Author: Michiel Mariën
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Category :
Languages : en
Pages :

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Author: Jasper Van Mullem
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Category :
Languages : en
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Author: Shao Heng
Publisher:
ISBN:
Category : Biomass conversion
Languages : en
Pages : 155

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Lignocellulosic biomass, such as agricultural and forest residues, is an inexpensive feedstock for bio-based products. Cost-effective production of bio-based products from lignocellulosic biomass requires simple conversion steps to break down carbohydrates to component mono-saccharides, and fermentation and/or chemical conversion of the sugars to final products. Lactic acid is one potential value-added product that could be produced economically from lignocellulosic biomass, if both the hexose and pentose sugars - derived from the cellulose and hemicellulose fractions, respectively - can be utilized completely with high efficiency. However, most natural lactic acid bacteria (LAB) cannot utilize xylose efficiently: the isomerization of xylose to xylulose in the phosphoketolase (PK) pathway constitutes a bottleneck step. Fortunately, it is possible to overcome this bottleneck via exogenous isomerization of xylose, thus allowing the microorganism to utilize xylulose as a viable alternative substrate for xylose. In this study, it has been demonstrated that this new approach could significantly improve the lactic acid yield. Lactobacillus pentosus and Lactobacillus casei (subspecies rhamnosus) were used in the fermentation of hexose, xylose, and xylulose to lactic acid. With L. pentosus, no preferential utilization of xylulose over xylose was seen, when both sugars were present in the medium. Sodium tetraborate and isomerization buffers, added to the fermentation broth to promote exogenous isomerization of xylose, strongly inhibited the growth of L. pentosus, which, in turn, led to poor utilization of xylulose. In contrast, with L. casei more robust growth and superior lactic acid yield were achieved from both glucose and xylulose, following exogenous isomerization with negligible xylose left at the end of fermentation. These results confirmed that, unlike L. pentosus, the exogenous isomerization additives do not inhibit L. casei and it is possible to maximize the utilization of both C6 and C5 sugars for lactic acid production by L. casei via the approach proposed in this study. In addition to lactic acid, succinic acid is a very important intermediary chemical building block that could constitute a viable alternative for petroleum-based bulk chemical precursors. Bio-based succinic acid produced from lignocellulosic biomass via microbial fermentation of the carbohydrate-derived sugars has the potential to reduce the cost of the product. However, inhibitors generated during the pretreatment and saccharification of biomass, especially lignin-derived phenolic compounds, could adversely affect the growth of succinic acid-producing microbes. Actinobacillus succinogenes - a promising strain that could be utilized for commercial succinic acid production - is strongly inhibited by the toxic compounds generated during pretreatment. In this study, some inexpensive commercially available enzymes were used to digest the chemical bonds between the glycoprotein in the cell wall and the polysaccharides of microalgae, which enabled the release of intracellular lipids and sugars. After removing the lipids using solvent extraction, the residue of the microalgae and the sugars remaining in the solution were successfully used as carbon and nitrogen sources for A. succinogenes fermentation for producing succinic acid. As such, the new process for fractionating microalgae developed in this study could significantly reduce the production costs of lipids and other bio-based products, because it allows the maximum utilization of every component in the micro-algal biomass.

Author: Yixing Zhang
Publisher:
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Category :
Languages : en
Pages :

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Lactic acid is an important platform chemical that has long history and wide applications in food, polymer, pharmaceutics and cosmetic industries. Lactic acid has two optical isomers; namely D-lactic acid and L-lactic acid. Racemic mixture of lactic acid are usually used as preservatives and ingredients in solvents, or as precursors for different chemicals. Currently there is an increasing demand of optical pure lactic acid as a feedstock for the production of poly-lactic acid (PLA). PLA is a biodegradable, biocompatible and environmental friendly alternative to plastics derived from petroleum based chemicals. Optically pure D or L-lactic acid is used for the synthesis of poly D or L- lactic acid (PDLA, PLLA). Blend of PDLA with PLLA results in a heat-resistant stereocomplex PLA with excellent properties. As a consequence, large quantity of cost effective D-lactic acid is required to meet the demand of stereocomplex PLA. Lignocellulosic biomass is a promising feedstock for lactic acid production because of its availability, sustainability and cost effectiveness compared to refined sugars and cereal grain-based sugars. Commercial use of lignocellulosic biomass for economic production of lactic acid requires microorganisms that are capable of using all sugars derived from lignocellulosic biomass. Therefore, the objectives of this study were: 1) to produce high level of optically pure D-lactic acid from lignocellulosic biomass-derived sugars using a homofermentative strain L. delbrueckii via simultaneous saccharification and fermentation (SSF); 2) to develop a co-culture fermentation system to produce lactic acid from both pentose and hexose sugars derived from lignocellulosic biomass; 3) to produce D-lactic acid by genetically engineered L. plantarum NCIMB 8826 [delta]ldhL1 and its derivatives; 4) to construct recombinant L. plantarum by introduction of a plasmid (pLEM415-xylAB) used for xylose assimilation and evaluate its ability to produce D-lactic acid from biomass sugars; and 5) to perform metabolic flux analysis of carbon flow in Lactobacillus strains used in our study. Our results showed that D-lactic acid yield from alkali-treated corn stover by L. delbrueckii and L. plantarum NCIMB 8826 [delta]ldhL1 via SSF were 0.50 g g−1 and 0.53 g g−1 respectively; however, these two D-lactic acid producing strains cannot use xylose from hemicellulose. Complete sugar utilization was achieved by co-cultivation of L. plantarum ATCC 21028 and L. brevis ATCC 367, and lactic acid yield increased to 0.78 g g−1 from alkali-treated corn stover, but this co-cultivation system produced racemic mixture of D and L lactic acid. Simultaneous utilization of hexose and pentose sugars derived from biomass was achieved by introduction of two plasmids pCU-PxylAB and pLEM415-xylAB carrying xylose assimilation genes into L. plantarum NCIMB 8826 [delta]ldhL1, respectively; the resulting recombinant strains [delta]ldhL1-pCU-PxylAB and [delta]ldhL1-pLEM415-xylAB used xylose and glucose simultaneously and produced high yield of optically pure D-lactic acid. Metabolic flux analysis verified the pathways used in these Lactobacillus strains and provided critical information to judiciously select the desired Lactobacillus strain to produce lactic acid catering to the composition of raw material and the optical purity requirement. This innovative study demonstrated strategies for low-cost biotechnological production of tailor-made lactic acid from specific lignocellulosic biomass, and thereby provides a foundational manufacturing route for a flexible and sustainable biorefinery to cater to the fuel and chemical industry.