Repeated batches as a strategy for high 2G ethanol production from undetoxified hemicellulose hydrolysate using immobilized cells of recombinant Saccharomyces cerevisiae in a fixed-bed reactor

Role of non-genetically modified or native pentose fermenting microorganisms in establishing viable lignocellulosic biorefineries in the Brazilian context

Brazil can play a pivotal role in the development of a circular bioeconomy as the country ranks among the top five major agricultural countries in the world producing a foreseeable lignocellulosic biomass from crops, such as sugarcane, soybean, corn, rice, coffee, and eucalyptus. Considering that pentose sugars (C5 sugars) represent 20%-35% of the amount of lignocellulosic biomass components, these sugars have a great potential in the development of carbon neutral economy. From the biomass conversion economic point of view, the conversion of hemicellulose into renewable products with a satisfactory yield is the most needed. However, the biochemical conversion of pentose sugars is challenging due to the scarcity of native pentose sugars fermenting microorganisms. While recent advances in metabolic engineering have been effective in developing a strong molecular chassis for efficient pentose sugars conversion, the yields, productivities, and stability of the genetically modified organisms (GMOs) are major limiting factors for industrial-scale applications. Native lignocellulosic sugars fermenting microorganisms are competent, robust, and inhibitor-tolerant but their lower productivities continue to be a big concern. This article explains the inherent characteristics of native pentose fermenting microorganisms in establishing viable lignocellulosic biorefineries in the Brazilian context, with a special focus on their isolation from Brazilian biodiversity, along with the evaluation of nongenetic engineering techniques to improve strains for biorefinery application.

Effective application of immobilized second generation industrial Saccharomyces cerevisiae strain on consolidated bioprocessing

Integrated bioprocessing strategies can facilitate ethanol production from both cellulose and hemicellulose fractions of lignocellulosic biomass. Consolidated bioprocessing (CBP) is an approach that combines enzyme production, biomass hydrolysis and sugar fermentation in a single step. However, technologies that propose the use of microorganisms together with solid biomass present the difficulty of the recovery and reuse of the biocatalyst, which can be overcome by cell immobilization. In this regard, this work applied immobilized cells of AC14 yeast, a recombinant yeast that secretes 7 hydrolytic enzymes, in the CBP process in a successful proof-of-concept for the enzyme access to the substrate polymers. The most appropriate cell load for CBP under the conditions studied with immobilized cells was selected among three optical densities (OD) 10, 55 and 100. These experiments were performed with free cells to ensure that the results were not biased by mass limitations effects. OD 10 achieved 100% of the sugar consumption and the higher specific production of enzymes, being selected for further studies. Diffusional effects were observed with immobilized cells under static conditions. However, mass transfer limitations were mitigated under agitation, with an 18.5% increase in substrate consumption rate (from 2.7 to 3.5 g/L/h), reaching the same substrate uptake rates as free cells. In addition, immobilized cells achieved 100% hydrolysis and consumption of all substrates offered within only 12 h. Overall, this is the first report of a successful application of immobilized yeast cells in CBP processes for bioethanol production, a promising technology that can be extended to other biorefinery bioproducts.

An overview on fermentation strategies to overcome lignocellulosic inhibitors in second-generation ethanol production using cell immobilization

The development of technologies to ferment carbohydrates (mainly glucose and xylose) obtained from the hydrolysis of lignocellulosic biomass for the production of second-generation ethanol (2G ethanol) has many economic and environmental advantages. The pretreatment step of this biomass is industrially performed mainly by steam explosion with diluted sulfuric acid and generates hydrolysates that contain inhibitory compounds for the metabolism of microorganisms, harming the next step of ethanol production. The main inhibitors are: organic acids, furan, and phenolics. Several strategies can be applied to decrease the action of these compounds in microorganisms, such as cell immobilization. Based on data published in the literature, this overview will address the relevant aspects of cell immobilization for the production of 2G ethanol, aiming to evaluate this method as a strategy for protecting microorganisms against inhibitors in different modes of operation for fermentation. This is the first overview to date that shows the relation between inhibitors, cells immobilization, and fermentation operation modes for 2G ethanol. In this sense, the state of the art regarding the main inhibitors in 2G ethanol and the most applied techniques for cell immobilization, besides batch, repeated batch and continuous fermentation using immobilized cells, in addition to co-culture immobilization and co-immobilization of enzymes, are presented in this work.

From Saccharomyces cerevisiae to Ethanol: Unlocking the Power of Evolutionary Engineering in Metabolic Engineering Applications

