Abstract
Several organic acids have been indicated among the top value chemicals from biomass. Lignocellulose is among the most attractive feedstocks for biorefining processes owing to its high abundance and low cost. However, its highly complex nature and recalcitrance to biodegradation hinder development of cost-competitive fermentation processes. Here, current progress in development of single-pot fermentation (i.e., consolidated bioprocessing, CBP) of lignocellulosic biomass to high value organic acids will be examined, based on the potential of this approach to dramatically reduce process costs. Different strategies for CBP development will be considered such as: (i) design of microbial consortia consisting of (hemi)cellulolytic and valuable-compound producing strains; (ii) engineering of microorganisms that combine biomass-degrading and high-value compound-producing properties in a single strain. The present review will mainly focus on production of organic acids with application as building block chemicals (e.g., adipic, cis,cis-muconic, fumaric, itaconic, lactic, malic, and succinic acid) since polymer synthesis constitutes the largest sector in the chemical industry. Current research advances will be illustrated together with challenges and perspectives for future investigations. In addition, attention will be dedicated to development of acid tolerant microorganisms, an essential feature for improving titer and productivity of fermentative production of acids.
Highlights
This route consists in carboxylation of pyruvate to oxalacetate, which is reduced to Malic Acid (MA) by NADH-dependent malate dehydrogenase (Mdh)
Additional applications of Fumaric Acid (FA) are in the pharmaceutical industry [126,127] and as supplement for cattle feed owing to its ability to dramatically reduce cattle emissions of greenhouse gas methane [128]
Proof-of-concept of Consolidated Bioprocessing (CBP) converting lignocellulose feedstocks to most of the top value organic acids indicated by a 2004 report of the US Department of Energy [10,11] was reported
Summary
Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. 35 US$/ton, while sugar costs about 460 US$/ton) [3,4], lignocellulose is among the most attractive substrates for biorefining strategies to produce high-value compounds (e.g., biofuels, bioplastics) through microbial fermentation [5]. There is increasing interest in using lignocellulose feedstocks for production of other commodity chemicals such as organic acids [7,8,9]. Application potential of microbial-derived organic acids strongly depends on their price, which in most cases currently limits their competitiveness with oil-derived compounds. Establishing cost-competitive microbial production processes (e.g., by using cheaper fermentation feedstock) is the key for fully expanding the market of these compounds [10], which is among the priority areas of economic development designated by the European Commission [15]
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