In situ extractive fermentation for the production of hexanoic acid from galactitol by Clostridium sp. BS-1

Exploiting the Thermotolerance of Clostridium Strain M1NH for Efficient Caproic Acid Fermentation from Ethanol and Acetic Acid

A novel thermotolerant caproic acid-producing bacterial strain, Clostridium M1NH, was successfully isolated from sewage sludge. Ethanol and acetic acid at a molar ratio of 4:1 proved to be the optimal substrates, yielding a maximum caproic acid production of 3.5 g/L. Clostridium M1NH exhibited remarkable tolerance to high concentrations of ethanol (up to 5% v/v), acetic acid (up to 5% w/v), and caproic acid (up to 2% w/v). The strain also demonstrated a wide pH tolerance range (pH 5.5-7.5) and an elevated temperature optimum between 35 and 40 °C. Phylogenetic analysis based on 16S rRNA gene sequences revealed that Clostridium M1NH shares a 98% similarity with Clostridium luticellarii DSM 29923 T. The robustness of strain M1NH and its efficient caproic acid production from low-cost substrates highlight its potential for sustainable bio-based chemical production. The maximum caproic acid yield achieved by Clostridium M1NH was 1.6-fold higher than that reported for C. kluyveri under similar fermentation conditions. This study opens new avenues for valorizing waste streams and advancing a circular economy model in the chemical industry.

Influence of pH and temperature on the performance and microbial community during the production of medium-chain carboxylic acids using winery effluents as substrate

Winery effluents containing high ethanol concentrations and diverse organic matter are ideal substrates for producing medium-chain carboxylic acids via fermentation and chain elongation. However, the process needs to be better understood. This study presents novel insights into the bioconversion mechanisms of medium-chain carboxylic acids by correlating fermentation and chain elongation kinetic profiles with the study of microbial communities at different pH (5 to 7) conditions and temperatures (30 to 40 °C). It was found that high productivities of MCCA were obtained using a native culture and winery effluents as a natural substrate. Minor pH variations significantly affected the metabolic pathway of the microorganisms for MCCA production. The maximal productivities of hexanoic (715 mg/L/d) and octanoic (350 mg/L/d) acids were found at pH 6 and 35 °C. Results evidence that the presence of Clostridium, Bacteroides, and Negativicutes promotes the high productions of MCCA. The formation of heptanoic acid was favor when Mogibacterium and Burkholderia were present.

Research on the Resource Recovery of Medium-Chain Fatty Acids from Municipal Sludge: Current State and Future Prospects

The production of municipal sludge is steadily increasing in line with the production of sewage. A wealth of organic contaminants, including nutrients and energy, are present in municipal sludge. Anaerobic fermentation can be used to extract useful resources from sludge, producing hydrogen, methane, short-chain fatty acids, and, via further chain elongation, medium-chain fatty acids. By comparing the economic and use values of these retrieved resources, it is concluded that a high-value resource transformation of municipal sludge can be achieved via the production of medium-chain fatty acids using anaerobic fermentation, which is a hotspot for future research. In this study, the selection of the pretreatment method, the method of producing medium-chain fatty acids, the influence of the electron donor, and the technique used to enhance product synthesis in the anaerobic fermentation process are introduced in detail. The study outlines potential future research directions for medium-chain fatty acid production using municipal sludge. These acids could serve as a starting point for investigating other uses for municipal sludge.

Recent advances in microbial production of medium chain fatty acid from renewable carbon resources: a comprehensive review

Microbial production of medium chain length fatty acids (MCFAs) from renewable resources is becoming increasingly important in establishing a sustainable and clean chemical industry. This review comprehensively summarizes current advances in microbial MCFA production from renewable resources. Detailed information is provided on two major MCFA production pathways using various renewable resources and other auxiliary pathways supporting MCFA production to help understand the fundamentals of bio-based MCFA production. In addition, conventional and well-studied MCFA producers are classified into two categories, natural and synthetic producers, and their characteristics on MCFA production are outlined. Moreover, various engineering strategies employed to achieve the highest MCFAs production up to date are showcased together with key enzymes suggested for MCFA overproduction. Finally, future challenges and perspectives are discussed towards more efficient production of bio-based MCFA production.

Bio-organic fertilizers improve Dendrocalamus farinosus growth by remolding the soil microbiome and metabolome

Organic and microbial fertilizers have potential advantages over inorganic fertilizers in improving soil fertility and crop yield without harmful side-effects. However, the effects of these bio-organic fertilizers on the soil microbiome and metabolome remain largely unknown, especially in the context of bamboo cultivation. In this study, we cultivated Dendrocalamus farinosus (D. farinosus) plants under five different fertilization conditions: organic fertilizer (OF), Bacillus amyloliquefaciens bio-fertilizer (Ba), Bacillus mucilaginosus Krassilnikov bio-fertilizer (BmK), organic fertilizer plus Bacillus amyloliquefaciens bio-fertilizer (OFBa), and organic fertilizer plus Bacillus mucilaginosus Krassilnikov bio-fertilizer (OFBmK). We conducted 16S rRNA sequencing and liquid chromatography/mass spectrometry (LC-MS) to evaluate the soil bacterial composition and soil metabolic activity in the different treatment groups. The results demonstrate that all the fertilization conditions altered the soil bacterial community composition. Moreover, the combination of organic and microbial fertilizers (i.e., in the OFBa and OFBmK groups) significantly affected the relative abundance of soil bacterial species; the largest number of dominant microbial communities were found in the OFBa group, which were strongly correlated with each other. Additionally, non-targeted metabolomics revealed that the levels of soil lipids and lipid-like molecules, and organic acids and their derivatives, were greatly altered under all treatment conditions. The levels of galactitol, guanine, and deoxycytidine were also markedly decreased in the OFBa and OFBmK groups. Moreover, we constructed a regulatory network to delineated the relationships between bamboo phenotype, soil enzymatic activity, soil differential metabolites, and dominant microbial. The network revealed that bio-organic fertilizers promoted bamboo growth by modifying the soil microbiome and metabolome. Accordingly, we concluded that the use of organic fertilizers, microbial fertilizers, or their combination regulated bacterial composition and soil metabolic processes. These findings provide new insights into how D. farinosus-bacterial interactions are affected by different fertilization regiments, which are directly applicable to the agricultural cultivation of bamboo.

