Mixed Culture Fermentation Research Articles

Study on Dynamic Model Construction and Analysis Method of Bacteria-algae Co-culture

To investigate the dynamic changes in growth and product synthesis of a mixed culture of Rhodotorula glutinis and Chlorella vulgaris, this study utilized fermentation data to analyze the rules and preliminary results of the microbial-algal symbiotic system. Employing Matlab, the author fitted a mathematical model proposed by predecessors, seeking parameters to establish a dynamic model for microbial fermentation. This model not only characterized parameter significance but also shed light on interactions, energy transfer mechanisms, and natural regulation balance during symbiotic fermentation. Results revealed that, under the influence of algae's photosynthetic carbon fixation, bacteria exhibited secondary growth, coupled with lipid synthesis and cell growth. The mixed culture model demonstrated a significant increase of 25.9% in biomass and 42.1% in oil content (reaching 30.694 g/L and 14.54 g/L, respectively). These findings suggest the model's potential as a theoretical guide for optimizing mixed culture fermentation processes.

Optimization of Enzyme-Assisted Extraction of Bioactive Compounds from Sea Buckthorn (Hippophae rhamnoides L.) Leaves: Evaluation of Mixed-Culture Fermentation.

Hippophae rhamnoides L. leaves possess a remarkable amount of polyphenols that could serve as a natural remedy in various applications. In comparison, numerous techniques, such as conventional and high-pressure techniques, are available for extracting the bioactive fractions from sea buckthorn leaves (SBL). However, enzyme-assisted extraction (EAE) of SBL has not been comprehensively studied. The aim of this study was to optimize critical EAE parameters of SBL using the cellulolytic enzyme complex, Viscozyme L, to obtain a high-yield extract with a high concentration of bioactive compounds. In order to determine the optimal conditions for EAE, the study employed a central composite design and response surface methodology to analyze the effects of four independent factors (pH, temperature, extraction time, and enzyme concentration) on two different responses. Our findings indicated that under optimal conditions (3:15 h extraction, temperature 45 °C, pH 4.9, and 1% Viscozyme L v/w of leaves DW), EAE yielded 28.90 g/100 g DW of the water-soluble fraction. Furthermore, the EAE-optimized liquid extract was continuously fermented using an ancient fermentation starter, Tibetan kefir grains, which possess lactic acid bacteria (LAB) and have significant potential for use in biopreservation. Interestingly, the results indicated various potential prebiotic characteristics of LAB. Additionally, alterations in the cell wall morphology of the SBL residue after EAE were examined using scanning electron microscopy (SEM). This study significantly optimized EAE parameters for sea buckthorn leaves, providing a promising natural source of bioactive compounds for various applications, such as nutraceuticals, functional foods, and high-value products.

Optimized Acetic Acid Production by Mixed Culture of Saccharomyces cerevisiae TISTR 5279 and Gluconobacter oxydans TBRC 4013 for Mangosteen Vinegar Fermentation Using Taguchi Design and Its Physicochemical Properties.

This research investigates the enhancement of acetic acid production in the mangosteen vinegar fermentation process through mixed-culture fermentation involving S. cerevisiae TISTR 5279 and G. oxydans TBRC 4013, alongside an analysis of the resulting mangosteen vinegar's qualities and properties using Taguchi Experimental Design (TED). It focuses on key parameters, such as the juice concentration, inoculum ratio, and pasteurization conditions, to optimize acetic acid production. The findings highlight that the unpasteurized condition exerts the most significant influence on acetic acid production yield (p < 0.01), followed by the 3:1 inoculum ratio of S. cerevisiae TISTR 5279 to G. oxydans TBRC 4013 and a 10% mangosteen concentration. The achieved theoretical maximum yield of acetic acid on day 21 was 85.23 ± 0.30%, close to the predicted 85.33% (p > 0.05). Furthermore, the highest recorded acetic acid concentration reached 5.34 ± 0.92%. On day 14 of fermentation, the maximum productivity and yield were 3.81 ± 0.10 g/L/h and 0.54 ± 0.22 g/g, respectively. The resulting mangosteen vinegar exhibited elevated levels of total phenolic content (359.67 ± 47.26 mg GAE/100 mL), total flavonoid content (12.96 ± 0.65 mg CAE/100 mL), and anti-DPPH radical activity (17.67 ± 0.22%), suggesting potential health benefits. Beyond these chemical aspects, the mangosteen vinegar displayed distinct physical and chemical characteristics from the original mangosteen juice, possibly conferring additional health advantages. These findings are promising for industrial vinegar fermentation models and propose the potential use of the product as a valuable dietary supplement.

