Comparative study of bioethanol production from sugarcane molasses by using Zymomonas mobilis and Saccharomyces cerevisiae

Adaptive Laboratory Evolution and Metabolic Engineering of Zymomonas mobilis for Bioethanol Production Using Molasses

Molasses with abundant sugars is widely used for bioethanol production. Although the ethanologenic bacterium Zymomonas mobilis can use glucose, fructose, and sucrose for ethanol production, levan production from sucrose reduces the ethanol yield of molasses fermentation. To increase ethanol production from sucrose-rich molasses, Z. mobilis was adapted in molasses, sucrose, and fructose in parallel. Adaptation in fructose is the most effective route to generate an evolved strain F74 with improved molasses utilization, which is majorly due to a G99S mutation in Glf for enhanced fructose import. Subsequent sacB deletion and sacC overexpression in F74 to divert sucrose metabolism from levan production to ethanol production further enhanced ethanol productivity 28.6% to 1.35 g/L/h. The efficient utilization of molasses by diverting sucrose metabolic flux through adaptation and genome engineering not only generated an excellent ethanol producer using molasses but also provided the strategy for developing microbial cell factories.

Investigating ethanol production using the Zymomonas mobilis crude extract

Cell-free systems have become valuable investigating tools for metabolic engineering research due to their easy access to metabolism without the interference of the membrane. Therefore, we applied Zymomonas mobilis cell-free system to investigate whether ethanol production is controlled by the genes of the metabolic pathway or is limited by cofactors. Initially, different glucose concentrations were added to the extract to determine the crude extract's capability to produce ethanol. Then, we investigated the genes of the metabolic pathway to find the limiting step in the ethanol production pathway. Next, to identify the bottleneck gene, a systemic approach was applied based on the integration of gene expression data on a cell-free metabolic model. ZMO1696 was determined as the bottleneck gene and an activator for its enzyme was added to the extract to experimentally assess its effect on ethanol production. Then the effect of NAD+ addition at the high concentration of glucose (1 M) was evaluated, which indicates no improvement in efficiency. Finally, the imbalance ratio of ADP/ATP was found as the controlling factor by measuring ATP levels in the extract. Furthermore, sodium gluconate as a carbon source was utilized to investigate the expansion of substrate consumption by the extract. 100% of the maximum theoretical yield was obtained at 0.01 M of sodium gluconate while it cannot be consumed by Z. mobilis. This research demonstrated the challenges and advantages of using Z. mobilis crude extract for overproduction.

Simultaneous Saccharification and Fermentation of Empty Fruit Bunches of Palm for Bioethanol Production Using a Microbial Consortium of S. cerevisiae and T. harzianum

A simultaneous saccharification and fermentation (SSF) optimization process was carried out on pretreated empty fruit bunches (EFBs) by employing the Response Surface Methodology (RSM). EFBs were treated using sequential acid-alkali pretreatment and analyzed physically by a scanning electron microscope (SEM). The findings revealed that the pretreatment had changed the morphology and the EFBs’ structure. Then, the optimum combination of enzymes and microbes for bioethanol production was screened. Results showed that the combination of S. cerevisiae and T. harzianum and enzymes (cellulase and β-glucosidase) produced the highest bioethanol concentration with 11.76 g/L and a bioethanol yield of 0.29 g/g EFB using 4% (w/v) treated EFBs at 30 °C for 72 h. Next, the central composite design (CCD) of RSM was employed to optimize the SSF parameters of fermentation time, temperature, pH, and inoculum concentration for higher yield. The analysis of optimization by CCD predicted that 9.72 g/L of bioethanol (0.46 g/g ethanol yield, 90.63% conversion efficiency) could be obtained at 72 h, 30 °C, pH 4.8, and 6.79% (v/v) of inoculum concentration using 2% (w/v) treated EFBs. Results showed that the fermentation process conducted using the optimized conditions produced 9.65 g/L of bioethanol, 0.46 g/g ethanol yield, and 89.56% conversion efficiency, which was in close proximity to the predicted CCD model.

