High-Titer Lactic Acid Production by Pediococcus acidilactici PA204 from Corn Stover through Fed-Batch Simultaneous Saccharification and Fermentation

Probing efficient microbial CO2 utilisation through metabolic and process modelling

Acetogenic gas fermentation is increasingly studied as a promising technology to upcycle carbon-rich waste gasses. Currently the product range is limited, and production yields, rates and titres for a number of interesting products do not allow for economically viable processes. By pairing process modelling and host-agnostic metabolic modelling, we compare fermentation conditions and various products to optimise the processes. The models were then used in a simulation of an industrial-scale bubble column reactor. We find that increased temperatures favour gas transfer rates, particularly for the valuable and limiting H2 , while furthermore predicting an optimal feed composition of 9:1 mol H2 to mol CO2 . Metabolically, the increased non-growth associated maintenance requirements of thermophiles favours the formation of catabolic products. To assess the expansion of the product portfolio beyond acetate, both a product volatility analysis and a metabolic pathway model were implemented. In-situ recovery of volatile products is shown to be within range for acetone but challenging due to the extensive evaporation of water, while the direct production of more valuable compounds by acetogens is metabolically unfavourable compared to acetate and ethanol. We discuss alternative approaches to overcome these challenges to utilise acetogenic CO2 fixation to produce a wider range of carbon negative chemicals.

Microbial production of lactic acid using organic wastes as low-cost substrates

Lactic acid is a natural organic acid with diverse of applications in food, pharmaceutical, cosmetics, and chemical industry. Recently, the demand of lactic acid has been grown due to its utilization for polylactic acid production. Microbial production of lactic acid production is preferable due to optical purity of product, utilization of low cost substrates, and low energy requirement. Lignocellulosic biomass and other organic wastes are considered potential raw materials for cost-effective production of lactic acid. The raw materials are either hydrolyzed by enzymes or dilute acids to release the reducing sugars that are fermented in to lactic acid. This review has been focussed on microbial production of lactic acid using different organic wastes as low cost substrate.

Enterococcus faecium s6 Enabled Efficient Homofermentative Lactic Acid Production from Xylan-Derived Sugars

A thermotolerant Enterococcus faecium s6 strain exhibited homoferementative lactic acid (LA) production at high xylose concentration (≥50 g/L). In batch fermentation at 45 °C and controlled pH of 6.5, strain E. faecium s6 produced up to 72.9 g/L of LA with a yield of 0.99 g/g-consumed xylose and productivity of 1.74 g/L.h from 75 g/L xylose. An increased LA concentration and productivities with high yields were obtained in repeated batch fermentation that was conducted for 24 runs. In this mode, the strain could produce LA up 81.1 g/L within 5 h fermentation at a high productivity of 13.5 g/L.h and a yield of 1.02 g/g-consumed xylose. The strain was unable to consume birchwood xylan as sole carbon source. However, it could efficiently utilize xylooligosaccharides of xylobiose, xylotriose, xylotetraose, xylopentaose, and xylohexaose. The intracellular xylosidase activity was induced by xylose. Using xylooligosaccharide (50 g/L)/xylose (5 g/L) mixtures, the strain was able to produce maximum LA at 48.2 g/L within 120 h at a yield of 1.0 g/g-consumed sugar and productivity of 0.331 g/L.h. These results indicate that E. faecium s6 is capable of directly utilizing xylan-hydrolyzate and will enable the development of a feasible and economical approach to the production of LA from hemicellulosic hydrolysate.

Fermentative Lactic Acid Production From Lignocellulosic Feedstocks: From Source to Purified Product

The second (lignocellulosic biomass and industrial wastes) and third (algal biomass) generation feedstocks gained substantial interest as a source of various value-added chemicals, produced by fermentation. Lactic acid is a valuable platform chemical with both traditional and newer applications in many industries. The successful fractionation, separation, and hydrolysis of lignocellulosic biomass result in sugars’ rich raw material for lactic acid fermentation. This review paper aims to summarize the investigations and progress in the last 5 years in lactic acid production from inexpensive and renewable resources. Different aspects are discussed—the type of raw materials, pretreatment and detoxification methods, lactic acid-producers (bacteria, fungi, and yeasts), use of genetically manipulated microorganisms, separation techniques, different approaches of process organization, as well as main challenges, and possible solutions for process optimization.

Microbial Secondary Metabolism and Biotechnology

In recent decades scientific research has demonstrated that the microbial world is infinitely richer and more surprising than we could have imagined. Every day, new molecules produced by microorganisms are discovered, and their incredible diversity has not yet delivered all of its messages. The current challenge of research is to select from the wide variety of characterized microorganisms and compounds, those which could provide rapid answers to crucial questions about human or animal health or more generally relating to society’s demands for medicine, pharmacology, nutrition or everyday well-being.

The advanced performance of microbial consortium for simultaneous utilization of glucose and xylose to produce lactic acid directly from dilute sulfuric acid pretreated corn stover

BACKGROUND

Lignocellulosic feedstocks have attracted much attention as a potential carbon source for lactic acid (LA) production because of their ready availability, sustainability, and renewability. However, there are at least two major technical challenges to producing LA from lignocellulose. Inhibitors derived from lignocellulose pretreatment have a negative impact on the growth of cells producing LA. Furthermore, pentose sugars produced from the pretreatment are difficultly utilized by most LA producers, which is known as the carbon catabolite repression (CCR) effect. This complex feedstock can be utilized by a robust microbial consortium with high bioconversion efficiency.