Simultaneous medium chain fatty acids production and process carbon emissions reduction in a continuous-flow reactor: Re-understanding of carbon flow distribution

Due to its wide application and high value, the production of medium chain fatty acids (MCFAs) from waste biomass has become one of the worldwide research hotspots. Increasing the carbon element participation from short-chain fatty acids to the form of MCFAs is also conductive to reduce the release of biogas from biological treatment process, because carbon is in the form of MCFAs instead of biogas which directly contribute to process carbon emissions reduction. However, many barriers limiting MCFAs production and application remain to be resolved. Aiming continuous MCFAs production from lactate-rich waste biomass, this study optimized the operation conditions and clarified the main limiting factors and possible mechanisms. The maximum caproic acid concentration of 2.757 g/L were obtained at the Upflow Velocity (ULV) of 1.15 m/h and pH 4.9-5.1. Caproiciproducens, Pseudoramibacter, norank_f_Eubacteriaceae, and Oscillibacter were identified to be the dominant microbial genus responsible for MCFAs production from lactate. The reduction of carbon emissions calculation was also studied in the present processes.

Caproic Acid-Producing Bacteria in Chinese Baijiu Brewing

Caproic acid can be used as spices, preservatives, animal feed additives, and biofuels. At the same time, caproic acid plays an important role in Chinese Baijiu. It is the precursor substance for the synthesis of ethyl caproate, which directly affects the quality of Chinese Baijiu. Caproic acid-producing bacteria are the main microorganisms that synthesize caproic acid in Chinese Baijiu, and the most common strain is Clostridium kluyveri. Caproic acid-producing bacteria synthesize n-caproic acid through reverse β-oxidation to extend the carboxylic acid chain. This method mainly uses ethanol and lactic acid as substrates. Ethanol and lactic acid are converted into acetyl-CoA, and acetyl-CoA undergoes a series of condensation, dehydrogenation, dehydration, and reduction to extend the carboxylic acid chain. This review addresses the important issues of caproic acid-producing bacteria in the brewing process of Baijiu: the common caproic acid-producing bacteria that have been reported metabolic pathways, factors affecting acid production, biological competition pathways, and the effect of mixed bacteria fermentation on acid production. It is hoped that this will provide new ideas for the study of caproic acid-producing bacteria in Chinese Baijiu.

Performance and Mechanism of Fe3O4 Improving Biotransformation of Waste Activated Sludge into Liquid High-Value Products

This study demonstrated that Fe₃O₄ simultaneously improves the total production and formation rate of medium-chain fatty acids (MCFAs) and long-chain alcohols (LCAs) from waste activated sludge (WAS) in anaerobic fermentation. Results revealed that when Fe₃O₄ increased from 0 to 5 g/L, the maximal MCFA and LCA production increased significantly, and the optimal fermentation time was also remarkably shortened from 24 to 9 days. Moreover, Fe₃O₄ also enhanced WAS degradation, and the corresponding degradation rate in the fermentation system increased from 43.86 to 72.38% with an increase in Fe₃O₄ from 0 to 5 g/L. Further analysis showed that Fe₃O₄ promoted the microbe activities of all the bioprocesses (including hydrolysis, acidogenesis, and chain elongation processes) involved in the MCFA and LCA production from WAS. Microbial community analysis indicated that Fe₃O₄ increased the abundances of key microbes involved in abovementioned bioprocesses correspondingly. Mechanistic investigations showed that Fe₃O₄ increased the conductivity of the fermented sludge, providing a better conductive environment for the anaerobic microbes. The redox cycle of Fe(II) and Fe(III) existed in the fermentation system with Fe₃O₄, which was likely to act as electron shuttles to conduct electron transfer (ET) from the electron donor to the acceptor, thus increasing ET efficiency. This study provides an effective method for enhancing the biotransformation of WAS into high-value products, potentially bringing economic benefits to WAS treatment.

Effect of substrate structure on medium chain fatty acids production and reactor microbiome

The medium chain fatty acids (MCFAs) produced from organic wastes can replace part fossil-fuel-based products to promote the sustainable development of economy and environment. However, the selection and collocation of feedstocks for MCFAs production are lack of reference basis. This study thereby aimed to investigate how the commonly used electron donor (ED) and substrate configuration affect MCFAs synthesis and then obtain the optimal substrate composition. It was found that the optimized ratios for ethanol/acetate, lactate/acetate, and ethanol/lactate/acetate were 3/1, 2/1, and 2/1/1, respectively, and the optimal substrate concentration was 400 mM C. Combining ethanol and lactate as co-EDs effectively concentrated substrate-carbon-flow (increased by 20-28% than sole ED) on MCFAs synthesis by promoting the elongation of butyrate and reutilization of by-products. As a result, the higher MCFAs yield of 646.22 mg COD/g COD and selectivity of 67.72% were obtained from co-EDs than those from sole ED. Moreover, the key functional bacteria enriched under different ED were also discrepant, which were Clostridium sensu stricto for ethanol, Corynebacterium for lactate, and Veillonella and Oscillibacter for ethanol-lactate, respectively. This study provided a basic but significant reference for the scale-up MCFAs production.

Metabolic cascade of complex organic wastes to medium-chain carboxylic acids: A review on the state-of-the-art multi-omics analysis for anaerobic chain elongation pathways

Medium-chain carboxylic acid (MCCA) production from organic wastes has attracted much attention because of their higher energy contents and diverse applications. Anaerobic reactor microbiomes are stable and resilient and have resulted in efficient performance during many years of operation for thousands of full-scale anaerobic digesters worldwide. The method underlying how the relevant microbial pathways contribute to elongate carbon chains in reactor microbiomes is important. In particular, the reverse β-oxidation pathway genes are critical to upgrading short-chain fermentation products to MCCAs via a chain elongation (CE) process. Diverse genomics and metagenomics studies have been conducted in various fields, ranging from intracellular metabolic pathways to metabolic cascades between different strains. This review covers taxonomic approach to culture processes depending on types of organic wastes and the deeper understanding of genome and metagenome-scale CE pathway construction, and the co-culture and multi-omics technology that should be addressed in future research.

