Mixed Sugars Research Articles

Metabolic engineering of Corynebacterium glutamicum for shikimate overproduction by growth-arrested cell reaction

Corynebacterium glutamicum with the ability to simultaneously utilize glucose/pentose mixed sugars was metabolically engineered to overproduce shikimate, a valuable hydroaromatic compound used as a starting material for the synthesis of the anti-influenza drug oseltamivir. To achieve this, the shikimate kinase and other potential metabolic activities for the consumption of shikimate and its precursor dehydroshikimate were inactivated. Carbon flux toward shikimate synthesis was enhanced by overexpression of genes for the shikimate pathway and the non-oxidative pentose phosphate pathway. Subsequently, to improve the availability of the key aromatics precursor phosphoenolpyruvate (PEP) toward shikimate synthesis, the PEP: sugar phosphotransferase system (PTS) was inactivated and an endogenous myo-inositol transporter IolT1 and glucokinases were overexpressed. Unexpectedly, the resultant non-PTS strain accumulated 1,3-dihydroxyacetone (DHA) and glycerol as major byproducts. This observation and metabolome analysis identified glyceraldehyde-3-phosphate dehydrogenase (GAPDH)-catalyzed reaction as a limiting step in glycolysis. Consistently, overexpression of GAPDH significantly stimulated both glucose consumption and shikimate production. Blockage of the DHA synthesis further improved shikimate yield. We applied an aerobic, growth-arrested and high-density cell reaction to the shikimate production by the resulting strain and notably achieved the highest shikimate titer (141g/l) and a yield (51% (mol/mol)) from glucose reported to date after 48h in minimal medium lacking nutrients required for cell growth. Moreover, comparable shikimate productivity could be attained through simultaneous utilization of glucose, xylose, and arabinose, enabling efficient shikimate production from lignocellulosic feedstocks. These findings demonstrate that C. glutamicum has significant potential for the production of shikimate and derived aromatic compounds.

Transcriptomes of a xylose-utilizing industrial flocculating Saccharomyces cerevisiae strain cultured in media containing different sugar sources

Lignocellulosic hydrolysates used for bioethanol production contain a mixture of sugars, with xylose being the second most abundant after glucose. Since xylose is not a natural substrate for Saccharomyces cerevisiae, recombinant S. cerevisiae strongly prefers glucose over xylose, and the fermentation rate and ethanol yield with xylose are both lower than those with glucose. To determine the molecular basis for glucose and xylose fermentation, we used microarrays to investigate the transcriptional difference of a xylose-utilizing industrial strain cultured in both single sugar media and a mixed sugar medium of glucose and xylose. The transcriptomes were nearly identical between glucose metabolizing cells in the glucose alone medium and those in the glucose fermentation phase in the mixed-sugar medium. Whereas the transcriptomes highly differed between the xylose metabolizing cells in the xylose alone medium and those in the xylose fermentation phase in the mixed sugar medium, and the differences mainly involved sulfur metabolism. When the transcriptional profiles were compared between glucose fermentation state and xylose fermentation state, we found the expression patterns of hexose transporters and glucose signaling pathway differed in response to different sugar sources, and the expression levels of the genes involved in gluconeogenesis, the glyoxylate and tricarboxylic acid cycles and respiration increased with xylose, indicating that the xylose-metabolizing cells had high requirements for maintenance energy and lacked the carbon catabolite repression capability. The effect of carbon catabolite repression by glucose lasted after glucose depletion for specific genes to different extents.Electronic supplementary materialThe online version of this article (doi:10.1186/s13568-016-0223-y) contains supplementary material, which is available to authorized users.

Succinic acid production by Actinobacillus succinogenes from batch fermentation of mixed sugars.

