactici and high L-lactic acid fermentation efficiency

L-lactic acid fermentation at low pH reduces the use of neutralizers during fermentation and the generation of solid wastes in purification processes. Most lactic acid bacteria exhibit weak tolerance and poor cell viability at low pH. This study proposes a pH shifting adaptive evolution method to improve the low-pH tolerance of an engineered Pediococcus acidilactici strain. In the first stage, cells were cultured at a moderate pH to maintain the cell viability, then shifted to a low pH to enhance low-pH tolerance. Long-term pH shifting evolution culture of the engineered P. acidilactici between the moderate and low pH resulted in a 43 % increase in L-lactic acid production at pH 4.6 (110.4 g/L) and a 2.1-fold increase at pH 4.4 (80.7 g/L) compared to the parental strain when using wheat straw as a feedstock. This pH-shifting adaptive evolution strategy provides an effective tool for improving the low-pH tolerance of lactic acid bacteria.

Review on synthesis of lactic acid and lactates from biomass derived carbohydrates via heterogeneous catalysis

The utilization of renewable lignocellulosic biomass resources is a promising solution to deal with the deficit of fossil resources and the associated environmental concerns. Among diverse biomass-derived products, lactic acid (LA) stands out as one of the most successful commodities and also a platform to connect raw biomass feedstocks with value-added chemicals and degradable polymers. Herein, we critically review the recent advances in the design and development of base, acid, and multifunctional catalytic systems for the conversion of different carbohydrates to LA and alkyl lactates via chemical routes. In addition to critically evaluating the advantages and disadvantages of different catalytic systems, we provide deep insights into the reaction mechanisms, including the reaction pathways of different feedstocks, the catalytic roles of different kinds of active sites, and the structure-activity relationship. Besides, we also highlight critical progress for the further upgrading of LA and alkyl lactates to important commodity products. We conclude with our perspective on the key challenges and future opportunities.

Balanced water and heat energy recycling by full evaporation of wastewater (FEW) in dry biorefining processes of lignocellulose biomass

Lignocellulosic biorefinery technology requires minimum energy consumption and wastewater generation to overcome challenges in industrial applications. This study established a rigorous model and a comprehensive physical property database of dry biorefining process on Aspen Plus platform for production including L-lactic acid, citric acid, sodium sugar acids, amino acid, and ethanol based on the experimental data. Full evaporation of wastewater (FEW) approach was proposed to completely replaced the external steam supply, and significantly reduced the freshwater input by 67% ∼ 85% and wastewater generation by 64% ∼ 89%, depending on the specific products. The carbon-neutral heat energy from lignin residue combustion generates an extra heat output of 1.098 ∼ 4.772 GJ per ton of dry wheat straw (DW) after all the heat energy needs of the biorefinery process and FEW treatments are satisfied, equivalent to a reduction of 0.219 ∼ 0.952 kg CO2 eq/kg DM emission. This study provided a self-consistent solution for water and energy balance in biorefinery processes.

Research advances on the consolidated bioprocessing of lignocellulosic biomass

Lignocellulosic biomass is an abundant and renewable bioresource for the production of biofuels and biochemical products. The classical biorefinery process for lignocellulosic degradation and conversion comprises three stages, i.e., pretreatment, enzymatic saccharification, and fermentation. However, the complicated pretreatment process, high cost of cellulase production, and insufficient production performance of fermentation strains have restricted the industrialization of biorefinery. Consolidated bioprocessing (CBP) technology combines the process of enzyme production, enzymatic saccharification, and fermentation in a single bioreactor using a specific microorganism or a consortium of microbes and represents another approach worth exploring for the production of chemicals from lignocellulosic biomass. The present review summarizes the progress made in research of CBP technology for lignocellulosic biomass conversion. In this review, different CBP strategies in lignocellulose biorefinery are reviewed, including CBP with natural lignocellulose-degrading microorganisms as the chassis, CBP with biosynthetic microorganisms as the chassis, and CBP with microbial co-culturing systems. This review provides new perspectives and insights on the utilization of low-cost feedstock lignocellulosic biomass for production of biochemicals.

