Brewing Strains Research Articles

Removal of heavy metals using a brewer's yeast strain of Saccharomyces cerevisiae: application to the treatment of real electroplating effluents containing multielements

AbstractBACKGROUND: Yeast cells have been recognized as an effective type of biomass for the treatment of wastewaters containing heavy metals. However, its capability to treat efficiently complex effluents loaded with several metals ions (Cr, Cu, Ni and Zn) has never been reported. The aim of this work was to study the feasibility of a hybrid technology, which combines chemical precipitation at pH 6.0 with a subsequent biotechnological‐based process (using heat‐killed cells of a flocculent brewing strain of Saccharomyces cerevisiae), to remove simultaneously several metals from real electroplating effluents.RESULTS: Two effluents containing Cu, Ni and Zn (effluent A) or Cr, Cu and Ni (effluent B) were treated. In both effluents, pH was adjusted to 6.0; in effluent B, Cr(VI) was previously reduced to Cr(III). Chemical speciation studies allowed defining the amount of biomass to be employed with a minimum number of batches. Subsequently to pH adjustment to 6.0, effluents were fully treated with a serial batch of biomass. After the third batch, metal concentrations were lowered to below the legal limits of discharge; removals ≥ 89, 91, 92 and 94% were attained for Ni, Cu, Cr and Zn, respectively.CONCLUSIONS: In the present work, the usefulness of using flocculent brewing yeast cells to treat complex industrial effluents loaded with several heavy metals was demonstrated. The hybrid process developed was shown to be an efficient alternative for the treatment of real electroplating effluents. Copyright © 2010 Society of Chemical Industry

Genetic improvement of brewer’s yeast: current state, perspectives and limits

Brewer's yeast strain optimisation may lead to a more efficient beer production process, better final quality or healthier beer. However, brewer's yeast genetic improvement is very challenging, especially true when it comes to lager brewer's yeast (Saccharomyces pastorianus) which contributes to 90% of the total beer market. This yeast is a genetic hybrid and allopolyploid. While early studies applying traditional genetic approaches encountered many problems, the development of rational metabolic engineering strategies successfully introduced many desired properties into brewer's yeast. Recently, the first genome sequence of a lager brewer's strain became available. This has opened the door for applying advanced omics technologies and facilitating inverse metabolic engineering strategies. The latter approach takes advantage of natural diversity and aims at identifying and transferring the crucial genetic information for an interesting phenotype. In this way, strains can be optimised by introducing "natural" mutations. However, even when it comes to self-cloned strains, severe concerns about genetically modified organisms used in the food and beverage industry are still a major hurdle for any commercialisation. Therefore, research efforts will aim at developing new sophisticated screening methods for the isolation of natural mutants with the desired properties which are based on the knowledge of genotype-phenotype linkage.

Influence of stressful fermentation conditions on neutral lipids of a Saccharomyces cerevisiae brewing strain

The neutral lipid fraction of the aerobically grown starter yeast culture of a Saccharomyces cerevisiae brewing strain, and three-first recycled yeast generations exposed to multiple stress factors during beer fermentation was studied. No pronounced changes in the cellular neutral lipid content between the non-stressed starter and stressed recycled cells were found. However, it was found that recycled yeast generations modulate their neutral lipid composition during fermentation. The ergosterol content was increased at the expense of steryl esters (SEs) and squalene, which resulted in a higher ergosterol/SEs molar ratio and a slightly higher ergosterol/squalene molar ratio. In addition, the proportion of unsaturated fatty acids, mainly palmitoleic acid increased in the neutral lipid fraction of the stressed recycled yeast generations. These results suggest that some specific neutral lipid species and fatty acids stored in the neutral lipid fraction are involved in the adaptive response of the brewer's yeast to stressful fermentation conditions. The striking finding was a high squalene content in the neutral lipid fraction of both the starter yeast culture and recycled yeast generations (22.4 vs. 19-20%, respectively), implying a possible biotechnological exploitation of this biologically active molecule from the yeast biomass.