Succinic acid production from the monosaccharides xylose, arabinose, glucose, mannose and galactose was studied using the bacterium Actinobacillus succinogenes. In Duran bottle cultures, containing 10g/L of each of sugar, succinic acid was produced from all sugars except for galactose. The highest succinate yield, 0.56g/g, was obtained with glucose, whereas the succinate yield was 0.42, 0.38 and 0.44g/g for xylose, mannose and arabinose, respectively. The specific succinate productivity was 0.7g/gh for glucose, but below 0.2g/gh for the other sugars. Batch bioreactor fermentations were carried out using a sugar mixture of the five sugars giving a total concentration of 50g/L, mimicking the distribution of sugars in spent sulfite liquor (SSL) from Eucalyptus which is rich in xylose. In this mixture, an almost complete conversion of all sugars (except galactose) was achieved resulting in a final succinate concentration of 21.8-26.8g/L and a total yield of 0.59-0.68g/g. There was evidence of co-consumption of glucose and xylose, whereas mannose was consumed after glucose. The main by-products were acetate 0.14-0.20g/g and formate 0.08-0.13g/g. NADH balance calculations suggested that NADH required for succinate production was not met solely from formate and acetate production, but other means of NADH production was necessary. Results from mixed sugar fermentations were verified using SSL as substrate resulting in a succinate yield of 0.60g/g. In addition, it was found that CO2 sparging could replace carbonate supply in the form of MgCO3 without affecting the succinate yield.

Negative frequency-dependent interactions can underlie phenotypic heterogeneity in a clonal microbial population.

Genetically identical cells in microbial populations often exhibit a remarkable degree of phenotypic heterogeneity even in homogenous environments. Such heterogeneity is commonly thought to represent a bet‐hedging strategy against environmental uncertainty. However, evolutionary game theory predicts that phenotypic heterogeneity may also be a response to negative frequency‐dependent interactions that favor rare phenotypes over common ones. Here we provide experimental evidence for this alternative explanation in the context of the well‐studied yeast GAL network. In an environment containing the two sugars glucose and galactose, the yeast GAL network displays stochastic bimodal activation. We show that in this mixed sugar environment, GAL‐ON and GAL‐OFF phenotypes can each invade the opposite phenotype when rare and that there exists a resulting stable mix of phenotypes. Consistent with theoretical predictions, the resulting stable mix of phenotypes is not necessarily optimal for population growth. We find that the wild‐type mixed strategist GAL network can invade populations of both pure strategists while remaining uninvasible by either. Lastly, using laboratory evolution we show that this mixed resource environment can directly drive the de novo evolution of clonal phenotypic heterogeneity from a pure strategist population. Taken together, our results provide experimental evidence that negative frequency‐dependent interactions can underlie the phenotypic heterogeneity found in clonal microbial populations.

Butyric acid production from softwood hydrolysate by acetate-consuming Clostridium sp. S1 with high butyric acid yield and selectivity

The aim of this work was to study the butyric acid production from softwood hydrolysate by acetate-consuming Clostridium sp. S1. Results showed that Clostridium sp. S1 produced butyric acid by simultaneously utilizing glucose and mannose in softwood hydrolysate and, more remarkably, it consumed acetic acid in hydrolysate. Clostridium sp. S1 utilized each of glucose, mannose, and xylose as well as mixed sugars simultaneously with partially repressed xylose utilization. When softwood (Japanese larch) hydrolysate containing glucose and mannose as the main sugars was used, Clostridium sp. S1 produced 21.17g/L butyric acid with the yield of 0.47g/g sugar and the selectivity of 1 (g butyric acid/g total acids) owing to the consumption of acetic acid in hydrolysate. The results demonstrate potential of Clostridium sp. S1 to produce butyric acid selectively and effectively from hydrolysate not only by utilizing mixed sugars simultaneously but also by converting acetic acid to butyric acid.