Comprehensive Review on Potential Contamination in Fuel Ethanol Production with Proposed Specific Guideline Criteria

Ethanol is a promising biofuel that can replace fossil fuel, mitigate greenhouse gas (GHG) emissions, and represent a renewable building block for biochemical production. Ethanol can be produced from various feedstocks. First-generation ethanol is mainly produced from sugar- and starch-containing feedstocks. For second-generation ethanol, lignocellulosic biomass is used as a feedstock. Typically, ethanol production contains four major steps, including the conversion of feedstock, fermentation, ethanol recovery, and ethanol storage. Each feedstock requires different procedures for its conversion to fermentable sugar. Lignocellulosic biomass requires extra pretreatment compared to sugar and starch feedstocks to disrupt the structure and improve enzymatic hydrolysis efficiency. Many pretreatment methods are available such as physical, chemical, physicochemical, and biological methods. However, the greatest concern regarding the pretreatment process is inhibitor formation, which might retard enzymatic hydrolysis and fermentation. The main inhibitors are furan derivatives, aromatic compounds, and organic acids. Actions to minimize the effects of inhibitors, detoxification, changing fermentation strategies, and metabolic engineering can subsequently be conducted. In addition to the inhibitors from pretreatment, chemicals used during the pretreatment and fermentation of byproducts may remain in the final product if they are not removed by ethanol distillation and dehydration. Maintaining the quality of ethanol during storage is another concerning issue. Initial impurities of ethanol being stored and its nature, including hygroscopic, high oxygen and carbon dioxide solubility, influence chemical reactions during the storage period and change ethanol’s characteristics (e.g., water content, ethanol content, acidity, pH, and electrical conductivity). During ethanol storage periods, nitrogen blanketing and corrosion inhibitors can be applied to reduce the quality degradation rate, the selection of which depends on several factors, such as cost and storage duration. This review article sheds light on the techniques of control used in ethanol fuel production, and also includes specific guidelines to control ethanol quality during production and the storage period in order to preserve ethanol production from first-generation to second-generation feedstock. Finally, the understanding of impurity/inhibitor formation and controlled strategies is crucial. These need to be considered when driving higher ethanol blending mandates in the short term, utilizing ethanol as a renewable building block for chemicals, or adopting ethanol as a hydrogen carrier for the long-term future, as has been recommended.

Zymomonas mobilis as an emerging biotechnological chassis for the production of industrially relevant compounds

Zymomonas mobilis is a well-recognized ethanologenic bacterium with outstanding characteristics which make it a promising platform for the biotechnological production of relevant building blocks and fine chemicals compounds. In the last years, research has been focused on the physiological, genetic, and metabolic engineering strategies aiming at expanding Z. mobilis ability to metabolize lignocellulosic substrates toward biofuel production. With the expansion of the Z. mobilis molecular and computational modeling toolbox, the potential of this bacterium as a cell factory has been thoroughly explored. The number of genomic, transcriptomic, proteomic, and fluxomic data that is becoming available for this bacterium has increased. For this reason, in the forthcoming years, systems biology is expected to continue driving the improvement of Z. mobilis for current and emergent biotechnological applications. While the existing molecular toolbox allowed the creation of stable Z. mobilis strains with improved traits for pinpointed biotechnological applications, the development of new and more flexible tools is crucial to boost the engineering capabilities of this bacterium. Novel genetic toolkits based on the CRISPR-Cas9 system and recombineering have been recently used for the metabolic engineering of Z. mobilis. However, they are mostly at the proof-of-concept stage and need to be further improved.

Adaptive laboratory evolution principles and applications in industrial biotechnology

Adaptive laboratory evolution (ALE) is an innovative approach for the generation of evolved microbial strains with desired characteristics, by implementing the rules of natural selection as presented in the Darwinian Theory, on the laboratory bench. New as it might be, it has already been used by several researchers for the amelioration of a variety of characteristics of widely used microorganisms in biotechnology. ALE is used as a tool for the deeper understanding of the genetic and/or metabolic pathways of evolution. Another important field targeted by ALE is the manufacturing of products of (high) added value, such as ethanol, butanol and lipids. In the current review, we discuss the basic principles and techniques of ALE, and then we focus on studies where it has been applied to bacteria, fungi and microalgae, aiming to improve their performance to biotechnological procedures and/or inspect the genetic background of evolution. We conclude that ALE is a promising and efficacious method that has already led to the acquisition of useful new microbiological strains in biotechnology and could possibly offer even more interesting results in the future.