Microbial Recycling of Polylactic Acid Food Packaging Waste into Carboxylates via Hydrolysis and Mixed-Culture Fermentation.

To establish a circular economy, waste streams should be used as a resource to produce valuable products. Biodegradable plastic waste represents a potential feedstock to be microbially recycled via a carboxylate platform. Bioplastics such as polylactic acid food packaging waste (PLA-FPW) are theoretically suitable feedstocks for producing carboxylates. Once feasible, carboxylates such as acetate, n-butyrate, or n-caproate can be used for various applications like lubricants or building blocks for making new bioplastics. In this study, pieces of industrial compostable PLA-FPW material (at 30 or 60 g/L) were added to a watery medium with microbial growth nutrients. This broth was exposed to 70 °C for a pretreatment process to support the hydrolysis of PLA into lactic acid at a maximum rate of 3.0 g/L×d. After 21 days, the broths of the hydrolysis experiments were centrifugated and a part of the supernatant was extracted and prepared for anaerobic fermentation. The mixed microbial culture, originating from a food waste fermentation bioprocess, successfully fermented the hydrolyzed PLA into a spectrum of new C2-C6 multi-carbon carboxylates. n-butyrate was the major product for all fermentations and, on average, 6.5 g/L n-butyrate was obtained from 60 g/L PLA-FPW materials. The wide array of products were likely due to various microbial processes, including lactate conversion into acetate and propionate, as well as lactate-based chain elongation to produce medium-chain carboxylates. The fermentation process did not require pH control. Overall, we showed a proof-of-concept in using real bioplastic waste as feedstock to produce valuable C2-C6 carboxylates via microbial recycling.

Impact of Candida railenensis during fermentation on the aromatic profile of Vidal blanc icewine.

Mixed culture fermentation with non-Saccharomyces yeasts and Saccharomyces cerevisiae as multi-starters has more advantages than spontaneous fermentation, and wine products with distinctive and pleasant aromas can fulfill the diverse demands of consumers. This study was carried out to illuminate the effect of sequential inoculation of indigenous Candida railenensis and S. cerevisiae on alcoholic fermentation behavior and chemical and aromatic characteristics of Vidal blanc icewine. During the mixed culture fermentation, C. railenensis was present in the initial and middle stages but was absent after 14 days. The results of basic chemical parameters showed that the glycerol content in the mixed culture-fermented icewine was higher than that of the pure fermented icewine, but the acetic acid content was the opposite. In terms of volatile aroma compounds, C. railenensis in the mixed culture fermentation reduced some metabolites such as lower alcohols, 1-hexanol, 3-methylthiopropanol, and their unpleasant notes and increased the production of some desired volatile aroma compounds such as benzaldehyde, β-damascenone, 2-furanmethanol, and 5-methyl furfural associated with rose, honey, nut, and caramel characteristics. Furthermore, C. railenensis also changed the sensory performance of icewine by participating in the fermentation of S. cerevisiae. These findings suggest that C. railenensis with positive enological properties has the ability to be used in icewine production, which has never been reported before.

Synergistic Fermentation with Functional Microorganisms Improves Safety and Quality of Traditional Chinese Fermented Foods.