Simultaneous effect of divalent cation in hydrolyzed cassava starch medium used by immobilized yeast for ethanol production

Response surface methodology was adopted in a central composite design to optimize ethanol production from cassava starch hydrolysate medium. Starch hydrolyzate was prepared from TMS 98/0581, a genetically developed cassava mosaic disease-resistant variety. The yeast whole cell, Saccharomyces pastorianus, a lager brewing strain (726 x 106 cells/ml, 98.78% viability) and fungamyl and termamyl (a-amylase enzymes), used for the 120 h fermentation, were immobilized by entrapment in calcium alginate gel. Effects of three divalent cationic concentrations Mg2+, Zn2+ and Ca2+ on ethanol yield were investigated at five variable combinations in 20 experimental runs in accordance with the experimental design. Maximum ethanol concentration of 12.53 %v/v was produced in the 96 h of fermentation when the divalent cationic combination was 64, 0.48 and 30 mg/l (Mg2+, Zn2+ and Ca2+), respectively. The study showed that effect of Zn2+ on ethanol yield was significantly (P≤0.05) quadratic. Keywords: Ethanol, immobilization, Saccharomyces pastorianus, divalent cations, optimization, response surface methodology, cassava mosaic disease.

High Expression Levels of Cell Wall Protein Cwp2p Decrease the Turbidity of Fresh Lager Beer by Reducing the Size of Haze Particles

Cwp2p is one of the major mannoproteins of the yeast cell wall. In this study, the CWP2 gene was placed under the constitutive TDH3 promoter and introduced into a lager production strain. When the obtained CWP2-enhanced strain was subjected to a fermentation trial, the resultant beer was less turbid and contained haze particles that were smaller than those in the control beer with the parental strain. Enhanced expression of CWP2 in the transformant strain resulted in a reduction in the size of haze particles, similar to a case in which the parental strain was propagated aerobically for the fermentation trial. The aerobically prepared parental pitching yeast cells exhibited a higher level of CWP2 mRNA expression during fermentation. These results suggest that the aerobic propagation of pitching yeast exerts a haze-reduction effect by changing the expression profiles of specific cell wall mannoproteins, including Cwp2p. Immunoblot analysis detected the green fluorescent protein-cell wall protein (GFP-Cwp2p) fusion protein in fresh beer when expressed in a lager brewing strain, indicating that a glycosylphosphatidylinositol (GPI)-anchored cell wall protein, such as Cwp2p, can be released into a solution under physiological conditions. The expression level of the aerobiosis-specific mannoproteins (e.g., Cwp2p) appeared to be a key factor in the determination of haze particle size and, accordingly, beer turbidity.

Physiological characterization of brewer’s yeast in high-gravity beer fermentations with glucose or maltose syrups as adjuncts

High-gravity brewing, which can decrease production costs by increasing brewery yields, has become an attractive alternative to traditional brewing methods. However, as higher sugar concentration is required, the yeast is exposed to various stresses during fermentation. We evaluated the influence of high-gravity brewing on the fermentation performance of the brewer's yeast under model brewing conditions. The lager brewer's strain Weihenstephan 34/70 strain was characterized at three different gravities by adding either glucose or maltose syrups to the basic wort. We observed that increased gravity resulted in a lower specific growth rate, a longer lag phase before initiation of ethanol production, incomplete sugar utilization, and an increase in the concentrations of ethyl acetate and isoamyl acetate in the final beer. Increasing the gravity by adding maltose syrup as opposed to glucose syrup resulted in more balanced fermentation performance in terms of higher cell numbers, respectively, higher wort fermentability and a more favorable flavor profile of the final beer. Our study underlines the effects of the various stress factors on brewer's yeast metabolism and the influence of the type of sugar syrups on the fermentation performance and the flavor profile of the final beer.

Accumulation and cellular distribution of zinc by brewing yeast

This study focused on the interactions between yeast and zinc in relation to beer fermentations. Yeast accumulation of zinc from growth media, including malt wort, was found to be rapid following inoculation with a brewing strain of Saccharomyces carlsbergensis. In contrast, at the onset of the fermentation, the uptake of other divalent cations such as magnesium and calcium was not as pronounced compared with zinc. At the end of fermentation, both growth media and yeast cells became zinc-depleted, the latter due to dilution of zinc to daughter cells following growth and cell division. In addition, in brewing fermenters, the levels of intracellular zinc were much higher in suspended yeast cells compared with cells that sedimented in the yeast cone at the end of fermentation. This may result in impaired yeast performance in subsequent fermentations if yeast is recycled into low zinc media and if the sub-population is composed by zinc-depleted daughter cells. Cellular uptake of zinc was mediated by a metabolism-dependent mechanism as evidenced by impaired uptake following heat shock. Zinc was thereafter localised in the yeast cell vacuole. As industrial fermentation processes may occasionally be suppressed due to zinc deficiencies, the findings of this study are pertinent for several yeast-based industries, especially beer production.