Production of fermentable sugars from corn fiber using soaking in aqueous ammonia (SAA) pretreatment and fermentation to succinic acid using Escherichia coli AFP184

Conversion of corn fiber (CF), a by-product from the corn-to-ethanol conversion process, into fermentable sugar and succinic acid was investigated using soaking in aqueous ammonia (SAA) pretreatment followed by biological conversions, including enzymatic hydrolysis and fermentation using genetically engineered E. coli (AFP184). The SAA pretreatment (using a 15% w/w NH4OH solution at a solid-to-liquid ratio of 1: 10 at 60 °C for 24 h) removed 20-38% of lignin and significantly improved the digestibility of the treated solid (85-99% of glucan digestibility). Following the enzymatic hydrolysis, the sugar-rich hydrolysate was subjected to dilute sulfuric acid treatment (1 wt% sulfuric acid and 120 °C for 1 h), which hydrolyzed the oligosaccharides in the hydrolysate into fermentable monomeric sugars. The mixed sugar hydrolysates containing hexose and pentose obtained from the two-step hydrolysis and SAA pretreatment were fermented to succinic acid using a genetically engineered microorganism, Escherichia coli AFP184, for evaluating the fermentability. Engineered E. coli AFP184 effectively converted soluble sugars in the hydrolysate to succinic acid (20.7 g/L), and the production rate and yield were further enhanced with additional nutrients; the highest concentration of succinic acid was 26.3 g/L for 48 h of fermentation.

Modelling succinic acid fermentation using a xylose based substrate

This study focuses on the development of unstructured models, including both substrate and product inhibition, that predict the cultivation of Actinobacillus succinogenes and Basfia succiniciproducens on a mixture of C5 and C6 sugars, similar to the sugar composition contained in spent sulphite liquor, the liquid waste stream from the sulphite pulping process. The main sugar monomer contained in the medium was xylose (72.6%) with galactose (12.2%), glucose (10.9%), mannose (4.2%) and arabinose (0.1%) making up the remaining sugar content. The growth inhibition caused by metabolic products (succinic, lactic, acetic, formic and mixed acids) and initial mixed sugar concentration was determined. The highest obtained succinic acid yield, final concentration and productivity in fermentations carried out in Duran bottles were 0.76g/g, 26.0g/L and 0.66g/L/h for B. succiniciproducens and 0.69g/g, 27.4g/L and 0.60g/L/h for A. succinogenes, respectively (the units in yield calculations are referred to grams of succinic acid produced per gram of total sugars consumed). The kinetic parameters for both strains were estimated from experimental results. The obtained R2 values for the fitted models were 0.96 for A. succinogenes and 0.94 for B. succiniciproducens. A sensitivity analysis on the obtained parameters showed that the maximum specific growth rates (μmax) and the growth associated substrate consumption parameters (γ) are the most influential model parameters for both microorganisms. The model was validated by fermentations conducted in lab-scale bioreactors showing good agreement between experimental data and model simulations.

Fermentative production of l-galactonate by using recombinant Saccharomyces cerevisiae containing the endogenous galacturonate reductase gene from Cryptococcus diffluens

The GAR1 gene, encoding d-galacturonate reductase in Cryptococcus diffluens, was isolated, and the GAR1-expression plasmid was constructed by insertion of GAR1 downstream of the yeast constitutive promoter in the yeast-integrating vector. Recombinant Saccharomyces cerevisiae expressing C.diffluensd-galacturonate reductase from a genome integrated copy of the gene was cultured for use the conversion of d-galacturonic acid to l-galactonic acid. The optimum conditions for l-galactonic acid production were determined in terms of the initial concentration of d-galacturonic acid, fermentation pH, and mixed sugars. The following conditions yielded high efficiency in the conversion of d-galacturonic acid to l-galactonic acid in large-scale cultures: 0.1% initial d-galacturonic acid concentration, pH 3.5, and glucose as additional sugar. The aerobic condition was necessary for the conversion of d-galacturonic acid. Subculture of that recombinant was not showing to decrease of the d-galacturonic acid conversion rate even though it was repeated in ten generations. Culturing in scale-up, the conversion rate of d-galacturonic acid to l-galactonic acid was increased.

