Medium-chain Carboxylic Acids Research Articles

Lactate and Ethanol Chain Elongation in the Presence of Lactose: Insight into Product Selectivity and Microbiome Composition

Organic waste streams rich in carbohydrates are costly to treat; however, they can be valorized to commodity chemicals such as medium-chain carboxylic acids (MCCAs), for example, caproic acid. Simple carbohydrates are easily fermentable, providing different intermediates for competing bacterial groups, which may lead to product diversification. For that reason, it is essential to understand the impact of sugar fermentation on chain elongation (CE) in the presence of key electron donors to control the biochemical pathways for MCCAs production. The research provides an insight into the impact of co-fermentation of lactose with lactate and ethanol as electron donors on CE and process selectivity in open culture fermentation. Co-fermentation of lactose with ethanol led mostly to acetogenesis and development of the Clostridium genus, while co-fermentation of lactose with solely lactate or with both electron donors activated the propionate production and diversified the process outcome to mix short- and medium-chain carboxylates. The highest caproate production efficiency and selectivity were achieved (53 and 65%, respectively) when lactose in the presence of lactate were used; CE improved with a higher initial lactose load. Interestingly, the microbiome was highly enriched in members of the Bacillaceae family, which was not reported before.

Recovery Techniques Enabling Circular Chemistry from Wastewater.

In an era where it becomes less and less accepted to just send waste to landfills and release wastewater into the environment without treatment, numerous initiatives are pursued to facilitate chemical production from waste. This includes microbial conversions of waste in digesters, and with this type of approach, a variety of chemicals can be produced. Typical for digestion systems is that the products are present only in (very) dilute amounts. For such productions to be technically and economically interesting to pursue, it is of key importance that effective product recovery strategies are being developed. In this review, we focus on the recovery of biologically produced carboxylic acids, including volatile fatty acids (VFAs), medium-chain carboxylic acids (MCCAs), long-chain dicarboxylic acids (LCDAs) being directly produced by microorganisms, and indirectly produced unsaturated short-chain acids (USCA), as well as polymers. Key recovery techniques for carboxylic acids in solution include liquid-liquid extraction, adsorption, and membrane separations. The route toward USCA is discussed, including their production by thermal treatment of intracellular polyhydroxyalkanoates (PHA) polymers and the downstream separations. Polymers included in this review are extracellular polymeric substances (EPS). Strategies for fractionation of the different fractions of EPS are discussed, aiming at the valorization of both polysaccharides and proteins. It is concluded that several separation strategies have the potential to further develop the wastewater valorization chains.

Development of an enhanced chain elongation process for caproic acid production from waste-derived lactic acid and butyric acid

Medium chain carboxylic acids (MCCA), such as caproic acid, are high-value chemicals with many industrial applications. The development of a biological mixed culture chain elongation (CE) process, using waste-derived lactic acid offers an opportunity to sustainably produce MCCA, such as caproic acid. However, the nature of the biological conversions in mixed communities is still poorly understood. In particular, inhibition of methanogens and MCCA competitors for the development of a specialised inoculum and optimisation of lactic acid conversion and selectivity toward caproic acid are required. Here, the impacts of physical and chemical treatments of the inoculum (anaerobic granular sludge) on the efficiency of CE were evaluated. Additionally, the effects of lactic acid/butyric acid concentrations and ratio (r LA/BA) on selectivity and concentration of caproic acid were investigated. Blended and acidified (pH 3) granular sludge yielded the highest caproic acid concentrations. Moreover, using pH 3 acidified and blended granular sludge, substrate concentrations of 250 mM (r LA/BA = 1.5:1) and 300 mM (r LA/BA = 1:1) were optimal for efficient caproic acid accumulation with a selectivity ≥90 and a mole of lactic to mole of butyric acid consumption (C r LA/BA) of 2:1. However, a similar outcome was achieved when using an enriched caproic acid producing culture with a lower substrate concentration of 200 mM (r LA/BA = 1:1). We report a lactic acid to butyric acid threshold concentration, below and above which the selectivity toward caproic acid is reduced. With these optimum substrate concentrations and ratios, the feasibility for efficient caproic acid synthesis using lactic acid as electron donor and butyric acid as electron acceptor was established. Caproic acid synthesis was completely inhibited when the substrate concentrations were increased to 400 mM (r LA/BA = 1:1). Finally, species affiliated with Ruminococcaceae were likely involved in the caproic acid synthesis. These findings have strong potential for process-design applications in continuous reactor systems, thus allowing for continuous, efficient and sustainable caproic acid production from biomass-derived lactic acid and butyric acid.