Pretreatment technologies for industrial effluents: Critical review on bioenergy production and environmental concerns

The implementation of different pretreatment techniques and technologies prior to effluent discharge is a direct result of the inefficiency of several existing wastewater treatment methods. A majority of the industrial sectors have known to cause severe negative effects on the environment. The five major polluting industries are the paper and pulp mills, coal manufacturing facilities, petrochemical, textile and the pharmaceutical sectors. Pretreatment methods have been widely used in order to lower the toxicity levels of effluents and comply with environmental standards. In this review, the possible environmental benefits and concerns of adopting different pretreatment technologies for renewable energy production and product/resource recovery has been reviewed and discussed.

Exploitation of olive oil mill wastewaters and molasses for ethanol production using immobilized cells of Saccharomyces cerevisiae

An alcoholic fermentation process is described, involving molasses, the main by-product of the sugar industry, blended with crude olive oil mill wastewaters (OOMWs) and immobilized Saccharomyces cerevisiae cells on delignified cellulosic material (DCM). For comparison, fermentations with free cells were also carried out. Initially, the optimum blending mixture for molasses dilution was sought after, while at a second step repeated batch fermentations at a temperature range 5-30 °C were performed to monitor the operational stability of the system. A 1/1 ratio of OOMWs/tap water blending mixture and cell immobilization resulted in higher fermentation parameters. Ethanol concentration and daily productivity values recorded at temperatures ≥ 20 °C (up to 67.8 g L^-1 and 67.6 g L^-1 d^-1, respectively) could be adopted by the industrial sector, although the decline in fermentation efficiency observed, probably due to the toxicity effects of OOMWs. Finally, the potential of OOMWs treatment for ethanol production is highlighted and assessed.

Bioprocessing Requirements for Bioethanol

This chapter discusses alternative energy sources and the advantages of biofuels over fossil fuels. It outlines the main steps of bioethanol production and suggests some alternative sources as potential feedstock. The core focus of this chapter is to examine new research which considers the use of agricultural waste as a feedstock for bioethanol production rather than conventional feedstocks such as sugarcane and corn. The advantages of sugarcane bagasse as a feedstock are discussed in detail and the bioprocessing requirements are studied in comparison to traditional methods that use sugarcane as the feedstock. The chapter concludes by briefly outlining further research that could potentially improve these processes.

Unconventional bacterial association for dough leavening

The purpose of the research was to obtain innovative yeast-free doughs leavened by Zymomonas mobilis and Lactobacillus sanfranciscensis. Z. mobilis, as well as Saccharomyces cerevisiae, produces an equimolar mixture of ethanol and CO₂ through glucose, fructose or sucrose fermentation. In the present work, the inability of Z. mobilis to metabolize maltose has been circumvented by the addition of L. sanfranciscensis in the formulation. Indeed, L. sanfranciscensis, a heterofermentative lactic acid bacterium (LAB) typical of sourdough environment, hydrolyzes maltose releasing glucose which can be used by Z. mobilis for its metabolism. Biomass samples of Z. mobilis subs. mobilis DSM 424 and L. sanfranciscensis DSM 20663 were grown separately in liquid media and then associated in a model dough. Leavening trials set up by using three different microbial combinations (Lactobacillus:Zymomonas 80+80mg, 15+145mg and 145+15mg biomass, i.e. 1:1, 1:10 and 10:1 respectively on cell dry weight basis) evidenced CO₂ production levels (mL) higher than the mathematical sum of CO₂ produced by the single bacteria. In particular, when the biomass combination of L. sanfranciscensis and Z. mobilis was 1:1 (80+80mg cdw) and 10:1 (145+15mg cdw) a CO₂ production of 46.3-41.4mL versus 26.7-28.5mL was achieved. The calculated productivity gain showed positive performances of the microbial combination up to 180-240min leavening. The subsequent efficiency loss may be due several factors, above all glucose shortage for Z. mobilis, as well as decrease of dough pH that can negatively affect both Lactobacillus and Zymomonas metabolism. As in traditional sourdoughs, L. sanfranciscensis was responsible for the souring activity with positive effects on both dough tasting and reduction of spoilage microbiota; Z. mobilis was instead responsible for most of the CO₂ production. A bakery product leavened with the unconventional association Z. mobilis - L. sanfranciscensis will be addressed to people having adverse responses to the ingestion of bakery goods, thus providing innovation in the area of yeast-free leavened food.