Development of Yogurts Enriched with Antioxidants from Wine

ABSTRACTThis study aimed to develop a new food product, attractive from the organoleptic point of view and with the advantage of being rich in phenolic compounds with antioxidant activity. For that, a phenolic-rich solution was obtained from wine and included in the recipe of the yogurts. The product developed was therefore a yogurt with wine antioxidants, prepared in different aroma variations. The results showed that the addition of the phenolic-rich solution caused an increase in the content of antioxidants without affecting acidity. On the other hand, based on sensory analysis, it was possible to choose two final products which were the favorite among the members of the sensorial panel, namely the yogurt with flavor of mixed fruits and sugar and the addition of the wine antioxidants solution and the yogurt with flavor of cinnamon and sugar and the addition of wine antioxidants.

Contrasted enzymatic cocktails reveal the importance of cellulases and hemicellulases activity ratios for the hydrolysis of cellulose in presence of xylans.

Various enzymatic cocktails were produced from two Trichoderma reesei strains, a cellulase hyperproducer strain and a strain with β-glucosidase activity overexpression. By using various carbon sources (lactose, glucose, xylose, hemicellulosic hydrolysate) for strains growth, contrasted enzymatic activities were obtained. The enzymatic cocktails presented various levels of efficiency for the hydrolysis of cellulose Avicel into glucose, in presence of xylans, or not. These latter were also hydrolyzed with different extents according to cocktails. The most efficient cocktails (TR1 and TR3) on Avicel were richer in filter paper activity (FPU) and presented a low ratio FPU/β-glucosidase activity. Cocktails TR2 and TR5 which were produced on the higher amount of hemicellulosic hydrolysate, possess both high xylanase and β-xylosidase activities, and were the most efficient for xylans hydrolysis. When hydrolysis of Avicel was conducted in presence of xylans, a decrease of glucose release occurred for all cocktails compared to hydrolysis of Avicel alone. Mixing TR1 and TR5 cocktails with two different ratios of proteins (1/1 and 1/4) resulted in a gain of efficiency for glucose release during hydrolysis of Avicel in presence of xylans compared to TR5 alone. Our results demonstrate the importance of combining hemicellulase and cellulase activities to improve the yields of glucose release from Avicel in presence of xylans. In this context, strategies involving enzymes production with carbon sources comprising mixed C5 and C6 sugars or combining different cocktails produced on C5 or on C6 sugars are of interest for processes developed in the context of lignocellulosic biorefinery.

Ethanol production from lignocellulosic hydrolysates using engineered Saccharomyces cerevisiae harboring xylose isomerase-based pathway

The efficient co-fermentation of glucose and xylose is necessary for the economically feasible bioethanol production from lignocellulosic biomass. Even with xylose utilizing Saccharomyces cerevisiae, the efficiency of the lignocellulosic ethanol production remains suboptimal mainly due to the low conversion yield of xylose to ethanol. In this study, we evaluated the co-fermentation performances of SXA-R2P-E, a recently engineered isomerase-based xylose utilizing strain, in mixed sugars and in lignocellulosic hydrolysates. In a high-sugar fermentation with 70g/L of glucose and 40g/L of xylose, SXA-R2P-E produced 50g/L of ethanol with an yield of 0.43gethanol/gsugars at 72h. From dilute acid-pretreated hydrolysates of rice straw and hardwood (oak), the strain produced 18–21g/L of ethanol with among the highest yield of 0.43–0.46gethanol/gsugars ever reported. This study shows a highly promising potential of a xylose isomerase-expressing strain as an industrially relevant ethanol producer from lignocellulosic hydrolysates.

L-Lactic acid production by combined utilization of agricultural bioresources as renewable and economical substrates through batch and repeated-batch fermentation of Enterococcus faecalis RKY1

Enterococcus faecalis RKY1 was used to produce l-lactic acid from hydrol, soybean curd residues (SCR), and malt. Hydrol was efficiently metabolized to l-lactic acid with optical purity of >97.5%, though hydrol contained mixed sugars such as glucose, maltose, maltotriose, and maltodextrin. Combined utilization of hydrol, SCR, and malt was enough to sustain lactic acid fermentation by E. faecalis RKY1. In order to reduce the amount of nitrogen sources and product inhibition, cell-recycle repeated-batch fermentation was employed, where a high cell mass (26.3g/L) was obtained. Lactic acid productivity was improved by removal of lactic acid from fermentation broth by membrane filtration and by linearly increased cell density. When the total of 10 repeated-batch fermentations were carried out using 100g/L hydrol, 150g/L SCR hydrolyzate, and 20g/L malt hydrolyzate as the main nutrients, lactic acid productivity was increased significantly from 3.20g/L/h to 6.37g/L/h.