Traditional fermented foods are favored by people around the world for their positive health and taste advantages. Many of the fermented foods, including Chinese traditional fermented foods, are produced through mixed-culture fermentation. Apart from reducing the formation of harmful compounds such as ethyl carbamate (EC) and biogenic amines (BAs) during food fermentation, it is also difficult to precisely control and regulate the fermentation process based on the control of environmental conditions alone, due to the complex microbiota and an unclarified fermentation mechanism. In this review, key microorganisms involved in Chinese fermented foods such as baijiu, soy sauce, and vinegar production are elaborated, and relations between microbial composition and the aroma or quality of food are discussed. This review focuses on the interpretation of functions and roles of beneficial (functional) microorganisms that participate in food fermentation and the discussion of the possibilities of the synergistic use of functional microorganisms to improve the safety and quality of Chinese fermented foods. Conducting work toward the isolation of beneficial microorganisms is a challenge for modern food fermentation technology. Thus, methods for the isolation and mutagenesis of functional microbial strains for synergistic food fermentation are summarized. Finally, the limitations and future prospects of the use of functional microorganisms in traditional Chinese fermented foods are reviewed. This review provides an overview of the applications of synergistic fermentation with functional microorganisms in the improvement of the safety or sensory qualities of fermented foods.

Insights into the grape must composition effect on Hanseniaspora vineae performance and metabolic aroma compounds in Chardonnay base wine for sparkling wine production

This study compares the performance of Hanseniaspora vineae (Hv) under mixed culture fermentation with Saccharomyces cerevisiae, with a conventional control (Sc) for the production of base wine. Hv exhibited fermentation kinetics comparable to controls when its nutritional requirements were satisfied. Volatile acidity in Hv from one batch was significantly lower (–0.3 g/L) compared to the controls, while in the other two batches the levels were equal. Interestingly, Hv trials showed a significant degradation of malic acid (–0.9 g/L on average) without variations of pH. Even though the matrix influenced the volatile composition, the characteristic features of H. vineae remained intact, with 24-fold higher levels of 2-phenylethyl acetate. There was no higher production of 2-aminoacetophenone, albeit tryptophan (known as a 2-aminoacetophenone precursor) is a preferred amino acid for H. vineae. Results suggest the potential of H. vineae to produce base wine with a broad spectrum of aroma compounds that increase flavor complexity, if obtained from nutrient-balanced grape musts.

Inhibition limits by undissociated acids in mixed culture fermentation and strategies to elevate process capacity

Using high substrate concentrations in mixed culture fermentation increases the products concentration achieved, making more cost-effective and sustainable its subsequent extraction. However, elevated levels of organic acids also result in inhibition by undissociated acids. This study investigates the concentration limits of undissociated acids that inhibit a lab-scale mixed culture carbohydrate fermenter operated under constant conditions (pH 5.4; HRT 20 h; 30 °C). Under these conditions, the conversion capacity was consistently inhibited at undissociated acids concentrations above 24.9 mM and substrate concentrations over 18.5 gCOD LFeed−1. This is attributed to the high maintenance costs incurred to maintain cell homeostasis at neutral intracellular pH in the presence of acids. This inhibition leads to lower yields and net biomass loss from the system, resulting in incomplete glucose conversion. Operating at a higher pH of 6 and utilizing granules proved to be effective strategies for expanding the conversion capacity.

Microbial Recycling of Bioplastics via Mixed-Culture Fermentation of Hydrolyzed Polyhydroxyalkanoates into Carboxylates.

Polyhydroxyalkanoates (PHA) polymers are emerging within biobased biodegradable plastic products. To build a circular economy, effective recycling routes should be established for these and other end-of-life bioplastics. This study presents the first steps of a potential PHA recycling route by fermenting hydrolyzed PHA-based bioplastics (Tianan ENMATTM Y1000P; PHBV (poly(3-hydroxybutyrate-co-3-hydroxyvalerate)) into carboxylates acetate and butyrate. First, three different hydrolysis pretreatment methods under acid, base, and neutral pH conditions were tested. The highest 10% (from 158.8 g COD/L to 16.3 g COD/L) of hydrolysate yield was obtained with the alkaline pretreatment. After filtration to remove the remaining solid materials, 4 g COD/L of the hydrolyzed PHA was used as the substrate with the addition of microbial nutrients for mixed culture fermentation. Due to microbial conversion, 1.71 g/L acetate and 1.20 g/L butyrate were produced. An apparent complete bioconversion from intermediates such as 3-hydroxybutyrate (3-HB) and/or crotonate into carboxylates was found. The overall yields of the combined processes were calculated as 0.07 g acetate/g PHA and 0.049 g butyrate/g PHA. These produced carboxylates can theoretically be used to reproduce PHA or serve many other applications as part of the so-called carboxylate platform.