Increased human population and the rapid decline of fossil fuels resulted in a global tendency to look for alternative fuel sources. Environmental concerns about fossil fuel combustion led to a sharp move towards renewable and environmentally friendly biofuels. Ethanol has been the primary fossil fuel alternative due to its low carbon emission rates, high octane content and comparatively facile microbial production processes. In parallel to the increased use of bioethanol in various fields such as transportation, heating and power generation, improvements in ethanol production processes turned out to be a global hot topic. Ethanol is by far the leading yeast output amongst a broad spectrum of bio-based industries. Thus, as a well-known platform microorganism and native ethanol producer, baker's yeast Saccharomyces cerevisiae has been the primary subject of interest for both academic and industrial perspectives in terms of enhanced ethanol production processes. Metabolic engineering strategies have been primarily adopted for direct manipulation of genes of interest responsible in mainstreams of ethanol metabolism. To overcome limitations of rational metabolic engineering, an alternative bottom-up strategy called inverse metabolic engineering has been widely used. In this context, evolutionary engineering, also known as adaptive laboratory evolution (ALE), which is based on random mutagenesis and systematic selection, is a powerful strategy to improve bioethanol production of S. cerevisiae. In this review, we focus on key examples of metabolic and evolutionary engineering for improved first- and second-generation S. cerevisiae bioethanol production processes. We delve into the current state of the field and show that metabolic and evolutionary engineering strategies are intertwined and many metabolically engineered strains for bioethanol production can be further improved by powerful evolutionary engineering strategies. We also discuss potential future directions that involve recent advancements in directed genome evolution, including CRISPR-Cas9 technology.

Cell Immobilization Using Alginate-Based Beads as a Protective Technique against Stressful Conditions of Hydrolysates for 2G Ethanol Production

The development of biorefineries brings the necessity of an efficient consumption of all sugars released from biomasses, including xylose. In addition, the presence of inhibitors in biomass hydrolysates is one of the main challenges in bioprocess feasibility. In this study, the application of Ca-alginate hybrid gels in the immobilization of xylose-consuming recombinant yeast was explored with the aim of improving the tolerance of inhibitors. The recombinant yeast Saccharomyces cerevisiae GSE16-T18SI.1 (T18) was immobilized in Ca-alginate and Ca-alginate–chitosan hybrid beads, and its performance on xylose fermentation was evaluated in terms of tolerance to different acetic acid concentrations (0–12 g/L) and repeated batches of crude sugarcane bagasse hemicellulose hydrolysate. The use of the hybrid gel improved yeast performance in the presence of 12 g/L of acetic acid, achieving 1.13 g/L/h of productivity and reaching 75% of the theoretical ethanol yield, with an improvement of 32% in the xylose consumption rate (1:1 V_beads/V_medium, 35 °C, 150 rpm and pH 5.2). The use of hybrid alginate–chitosan gel also led to better yeast performance at crude hydrolysate, yielding one more batch than the pure-alginate beads. These results demonstrate the potential of a hybrid gel as an approach that could increase 2G ethanol productivity and allow cell recycling for a longer period.

Lipozyme 435-Mediated Synthesis of Xylose Oleate in Methyl Ethyl Ketone

In this paper, we have performed the Lipozyme 435-catalyzed synthesis of xylose oleate in methyl ethyl ketone (MEK) from xylose and oleic acid. The effects of substrates’ molar ratios, reaction temperature, reaction time on esterification rates, and Lipozyme 435 reuse were studied. Results showed that an excess of oleic acid (xylose: oleic acid molar ratio of 1:5) significantly favored the reaction, yielding 98% of xylose conversion and 31% oleic acid conversion after 24 h-reaction (mainly to xylose mono- and dioleate, as confirmed by mass spectrometry). The highest Lipozyme 435 activities occurred between 55 and 70 °C. The predicted Ping Pong Bi Bi kinetic model fitted very well to the experimental data and there was no evidence of inhibitions in the range assessed. The reaction product was purified and presented an emulsion capacity close to that of a commercial sugar ester detergent. Finally, the repeated use of Lipozyme 435 showed a reduction in the reaction yields (by 48 and 19% in the xylose and oleic acid conversions, respectively), after ten 12 h-cycles.

Hemicellulosic Bioethanol Production from Fast-Growing Paulownia Biomass

In order to exploit a fast-growing Paulownia hardwood as an energy crop, a xylose-enriched hydrolysate was obtained in this work to increase the ethanol concentration using the hemicellulosic fraction, besides the already widely studied cellulosic fraction. For that, Paulownia elongata x fortunei was submitted to autohydrolysis treatment (210 °C or S0 of 4.08) for the xylan solubilization, mainly as xylooligosaccharides. Afterwards, sequential stages of acid hydrolysis, concentration, and detoxification were evaluated to obtain fermentable sugars. Thus, detoxified and non-detoxified hydrolysates (diluted or not) were fermented for ethanol production using a natural xylose-consuming yeast, Scheffersomyces stipitis CECT 1922, and an industrial Saccharomyces cerevisiae MEC1133 strain, metabolic engineered strain with the xylose reductase/xylitol dehydrogenase pathway. Results from fermentation assays showed that the engineered S. cerevisiae strain produced up to 14.2 g/L of ethanol (corresponding to 0.33 g/g of ethanol yield) using the non-detoxified hydrolysate. Nevertheless, the yeast S. stipitis reached similar values of ethanol, but only in the detoxified hydrolysate. Hence, the fermentation data prove the suitability and robustness of the engineered strain to ferment non-detoxified liquor, and the appropriateness of detoxification of liquor for the use of less robust yeast. In addition, the success of hemicellulose-to-ethanol production obtained in this work shows the Paulownia biomass as a suitable renewable source for ethanol production following a suitable fractionation process within a biorefinery approach.