Evolved hexose transporter enhances xylose uptake and glucose/xylose co-utilization in Saccharomyces cerevisiae.

Enhancing xylose utilization has been a major focus in Saccharomyces cerevisiae strain-engineering efforts. The incentive for these studies arises from the need to use all sugars in the typical carbon mixtures that comprise standard renewable plant-biomass-based carbon sources. While major advances have been made in developing utilization pathways, the efficient import of five carbon sugars into the cell remains an important bottleneck in this endeavor. Here we use an engineered S. cerevisiae BY4742 strain, containing an established heterologous xylose utilization pathway, and imposed a laboratory evolution regime with xylose as the sole carbon source. We obtained several evolved strains with improved growth phenotypes and evaluated the best candidate using genome resequencing. We observed remarkably few single nucleotide polymorphisms in the evolved strain, among which we confirmed a single amino acid change in the hexose transporter HXT7 coding sequence to be responsible for the evolved phenotype. The mutant HXT7(F79S) shows improved xylose uptake rates (Vmax = 186.4 ± 20.1 nmol•min−1•mg−1) that allows the S. cerevisiae strain to show significant growth with xylose as the sole carbon source, as well as partial co-utilization of glucose and xylose in a mixed sugar cultivation.

Co-utilization of glycerol and lignocellulosic hydrolysates enhances anaerobic 1,3-propanediol production by Clostridium diolis.

Anaerobic fermentation using lignocellulosic hydrolysates as co-substrates is an economically attractive method to enhance 1,3-propanediol (1,3-PD) production by increasing the conversion yield from glycerol. Lignocellulosic hydrolysates contain the mixed sugars that are primarily glucose, xylose, and arabinose. Therefore, these three individual sugars were used, separately, as co-substrates with glycerol, in 1,3-PD production by a Clostridium diolis strain DSM 15410, resulting in an 18%–28% increase in the 1,3-PD yield. Co-fermentation of the mixed sugars and glycerol obtained a higher intracellular NADH/NAD+ ratio and increased the 1,3-PD yield by 22% relative to fermentation of glycerol alone. Thereafter, two kinds of lignocellulosic hydrolysates, corn stover hydrolysate and corncob molasses, were individually co-fermented with glycerol. The maximum 1,3-PD yield from glycerol reached 0.85 mol/mol. Fed-batch co-fermentation was also performed, improving the 1,3-PD yield (from 0.62 mol/mol to 0.82 mol/mol). These results demonstrate that the co-fermentation strategy is an efficient and economical way to produce 1,3-PD from glycerol.

Effect of different mixed starter cultures and fermentation conditions on biological safety of ‘Zha fish'

The purpose of this paper was to explore the influence of different starter cultures and fermentation conditions on biological safety of ‘Zha fish '(a fermented grass carp). Taken Lactobacillus plantarum(Lp),Pediococcus pentosaceus(Pp),Staphylococcus xylose(Sx),Saccharomyces cerevisiae(Sc) as starter culture,which was mixed with methods of Lp+Sx+Sc,Lp+Pp+Sc,and Sx+Pp+Sc under a ratio of 1∶1∶1(v/v/v). Tthe harmful bacterial count and biogenic amine(BA) content of ‘Zha fish' was assayed periodically under different fermentation conditions. The results showed that inoculation fermentation( IF) exhibited better antibacterial effects against harmful bacteria than that of the natural fermentation(NF)(p<0.01). Among three mixed starter cultures,Lp+Sx+Sc exhibited the best effect on inhibiting the formation of BA(p<0.05). With 20%(m/m) KCl substituted for Na Cl,it showed better inhibition effect on BA formation than that of samples no KCl substitution(p <0.05). The suitable content of composited salt was 2.4% Na Cl +0.6% KCl. From the point of view of BA control and production efficiency,20 ℃ was a suitable one. As to the sensory quality and BA control,5.0%mixed sugar was preferred. The total content of BA in ‘Zha fish' was as low as 43.50 mg/kg,and an excellent sensory quality could be achieved using Lp +Sx +Sc as starter culture to fermentation for 50 d under above-mentioned optimal conditions. The data obtained in this study suggested that biological safety of ‘Zha fish 'products could be ensured if Lp +Sx +Sc was taken as starter culture under an appropriate fermentation conditions.