Reactor microbiome enriches vegetable oil with n-caproate and n-caprylate for potential functionalized feed additive production via extractive lactate-based chain elongation

BACKGROUND

Biotechnological processes for efficient resource recovery from residual materials rely on complex conversions carried out by reactor microbiomes. Chain elongation microbiomes produce valuable medium-chain carboxylates (MCC) that can be used as biobased starting materials in the chemical, agriculture and food industry. In this study, sunflower oil is used as an application-compatible solvent to accumulate microbially produced MCC during extractive lactate-based chain elongation. The MCC-enriched solvent is harvested as a potential novel product for direct application without further MCC purification, e.g., direct use for animal nutrition. Sunflower oil biocompatibility, in situ extraction performance and effects on chain elongation were evaluated in batch and continuous experiments. Microbial community composition and dynamics of continuous experiments were analyzed based on 16S rRNA gene sequencing data. Potential applications of MCC-enriched solvents along with future research directions are discussed.

RESULTS

Sunflower oil showed high MCC extraction specificity and similar biocompatibility to oleyl alcohol in batch extractive fermentation of lactate and food waste. Continuous chain elongation microbiomes produced the MCC n-caproate (nC6) and n-caprylate (nC8) from L-lactate and acetate at pH 5.0 standing high undissociated n-caproic acid concentrations (3 g L^-1). Extractive chain elongation with sunflower oil relieved apparent toxicity of MCC and production rates and selectivities reached maximum values of 5.16 ± 0.41 g nC6 L^-1 d^-1 (MCC: 11.5 g COD L^-1 d^-1) and 84 ± 5% (e^- eq MCC per e^- eq products), respectively. MCC were selectively enriched in sunflower oil to concentrations up to 72 g nC6 L^-1 and 3 g nC8 L^-1, equivalent to 8.3 wt% in MCC-enriched sunflower oil. Fermentation at pH 7.0 produced propionate and n-butyrate instead of MCC. Sunflower oil showed stable linoleic and oleic acids composition during extractive chain elongation regardless of pH conditions. Reactor microbiomes showed reduced diversity at pH 5.0 with MCC production linked to Caproiciproducens co-occurring with Clostridium tyrobutyricum, Clostridium luticellarii and Lactobacillus species. Abundant taxa at pH 7.0 were Anaerotignum, Lachnospiraceae and Sporoanaerobacter.

CONCLUSIONS

Sunflower oil is a suitable biobased solvent to selectively concentrate MCC. Extractive reactor microbiomes produced MCC with improved selectivity and production rate, while downstream processing complexity was reduced. Potential applications of MCC-enriched solvents may include feed, food and biofuels purposes.

Biological production of medium-chain carboxylates through chain elongation: An overview

Medium chain carboxylates (MCCs) have wide applications in various industries, but the traditional MCCs production methods are costly and unsustainable. Anaerobic fermentation offers a more scalable, economical and eco-friendly platform for producing MCCs through chain elongation which converts short chain carboxylates and electron donor into more valuable MCCs. However, the underlying microbial pathways are not well understood. In this review, biological production of MCCs through chain elongation is introduced elaborately, including the metabolic pathways, electron donor and substrates, microorganisms and influencing factors. Then, the strategies for enhancing MCCs production are extensively analyzed and summarized, along with the technologies for MCCs separation from the fermentation broth. Finally, challenges and perspectives concerning the large-scale MCCs production are proposed, providing suggestions for the future research. Extensive review demonstrated that anaerobic fermentation has great potential in achieving economical and sustainable MCCs production from complex organic substrates, including organic waste streams, which would significantly broaden the application of MCCs, especially in the renewable energy field. An interdisciplinary approach with knowledge from microbiology and biochemistry to chemical separations and environmental engineering is required to use this promising technology as a valorization method for converting organic biomass or organic wastes into valuable MCCs.

Opportunities and challenges in microbial medium chain fatty acids production from waste biomass

Medium chain fatty acids (MCFAs) that produced from affordable waste biomass via chain elongation (CE) technology are recognized as the potential alternatives to part fossil-derived chemicals, contributing to the sustainable development of economy and environment. The purpose of this review is to provide comprehensive analyses on the opportunities and challenges of MCFAs production and application. First, both two microbial MCFAs synthesis pathways of reverse β-oxidation and fatty acid biosynthesis were introduced/compared in detail to give readers a thorough understanding of the CE process, with the expectation of further boosting MCFAs production by well distinguishing them. Furthermore, the six key MCFAs production bottlenecks, corresponding research progresses, and possible solutions were analyzed. Five major MCFAs production strategies with their production mechanism, performances, and characteristics were also critically assessed. Additionally, the commercial production status was introduced, and future alternative production mode and research priorities were also recommended.

Hexanol biosynthesis from syngas by Clostridium carboxidivorans P7 - product toxicity, temperature dependence and in situ extraction

Clostridium carboxidivorans converts syngas into industrial alcohols like hexanol, but titers may be limited by product toxicity. Investigation of IC₅₀ at 30 °C (17.5 mM) and 37 °C (11.8 mM) revealed increased hexanol tolerance at lower temperatures. To avoid product toxicity, oleyl alcohol was added as an extraction solvent, increasing hexanol production nearly 2.5-fold to 23.9 mM (2.4 g/L) at 30 °C. This titer exceeds the concentration that is acutely toxic in the absence of a solvent, confirming the hypothesis that current hexanol production is limited by product toxicity. The solvent however had no positive effect at 37 °C. Furthermore, C. carboxidivorans cell membranes adapted to the higher temperature by incorporating more saturated fatty acids, but surprisingly not to hexanol. Corn oil and sunflower seed oil were tested as alternative, inexpensive extraction solvents. Hexanol titers were similar with all solvents, but oleyl alcohol achieved the highest extraction efficiency.

Anaerobic digestion of organic waste: Recovery of value-added and inhibitory compounds from liquid fraction of digestate

Anaerobic digestion, as an eco-friendly waste treatment technology, is facing the problem of low stability and low product value. Harvesting value-added products beyond methane and removing the inhibitory compounds will unleash new vitality of anaerobic digestion, which need to be achieved by selective separation of certain compounds. Various methods are reviewed in this study for separating valuable products (volatile fatty acids, medium-chain carboxylic acids, lactic acid) and inhibitory substance (ammonia) from the liquid fraction of digestate, including their performance, applicability, corresponding limitations and roadmaps for improvement. In-situ extraction that allows simultaneous production and extraction is seen as promising approach which carries good potential to overcome the barriers for continuous production. The prospects and challenges of the future development are further analyzed based on in-situ extraction and economics.