The inheritance of mtDNA in lager brewing strains

In this work, we compared the mtDNA of a number of interspecific Saccharomyces hybrids (Saccharomyces cerevisiae x Saccharomyces uvarum and S. cerevisiae x Saccharomyces bayanus) to the mtDNA of 22 lager brewing strains that are thought to be the result of a natural hybridization between S. cerevisiae and another Saccharomyces yeast, possibly belonging to the species S. bayanus. We detected that in hybrids constructed in vitro, the mtDNA could be inherited from either parental strain. Conversely, in the lager strains tested, the mtDNA was never of the S. cerevisiae type. Moreover, the nucleotide sequence of lager brewing strains COXII gene was identical to S. bayanus strain NBRC 1948 COXII gene. MtDNA restriction analysis carried out with three enzymes confirmed this finding. However, restriction analysis with a fourth enzyme (AvaI) provided restriction patterns for lager strains that differed from those of S. bayanus strain NBRC 1948. Our results raise the hypothesis that the human-driven selection carried out on existing lager yeasts has favored only those bearing optimal fermentation characteristics at low temperatures, which harbor the mtDNA of S. bayanus.

Targeting of mitochondrial Saccharomyces cerevisiae Ilv5p to the cytosol and its effect on vicinal diketone formation in brewing

Vicinal diketones (VDK) cause butter-like off-flavors in beer and are formed by a non-enzymatic oxidative decarboxylation of alpha-aceto-alpha-hydroxybutyrate and alpha-acetolactate, which are intermediates in isoleucine and valine biosynthesis taking place in the mitochondria. On the assumption that part of alpha-acetolactate can be formed also in the cytosol due to a mislocalization of the responsible acetohydroxyacid synthase encoded by ILV2 and ILV6, functional expression in the cytosol of acetohydroxyacid reductoisomerase (Ilv5p) was explored. Using the cytosolic Ilv5p, I aimed to metabolize the cytosolically formed alpha-aetolactate, thereby lowering the total VDK production. Among mutant Ilv5p enzymes with varying degrees of N-terminal truncation, one with a 46-residue deletion (Ilv5pDelta46) exhibited an unequivocal localization in the cytosol judged from microscopy of the Ilv5pDelta46-green fluorescent protein fusion protein and the inability of Ilv5pDelta46 to remedy the isoleucine/valine requirement of an ilv5Delta strain. When introduced into an industrial lager brewing strain, a robust expression of Ilv5pDelta46 was as effective as that of a wild-type Ilv5p in lowering the total VDK production in a 2-l scale fermentation trial. Unlike the case of the wild-type Ilv5p, an additional expression of Ilv5pDelta46 did not alter the quality of the resultant beer in terms of contents of aromatic compounds and organic acids.

The adenylate energy charge and specific fermentation rate of brewer's yeasts fermenting high‐ and very high‐gravity worts

Intracellular and extracellular ATP, ADP and AMP (i.e. 5'-AMP) were measured during fermentations of high- (15 degrees P) and very high-gravity (VHG, 25 degrees P) worts by two lager yeasts. Little extracellular ATP and ADP but substantial amounts of extracellular AMP were found. Extracellular AMP increased during fermentation and reached higher values (3 microM) in 25 degrees P than 15 degrees P worts (1 microM). More AMP (13 microM at 25 degrees P) was released during fermentation with industrially cropped yeast than with the same strain grown in the laboratory. ATP was the dominant intracellular adenine nucleotide and the adenylate energy charge (EC = ([ATP] + 0.5*[ADP])/([ATP] + [ADP] + [AMP])) remained high (>0.8) until residual sugar concentrations were low and specific rates of ethanol production were < 5% of the maximum values in early fermentation. The high ethanol concentrations (>85 g/l) reached in VHG fermentations did not decrease the EC below values that permit synthesis of new proteins. The results suggest that, during wort fermentations, the ethanol tolerance of brewer's strains is high so long as fermentation continues. Under these conditions, maintenance of the EC seems to depend upon active transport of alpha-glucosides, which in turn depends upon maintenance of the EC. Therefore, the collapse of the EC and cell viability when residual alpha-glucoside concentrations no longer support adequate rates of fermentation can be very abrupt. This emphasizes the importance of early cropping of yeast for recycling.