RESULTS

In this study, a thermophilic consortium DUT50 producing LA was enriched and employed to improve corn stover (CS) utilization. Enterococcus was the dominant family in the consortium DUT50, accounting for 93.66% of the total abundance, with Lactobacillus, Bacillus, Lactococcus, and Trichococcus accounted for the remaining 2.68%. This consortium could be resistant to inhibitors concentration up to 9.74 g/L (2.88 g/L acetic acid, 2.46 g/L furfural, 2.20 g/L 5-HMF, and 2.20 g/L vanillin derived from pretreatment of CS), and simultaneously metabolizes hexose and pentose without CCR effect. Based on the promising consortium features, an efficient process of simultaneous saccharification and co-fermentation (SSCF) was developed to produce LA from acid pretreated corn stover, in which solid-liquid separation and detoxification were avoided. The key influencing factors were investigated and optimized, including dry biomass and cellulase loading, corn steep liquor powder concentration, and the pre-hydrolysis time. The highest LA titer of 71.04 g/L with a yield of 0.49 g/g-CS was achieved at a dry biomass loading of 20% (w/v), which is the highest LA production from non-detoxified acid pretreated corn stover via the SSCF process without wastewater generation reported to date. The simultaneous metabolism of hexose and pentose revealed collaboration between Enterococcus in the consortium, whereas xylose may be efficiently metabolized by Lactobacillus and Bacillus with low abundance via the pentose phosphate pathway.

CONCLUSIONS

The experimental results demonstrated the potential advantage of symbiosis in microbial consortia used for LA production from lignocellulosic biomass.

Two-Stage Fermentation of Lipomyces starkeyi for Production of Microbial Lipids and Biodiesel

The high operating cost is currently a limitation to industrialize microbial lipids production by the yeast Lipomyces starkeyi. To explore economic fermentation technology, the two-stage fermentation of Lipomyces starkeyi using yeast extract peptone dextrose (YPD) medium, orange peel (OP) hydrolysate medium, and their mixed medium were investigated for seven days by monitoring OD₆₀₀ values, pH values, cell growth status, C/N ratios, total carbon concentration, total nitrogen concentration, residual sugar concentration, lipid content, lipid titer, and fatty acids profiles of lipids. The results showed that two-stage fermentation with YPD and 50% YPD + 50% OP medium contributed to lipid accumulation, leading to larger internal lipid droplets in the yeast cells. However, the cells in pure OP hydrolysate grew abnormally, showing skinny and angular shapes. Compared to the one-stage fermentation, the two-stage fermentation enhanced lipid contents by 18.5%, 27.1%, and 21.4% in the flasks with YPD medium, OP medium, and 50%YPD + 50%OP medium, and enhanced the lipid titer by 77.8%, 13.6%, and 63.0%, respectively. The microbial lipids obtained from both one-stage and two-stage fermentation showed no significant difference in fatty acid compositions, which were mainly dominated by palmitic acid (33.36–38.43%) and oleic acid (46.6–48.12%). Hence, a mixture of commercial medium and lignocellulosic biomass hydrolysate could be a promising option to balance the operating cost and lipid production.

An aptly industrialized bioprocess for lactic acid production from corn stover using thermotolerant microbial consortia

Chemical pretreatment of lignocellulosic biomass is a critical step in the conversion of lignocellulose to biofuels and biochemical. The main drawback of this pretreatment process is the formation of inhibitors which exhibit combined toxicity to microorganisms and result to low product concentrations and yields. In this study, the selection of microbial consortia by enrichment on hydrolysate of H₂SO₄-pretreated corn stover (pre-CS) without detoxification has been investigated as an efficient way to develop new strategies for lignocellulose utilization. The analysis of cattle stomach-dervied microbial consortia domesticated to degrade hydrolysate of pre-CS to produce lactic acid (LA) at different temperatures was investigated. Bacterial 16S rRNA gene amplicon sequencing analyses indicated that the three microbial consortia were taxonomically distinct and Enterococcus became dominant at high temperature. The highest glucose consumption rate was observed at 45 °C, while the three microbial consortia showed similar consumption rates of xylose and arabinose. The selected microbial consortia DUT37, DUT45 and DUT47 showed preferable resistances to inhibitors in hydrolysate of pre-CS and abilities of xylose utilization. A batch simultaneous saccharification and fermentation (SSF) process was developed by microbial consortium DUT47 at 47 °C to produce LA from pre-CS under non-detoxified and non-sterile conditions. The LA concentration and yield were 43.73 g/L and 0.50 g/g-corn stover (CS), respectively. Microbial consortium DUT47 has been shown to be suitable for LA production from H₂SO₄-pretreated corn stover without detoxification due to its thermophilic growth characteristics, robust tolerance of inhibitors, and the simultaneous utilization of glucose and xylose.

High-efficient L-lactic acid production from inedible starchy biomass by one-step open fermentation using thermotolerant Lactobacillus rhamnosus DUT1908

The purpose of this study was to establish a simplified operational process for lactic acid (LA) production from inedible starchy biomass by open fermentation using thermotolerant Lactobacillus rhamnosus DUT1908. One step simultaneous liquefaction, saccharification and fermentation (SLSF) was proposed to produce LA using aging paddy rice with hull (APRH) as feedstock. First, a robust microbial strain was obtained by adaptive laboratory evolution under high temperature stress. As a result, L. rhamnosus DUT1908 showed high thermotolerance up to 50 °C and high efficiency of substrate utilization. Then, the performance of this thermotolerant L-lactic acid producing strain was demonstrated. Finally, various fermentation strategies were compared for LA production from APRH, including simultaneous saccharification and fermentation (SSF) and SLSF. In one-step open SLSF process, 107.8 g/L lactic acid was obtained with a productivity of 3.4 g/(L.h) and a yield to theoretical glucose of 0.89 g/g. This is the highest yield and productivity of lactic acid reported on starchy residues, and provides an efficient route for the development of high value-added products.