Fate of influent microbial populations during medium chain carboxylic acid recovery from brewery and pre-fermented food waste streams

Waste streams continuously introduce active and inactive microbial populations that can influence assembly of microbial communities in chain elongation systems.

Production of Medium-Chain Carboxylic Acids by Co-Fermentation of Antibiotic Fermentation Residue with Fallen Ginkgo Leaves

Production of Medium-Chain Carboxylic Acids by Co-Fermentation of Antibiotic Fermentation Residue with Fallen Ginkgo Leaves

Prediction and Optimization of Medium-Chain Carboxylic Acids Production from Food Waste Using Machine Learning Models

Prediction and Optimization of Medium-Chain Carboxylic Acids Production from Food Waste Using Machine Learning Models

Chain elongation process for caproate production using lactate as electron donor in Megasphaera hexanoica

Megasphaera hexnaoica is anaerobic bacteria who has well running reverse β-oxidation pathway. In previous study, the strain showed excellent production of medium chain carboxylic acids (MCCAs) using fructose as electron donor. In this study, chain elongation process study using lactate instead of fructose was conducted in M. hexnaoica fermentation. It was found that M. hexanoica can use lactate as electron donor in chain elongation process. 8.9 g/L caproate production was achieved in fermentation using lactate as sole electron donor. Compare to fructose condition, lactate as electron donor showed more than 3 times higher specific titer and specific productivity. In addition, when fructose and lactate were used as electron donor simultaneously, further improvement of MCCAs production was observed to achieve maximum caproate productivity of 20.9 g/L/day. Utilization of lactate as electron donor in M. hexanoica showed potential opportunity in chain elongation process.

Anaerobic Dynamic Membrane Bioreactor Development to Facilitate Organic Waste Conversion to Medium-Chain Carboxylic Acids and Their Downstream Recovery

Platform chemicals such as medium-chain carboxylic acids (MCCAs) can be produced from organic waste streams via chain elongation in anaerobic mixed-culture bioreactors. A product recovery system is needed to collect MCCAs from the bioreactor effluent. Membrane-based liquid–liquid extraction, the most commonly used product recovery approach, requires suspended solids removal from the bioreactor effluent to avoid membrane fouling. An anaerobic dynamic membrane bioreactor (AnDMBR) was developed to evaluate MCCA production from brewery and prefermented food waste and to produce a permeate with low suspended solids to facilitate downstream product recovery. The AnDMBR employed an inexpensive stainless-steel mesh as the support material for the development of a biofilm or dynamic membrane, which was responsible for solids–liquid separation. The AnDMBR produced a permeate quality with an average total suspended solids (TSS) concentration of 0.12 g L–1, while the average bioreactor TSS concentration was two orders of magnitude higher (21.6 g L–1). A maximum solids removal efficiency of ≥99% was achieved and good permeate quality was sustained for over 200 days without fouling control or cleaning the support material. In addition to solids–liquid separation, the dynamic membrane was responsible for a substantial fraction of the biological activity of the AnDMBR. The relative activity of Clostridiales, as determined by 16S rRNA sequencing, correlated with MCCA production and was higher in the dynamic membrane (20.0 ± 4.9%) than in the suspended biomass (5.2 ± 2.7%) in the bioreactor. This observation was consistent with MCCA production data as the permeate MCCA concentrations were significantly (p = 8.2 × 10–5) higher than that in the bioreactor, suggesting that the dynamic membrane biofilm contributed to chain elongation.