Detection of Mannan from Saccharomyces Cerevisiae by Flow Cytometry

Mannan plays an important role in the formation of some beer hazes. The effects of mechanical agitation of Saccharomyces cerevisiae brewing strains and subsequent impact on beer quality were investigated. Six different brewing yeast strains (ale and lager) were subjected to mechanical agitation in a temperature-controlled bioreactor. As a result of this study, a protocol is described for the determination of mannan residues from S. cerevisiae cells in nearly real time. Flow cytometric methods permitted the quantitative analysis of mannan residues based on particle size, relative granularity, and florescence intensity. The mannan concentrations and optical density of its supernatant increased as the yeast slurry was agitated. It was concluded that hydrodynamic stresses from agitation of yeast slurries cause mannan, an unfilterable haze constituent, to be released from the yeast cell wall. Furthermore, flow cytometry provides an innovative, rapid, and viable option for quantifying the influence that beer processing conditions have on beer stability.

Extracting the hidden features in saline osmotic tolerance in Saccharomyces cerevisiae from DNA microarray data using the self-organizing map: biosynthesis of amino acids

During saline stress, Saccharomyces cerevisiae changes its metabolic fluxes through the direct accumulation of metabolites such as glycerol and trehalose, which in turn provide tolerance to the cell against stress. Previous research shows that the various controls at both transcriptional and translational levels decide the phenomenon of stress, but details about such transition is still not very clear. This paper attempts to extract some hidden features through the information extraction approach from DNA microarray data during transition to osmotic tolerance, which are expected to be important in directing to the tolerance stage upon encountering osmotic stress in yeast. Time course of DNA microarray data during osmotic tolerance was analyzed by computational approach 'self-organizing map (SOM) extended with hierarchical clustering'. Since eukaryotic gene expression is governed by short regulatory sequences found upstream in promoter regions, therefore clusters containing the similar profiles obtained by SOM were further analyzed for overrepresentation of known regulatory binding sites in promoter region. It was found that apart from known and expected 'STRE' during osmotic stress, the 'GCN4' binding site is also found to be significant. Hence, it was suggested that the process of osmotic tolerance proceeds through a stage of amino acid starvation. The intracellular amino acids pool also found to be depleted during transition and restoration is faster in brewing strain than laboratory strain. Experiments involving supplementation of amino acids helps in reducing the lag time for recovery, which was found to be similar to that of brewing strain.

INVERSE METABOLIC ENGINEERING BY INTEGRATION OF MULTIPLE OMICS ANALYSES

Abstract An inverse metabolic engineering method with “multiple omics analyses” was applied to creation of a stress tolerant strain of Saccharomyces cerevisiae. DNA microarray data of laboratory and brewing strains under high ethanol concentration condition (transcriptomics data) were compared and analyzed by a clustering method. Sensitivity analysis of gene knockout mutant library was further performed (phenomics data). The selection of candidate genes for conferring stress tolerance was successfully performed.

Comparison of transcriptional responses to osmotic stresses induced by NaCl and sorbitol additions in Saccharomyces cerevisiae using DNA microarray

Transcriptional responses of laboratory and brewing strains of Saccharomyces cerevisiae to osmotic stresses induced by adding either NaCl or sorbitol to their cultures were compared by clustering DNA microarray data. Our results suggest that the difference in the transcriptional responses of the two strains between NaCl and sorbitol additions is small when the dynamics of the total change in gene expression are similar.

Pure and Mixed Genetic Lines of Saccharomyces bayanus and Saccharomyces pastorianus and Their Contribution to the Lager Brewing Strain Genome