Direct Conversion of Food Waste Extract into Caproate: Metagenomics Assessment of Chain Elongation Process

In a circular economy strategy, waste resources can be used for the biological production of high added-value substances, such as medium chain fatty acids (MCFAs), thus minimising waste and favouring a sustainable process. This study investigates single-stage fermentation processes for the production of MCFAs in a semi-continuous reactor treating the extract of real food waste (FW), without the addition of external electron donors. Two sequential acidogenic fermentation tests were carried out at an organic loading rate (OLR) of 5 and 15 gCOD L⁻¹d⁻¹ with a hydraulic retention time of 4 days and pH controlled at 6 ± 0.2. The highest level of caproate (4.8 g L⁻¹) was observed at OLR of 15 gCOD L⁻¹d⁻¹ with a microbiome mainly composed by lactate-producing Actinomyces, Atopobium, and Olsenella species and caproate-producing Pseudoramibacter. Metagenomic analysis revealed the presence of key enzymes for the production of lactate, such as lactate dehydrogenase and pyruvate ferredoxin oxidoreductase, as well as several enzymes involved in the reverse β-oxidation pathway, thus suggesting the occurrence of a lactate-based chain elongation process.

Potential of medium chain fatty acids production from municipal solid waste leachate: Effect of age and external electron donors

A large quantity of leachate is generated during municipal solid waste collection operation and in landfills due to the large amount of organic waste and high humidity. The content of medium chain fatty acids (MCFAs) in the leachate is a low cost feedstock for bio-based chemical and fuel production processes. The aim of this study is to investigate the MCFA production potential of three leachate ages through chain elongation process under uncontrolled pH batch test. Moreover, the effect of using different external electron donors (ethanol, methanol and a mixture of both) is studied. The experiment consists of characterizing the samples then adding external electron donors with a specific ratio to leachate samples under mesophilic temperature. For this investigation, also a statistical analysis is done, which shows the production of MCFAs is highly influenced by leachate age. The results indicate that the production of even-numbered acids increase from 600 to 1,000 mg/L by the end of the ethanol chain elongation experiment for young leachate. However, a higher MCFA production of more than 1,000 mg/L is achieved by using the mixture of methanol and ethanol as electron donor. Furthermore, all methanol chain elongation experiments lead to an odd-numbered production of MCFAs, such as pentanoic and heptanoic acids. These results confirm the potential improvement of MCFA production from leachate through choosing the optimal leachate age and electron donor. Overall, producing MCFAs from leachate is a good example of circular bio-economy because waste is used to produce biochemicals, which closes the material cycle.

The Isolate Caproiciproducens sp. 7D4C2 Produces n-Caproate at Mildly Acidic Conditions From Hexoses: Genome and rBOX Comparison With Related Strains and Chain-Elongating Bacteria

Bulk production of medium-chain carboxylates (MCCs) with 6-12 carbon atoms is of great interest to biotechnology. Open cultures (e.g., reactor microbiomes) have been utilized to generate MCCs in bioreactors. When in-line MCC extraction and prevention of product inhibition is required, the bioreactors have been operated at mildly acidic pH (5.0-5.5). However, model chain-elongating bacteria grow optimally at neutral pH values. Here, we isolated a chain-elongating bacterium (strain 7D4C2) that grows at mildly acidic pH. We studied its metabolism and compared its whole genome and the reverse β-oxidation (rBOX) genes to other bacteria. Strain 7D4C2 produces lactate, acetate, n-butyrate, n-caproate, biomass, and H2/CO2 from hexoses. With only fructose as substrate (pH 5.5), the maximum n-caproate specificity (i.e., products per other carboxylates produced) was 60.9 ± 1.5%. However, this was considerably higher at 83.1 ± 0.44% when both fructose and n-butyrate (electron acceptor) were combined as a substrate. A comparison of 7D4C2 cultures with fructose and n-butyrate with an increasing pH value from 4.5 to 9.0 showed a decreasing n-caproate specificity from ∼92% at mildly acidic pH (pH 4.5-5.0) to ∼24% at alkaline pH (pH 9.0). Moreover, when carboxylates were extracted from the broth (undissociated n-caproic acid was ∼0.3 mM), the n-caproate selectivity (i.e., product per substrate fed) was 42.6 ± 19.0% higher compared to 7D4C2 cultures without extraction. Based on the 16S rRNA gene sequence, strain 7D4C2 is most closely related to the isolates Caproicibacter fermentans (99.5%) and Caproiciproducens galactitolivorans (94.7%), which are chain-elongating bacteria that are also capable of lactate production. Whole-genome analyses indicate that strain 7D4C2, C. fermentans, and C. galactitolivorans belong to the same genus of Caproiciproducens. Their rBOX genes are conserved and located next to each other, forming a gene cluster, which is different than for other chain-elongating bacteria such as Megasphaera spp. In conclusion, Caproiciproducens spp., comprising strain 7D4C2, C. fermentans, C. galactitolivorans, and several unclassified strains, are chain-elongating bacteria that encode a highly conserved rBOX gene cluster. Caproiciproducens sp. 7D4C2 (DSM 110548) was studied here to understand n-caproate production better at mildly acidic pH within microbiomes and has the additional potential as a pure-culture production strain to convert sugars into n-caproate.

Factors influencing volatile fatty acids production from food wastes via anaerobic digestion

Volatile fatty acids (VFAs) are intermediate products in anaerobic digestion. The effect of substrate loading or inoculum to substrate ratio (ISR), the addition of methanogen inhibitor, O₂ presence, control the reactor’s pH, and inoculum adaptation on the VFAs production from food waste through acidogenesis process was investigated in this study. Addition of 2-bromoethane sulfonic (BES) as methanogen inhibitor suppressed VFA consumption by methanogens at ISR 1:1. At higher substrate loading (ISR 1:3), methane production can be suppressed even without the addition of BES. However, at high substrate loading, controlling the pH during acidogenesis is important to achieve high VFAs yield. Acclimatization of inoculum is also one of the strategies to achieve high VFA yield. The highest VFAs yield obtained in this work was 0.8 g VFA/g VS _added at ISR 1:3, controlled pH at 6, with the presence of initial O₂ (headspace unflushed).

Sugar alcohols derived from lactose: lactitol, galactitol, and sorbitol

Lactose is a common natural disaccharide composed of galactose and glucose molecules. It is mainly found in the whey, the by-product of cheese and casein industries. As the supply of lactose far exceeds demand, a lot of lactose was discarded as the waste every year, which not only leads to resource waste, but also causes environmental pollution. Therefore, the deep processing of lactose as the feedstock has become a hot research topic. The lactose-derived sugar alcohols, including lactitol, sorbitol, and galactitol, have shown great potential applications not only in food manufacture, but also in pharmaceutical, cosmetic, and material fields. In this paper, we focus on the property, physiological effect, production, and application of the lactose-derived sugar alcohols. KEY POINTS: • The deep processing of lactose as the feedstock has become a hot research topic. • The lactose-derived sugar alcohols show great application values. • Recent advances in the lactose-derived sugar alcohols are reviewed.