Cereal Crops Are not Created Equal: Wheat Consumption Associated with Obesity Prevalence Globally and Regionally.

Background: Cereals have been extensively advocated as the beneficial food group in terms of body weight management, but each staple cereal crop may contribute in different ways. Studies of the association between wheat availability and risk of obesity are controversial. This study aimed to test the global and regional association between wheat availability as reported by FAO and obesity prevalence at a population level. FAO does not distinguish between whole grain wheat and refined wheat. Methods: Population-specific data from 170 countries on prevalence of obesity, availabilities of mixed cereals, wheat, rice, maize, meat, sugar, fat, soy and calories and GDP are obtained from the UN agencies. All variables were measured as per capita per day (or per year). Each country is treated as an individual subject. SPSS v. 22 is used to analyse these data for all the 170 countries and official country groupings (regions) using non parametric and parametric correlations, including partial correlation analysis. Results: Pearson's correlation coefficient analysis showed that obesity prevalence is positively associated with wheat availability (r = 0.500, p < 0.001), but is inversely associated with availabilities of total cereals (r = −0.132, p = 0.087), rice (r = −0.405, p < 0.001) and maize (r = −0.227, p = 0.004). These associations remain in partial correlation model when we keep availabilities of meat, fat, sugar, soy, caloric intake and GDP statistically constant. Overall, positive associations between wheat availability and obesity prevalence remain in different regions. Maize and mixed cereal availabilities do not show independent associations with the obesity prevalence. Conclusions: Our study suggests that wheat availability is an independent predictor of the obesity prevalence both worldwide and with special regard to the regions of Africa, Americas and Asia. Future studies should distinguish between possible influence of whole grain and ultra-processed refined wheat products.

Metabolic Engineering and Comparative Performance Studies of Synechocystis sp. PCC 6803 Strains for Effective Utilization of Xylose.

Wood sugars such as xylose can be used as an inexpensive carbon source for biotechnological applications. The model cyanobacterium Synechocystis sp. PCC 6803 lacks the ability to catabolize wood sugars as an energy source. Here, we generated four Synechocystis strains that heterologously expressed XylAB enzymes, which mediate xylose catabolism, either in combination with or without one of three xylose transporters, namely XylE, GalP, or Glf. Except for glf, which is derived from the bacterium Zymomonas mobilis ZM4, the heterologous genes were sourced from Escherichia coli K-12. All of the recombinant strains were able to utilize xylose in the absence of catabolite repression. When xylose was the lone source of organic carbon, strains possessing the XylE and Glf transporters were most efficient in terms of dry biomass production and xylose consumption and the strain lacking a heterologous transporter was the least efficient. However, in the presence of a xylose-glucose mixed sugar source, the strains exhibited similar levels of growth and xylose consumption. This study demonstrates that various bacterial xylose transporters can boost xylose catabolism in transgenic Synechocystis strains, and paves the way for the sustainable production of bio-compounds and green fuels from lignocellulosic biomass.