The yeast species Saccharomyces bayanus and Saccharomyces pastorianus are of industrial importance since they are involved in the production process of common beverages such as wine and lager beer; however, they contain strains whose variability has been neither fully investigated nor exploited in genetic improvement programs. We evaluated this variability by using PCR-restriction fragment length polymorphism analysis of 48 genes and partial sequences of 16. Within these two species, we identified "pure" strains containing a single type of genome and "hybrid" strains that contained portions of the genomes from the "pure" lines, as well as alleles termed "Lager" that represent a third genome commonly associated with lager brewing strains. The two pure lines represent S. uvarum and S. bayanus, the latter a novel group of strains that may be of use in strain improvement programs. Hybrid lines identified include (i) S. cerevisiae/S. bayanus/Lager, (ii) S. bayanus/S. uvarum/Lager, and (iii) S. cerevisiae/S. bayanus/S. uvarum/Lager. The genome of the lager strains may have resulted from chromosomal loss, replacement, or rearrangement within the hybrid genetic lines. This study identifies brewing strains that could be used as novel genetic sources in strain improvement programs and provides data that can be used to generate a model of how naturally occurring and industrial hybrid strains may have evolved.

Comparative analysis of transcriptional responses to saline stress in the laboratory and brewing strains of Saccharomyces cerevisiae with DNA microarray

To construct yeast strains showing tolerance to high salt concentration stress, we analyzed the transcriptional response to high NaCl concentration stress in the yeast Saccharomyces cerevisiae using DNA microarray and compared between two yeast strains, a laboratory strain and a brewing one, which is known as a stress-tolerant strain. Gene expression dynamically changed following the addition of NaCl in both yeast strains, but the degree of change in the gene expression level in the laboratory strain was larger than that in the brewing strain. The response of gene expression to the low NaCl concentration stress was faster than that to the high NaCl concentration stress in both strains. Expressions of the genes encoding enzymes involved in carbohydrate metabolism and energy production in both strains or amino acid metabolism in the brewing strain were increased under high NaCl concentration conditions. Moreover, the genes encoding sodium ion efflux pump and copper metallothionein proteins were more highly expressed in the brewing strain than in the laboratory strain. According to the results of transcriptome analysis, candidate genes for the creation of stress-tolerant strain were selected, and the effect of overexpression of candidate genes on the tolerance to high NaCl concentration stress was evaluated. Overexpression of the GPD1 gene encoding glycerol-3-phosphate dehydrogenase, ENA1 encoding sodium ion efflux protein, and CUP1 encoding copper metallothionein conferred high salt stress tolerance to yeast cells, and our selection of candidate genes for the creation of stress-tolerant yeast strains based on the transcriptome data was validated.

Molecular species of phosphatidylethanolamine from continuous cultures of Saccharomyces pastorianus syn. carlsbergensis strains

AbstractSaccharomyces pastorianus syn. carlsbergensis strain 34/70 is well known to be the most used strain for lager beer production. The difference between this strain and very closely related strain 34/78 is the latter's greater flocculating character. This single physiological trait can cause technical difficulties in beer production. The aim of this study was to determine whether lipid analysis by a combination of thin layer chromatography (TLC) with electrospray ionization mass spectrometry (ESI–MS) could be used as a strain‐typing technique in order to distinguish S. pastorianus syn. carlsbergensis strain 34/70 from strain 34/78. Both strains (34/70 and 34/78) were harvested after continuous culture under standard conditions. Polar lipids were then extracted from lyophilized cultures and analysed by TLC in order to separate phospholipid families. Phosphatidylethanolamine (PE) was extracted and investigated using ESI–MS, to gain further information on individual molecular species. Using TLC analysis, lipids were separated corresponding to standards for PE, phosphatidylcholine (PC), phosphatidylglycerol (PG), cardiolipin (CL), phosphatidylserine (PS), phosphatidylinositol (PI), phosphatidic acid (PA) and sphingomyelin (SM). ESI–MS of the PE band, separated by TLC, showed that electrospray mass spectra were highly reproducible for repeat cultures. Novel findings were that both brewing strains displayed major phospholipid peaks with m/z 714, PE (34 : 2) m/z 742, PE (36 : 2) and m/z 758, PE (37 : 1). However, strain 34/78 had additional peaks of m/z 700, PE (33 : 2) and m/z 728, PE (35 : 2). Strain 34/70 had an extra peak with m/z 686 PE (32 : 2). We conclude that combined TLC/ESI–MS can distinguish between S. pastorianus syn. carlsbergensis 34/70 and 34/78 and may be a useful typing technique for differentiation of closely related yeast strains. This novel approach may aid quality assurance and could be suitable for yeast collections and larger industrial companies. Copyright © 2006 John Wiley &amp; Sons, Ltd.