Competition between biodetoxification fungus and lactic acid bacterium in the biorefinery processing chain for production of cellulosic L-lactic acid

Biodetoxification fungus selectively degrades toxic inhibitors generated from pretreatment of lignocellulose without consuming fermentable sugars. However, one barrier for practical application is the sustained cell viability in the consequent fermentation step to compete the fermentable sugars with fermenting strains, resulting in sugar loss and reduced target product yield. This study investigated the competitive growth property between the biodetoxification fungus Paecilomyces variotii FN89 and the L-lactic acid bacterium Pediococcus acidilactici ZY271 under varying temperature and lactic acid osmatic stress. The results show that the L-lactic acid bacterium Ped. acidilactici ZY271 showed less thermotolerance to Pae. variotii FN89 at high temperature of 45 °C to 50 °C in both synthetic medium and wheat straw hydrolysate. In the higher temperature environment, the growth of the biodetoxification strian failed to compete with the lactic acid fermentation strain and was quickly eliminated from the fermentation system. The high temperature fermentation facilitated a fast transition from the detoxification stage to the fermentation stage for higher production of L-lactic acid.

Third-generation D-lactic acid production using red macroalgae Gelidium amansii by co-fermentation of galactose, glucose and xylose

Macroalgae biomass has been considered as a promising renewable feedstock for lactic acid production owing to its lignin-free, high carbohydrate content and high productivity. Herein, the D-lactic acid production from red macroalgae Gelidium amansii by Pediococcus acidilactici was investigated. The fermentable sugars in G. amansii acid-prehydrolysate were mainly galactose and glucose with a small amounts of xylose. P. acidilactici could simultaneously ferment the mixed sugars of galactose, glucose and xylose into D-lactic acid at high yield (0.90 g/g), without carbon catabolite repression (CCR). The assimilating pathways of these sugars in P. acidilactici were proposed based on the whole genome sequences. Simultaneous saccharification and co-fermentation (SSCF) of the pretreated and biodetoxified G. amansii was also conducted, a record high of D-lactic acid (41.4 g/L) from macroalgae biomass with the yield of 0.34 g/g dry feedstock was achieved. This study provided an important biorefinery strain for D-lactic acid production from macroalgae biomass.

Recycling of waste calcium carbonate in lignocellulosic biorefining chain for chiral lactic acid production

One of the major end-products of lignocellulosic biorefining chain is the solid residues containing various compounds. The present approach to solid residues treatment is combustion for generation of heat and electricity. This study investigated the potential for recycling of the combustion ash from the solid residues after lignocellulosic dry biorefining process. A range of characterizations showed that the combustion ash contained a high amount of calcium carbonate. By recycling the ash as the neutralizer in biorefining process, the waste calcium carbonate in the ash was efficiently utilized for pretreated biomass neutralization and can replace 40 % of calcium hydroxide for lactic acid production. The chiral L-lactic acid titer reached 102.4 ± 3.6 g/L from 20 % (w/w) solids loading of wheat straw. Three feasible strategies of ash recycling for the investigated biorefinery concept were further proposed base on the rigorous calcium mass calculation, which can efficiently reduce the consumption of neutralizers.

The chromatin remodeler Ino80 regulates yeast stress tolerance and cell metabolism through modulating nitrogen catabolite repression