Organosolv pretreated birch sawdust for the production of green hydrogen and renewable chemicals in an integrated biorefinery approach

Sustainable production of fuels and chemicals is the most important way to reduce the carbon footprint in the environment. Forest based abundant lignocellulosic biomass as a renewable feedstock can be an attractive source of biofuels and biochemicals. This study evaluated the production of hydrogen (H2) along with platform chemicals from an organosol pretreated birch sawdust (SD). Acidogenic fermentation (AF) of pretreated SD resulted in production of green H2 (121.4 mL/gVS) along with short (17.8 g/L) and medium (2.64 g/L) chain carboxylic acids. Further integration of AF with anaerobic digestion (AD) in a biorefinery framework offered production of biomethane (bioCH4: 246 mL/gVS) from the leftover SD from AF. Integration of bioH2 with bioCH4 at different time interval of digestion showed 8–14 L biohythane formation ran with a H2 fraction of 1.6–0.3 H2/(H2 + CH4) documenting energy content of 8–9.08 kJ/gVS.

Volatile and medium chain fatty acid production from cabbage residues using mixed culture fermentations and in-situ product removal by electrodialysis

Volatile fatty acids (VFA) and medium chain carboxylic acids (MCCAs) are needed in several branches of the chemical industry such as in pharmaceutics, food, and feed, making a biorefinery inspired production that starts from low-grade biomass an interesting alternative to conventional petrochemical processes. In this study, butyric, valeric and caproic acid were produced with a defined co-culture of Megasphaera cerevisiae and Pediococcus pentosaceus by utilizing food waste residue, hydrolyzed cabbage, as the sole carbon source. The developed process was scaled-up to 25 L and coupled with a modified electrodialysis for in-situ product removal to limit the concentrations of accumulating VFAs and MCCAs in the fermentation broth. This process resulted in titers of 10.98 g L-1, 6.19 g L-1 and 0.23 g L-1 of butyric, valeric acid and of caproic acid, respectively, showing that MCCAs and VFAs of growing demand can be produced via valorization of organic residues from waste streams.

Long-Term Continuous Extraction of Medium-Chain Carboxylates by Pertraction With Submerged Hollow-Fiber Membranes.

Medium-chain carboxylic acids (MCCAs), which can be generated from organic waste and agro-industrial side streams through microbial chain elongation, are valuable chemicals with numerous industrial applications. Membrane-based liquid-liquid extraction (pertraction) as a downstream separation process to extract MCCAs has been applied successfully. Here, a novel pertraction system with submerged hollow-fiber membranes in the fermentation bioreactor was applied to increase the MCCA extraction rate and reduce the footprint. The highest average surface-corrected MCCA extraction rate of 655.2 ± 86.4 mmol C m−2 d−1 was obtained, which was higher than any other previous reports, albeit the relatively small surface area removed only 11.6% of the introduced carbon via pertraction. This submerged extraction system was able to continuously extract MCCAs with a high extraction rate for more than 8 months. The average extraction rate of MCCA by internal membrane was 3.0- to 4.7-fold higher than the external pertraction (traditional pertraction) in the same bioreactor. A broth upflow velocity of 7.6 m h−1 was more efficient to extract MCCAs when compared to periodic biogas recirculation operation as a means to prevent membrane fouling. An even higher broth upflow velocity of 40.5 m h−1 resulted in a significant increase in methane production, losing more than 30% of carbon conversion to methane due to a loss of H2, and a subsequent drop in the H2 partial pressure. This resulted in the shift from a microbial community with chain elongators as the key functional group to methanogens, because the drop in H2 partial pressure led to thermodynamic conditions that oxidizes ethanol and carboxylic acids to acetate and H2 with methanogens as the syntrophic partner. Thus, operators of chain elongating systems should monitor the H2 partial pressure when changes in operating conditions are made.

Selective Extraction of Medium-Chain Carboxylic Acids by Electrodialysis and Phase Separation.

Carboxylic acids obtained via the microbial electrochemical conversion of waste gases containing carbon dioxide (i.e., microbial electrosynthesis) can be used in lieu of nonrenewable building-block chemicals in the manufacture of a variety of products. When targeting valuable medium-chain carboxylic acids such as caproic acid, electricity-driven fermentations can be limited by the accumulation of fermentation products in the culturing media, often resulting in low volumetric productivities and titers due to direct toxicity or inhibition of the biocatalyst. In this study, we tested the effectiveness of a simple electrodialysis system in upconcentrating carboxylic acids from a model solution mimicking the effluent of a microbial electrochemical system producing short- and medium-chain carboxylic acids. Under batch extraction conditions, the electrodialysis scheme enabled the recovery of 60% (mol mol–1) of the total carboxylic acids present in the model fermentation broth. The particular arrangement of conventional monopolar ion exchange membranes and hydraulic recirculation loops allowed the progressive acidification of the extraction solution, enabling phase separation of caproic acid as an immiscible oil with 76% purity.