PCR-Based Differentiation and Homology of Brewing and Type Strains of the Genus Saccharomyces

Restriction fragment length polymorphism (RFLP) patterns of PCR-amplified ribosomal RNA gene fragments (rDNA) and randomly amplified polymorphic DNA (RAPD) were applied for the analysis of 15 brewing and 6 related yeast strains of the genus Saccharomyces. One five-base (ScrFI) and two four-base cutting (HaeIII, MspI) restriction enzymes were used. The primers 21 and M13 core sequence were selected for RAPD analysis. PCR-RFLP rDNA analysis with HaeIII, ScrFI and MspI differentiated the strains tested into four, five and four types of patterns, respectively and the analyses of the profiles showed 100% homology, between the yeast strains. One strain was an exception. Homological groups were observed for strains used in breweries globally, from a local production strain and from the isolates identified as S. cerevisiae. Using RAPD analysis, and according to discrete differences in the profiles, it was possible to divide twenty one strains into 15 and 20 groups with primer 21 and M13 respectively. RFLP-PCR rDNA analysis was used to show similarities in closely related brewing strains, while RAPD analysis was used for differentiation of strains.

Expression levels of the yeast alcohol acetyltransferase genes ATF1, Lg-ATF1, and ATF2 control the formation of a broad range of volatile esters.

Volatile aroma-active esters are responsible for the fruity character of fermented alcoholic beverages such as beer and wine. Esters are produced by fermenting yeast cells in an enzyme-catalyzed intracellular reaction. In order to investigate and compare the roles of the known Saccharomyces cerevisiae alcohol acetyltransferases, Atf1p, Atf2p and Lg-Atf1p, in volatile ester production, the respective genes were either deleted or overexpressed in a laboratory strain and a commercial brewing strain. Subsequently, the ester formation of the transformants was monitored by headspace gas chromatography and gas chromatography combined with mass spectroscopy (GC-MS). Analysis of the fermentation products confirmed that the expression levels of ATF1 and ATF2 greatly affect the production of ethyl acetate and isoamyl acetate. GC-MS analysis revealed that Atf1p and Atf2p are also responsible for the formation of a broad range of less volatile esters, such as propyl acetate, isobutyl acetate, pentyl acetate, hexyl acetate, heptyl acetate, octyl acetate, and phenyl ethyl acetate. With respect to the esters analyzed in this study, Atf2p seemed to play only a minor role compared to Atf1p. The atf1Delta atf2Delta double deletion strain did not form any isoamyl acetate, showing that together, Atf1p and Atf2p are responsible for the total cellular isoamyl alcohol acetyltransferase activity. However, the double deletion strain still produced considerable amounts of certain other esters, such as ethyl acetate (50% of the wild-type strain), propyl acetate (50%), and isobutyl acetate (40%), which provides evidence for the existence of additional, as-yet-unknown ester synthases in the yeast proteome. Interestingly, overexpression of different alleles of ATF1 and ATF2 led to different ester production rates, indicating that differences in the aroma profiles of yeast strains may be partially due to mutations in their ATF genes.

Effect of Sugar Catabolite Repression in Correlation with the Structural Complexity of the Nitrogen Source on Yeast Growth and Fermentation

Biomass and ethanol production by industrial Saccharomyces cerevisiae strains were strongly affected by the structural complexity of the nitrogen source during fermentation in media containing galactose, and supplemented with a nitrogen source varying from a single ammonium salt (ammonium sulfate) to free amino acids (casamino acids) and peptides (peptone). Diauxie was observed at low galactose concentrations independent of nitrogen supplementation. At high sugar concentrations altered patterns of galactose utilisation were observed. Biomass accumulation and ethanol production depended on the nature of the nitrogen source and were different for baking and brewing ale and lager strains. Baking yeast showed improved galactose fermentation performance in the medium supplemented with casamino acids. High biomass production was observed with peptone and casamino acids for the ale brewing strain, however high ethanol production was observed only in the presence of casamino acids. Conversely, peptone was the nitrogen supplement that induced higher biomass and ethanol production for the lager brewing strain. Ammonium salts always induced poor yeast performance. The results with galactose differed from those obtained with glucose and maltose which indicated that supplementation with a nitrogen source in the peptide form (peptone) was more positive for yeast metabolism, suggesting that sugar catabolite repression has a central role in yeast performance in a medium containing nitrogen sources with differing levels of structural complexity.