Improvement of soybean meal quality by one-step fermentation with mixed-culture based on protease activity

Microbial fermentation can improve the quality of soybean meal by reducing the level of anti-nutrition factors and increasing the content of protein and small peptides. In this study, Bacillus velezensis, Enterococcus faecium and Saccharomyces boulardii were used for one-step mixed fermentation of soybean meal. The fermentation of soybean meal was carried out in an anaerobic bag using fermentation liquid of strains with maximum protease activity as inoculum and water. The results of process optimization showed that the mixture ratio of B. velezensis: E. faecium: S. boulardii was 37.5%:25%:37.5%. The three microorganisms added together as a mixture. Under the conditions of 55% inoculation with microorganisms mixture, 0.8% acid protease addition, 46 ℃ and 6 days, the quality of fermented soybean meal was significantly improved. The small peptides increased from 4.48% to 47.4%, the TCA solution protein increased from 44.3 g/kg to 255.2 g/kg, the antigenic protein glycinin decreased by 94.37% from 320.12 μg/mL to 18.02 μg/mL, and β-conglycinin decreased by 85.83% from 232.11 μg/mL to 32.89 μg/mL. Other anti-nutritional factors urease, phytic acid and stachystryose all decreased. The result indicated that it was feasible to improve the feed quality of soybean meal by one-step mixed fermentation based on protease activity. Industrial relevanceFermentation is the main method to improve the feed quality of soybean meal. Our study revealed that one-step mixed-culture fermentation based on microbial protease activity showed a significant effect on increasing beneficial small peptides, reducing anti-nutrient factors and enhancing palatability of soybean meal. This process simplified the operation of mixed fermentation and could be implemented in solid fermentation system of feed and food industry to improve the beneficial properties of products.

Steering the product spectrum in high-pressure anaerobic processes: CO2 partial pressure as a novel tool in biorefinery concepts

BackgroundElevated CO2 partial pressure (pCO2) has been proposed as a potential steering parameter for selective carboxylate production in mixed culture fermentation. It is anticipated that intermediate product spectrum and production rates, as well as changes in the microbial community, are (in)directly influenced by elevated pCO2. However, it remains unclear how pCO2 interacts with other operational conditions, namely substrate specificity, substrate-to-biomass (S/X) ratio and the presence of an additional electron donor, and what effect pCO2 has on the exact composition of fermentation products. Here, we investigated possible steering effects of elevated pCO2 combined with (1) mixed substrate (glycerol/glucose) provision; (2) subsequent increments in substrate concentration to increase the S/X ratio; and (3) formate as an additional electron donor.ResultsMetabolite predominance, e.g., propionate vs. butyrate/acetate, and cell density, depended on interaction effects between pCO2–S/X ratio and pCO2–formate. Individual substrate consumption rates were negatively impacted by the interaction effect between pCO2–S/X ratio and were not re-established after lowering the S/X ratio and adding formate. The product spectrum was influenced by the microbial community composition, which in turn, was modified by substrate type and the interaction effect between pCO2–formate. High propionate and butyrate levels strongly correlated with Negativicutes and Clostridia predominance, respectively. After subsequent pressurized fermentation phases, the interaction effect between pCO2–formate enabled a shift from propionate towards succinate production when mixed substrate was provided.ConclusionsOverall, interaction effects between elevated pCO2, substrate specificity, high S/X ratio and availability of reducing equivalents from formate, rather than an isolated pCO2 effect, modified the proportionality of propionate, butyrate and acetate in pressurized mixed substrate fermentations at the expense of reduced consumption rates and increased lag-phases. The interaction effect between elevated pCO2 and formate was beneficial for succinate production and biomass growth with a glycerol/glucose mixture as the substrate. The positive effect may be attributed to the availability of extra reducing equivalents, likely enhanced carbon fixating activity and hindered propionate conversion due to increased concentration of undissociated carboxylic acids.

