Optimizing ethanol production by thermotolerant Kluyveromyces marxianus CCT 7735 in a mixture of sugarcane bagasse and ricotta whey

Stress response and adaptation mechanisms in Kluyveromyces marxianus

Kluyveromyces marxianus is a non-Saccharomyces yeast that has gained importance due to its great potential to be used in the food and biotechnology industries. In general, K. marxianus is a known yeast for its ability to assimilate hexoses and pentoses; even this yeast can grow in disaccharides such as sucrose and lactose and polysaccharides such as agave fructans. Otherwise, K. marxianus is an excellent microorganism to produce metabolites of biotechnological interest, such as enzymes, ethanol, aroma compounds, organic acids, and single-cell proteins. However, several studies highlighted the metabolic trait variations among the K. marxianus strains, suggesting genetic diversity within the species that determines its metabolic functions; this diversity can be attributed to its high adaptation capacity against stressful environments. The outstanding metabolic characteristics of K. marxianus have motivated this yeast to be a study model to evaluate its easy adaptability to several environments. This chapter will discuss overview characteristics and applications of K. marxianus and recent insights into the stress response and adaptation mechanisms used by this non-Saccharomyces yeast.

Enhancing bioflavor production by solid-state fermentation using Kluyveromyces marxianus and l-phenylalanine

This study includes the utilization of sweet lemon peel (SLP) and sugarcane bagasse (SB) in solid-state fermentation using Kluyveromyces marxianus for bioflavor compounds production adopting response surface methodology. The major flavor compounds, 2-phenylethanol (2-PE) and 2-phenylethyl acetate (2-PEA) were quantified using gas chromatography-mass spectrometry with and without adding any supplements. Quantification of flavor compounds indicated that without adding any accessory in the substrate, the concentration of 2-PE using SLP and SB was 0.15 ± 0.003 mg/g and 0.14 ± 0.002 mg/g, respectively. Whereas 2-PEA concentration using SLP and SB was observed as 0.01 ± 0.008 mg/g and 0.02 ± 0.001 mg/g, respectively. The addition of l-phenylalanine (l-phe) in the substrates showed 30%-75% enhancement in the production of 2-PE and 2-PEA. The present study indicates that the K. marxianus is a potential microbial cell factory for the production of 2-PE and 2-PEA with the addition of synthetic l-phe having a plethora of applications in food and pharmaceutical industries.

Kluyveromyces marxianus as a microbial cell factory for lignocellulosic biomass valorisation

The non-conventional yeast Kluyveromyces marxianus is widely used for several biotechnological applications, mainly due to its thermotolerance, high growth rate, and ability to metabolise a wide range of sugars. These cell traits are strategic for lignocellulosic biomass valorisation and strain diversity prompts the development of robust chassis, either with improved tolerance to lignocellulosic inhibitors or ethanol. This review summarises bioethanol and value-added chemicals production by K. marxianus from different lignocellulosic biomasses. Moreover, metabolic engineering and process optimization strategies developed to expand K. marxianus potential are also compiled, as well as studies reporting cell mechanisms to cope with lignocellulosic-derived inhibitors. The main lignocellulosic-based products are bioethanol, representing 71% of the reports, and xylitol, representing 17% of the reports. K. marxianus also proved to be a good chassis for lactic acid and volatile compounds production from lignocellulosic biomass, although the literature on this matter is still scarce. The increasing advances in genome editing tools and process optimization strategies will widen the K. marxianus-based portfolio products.