In situ product recovery techniques aiming to obtain biotechnological products: A glance to current knowledge

Biotechnology and bioengineering techniques have been widely used in the production of biofuels, chemicals, pharmaceuticals, and food additives, being considered a "green" form of production because they use renewable and nonpolluting energy sources. On the other hand, in the traditional processes of production, the target product obtained by biotechnological routes must undergo several stages of purification, which makes these processes more expensive. In the past few years, some works have focused on processes that integrate fermentation to the recovery and purification steps necessary to obtain the final product required. This type of process is called in situ product recovery or extractive fermentation. However, there are some differences in the concepts of the techniques used in these bioprocesses. In this way, this review sought to compile relevant content on considerations and procedures that are being used in this field, such as evaporation, liquid-liquid extraction, permeation, and adsorption techniques. Also, the objective of this review was to approach the different configurations in the recent literature of the processes employed and the main bioproducts obtained, which can be used in the food, pharmaceutical, chemical, and/or fuel additives industry. We intended to elucidate concepts of these techniques, considered very recent, but which emerge as a promising alternative for the integration of bioprocesses.

Review on the production of medium and small chain fatty acids through waste valorization and CO2 fixation

The developing approaches in the recovery of resources from biowastes for the production of renewable value-added products and fuels, using microbial cultures as bio-catalyst have now became promising aspect. In the path of anaerobic digestion, the microorganisms are assisting transformation of a complex organic feedstock/waste to biomass and biogas. This potentiality consequently leads to the production of intermediate precursors of renewable value-added products. Particularly, a set of anaerobic pathways in the fermentation process, yields small-chain fatty acids (SCFA), and medium-chain fatty acids (MCFA) via chain elongation pathways from waste valorization and CO₂ fixation. This review focuses on the production of SCFA and MCFA from CO₂, synthetic substrates and waste materials. Moreover, the review introduces the metabolic engineering of Escherichia coli and Saccharomyces cerevisiae for SCFAs/MCFAs production. Furtherly, it concludes that future critical research might target progress of this promising approach as a valorization of complex organic wastes.

Propionic Acid: Method of Production, Current State and Perspectives

During the past years, there has been a growing interest in the bioproduction of propionic acid by Propionibacterium. One of the major limitations of the existing models lies in their low productivity yield. Hence, many strategies have been proposed in order to circumvent this obstacle. This article provides a comprehensive synthesis and review of important biotechnological aspects of propionic acid production as a common ingredient in food and biotechnology industries. We first discuss some of the most important production processes, mainly focusing on biological production. Then, we provide a summary of important propionic acid producers, including Propionibacterium freudenreichii and Propionibacterium acidipropionici, as well as a wide range of reported growth/production media. Furthermore, we describe bioprocess variables that can have impact on the production yield. Finally, we propose methods for the extraction and analysis of propionic acid and put forward strategies for overcoming the limitations of competitive microbial production from the economical point of view. Several factors influence the propionic acid concentration and productivity such as culture conditions, type and bioreactor scale; however, the pH value and temperature are the most important ones. Given that there are many reports about propionic acid production from glucose, whey permeate, glycerol, lactic acid, hemicelluloses, hydrolyzed corn meal, lactose, sugarcane molasses and enzymatically hydrolyzed whole wheat flour, only few review articles evaluate biotechnological aspects, i.e. bioprocess variables.

Impact of feedstocks and downstream processing technologies on the economics of caproic acid production in fermentation by Megasphaera elsdenii T81

Caproic acid (CA) was produced by Megasphaera elsdenii T81 with Jerusalem artichoke tubers (JA) as a feedstock. More CA was produced under the medium with the acid hydrolysate of JA than the comparative medium with a carbon composition similar to that of JA. CA was produced up to 13.0 g/L and 0.52 g/L/h with extractive fermentation using a mixed solvent of alamine 336 in oleyl alcohol at 37 °C. The JA cost to produce 1 ton of CA is only 505 USD, which is much lower than that required for purchasing sucrose (860 USD) in CA production. As a result of the analysis performed using SuperPro Designer, including the cost of distillation to obtain pure CA, the estimated production cost for CA from dry JA is 1869 USD/ton CA at the production scale of 2000 ton/year, which is lower than the current market price for petroleum-derived CA (~2500 USD/ton).

Road to full bioconversion of biowaste to biochemicals centering on chain elongation: A mini review

Production of biochemicals from waste streams has been attracting increasing worldwide interest to achieve climate protection goals. Chain elongation (CE) for production of medium-chain carboxylic acids (MCCAs, especially caproate, enanthate and caprylate) from diverse biowaste has emerged as a potential economic and environmental technology for a sustainable society. The present mini review summarizes the research utilizing various synthetic or real waste-derived substrates available for MCCA production. Additionally, the microbial characteristics of the CE process are surveyed and discussed. Considering that a large proportion of recalcitrantly biodegradable biowaste and residues cannot be further utilized by CE systems and remain to be treated and disposed, we propose here a loop concept of bioconversion of biowaste to MCCAs making full use of the biowaste with zero emission. This could make possible an alternative technology for synthesis of value-added products from a wide range of biowaste, or even non-biodegradable waste (such as, plastics and rubbers). Meanwhile, the remaining scientific questions, unsolved problems, application potential and possible developments for this technology are discussed.

An Efficient New Process for the Selective Production of Odd-Chain Carboxylic Acids by Simple Carbon Elongation Using Megasphaera hexanoica

The caproate-producing bacterium, Megasphaera hexanoica, metabolizes fructose to produce C₂~C₈ carbon-chain carboxylic acids using various electron acceptors. In particular, odd-chain carboxylic acids (OCCAs) such as valerate (C₅) and heptanoate (C₇), were produced at relatively high concentrations upon propionate supplementation. Using a statistical experimental design method, the optimal culture medium was established for the selective production of OCCAs among the total produced acids. In a medium containing 2.42 g L⁻¹ sodium acetate and 18.91 g L⁻¹ sodium propionate, M. hexanoica produced 9.48 g L⁻¹ valerate, 2.48 g L⁻¹ heptanoate, and 0.12 g L⁻¹ caproate. To clarify the metabolism of the exogenous added propionate for OCCAs production, ¹³C tracer experiments were performed by supplementing the culture broth with [1,2,3-¹³C₃] propionate. The metabolites analysis based on mass spectrometry showed that the propionate was only used to produce valerate and heptanoate without being participated in other metabolic pathways. Furthermore, the carbon elongation pathway in M. hexanoica was explained by the finding that the incorporation of propionate and acetate in the produced valerate occurred in only one orientation.