A novel image-based method for simultaneous counting of Lactobacillus and Saccharomyces in mixed culture fermentation.

Mixed microorganism cultures are prevalent in the food industry. From sour beer to sourdough bread, mixed culture popularity has been growing steadily. A variety of microbiological mixtures have been used in these unique fermenting processes to create distinctive flavor profiles and potential health benefits. Although mixed cultures may seem ubiquitous, they are typically not well characterized, which may be due to the lack of user-friendly and simple measurement tools. Image-based cytometry systems have been employed to count bacteria or yeast cells directly and rapidly in plastic chambers, as opposed to traditional cell counting methods that can take days to grow and count. In this work, we aim to develop a novel image cytometry method to distinguish and enumerate mixed cultures of yeast and bacteria in beer products. We demonstrate that the proposed image cytometry method can be used to rapidly distinguish and simultaneously count the bacteria and yeast cells mixed cultures. The Cellometer X2 from Nexcelom was used to automate the counting of Lactobacillus plantarum and Saccharomyces cerevisiae in a mixed culture using fluorescent dyes and size exclusion image analysis algorithm. The proposed image cytometry method was tested with a series of experiments. (1) Yeast and bacteria monoculture titration, (2) mixed culture with various ratios, and (3) monitoring a Berliner Weisse mixed culture fermentation. All experiments were validated by comparing directly to manual counting of yeast and bacteria colony formation on agar plates. The two counting methods were highly comparable with ANOVA analysis showing p-value > 0.05 for all trials. The image cytometry method was the most accurate for yeast concentrations between 105-107 cells/mL and 106-108 cells/mL for lactic acid bacteria. Overall, the novel image cytometry method was able to distinguish and count mixed cultures consistently and accurately, which may provide better characterization and assessment of mixed culture brewing applications that may produce more consistent and higher quality products.

Optimization of strains for fermentation of kiwifruit juice and effects of mono- and mixed culture fermentation on its sensory and aroma profiles

In this study, a quality evaluation model of fermented kiwifruit juice (KJ) based on strain growth characteristics, sensory quality and functional characteristics was established by PCA, and the effects of mono- and mixed culture fermentation on the sensory and aroma profiles of KJ were comparatively studied. Experiments determined that L. brevis (LB) was the optimal strain for monoculture fermentation, and L. plantarum (LP2):LB = 1:2 was the optimum ratio for mixed fermentation. The results showed that lactic acid bacteria (LAB) fermentation significantly reduced the pH, soluble solid content and lightness, and improved its functional characteristics and viscosity. Mixed culture fermentation was superior to monoculture fermentation in terms of colony counts, sensory quality and viscosity. In general, after LAB fermentation, the concentrations of esters, ketones, alcohols and terpenoids in KJ increased significantly, while the concentrations of aldehydes decreased significantly. The production of esters and terpenoids was more strongly promoted by monoculture fermentation, while mixed culture fermentation promoted the production of more ketones and alcohols. 2,5-octanedione and 1-octen-3-ol could be the characteristic aroma compounds of mixed fermented KJ.

Quality Improvement in Apple Ciders during Simultaneous Co-Fermentation through Triple Mixed-Cultures of Saccharomyces cerevisiae, Pichia kudriavzevii, and Lactiplantibacillus plantarum.