Past, Present, and Future Perspectives on Whey as a Promising Feedstock for Bioethanol Production by Yeast

Concerns about fossil fuel depletion and the environmental effects of greenhouse gas emissions have led to widespread fermentation-based production of bioethanol from corn starch or sugarcane. However, competition for arable land with food production has led to the extensive investigation of lignocellulosic sources and waste products of the food industry as alternative sources of fermentable sugars. In particular, whey, a lactose-rich, inexpensive byproduct of dairy production, is available in stable, high quantities worldwide. This review summarizes strategies and specific factors essential for efficient lactose/whey fermentation to ethanol. In particular, we cover the most commonly used strains and approaches for developing high-performance strains that tolerate fermentation conditions. The relevant genes and regulatory systems controlling lactose utilization and sources of new genes are also discussed in detail. Moreover, this review covers the optimal conditions, various feedstocks that can be coupled with whey substrates, and enzyme supplements for increasing efficiency and yield. In addition to the historical advances in bioethanol production from whey, this review explores the future of yeast-based fermentation of lactose or whey products for beverage or fuel ethanol as a fertile research area for advanced, environmentally friendly uses of industrial waste products.

Screening and Growth Characterization of Non-conventional Yeasts in a Hemicellulosic Hydrolysate

Lignocellulosic biomass is an attractive raw material for the sustainable production of chemicals and materials using microbial cell factories. Most of the existing bioprocesses focus on second-generation ethanol production using genetically modified Saccharomyces cerevisiae, however, this microorganism is naturally unable to consume xylose. Moreover, extensive metabolic engineering has to be carried out to achieve high production levels of industrially relevant building blocks. Hence, the use of non-Saccharomyces species, or non-conventional yeasts, bearing native metabolic routes, allows conversion of a wide range of substrates into different products, and higher tolerance to inhibitors improves the efficiency of biorefineries. In this study, nine non-conventional yeast strains were selected and screened on a diluted hemicellulosic hydrolysate from Birch. Kluyveromyces marxianus CBS 6556, Scheffersomyces stipitis CBS 5773, Lipomyces starkeyi DSM 70295, and Rhodotorula toruloides CCT 7815 were selected for further characterization, where their growth and substrate consumption patterns were analyzed under industrially relevant substrate concentrations and controlled environmental conditions in bioreactors. K. marxianus CBS 6556 performed poorly under higher hydrolysate concentrations, although this yeast was determined among the fastest-growing yeasts on diluted hydrolysate. S. stipitis CBS 5773 demonstrated a low growth and biomass production while consuming glucose, while during the xylose-phase, the specific growth and sugar co-consumption rates were among the highest of this study (0.17 h^–1 and 0.37 g/gdw*h, respectively). L. starkeyi DSM 70295 and R. toruloides CCT 7815 were the fastest to consume the provided sugars at high hydrolysate conditions, finishing them within 54 and 30 h, respectively. R. toruloides CCT 7815 performed the best of all four studied strains and tested conditions, showing the highest specific growth (0.23 h^–1), substrate co-consumption (0.73 ± 0.02 g/gdw*h), and xylose consumption (0.22 g/gdw*h) rates. Furthermore, R. toruloides CCT 7815 was able to produce 10.95 ± 1.37 gL^–1 and 1.72 ± 0.04 mgL^–1 of lipids and carotenoids, respectively, under non-optimized cultivation conditions. The study provides novel information on selecting suitable host strains for biorefinery processes, provides detailed information on substrate consumption patterns, and pinpoints to bottlenecks possible to address using metabolic engineering or adaptive evolution experiments.