Genome Sequence of the Caproic Acid-Producing Bacterium Caproiciproducens galactitolivorans BS-1T (JCM 30532)

Caproiciproducens galactitolivorans BS-1^T is an anaerobic bacterium that produces acetate, butyrate, and caproate. The genome has a size of 2.57 Mbp and harbors 2,439 predicted protein-coding genes.

Caproiciproducens galactitolivorans BS-1^T is an anaerobic bacterium that produces acetate, butyrate, and caproate. The genome has a size of 2.57 Mbp and harbors 2,439 predicted protein-coding genes.

Medium chain carboxylic acids production from waste biomass: Current advances and perspectives

Alternative chemicals to diverse fossil-fuel-based products is urgently needed to mitigate the adverse impacts of fossil fuel depletion on human development. To this end, researchers have focused on the production of biochemical from readily available and affordable waste biomass. This is consistent with current guidelines for sustainable development and provides great advantages related to economy and environment. The search for suitable biochemical products is in progress worldwide. Therefore, this review recommends a biochemical (i.e., medium chain carboxylic acids (MCCAs)) utilizing an emerging biotechnological production platform called the chain elongation (CE) process. This work covers comprehensive introduction of the CE mechanism, functional microbes, available feedstock types and corresponding utilization strategies, major methods to enhance the performance of MCCAs production, and the challenges that need to be addressed for practical application. This work is expected to provide a thorough understanding of the CE technology, to guide and inspire researchers to solve existing problems in depth, and motivate large-scale MCCAs production.

Food waste-derived volatile fatty acids platform using an immersed membrane bioreactor

Volatile fatty acids (VFAs) are the key intermediates from anaerobic digestion (AD) process that can be a platform to synthesize products of higher value than biogas. However, some obstacles still exist that prevent large-scale production and application of VFAs, key among them being the difficulty in recovering the acids from the fermentation medium and low product yields. In this study, a novel anaerobic immersed membrane bioreactor (iMBR) with robust cleaning capabilities, which incorporated frequent backwashing to withstand the complex AD medium, was designed and applied for production and in situ recovery of VFAs. The iMBR was fed with food waste and operated without pH control, achieving a high yield of 0.54 g VFA/g VS_added. The continuous VFA recovery process in the iMBR was investigated for 40 days at OLRs of 2 gVS/L/d and 4 gVS/L/d without significant change in the permeate flux at a maximum suspended solids concentration of 31 g/L.

Comparison of Three Deoxidation Agents for Ozonated Broths Used in Anaerobic Biotechnological Processes

Anaerobic fermentation of organic compounds is used in many biotechnological processes and has been the subject of much research. A variety of process conditions and different growth media can be used to obtain microbial metabolites. The media must be free from contamination before fermentation. Sterilization is most often achieved by applying heat or other treatments, such as ozonation. Sterilization of liquid media using ozone can be very beneficial, but this method introduces high concentrations of residual oxygen, which inhibit anaerobic processes. Deoxidation is therefore necessary to remove the oxygen from ozonated broths. This study evaluates the effectiveness of three deoxidation agents for two kinds of fermentation media based on malt or molasses: ultrasound, iron(II) sulfate, and Metschnikowia sp. yeast. The time needed for deoxidation varied, depending on the kind of broth and the deoxidation agent. In general, the dynamics of oxygen removal were faster in malt broth. A comparative analysis showed that yeast biomass was the most effective agent, achieving deoxidation in the shortest time. Moreover, the fully deoxidated broth was supplemented with yeast biomass, which is rich in biogenic substrates, expressed as a protein content of 0.13–0.73 g/L. Application of Metschnikowia sp. may therefore be considered as an effective strategy for simultaneous deoxidation and nutrient supplementation of broths used in anaerobic biotechnological processes.

Medium Chain Carboxylic Acids from Complex Organic Feedstocks by Mixed Culture Fermentation

Environmental pressures caused by population growth and consumerism require the development of resource recovery from waste, hence a circular economy approach. The production of chemicals and fuels from organic waste using mixed microbial cultures (MMC) has become promising. MMC use the synergy of bio-catalytic activities from different microorganisms to transform complex organic feedstock, such as by-products from food production and food waste. In the absence of oxygen, the feedstock can be converted into biogas through the established anaerobic digestion (AD) approach. The potential of MMC has shifted to production of intermediate AD compounds as precursors for renewable chemicals. A particular set of anaerobic pathways in MMC fermentation, known as chain elongation, can occur under specific conditions producing medium chain carboxylic acids (MCCAs) with higher value than biogas and broader applicability. This review introduces the chain elongation pathway and other bio-reactions occurring during MMC fermentation. We present an overview of the complex feedstocks used, and pinpoint the main operational parameters for MCCAs production such as temperature, pH, loading rates, inoculum, head space composition, and reactor design. The review evaluates the key findings of MCCA production using MMC, and concludes by identifying critical research targets to drive forward this promising technology as a valorisation method for complex organic waste.

New coculture system of Clostridium spp. and Megasphaera hexanoica using submerged hollow-fiber membrane bioreactors for caproic acid production

In this study, a coculture bioprocess was developed with Clostridium strains producing butyric acid and Megasphaera hexanoica producing caproic acid from the butyric acid. The two bacterial strains were each cultivated in two submerged hollow-fiber membrane bioreactors (s-HF/MBRs), separately. Each fermentation broth was filtered through the membrane modules, and the filtered broth was either interchanged on another reactor or obtained sequentially through. Using s-HF/MBRs, the caproic acid concentration increased to 10.08 g L^-1, with the fastest productivity of 0.69 g L^-1 h^-1, which higher than that previously reported.