This study explored the effect of the combination of Saccharomyces yeast, non-Saccharomyces yeast (Pichia kudriavzevii), and Lactiplantibacillus plantarum during cider fermentation on physicochemical properties, antioxidant activities, flavor and aroma compounds, as well as sensory qualities. Ciders fermented with the triple mixed-cultures of these three species showed lower acid and alcohol content than those fermented with the single-culture of S. cerevisiae. The antioxidant activities were enhanced by the triple mixed-culture fermentation, giving a higher 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging rate and total antioxidant capacity; specifically, the SPL5 cider showed the highest DPPH radical scavenging rate (77.28%), while the SPL2 gave the highest total antioxidant capacity (39.57 mmol/L). Additionally, the triple mixed-culture fermentation resulted in improved flavor and aroma with a lower acidity (L-malic acid) and higher aroma compounds (Esters), when compared with the single-culture fermented ciders (Saccharomyces cerevisiae); more specifically, the SPL4 cider resulted in the highest total flavor and aroma compounds. In addition, sensory evaluation demonstrated that ciders produced using the triple mixed-cultures gained higher scores than those fermented using the single-culture of S. cerevisiae, giving better floral aroma, fruity flavor, and overall acceptability. Therefore, our results indicated that the triple mixed-cultures (S. cerevisiae, P. kudriavzevii, and L. plantarum) were found to make up some enological shortages of the single S. cerevisiae fermented cider. This study is believed to provide a potential strategy to enhance cider quality and further give a reference for new industrial development protocols for cider fermentation that have better sensory qualities with higher antioxidant properties.

Aroma Profiles of Vitis vinifera L. cv. Gewürztraminer Must Fermented with Co-Cultures of Saccharomyces cerevisiae and Seven Hanseniaspora spp.

In this study, the aroma-production profiles of seven different Hanseniaspora strains, namely H. guilliermondii, H. meyeri, H. nectarophila, H. occidentalis, H. opuntiae, H. osmophila and H. uvarum were determined in a simultaneous co-inoculation with the wine yeast Saccharomyces cerevisiae Champagne Epernay Geisenheim (Uvaferm CEG). All co-inoculated fermentations with Hanseniaspora showed a dramatic increase in ethyl acetate levels except the two (H. occidentalis and H. osmophila) that belong to the so-called slow-evolving clade, which had no meaningful difference, compared to the S. cerevisiae control. Other striking observations were the almost complete depletion of lactic acid in mixed-culture fermentations with H. osmophila, the more than 3.7 mg/L production of isoamyl acetate with H. guilliermondii, the significantly lower levels of glycerol with H. occidentalis and the increase in certain terpenols, such as citronellol with H. opuntiae. This work allows for the direct comparison of wines made with different Hanseniapora spp. showcasing their oenological potential, including two (H. meyeri and H. nectarophila) previously unexplored in winemaking experiments.

Enhancing Biobased Volatile Fatty Acids Production from Olive Mill Solid Waste by Optimization of pH and Substrate to Inoculum Ratio

The pH and substrate-to-inoculum ratio (S/I) are important parameters in the anaerobic fermentation of agroindustrial residues, and therefore the optimization of these two parameters is needed for a stable, efficient, and sustainable reactor operation. In this work, the parameters pH (5–9) and S/I (0.5–3 gVS gVS−1) were optimized to produce biobased volatile fatty acids (VFAs) from hydrothermally pretreated olive mill solid waste (HPOMSW). The response variables evaluated in the Doehlert design were total VFAs concentration (tVFAs) (mg L−1) and amounts (%) of isobutyric, butyric, isovaleric, and valeric acids on the VFAs profile. The pH was the variable that most influenced the mixed culture fermentation of HPOMSW, proving to be a key parameter in the process. Microbial community analyses of conditions 1 (S/I = 3 gVS gVS−1 and pH = 7) and 4 (S/I = 1.13 gVS gVS−1 and pH = 5) showed that Proteobacteria and Firmicutes accounted for more than 87% of the total microorganisms identified for both conditions. In addition, the second-order model best fitted the experimental data for the VFAs production at the desirable condition (S/I = 3 gVS gVS−1 and pH = 8).

Schizosaccharomyces pombe in the Brewing Process: Mixed-Culture Fermentation for More Complete Attenuation of High-Gravity Wort

High-gravity brewing is a method that maximises brewhouse capacity and reduces energy consumption per unit of beer produced. The fermentation of wort with high sugar content is known to impact the fermentation characteristics and production of aroma-active volatiles, and as such, cultures that are adapted to this method are industrially valuable. Mixed-culture fermentation offers brewers the opportunity to combine desirable features from multiple strains of yeast and to take advantage of the interactions between those strains. In this study, a highly attenuative strain of Schizosaccharomyces pombe is paired with a fast-fermenting brewing strain of Saccharomyces cerevisiae in the fermentation of wort at both standard and high gravity at centilitre scale. Mixed cultures were found to produce several esters and higher alcohols in higher concentration than in either of the parent monocultures at both standard and high gravity. The mixed culture also represented a compromise between fermentation length (modelled by the logistic equation), which was extended by the inclusion of S. pombe, and ethanol yield, which was increased. The application of mixed-culture strategies to high-gravity brewing practices may allow brewers greater flexibility in achieving desired flavour profiles whilst increasing brewhouse efficiency.