Chromatin remodelers are important in maintaining the dynamic chromatin state in eukaryotic cells, which is essential for epigenetic regulation. Among the remodelers, the multi-subunits complex INO80 plays crucial roles in transcriptional regulation. However, current knowledge of chromatin regulation of the core subunit Ino80 on stress adaptation remains mysterious. Here we revealed that overexpressing the chromatin remodeler Ino80 elevated tolerance to multiple stresses in budding yeast Saccharomyces cerevisiae. Analyses of differential chromatin accessibility and global transcription levels revealed an enrichment of genes involved in NCR (nitrogen catabolite repression) under acetic acid stress. We demonstrated that Ino80 overexpression reduced the histone H3 occupancy in the promoter region of the glutamate dehydrogenase gene GDH2 and the allantoinase gene DAL1. Consistently, the decreased occupancy of nucleosome was revealed in the Ino80-inactivation mutant. Further analyses showed that Ino80 was recruited to the specific DNA locus in the promoter region of GDH2. Consistently, Ino80 overexpression facilitated the utilization of non-preferred nitrogen source to enhance ethanol yield under prolonged acetic acid stress. These results demonstrate that Ino80 plays a crucial role in coordinating carbon and nitrogen metabolism during stress adaptation.

Upgrading dry acid pretreatment by post-hydrolysis for carbon efficient conversion of lignocellulose

Dry acid pretreatment (DAP) as a promising process for industrial biorefinery provide an efficient bioconversion of cellulose without free wastewater, although the partial xylan and lignin degrade to inhibitors or recondense. A biorefinery strategy for carbon efficient conversion of lignocellulose into bioethanol, xylose, and reactive lignin was developed by upgrading DAP with post-hydrolysis. The results showed that lignocellulose after mild DAP (175 °C, acid dosage of 15 mg/g dry material) obtained higher xylan recovery and lower inhibitors than that of general DAP. Subsequently, post-hydrolysis, simultaneous saccharification and ethanol fermentation were performed at solids loading of 20 wt% without detoxification and sterilization, resulting in xylose and ethanol yield of 71.8 % and 67.6 %. The fractionated lignin presented more reactive β-aryl ether linkages and less condensation than that from DAP. 66 % of lignocellulose carbon was recovered as ethanol, xylose and reactive lignin. This upgrading biorefinery strategy provided an easy-to-operate process for integrated utilization of lignocellulose.

Detection and elimination of trace d-lactic acid in lignocellulose biorefining chain: Generation, flow, and impact on chiral lactide synthesis

High chiral purity of lactic acid is a crucial indicator for the synthesis of chiral lactide as the primary intermediate chemical for ring-open polymerization of high molecular weight polylactic acid (PLA). Lignocellulose biomass is the most promising carbohydrate feedstock for commercial production of PLA, but the presence of trace d-lactic acid in the biorefinery chain adversely affects the synthesis and quality of chiral lactide. This study analyzed the fingerprint of trace  d-lactic acid in the biorefinery chain and found that the major source of  d-lactic acid comes from lignocellulose feedstock. The naturally occurring lactic acid bacteria and water-soluble carbohydrates in lignocellulose feedstock provide the necessary conditions for  d-lactic acid generation. Three strategies were proposed to eliminate the generation pathway of  d-lactic acid, including reduction of moisture content, conversion of water-soluble carbohydrates to furan aldehydes in pretreatment, and conversion to  l-lactic acid by inoculating engineered  l-lactic acid bacteria. The natural reduction of lactic acid content in lignocellulose feedstock during storage was observed due to the lactate oxidase-catalyzed oxidation of  l- and  d-lactic acids. This study provided an important support for the production of cellulosic  l-lactic acid with high chiral purity.

High titer (>200 g/L) lactic acid production from undetoxified pretreated corn stover

Lignocellulosic biomass is a reliable feedstock for lactic acid fermentation, low product titers hamper the scale production of cellulosic lactic acid. In this study, a Densifying Lignocellulosic biomass with Chemicals (sulfuric acid) pretreatment based cellulosic lactic acid biorefinery system was developed and demonstrated from multi-dimensions of producing bacteria, fermentation modes, corn stover solid loadings, fermentation vessels, and product purification. Results suggested that several lactic acid bacteria exhibited high fermentation activity in high solid loading corn stover hydrolysates. Remarkably, simultaneous saccharification co-fermentation performed in 100-mL flasks enabled 210.1 g/L lactic acid from 40% solid loading corn stover hydrolysate. When simultaneous saccharification co-fermentation was performed in 3-L bioreactors, 157.4 g/L lactic acid was obtained from 35% solid loading corn stover hydrolysate. These obtained lactic acid titers are the highest reports until now when lignocellulosic biomasses are used as substrates, making it efficient for scale production of cellulosic lactic acid.