Quality of ultra-high temperature treated chocolate milk manufactured in Bangladesh

Quality of ultra-high temperature (UHT) treated chocolate milk manufactured by different manufacturing companies of Bangladesh was evaluated. The milk samples were collected from retail shops of different markets supplied by three major dairy brands namely Aarong, Starship and Milkman. From the organoleptic tests it was found that there were no significant variation in consistency, but significant variations (p?0.05) were found in case of flavor and colour and appearance, pH and solids-not-fat (SNF) content showed insignificant difference, while significant variations at different levels (p&lt;0.01 to 0.05) were found in all other chemical parameters like contents of total solids, fat, protein, ash and carbohydrate (mixed sugar). There was no evidence of presence of bacteria (total viable bacteria and coliform bacteria) in any of the samples. Considering all of organoleptic, chemical and microbiological properties of the samples of UHT chocolate milks, it could be concluded that Starship UHT chocolate milk was better and more acceptable followed by that supplied by Milkman. It could be suggested that to obtain quality UHT chocolate milk proper method, composition of ingredients and also the strict hygienic and sanitary measures should be followed.Bang. J. Anim. Sci. 2015. 44(2): 128-131

Activating and Elucidating Metabolism of Complex Sugars in Yarrowia lipolytica.

The oleaginous yeast Yarrowia lipolytica is an industrially important host for production of organic acids, oleochemicals, lipids, and proteins with broad biotechnological applications. Albeit known for decades, the unique native metabolism of Y. lipolytica for using complex fermentable sugars, which are abundant in lignocellulosic biomass, is poorly understood. In this study, we activated and elucidated the native sugar metabolism in Y. lipolytica for cell growth on xylose and cellobiose as well as their mixtures with glucose through comprehensive metabolic and transcriptomic analyses. We identified 7 putative glucose-specific transporters, 16 putative xylose-specific transporters, and 4 putative cellobiose-specific transporters that are transcriptionally upregulated for growth on respective single sugars. Y. lipolytica is capable of using xylose as a carbon source, but xylose dehydrogenase is the key bottleneck of xylose assimilation and is transcriptionally repressed by glucose. Y. lipolytica has a set of 5 extracellular and 6 intracellular β-glucosidases and is capable of assimilating cellobiose via extra- and intracellular mechanisms, the latter being dominant for growth on cellobiose as a sole carbon source. Strikingly, Y. lipolytica exhibited enhanced sugar utilization for growth in mixed sugars, with strong carbon catabolite activation for growth on the mixture of xylose and cellobiose and with mild carbon catabolite repression of glucose on xylose and cellobiose. The results of this study shed light on fundamental understanding of the complex native sugar metabolism of Y. lipolytica and will help guide inverse metabolic engineering of Y. lipolytica for enhanced conversion of biomass-derived fermentable sugars to chemicals and fuels.

Ethanol Fermentation on Mixed Sugars Using Mixed Culture of Two Yeast Strains

The objective of this study was to evaluate the use of mixed cultures of the recommended yeast strainsfrom a previous study on ethanol fermentation using a substrate mixture consisting of sucrose, glucose, andfructose. There were three mixed (combination) cultures namely OUT7096/OUT7913, OUT7096/OUT7921,and OUT7913/OUT7921. The fermentation medium contained sugar mixture consisting of glucose, fructose,and sucrose with a composition generally close to the composition of sugars in sweet sorghum juice. Overall,fermentation is carried out in two stages. First fermentation was performed using the three mixed culturesto determine the best combination based on the concentration of ethanol produced and the concentration ofresidual sugar. Second fermentation was conducted using the best mixed culture obtained from the fi rst stage.This second stage was intended to describe the pattern of the changes in the concentration of ethanol, sugarsand biomass during the fermentation progresses and to determine some kinetic parameters namely ethanolyield (Yp/s), growth yield (Yx/s) and specifi c growth rate (μ). The results of the fi rst fermentation showed thatthe best mixed culture was OUT7913/OUT7921 and the subsequent fermentation using this culture providethe highest ethanol yield (Yp/s) = 0.47 g.g-1 that was reached at 53rd hour, growth yield (Yx/s) = 0.425 g.g-1, andμ = 0.12 hour-1.Keywords : fermentation, ethanol, mixed culture, mixed sugar