Upgrading liquor-making wastewater into medium chain fatty acid: Insights into co-electron donors, key microflora, and energy harvest

Ethanol and lactate are considered suitable electron donors (EDs) for chain elongation (CE); however, their respective shortcomings still limit the substrate conversion ratio and medium chain fatty acid (MCFA) production. To address this limitation, different EDs and electron acceptors (EAs) were combined to compare their CE performances, and to investigate whether the combination of ethanol and lactate could further enhance the MCFA production based on the complementary characteristics of ethanol and lactate. The results verified, for the first time, ethanol and lactate as the co-EDs formed a cooperative relationship to largely promote the conversion of substrates into MCFA. The co-EDs of ethanol and lactate stimulated the transformation of dispersive lactate-carbon flux from the competing acrylate pathway into n-heptylate. Additionally, the coexisting by-products (H₂ and CO₂) from ethanol and lactate also contributed to the supererogatory MCFA generation. The key microbial taxa that distinguished the co-EDs from their single action were the preponderant species from class Negativicutes and family Ruminococcaceae. In addition, the co-EAs of acetate, n-butyrate, and n-caproate also promoted MCFA generation. Low concentration of n-caproate could be directly elongated into n-caprylate, while n-caproate concentration exceeding the toxic limit was unsuitable as an EA. This research provided a guide for substrate selection and collocation for CE technology. Chinese liquor-making wastewater (CLMW) was subsequently used as a substrate for MCFA production since it contains abundant lactate, ethanol, and short-chain fatty acids. In this study, a MCFA selectivity of 80.34 ± 5.26%, a slightly higher selectivity which is in the range of previously reported ones, was obtained. This study paves a way for the sustainable development of Chinese liquor industry by recycling the high-output CLMW into MCFA.

Improvement of n-caproic acid production with Ruminococcaceae bacterium CPB6: selection of electron acceptors and carbon sources and optimization of the culture medium

Background

Global energy and resource shortages make it necessary to quest for renewable resources. n-Caproic acid (CA) production based on carboxylate platform by anaerobic fermentation is booming. Recently, a novel Ruminococcaceae bacterium CPB6 is shown to be a potential biotransformation factory for CA production from lactate-containing wastewater. However, little is known about the effects of different electron acceptors (EAs) on the fermentative products of strain CPB6, as well as the optimum medium for CA production.

Results

In this study, batch experiments were performed to investigate the fermentative products of strain CPB6 in a lactate medium supplemented with different EAs and sugars. Supplementation of acetate, butyrate and sucrose dramatically increased cell growth and CA production. The addition of propionate or pentanoate resulted in the production of C5 or C7 carboxylic acid, respectively. Further, a Box–Behnken experiment was conducted to optimize the culture medium for CA production. The result indicated that a medium containing 13.30 g/L sucrose, 22.35 g/L lactate and 16.48 g/L butyrate supported high-titer CA production (16.73 g/L) with a maximum productivity of 6.50 g/L/day.

Conclusions

This study demonstrated that strain CPB6 could produce C6–C7 carboxylic acids from lactate (as electron donor) with C2–C5 short-chain carboxylic acids (as EAs), but CA (C6 carboxylic acid) was the most major and potential product. Butyrate and sucrose were the most significant EA and carbon source respectively for CA production from lactate by strain CPB6. High titer of CA can be produced from a synthetic substrate containing sucrose, lactate and butyrate. The work provided significant implications for improving CA production in industry-scale.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0946-3) contains supplementary material, which is available to authorized users.

Integrating electrochemical, biological, physical, and thermochemical process units to expand the applicability of anaerobic digestion

Anaerobic digestion (AD) is a mature biotechnology-production platform with millions of installations at homes, farms, and industrial/municipal settings. Large-scale industrial, agricultural, and municipal waste-treatment systems may observe novel integration with electrochemical, biological, physical, and thermochemical process units to make AD more attractive. Without governmental subsidies, AD has often only a relatively low economic return or none at all. Diversification of products besides methane in biogas may help to change this. Here, several sections discuss different process units to: 1) upgrade biogas into biomethane; 2) convert carbon dioxide in biogas to more biomethane; 3) generate cooling power from process heat; 4) produce bio-crude oil (bio-oil) from organic matter; and 5) produce a liquid biochemical product from organic matter. This is not meant to be an exhaustive list, but rather a selection of particularly promising process units from a technological view, which are already integrated with AD or close to full-scale integration.

Biological caproate production by Clostridium kluyveri from ethanol and acetate as carbon sources

Caproate is a valuable industrial product and chemical precursor. In this study, batch tests were conducted to investigate the fermentative caproate production through chain elongation from acetate and ethanol. The effect of acetate/ethanol ratio and initial ethanol concentration on caproate production was examined. When substrate concentration was controlled at 100mM total carbon, hydrogen was used as an additional electron donor. The highest caproate concentration of 3.11g/L was obtained at an ethanol/acetate ratio of 7:3. No additional electron donor was needed upon an ethanol/acetate ratio ≥7:3. Caproate production increased with the increase of carbon source until ethanol concentration over 700mM, which inhibited the fermentation process. The highest caproate concentration of 8.42g/L was achieved from high ethanol strength wastewater with an ethanol/acetate ratio of 10:1 (550mM total carbon). Results obtained in this study can pave the way towards efficient chain elongation from ethanol-rich wastewater.

Profiles of Volatile Flavor Compounds in Milk Fermented with Different Proportional Combinations of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus

Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus are key factors in the fermentation process and the final quality of dairy products worldwide. This study was performed to investigate the effects of the proportions of Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus isolated from traditionally fermented dairy products in China and Mongolia on the profile of volatile compounds produced in samples. Six proportional combinations (1:1, 1:10, 1:50, 1:100, 1:1000, and 1:10,000) of L. delbrueckii subsp. bulgaricus IMAU20401 to S. thermophilus ND03 were considered, and the volatiles were identified and quantified by solid-phase microextraction and gas chromatography-mass spectrometry (SPME-GC-MS) against an internal standard. In total, 89 volatile flavor compounds, consisting of aldehydes, ketones, acids, alcohols, esters, and aromatic hydrocarbons, were identified. Among these, some key flavor volatile compounds were identified, including acetaldehyde, 3-methylbutanal, acetoin, 2-heptanone, acetic acid, butanoic acid, and 3-methyl-1-butanol. The of L. delbrueckii subsp. bulgaricus IMAU20401 to S. thermophilus ND03 influenced the type and concentration of volatiles produced. In particular, aldehydes and ketones were present at higher concentrations in the 1:1000 treatment combination than in the other combinations. Our findings emphasize the importance of selecting the appropriate proportions of L. delbrueckii subsp. bulgaricus and S. thermophilus for the starter culture in determining the final profile of volatiles and the overall flavor of dairy products.