Coproduction of single cell protein and lipid from lignocellulose derived carbohydrates and inorganic ammonia salt with soluble ammonia recycling

Co-production of single cell protein (SCP) and lipid from lignocellulose-derived carbohydrates and inorganic ammonia offers a promising alternative for poultry or aquaculture feeds. An engineered oleaginous yeast Trichosporon cutaneum MP11 showed great potential for producing SCP and lipid from wheat straw and ammonia sulfate with minimum nutrient input. Trichosporon cutaneum MP11 showed stronger SCP and lipid fermentability using dry acid pretreated and biodetoxified wheat straw than using pure sugars. The residual ammonium sulfate in fermentation broth was recycled up to five times, resulting in ∼70% of nitrogen fixation into SCP. The overall yield of SCP and lipid from lignocellulose-derived sugars was 0.15 g/g and 0.11 g/g, respectively. This translates to the production of one ton of SCP (0.56 ton) and lipid (0.44 ton) from 6.6 tons of wheat straw, or one ton of SCP and lipid containing yeast cells (dry) from 4.8 tons of wheat straw.

Simultaneous and rate-coordinated conversion of lignocellulose derived glucose, xylose, arabinose, mannose, and galactose into D-lactic acid production facilitates D-lactide synthesis

D-lactide is the precursor of poly(D-lactide) (PDLA) or stereo-complex with poly(L-lactide) (PLLA). Lignocellulosic biomass provides the essential feedstock option to synthesize D-lactic acid and D-lactide. The residual sugars in D-lactic acid fermentation broth significantly blocks the D-lactide synthesis. This study showed a simultaneous and rate-coordinated conversion of lignocellulose derived glucose, xylose, arabinose, mannose, and galactose into D-lactic acid by adaptively evolved Pediococcus acidilactici ZY271 by simultaneous saccharification and co-fermentation (SSCF) of wheat straw. The produced D-lactic acid achieved minimum residual sugars (∼1.7 g/L), high chirality (∼99.1%) and high titer (∼128 g/L). A dry acid pretreatment eliminated the wastewater stream generation and the biodetoxification by fungus Amorphotheca resinae ZN1 removed the inhibitors from the pretreatment. The removal of the sugar residues and inhibitor impurities in D-lactic acid production from lignocellulose strongly facilitated the D-lactide synthesis. This study filled the gap in cellulosic D-lactide production from lignocellulose-derived D-lactic acid.

Microbial tolerance engineering for boosting lactic acid production from lignocellulose

Lignocellulosic biomass is an attractive non-food feedstock for lactic acid production via microbial conversion due to its abundance and low-price, which can alleviate the conflict with food supplies. However, a variety of inhibitors derived from the biomass pretreatment processes repress microbial growth, decrease feedstock conversion efficiency and increase lactic acid production costs. Microbial tolerance engineering strategies accelerate the conversion of carbohydrates by improving microbial tolerance to toxic inhibitors using pretreated lignocellulose hydrolysate as a feedstock. This review presents the recent significant progress in microbial tolerance engineering to develop robust microbial cell factories with inhibitor tolerance and their application for cellulosic lactic acid production. Moreover, microbial tolerance engineering crosslinking other efficient breeding tools and novel approaches are also deeply discussed, aiming to providing a practical guide for economically viable production of cellulosic lactic acid.