Optimization of ethanol production using newly isolated ethanologenic yeasts

Yeasts are important microorganisms used for ethanol production; however, they are not equally efficient in the amount of ethanol production under different environmental conditions. It is, therefore, necessary to screen for elite strains to utilize them for commercial production of these commodities. In this study, yeasts were isolated from different Ethiopian traditional fermented alcoholic beverages (teji, tella, shamiata and areqe tinisis), milk and ergo, teff and maize dough, soil and compost, flowers, and fruits to evaluate their potential use for ethanol fermentation process. Isolates were screened for efficient ethanol production and the selected ones were identified using phenotypic and genetic characters using D1/D2 region of LSU rDNA sequence analysis. The yeast isolates were evaluated based on their growth and fermentation of different carbon sources. Response surface methodology (RSM) was applied to optimize temperature, pH and incubation time using central composite design (CCD) in Design-Expert 7.0.0. A total of 211 yeasts colonies were isolated of which 60% were ethanologenic yeasts (ethanol producers) and 40% were non-ethanol producers. The yeast population detected from various sources was in the range of 10 ⁵ CFU from traditional foods and beverages to that of 10 ³ CFU from fruits and soil samples. The data also showed that the number of colony types (diversity) did not correlate with population density. The highly fermentative isolates were taxonomically characterized into four genera, of which 65% of the isolates (ETP37, ETP50; ETP53, ETP89, ETP94) were categorized under Saccharomyces cerevisiae, and the remaining were Pichia fermentans ETP22, Kluyveromyces marxianus ETP87, and Candida humilis ETP122. The S. cerevisiae isolates produced ethanol (7.6-9.0 g/L) similar with K. marxianus ETP87 producing 7.97 g/L; comparable to the ethanol produced from commercial baker's yeast (8.43 g/L) from 20 g/L dextrose; whereas C. humilis ETP122 and P. fermentans ETP22 produced 5.37 g/L and 6.43 g/L ethanol, respectively. S. cerevisiae ETP53, K. marxianus ETP87, P. fermentans ETP22 and C. humilis ETP122 tolerated 10% extraneous ethanol but the percentage of ethanol tolerance considerably decreased upon 15%. S. cerevisiae ETP53 produced ethanol optimally at pH 5.0, 60 h, and 34 ^o C. pH 4.8, temperature 36 ^o C, and 65 h of time were optimal growth conditions of ethanol fermentation by K. marxianus ETP87. The ethanol fermentation conditions of P. fermentans ETP22 was similar to S. cerevisiae ETP53 though the ethanol titer of S. cerevisiae ETP53 was higher than P. fermentans ETP22. Therefore, S. cerevisiae ETP53, K. marxianus and P. fermentans ETP22 are good candidates for ethanol production.

Kluyveromyces marxianus: Current State of Omics Studies, Strain Improvement Strategy and Potential Industrial Implementation

Bioethanol is considered an excellent alternative to fossil fuels, since it importantly contributes to the reduced consumption of crude oil, and to the alleviation of environmental pollution. Up to now, the baker yeast Saccharomyces cerevisiae is the most common eukaryotic microorganism used in ethanol production. The inability of S. cerevisiae to grow on pentoses, however, hinders its effective growth on plant biomass hydrolysates, which contain large amounts of C5 and C12 sugars. The industrial-scale bioprocessing requires high temperature bioreactors, diverse carbon sources, and the high titer production of volatile compounds. These criteria indicate that the search for alternative microbes possessing useful traits that meet the required standards of bioethanol production is necessary. Compared to other yeasts, Kluyveromyces marxianus has several advantages over others, e.g., it could grow on a broad spectrum of substrates (C5, C6 and C12 sugars); tolerate high temperature, toxins, and a wide range of pH values; and produce volatile short-chain ester. K. marxianus also shows a high ethanol production rate at high temperature and is a Crabtree-negative species. These attributes make K. marxianus promising as an industrial host for the biosynthesis of biofuels and other valuable chemicals.

Technical integrative approaches to cheese whey valorization towards sustainable environment

Whey, a byproduct of cheese production, is often treated as an industrial dairy waste. A large volume of this product is disposed of annually due to inadequate bioconversion approaches. With its high pollutant load, disposal without pretreatment has raised a lot of environmental concerns alerting the need to seek optimal methods for adequately extracting and utilizing its organic content. In recent years, several techniques for whey valorization have emerged which may serve as interventionary measures against its environmental effects after disposal. In this review, we discuss five major approaches, by which whey can be converted into eco-friendly products, to significantly cut whey wastage. The approaches to whey valorization are therefore examined under the following perspectives: whey as a raw material for the production of bioethanol and prebiotic oligosaccharides via β-galactosidase and microbe catalyzed reactions, for the production of refined lactose as an excipient for pharmaceutical purposes, and the clinical significance of whey hydrolysates and their antifungal activity in food processing.