Significant enhancement by biochar of caproate production via chain elongation

In this study, biochar was introduced into a chain elongation system to enhance the bioproduction of caproate and caprylate. The concentration of caproate increased to 21.1 g/L upon the addition of biochar, which is the highest level of caproate reported for such a system to date when ethanol was used as electron donor. The addition of biochar created a tougher system with more stable microorganism community structure for chain elongation, in which no obvious inhibition by products or substrates was observed, moreover, the lag phase was reduced 2.3-fold compared to the system without biochar. These reinforcement effect of biochar are attributed to the enhanced conductivity due to the significant enrichment of functional microorganisms via the microbial network surrounding smaller biochar particles, and via the adsorption on the rough surfaces or pores of larger particles, which facilitated electron transfer. Higher amounts of extracellular polymer substances and higher conductivity induced by biochar could contribute to the reinforcement effect in chain elongation.

Complete genome sequence of Ruminococcaceae bacterium CPB6: A newly isolated culture for efficient n-caproic acid production from lactate

n-caproic acid (CA) is a valuable chemical feedstock for various industrial applications. Biological production of CA from renewable carbon sources has attracted a lot of attentions recently. We lately reported the new culture Ruminococcaceae bacterium CPB6, which was isolated from a microbiome for efficient CA production from lactate. To further elucidate its metabolism, we sequenced the whole genome of the strain. The size of the complete genome is 2,069,994bp with 50.58% GC content; no plasmid was identified. Sets of genes involved in the fatty acid biosynthesis via acyl carrier protein (ACP) and coenzyme A (CoA) as well as lactate oxidation/reduction pathways were identified in the genome. These genes were inferred to be correlated with the CA production. The complete genome sequence provides essential information for the elucidation of the metabolism for CA production from lactate, and further improvement of the strain through genetic engineering for enhanced CA production and other biotechnological purposes.

Megasphaera hexanoica sp. nov., a medium-chain carboxylic acid-producing bacterium isolated from a cow rumen

Strain MHT, a strictly anaerobic, Gram-stain-negative, non-spore-forming, spherical coccus or coccoid-shaped microorganism, was isolated from a cow rumen during a screen for hexanoic acid-producing bacteria. The microorganism grew at 30-40 °C and pH 5.5-7.5 and exhibited production of various short- and medium-chain carboxylic acids (acetic acid, butyric acid, pentanoic acid, isobutyric acid, isovaleric acid, hexanoic acid, heptanoic acid and octanoic acid), as well as H2 and CO2 as biogas. Phylogenetic analysis based on 16S rRNA gene sequencing demonstrated that MHT represents a member of the genus Megasphaera, with the closest relatives being Megapsphaera indica NMBHI-10T (94.1 % 16S rRNA sequence similarity), Megasphaera elsdenii DSM 20460T (93.8 %) and Megasphaera paucivorans DSM 16981T (93.8 %). The major cellular fatty acids produced by MHT included C12 : 0, C16 : 0, C18 : 1cis 9, and C18 : 0, and the DNA G+C content of the MHT genome is 51.8 mol%. Together, the distinctive phenotypic and phylogenetic characteristics of MHT indicate that this microorganism represents a novel species of the genus Megasphaera, for which the name Megasphaera hexanoica sp. nov. is herein proposed. The type strain of this species is MHT (=KCCM 43214T=JCM 31403T).

Function of bacterial community dynamics in the formation of cadaveric semiochemicals during in situ carcass decomposition

The decomposition of dead mammalian tissue involves a complex temporal succession of epinecrotic bacteria. Microbial activity may release different cadaveric volatile organic compounds which in turn attract other key players of carcass decomposition such as scavenger insects. To elucidate the dynamics and potential functions of epinecrotic bacteria on carcasses, we monitored bacterial communities developing on still-born piglets incubated in different forest ecosystems by combining high-throughput Illumina 16S rRNA sequencing with gas chromatography-mass spectrometry of volatiles. Our results show that the community structure of epinecrotic bacteria and the types of cadaveric volatile compounds released over the time course of decomposition are driven by deterministic rather than stochastic processes. Individual cadaveric volatile organic compounds were correlated with specific taxa during the first stages of decomposition which are dominated by bacteria. Through best-fitting multiple linear regression models, the synthesis of acetic acid, indole and phenol could be linked to the activity of Enterobacteriaceae, Tissierellaceae and Xanthomonadaceae, respectively. These conclusions are also commensurate with the metabolism described for the dominant taxa identified for these families. The predictable nature of in situ synthesis of cadaveric volatile organic compounds by epinecrotic bacteria provides a new basis for future chemical ecology and forensic studies.

Electricity-assisted production of caproic acid from grass

BACKGROUND

Medium chain carboxylic acids, such as caproic acid, are conventionally produced from food materials. Caproic acid can be produced through fermentation by the reverse β-oxidation of lactic acid, generated from low value lignocellulosic biomass. In situ extraction of caproic acid can be achieved by membrane electrolysis coupled to the fermentation process, allowing recovery by phase separation.

RESULTS

Grass was fermented to lactic acid in a leach-bed-type reactor, which was then further converted to caproic acid in a secondary fermenter. The lactic acid concentration was 9.36 ± 0.95 g L^-1 over a 33-day semi-continuous operation, and converted to caproic acid at pH 5.5-6.2, with a concentration of 4.09 ± 0.54 g L^-1 during stable production. The caproic acid product stream was extracted in its anionic form, concentrated and converted to caproic acid by membrane electrolysis, resulting in a >70 wt% purity solution. In a parallel test exploring the upper limits of production rate through cell retention, we achieved the highest reported caproic acid production rate to date from a lignocellulosic biomass (grass, via a coupled process), at 0.99 ± 0.02 g L^-1 h^-1. The fermenting microbiome (mainly consisting of Clostridium IV and Lactobacillus) was capable of producing a maximum caproic acid concentration of 10.92 ± 0.62 g L^-1 at pH 5.5, at the border of maximum solubility of protonated caproic acid.

CONCLUSIONS

Grass can be utilized as a substrate to produce caproic acid. The biological intermediary steps were enhanced by separating the steps to focus on the lactic acid intermediary. Notably, the pipeline was almost completely powered through electrical inputs, and thus could potentially be driven from sustainable energy without need for chemical input.Graphical abstractMicrobial and electrochemical production of lactic acid, caproic acid and decane from grass.