Conversion of Carbon Monoxide to Chemicals Using Microbial Consortia.

Syngas, a gaseous mixture of CO, H2 and CO2, can be produced by gasification of carbon-containing materials, including organic waste materials or lignocellulosic biomass. The conversion of bio-based syngas to chemicals is foreseen as an important process in circular bioeconomy. Carbon monoxide is also produced as a waste gas in many industrial sectors (e.g., chemical, energy, steel). Often, the purity level of bio-based syngas and waste gases is low and/or the ratios of syngas components are not adequate for chemical conversion (e.g., by Fischer-Tropsch). Microbes are robust catalysts to transform impure syngas into a broad spectrum of products. Fermentation of CO-rich waste gases to ethanol has reached commercial scale (by axenic cultures of Clostridium species), but production of other chemical building blocks is underexplored. Currently, genetic engineering of carboxydotrophic acetogens is applied to increase the portfolio of products from syngas/CO, but the limited energy metabolism of these microbes limits product yields and applications (for example, only products requiring low levels of ATP for synthesis can be produced). An alternative approach is to explore microbial consortia, including open mixed cultures and synthetic co-cultures, to create a metabolic network based on CO conversion that can yield products such as medium-chain carboxylic acids, higher alcohols and other added-value chemicals.

Direct Medium-Chain Carboxylic Acid Oil Separation from a Bioreactor by an Electrodialysis/Phase Separation Cell.

Medium-chain carboxylic acids (MCCAs) are valuable platform chemicals and can be produced from waste biomass sources or syngas fermentation effluent through microbial chain elongation. We have previously demonstrated successful approaches to separate >90% purity oil with different MCCAs (MCCA oil) by integrating the anaerobic bioprocess with membrane-based liquid-liquid extraction (pertraction) and membrane electrolysis. However, two-compartment membrane electrolysis unit without pertraction was not able to separate MCCA oil. Therefore, we developed a five-compartment electrodialysis/phase separation cell (ED/PS). First, we tested an ED/PS cell in series with pertraction and achieved a maximum MCCA-oil flux of 1.7 × 103 g d-1 per projected area (m2) (19 mL oil d-1) and MCCA-oil transfer efficiency [100% × moles MCCA-oil moles electrons-1] of 74% at 15 A m-2. This extraction system at 15 A m-2 demonstrated a ∼10 times lower electric-power consumption (1.1 kWh kg-1 MCCA oil) than membrane electrolysis in series with pertraction (9.9 kWh kg-1 MCCA oil). Second, we evaluated our ED/PS as a stand-alone unit when integrated with the anaerobic bioprocess and demonstrated that we can selectively extract and separate MCCA oil directly from chain-elongating bioreactor broth with just an abiotic electrochemical cell. However, the electric-power consumption increased considerably due to the lower MCCA concentrations in the bioreactor broth compared to the pertraction broth.

Production and extraction of medium chain carboxylic acids at a semi-pilot scale

Abstract Microbial chain elongation enables the conversion of organic wastes and agroindustrial sidestreams into medium chain carboxylic acids (MCCAs), with application in the production of plasticizers, personal care products, functional feed additives, and fuels. This work describes the up-scaling of the production of MCCAs from thin stillage, and their separation into an oil at kilogram scale with a MCCA content of more than 72%. The chain elongation occurred at a maximum rate of 1.7 ± 0.6 g MCCA L−1 d−1 without the use of ethanol as electron donor, even when it was supplied. Two in-line extraction trains are compared: (1) solvent extraction based on pertraction followed by electrochemical phase separation of the acids through a 2-compartment membrane electrolysis (ME) cell, and (2) direct electrochemical extraction from the fermentation broth using a 3-compartment ME cell. Using the first treatment train, n-caproic acid was extracted at an efficiency of 73%. The decrease in the fermentation pH from 5.5 to 5.0 enabled doubling the MCCA and oil production rate without a concomitant increase in NaOH dosage for pH stabilization. The selectivity brought about by pertraction and its capacity to up-concentrate MCCAs in an alkaline-pH extract enabled a more efficient use of the electricity for ME (as low as 6.2 kWh kg−1 oil) when compared to a period when ME was applied directly to the filtered fermentation broth (160 kWh kg−1 oil). The 3-compartment ME cell was able to phase separate oil extracting directly from the fermentation broth but it was not selective for longer-chain carboxylic acids due to co-extraction of short chain carboxylic acids and inorganic anions.