Lactate addition boosts valerate yields in granular mixed culture carbohydrate fermentation

Valerate is a valuable volatile fatty acid with multiple industrial applications that can be sustainably produced from biowaste using odd-chain elongation processes in open mixed culture fermentation (MCF). The selective production of valerate in MCF has not been well explored to date. This preliminary study aims to improve the valerate yields achievable in a lab-scale granular MCF from glucose, by exogenous addition of lactate. Constant operating conditions were maintained (HRT 20 h; 30 ◦C; pH 5.4) with increasing lactate:glucose ratios in the influent while the total substrate COD concentration remained constant at 36 g COD Lfeed-1. Increasing ratios led to higher valerate production, achieving a maximum concentration and yield of 12.7 g COD L-1 and 34 % COD, respectively, at a lac:glu molar ratio of 9.9:1. This yield is 65 % higher than obtained without lactate addition. The higher valerate yields were concomitant with a decrease in propionate, suggesting a propionate plus lactate pathway. The abundance of Megasphaera, well-known chain elongator, increased (up to 52 %) with increasing lac:glu ratios. In contrast, higher ratios (23.6:1) resulted in system collapse which was attributed to the low metabolic energy available from the low glucose fed, insufficient to sustain biomass growth and the expected high maintenance in a medium with high concentrations of undissociated acids.

Kinetic modelling of a dark fermentation process using tequila vinasses as substrate for hydrogen production

In this work, a search methods strategy based on optimization algorithms is proposed for the kinetic modelling of a dark fermentation process using tequila stillage substrate for hydrogen production. Kinetic model based on the ADM1 approach is fitted to the experimental data to evaluate the dynamic behaviour of the metabolites produced and consumed during the mixed-culture fermentation. This batch fermentative process is based on mass balance analysis and correlated kinetic expressions following a Monod type dependence. In previous works, batch experiments were performed using different substrates to characterize the metabolic transformations involved in hydrogen production. Where, a kinetic model was developed and fitted to the obtained temporal profiles using Matlab and Excel solvers, allowing a satisfactory description of the metabolic pathways leading to hydrogen production. The aim of this research is to show the efficacy of the proposed optimization strategy to find the kinetic parameters of the model that will allow predictions on the behaviour of hydrogen production.

Highly selective butyric acid production by coupled acidogenesis and ion substitution electrodialysis

Selective production of carboxylic acids (CAs) from mixed culture fermentation remains a difficult task in organic waste valorization. Herein, we developed a facile and sustainable carbon loop strategy to regulate the fermentation micro-environment and steer acidogenesis towards selective butyric acid production. This new ion substitution electrodialysis-anaerobic membrane bioreactor (ISED-AnMBR) integrated system demonstrated a high butyric acid production at 11.19 g/L with a mass fraction of 76.05%. In comparison, only 1.04 g/L with a mass fraction of 30.56% was observed in the uncoupled control reactor. The carbon recovery reached a maximum of 96.09% with the assistance of ISED. Inorganic carbon assimilation was believed to be an important contributor, which was verified by 13C isotopic tracing. Microbial community structure shows the dominance of Clostridia (80.16%) in the unique micro-environment (e.g., pH 4.80–5.50) controlled by ISED, which is believed beneficial to the growth of such fermentative bacteria with main products of butyric acid and acetic acid. In addition, the emergence of chain elongators such as Clostridium sensu stricto 12 was observed to have a great influence on butyric acid production. This work provides a new approach to generate tailored longer chain carboxylic acids from organic waste with high titer thus contributing to a circular economy.