Hierarchical Emulsion-Templated Monoliths (polyHIPEs) as Scaffolds for Covalent Immobilization of P. acidilactici

The immobilized cell fermentation technique (IMCF) has gained immense popularity in recent years due to its capacity to enhance metabolic efficiency, cell stability, and product separation during fermentation. Porous carriers used as cell immobilization facilitate mass transfer and isolate the cells from an adverse external environment, thus accelerating cell growth and metabolism. However, creating a cell-immobilized porous carrier that guarantees both mechanical strength and cell stability remains challenging. Herein, templated by water-in-oil (w/o) high internal phase emulsions (HIPE), we established a tunable open-cell polymeric P(St-co-GMA) monolith as a scaffold for the efficient immobilization of Pediococcus acidilactici (P. acidilactici). The porous framework's mechanical property was substantially improved by incorporating the styrene monomer and cross-linker divinylbenzene (DVB) in the HIPE's external phase, while the epoxy groups on glycidyl methacrylate (GMA) supply anchoring sites for P. acidilactici, securing the immobilization to the inner wall surface of the void. For the fermentation of immobilized P. acidilactici, the polyHIPEs permit efficient mass transfer, which increases along with increased interconnectivity of the monolith, resulting in higher _L-lactic acid yield compared to that of suspended cells with an increase of 17%. The relative _L-lactic acid production is constantly maintained above 92.9% of their initial relative production after 10 cycles, exhibiting both its great cycling stability and the durability of the material structure. Furthermore, the procedure during recycle batch also simplifies downstream separation operations.

High solids loading pretreatment: The core of lignocellulose biorefinery as an industrial technology - An overview

Pretreatment is the first and most determinative, yet the least mature step of lignocellulose biorefinery chain. The current stagnation of biorefinery commercialization indicates the barriers of the existing pretreatment technologies are needed to be unlocked. This review focused on one of the core factors, the high lignocellulose solids loading in pretreatment. The high solids loading of pretreatment significantly reduces water input, energy requirement, toxic compound discharge, solid/liquid separation costs, and carbon dioxide emissions, improves the titers of sugars and biproducts to meet the industrial requirements. Meanwhile, lignocellulose feedstock after high solids loading pretreatment is compatible with the existing logistics system for densification, packaging, storage, and transportation. Both the technical-economic analysis and the cellulosic ethanol conversion performance suggest that the solids loading in the pretreatment step need to be further elevated towards an industrial technology and the effective solutions should be proposed to the technical barriers in high solids loading pretreatment operations.

One-pot d-lactic acid production using undetoxified acid-pretreated corncob slurry by an adapted Pediococcus acidilactici

Inhibitor tolerance is still a bottleneck for lactic acid bacteria in lignocellulose biorefinery, while it is hard to obtain one engineered strain with strong tolerance to all inhibitors. Herein, a robust adapted d-lactic acid producing strain Pediococcus acidilactici XH11 was obtained by 111 days' long-term adaptive evolution in undetoxified corncob prehydrolysates. The adapted strain had higher inhibitors tolerance compared to the parental strain, primarily due to its increased conversion capacities of four typical aldehyde inhibitors (furfural, HMF, vanillin, and 4-hydroxybenzaldehyde). One-pot simultaneous saccharification and co-fermentation was successfully achieved using the whole slurry of acid-pretreated corncob without solid-liquid separation and detoxification, by applying the adapted P. acidilactici XH11. Finally, 61.9 g/L of d-lactic acid was generated after 96 h' fermentation (xylose conversion of 89.9 %) with the overall yield of 0.48 g/g dry corncob. This study gave an important option for screening of industrial strains in cellulosic lactic acid production processes.

Myco-biorefinery approaches for food waste valorization: Present status and future prospects

Increases in population and urbanization leads to generation of a large amount of food waste (FW) and its effective waste management is a major concern. But putrescible nature and high moisture content is a major limiting factor for cost effective FW valorization. Bioconversion of FW for the production of value added products is an eco-friendly and economically viable strategy for addressing these issues. Targeting on production of multiple products will solve these issues to greater extent. This article provides an overview of bioconversion of FW to different value added products.