Carboxylic acids production using residual glycerol as a substrate in anaerobic fermentation: A kinetic modeling study

This work aimed to analyze the carboxylic acids (CA) production using residual glycerol (RG) from biodiesel as a substrate in the anaerobic fermentation and perform a kinetic study of this bioprocess. RG appeared to be a readily biodegradable carbon source to the acidogenic microorganisms. A yield of 0.62 mg acids mg CODA−1 was achieved, corresponding to 0.82 mg CODCA mgCODA−1. Caproic acid, a medium-chain CA, was produced even without external supplementation of electron donors. There was variation between the best fit models to describe the CA kinetics production. Most of the acids produced were better explained by exponential models, emphasizing the Cone and Fitzhugh models. Therefore, RG is a potential agro-industrial waste for CA production from a resource recovery perspective.

Adjusting Organic Load as a Strategy to Direct Single-Stage Food Waste Fermentation from Anaerobic Digestion to Chain Elongation

Production of medium chain carboxylic acids (MCCA) as renewable feedstock bio-chemicals, from food waste (FW), requires complicated reactor configurations and supplementation of chemicals to achieve product selectivity. This study evaluated the manipulation of organic loading rate in an un-supplemented, single stage stirred tank reactor to steer an anaerobic digestion (AD) microbiome towards acidogenic fermentation (AF), and thence to chain elongation. Increasing substrate availability by switching to a FW feedstock with a higher COD stimulated chain elongation. The MCCA species n-caproic (10.1 ± 1.7 g L−1) and n-caprylic (2.9 ± 0.8 g L−1) acid were produced at concentrations comparable to more complex reactor set-ups. As a result, of the adjusted operating strategy, a more specialised microbiome developed containing several MCCA-producing bacteria, lactic acid-producing Olsenella spp. and hydrogenotrophic methanogens. By contrast, in an AD reactor that was operated in parallel to produce biogas, the retention times had to be doubled when fed with the high-COD FW to maintain biogas production. The AD microbiome comprised a diverse mixture of hydrolytic and acidogenic bacteria, and acetoclastic methanogens. The results suggest that manipulation of organic loading rate and food-to-microorganism ratio may be used as an operating strategy to direct an AD microbiome towards AF, and to stimulate chain elongation in FW fermentation, using a simple, un-supplemented stirred tank set-up. This outcome provides the opportunity to repurpose existing AD assets operating on food waste for biogas production, to produce potentially higher value MCCA products, via simple manipulation of the feeding strategy.

Performance of native open cultures (winery effluents, ruminal fluid, anaerobic sludge and digestate) for medium-chain carboxylic acid production using ethanol and acetate

The production of medium-chain carboxylic acids (MCCA) was studied by using five different native microbiomes (ruminal fluid, endogenous red and white winery effluents, acidogenic digestate and anaerobic granular sludge). The presence of Clostridium was found to be essential in an efficient endogenous open culture inocula. The endogenous winery effluent microbiota produced the highest hexanoic acid concentration, followed by anaerobic sludge inoculum. Similarly, the highest volumetric and specific rates of hexanoic acid production (263 g hexanoic acid/L/d and 13 g/g of total solids/d) were observed for the inoculum harvested from the red wine winery effluent. However, the microbiota present in the ruminal fluid inoculum was found to be more versatile, allowing the production of longer-chain carboxylic acids such as heptanoic and octanoic acids. The fermentative digestate inoculum produced the lowest concentration of hexanoic acid at the lowest rate.

Multi-omics study on chain elongation processes for medium chain carboxylic acids

Contribution to the International Chain Elongation Conference 2020 | ICEC 2020. An abstract can be found in the right column.

Bio-electro CO2 recycling to medium chain carboxylic acids in two steps

Contribution to the International Chain Elongation Conference 2020 | ICEC 2020. An abstract can be found in the right column.