Genomic analysis and lactose transporter expression in Kluyveromyces marxianus CCT 7735

Kluyveromyces marxianus CCT 7735 has been used to produce ethanol, aromatic compounds, enzymes and heterologous proteins besides assimilates lactose as carbon source. Its genome has 10.7 Mb and encodes 4787 genes distributed in 8 nuclear chromosomes and one mitochondrial. Contrary to Kluyveromyces lactis, which has a unique LAC12 gene (encodes lactose permease), K. marxianus possesses four. The presence of degenerated copies and Solo-LTRs related to retrotransposon TKM close to the LAC12 genes in K. marxianus indicates ectopic recombinations. The Lac12 permeases of K. marxianus and K. lactis are conserved, however the conservation is higher between the copy of the left side of the chromosome three and the unique copy of K. lactis, indicating that this copy is the ancestor. The expression of the four LAC12 genes occurred in aerobiosis and hypoxia. Notably, the high lactose consumption in hypoxia seems to be related to the high expression of the LAC12 genes.

Valuation of agro-industrial wastes as substrates for heterologous production of α-galactosidase

Background

The recycling of agro-industrial wastes is at present limited by the availability of efficient and low-cost enzyme cocktails. The use of these materials as culture media to produce the enzymes can contribute to the profitability of the recycling process and to the circular economy. The aim of this work is the construction of a recombinant yeast strain efficient to grow in mixed whey (residue of cheese making) and beet molasses (residue of sugar manufacture) as culture medium, and to produce heterologous α-galactosidase, an enzyme with varied industrial applications and wide market.

Results

The gene MEL1, encoding the α-galactosidase of Saccharomyces cerevisiae, was integrated (four copies) in the LAC4 locus of the Kluyveromyces lactis industrial strain GG799. The constructed recombinant strain produces high levels of extracellular α-galactosidase under the control of the LAC4 promoter, inducible by lactose and galactose, and the native MEL1 secretion signal peptide. K. lactis produces natively beta-galactosidase and invertase thus metabolizing the sugars of whey and molasses. A culture medium based on whey and molasses was statistically optimized, and then the cultures scaled-up at laboratory level, thus obtaining 19 U/mL of heterologous α-galactosidase with a productivity of 0.158 U/L h, which is the highest value reported hitherto from a cheap waste-based medium.

Conclusions

A K. lactis recombinant strain was constructed and a sustainable culture medium, based on a mixture of cheese whey and beet molasses, was optimized for high productivity of S. cerevisiae α-galactosidase, thus contributing to the circular economy by producing a heterologous enzyme from two agro-industrial wastes.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0988-6) contains supplementary material, which is available to authorized users.

Boosting bioethanol production from Eucalyptus wood by whey incorporation

The mixture of Eucalyptus globulus wood (EGW) and cheese whey powder (CWP) was proposed for intensification of simultaneous saccharification and fermentation (SSF) at high temperature and solid loadings using the industrial Saccharomyces cerevisiae Ethanol Red® strain. High ethanol concentration (93 g/L), corresponding to 94% ethanol yield, was obtained at 35 °C from 37% of solid mixture using cellulase and β-galactosidase enzymes (24.2 FPU/g and 20.0 U/g, respectively). The use of CWP mixed with pretreated EGW increased the ethanol concentration in 1.5-fold, in comparison with SSF experiments without CWP for both Ethanol Red® and CEN.PK113-7D strains. Moreover, 1.4-fold higher ethanol concentration was obtained with Ethanol Red®, in comparison with CEN.PK113-7D strain. Ethanol Red® strain was genetically engineered for β-galactosidase production in order to advance towards a fully integrated process. This work shows the feasibility of attaining high ethanol concentrations in second generation bioprocesses by a multi-waste valorization approach.