A Survey of Industrial Strains ofSaccharomyces cerevisiaeReveals Numerous Altered Patterns of Maltose and Sucrose Utilisation

Carbon and nitrogen sources consumption by ale and lager yeast strains: a comparative study during fermentation

The rapid and efficient consumption of carbon and nitrogen sources by brewer's yeast is critical for the fermentation process in the brewing industry. The comparison of the growth characterizations of typical ale and lager yeast, as well as their consumption preference to carbon and nitrogen sources were investigated in this study. Results showed that the ale strain grew faster and had a more extended stationary phase than the lager strain. However, the lager strain was more tolerant to the stressful environment in the later stage of fermentation. Meanwhile, the ale and lager yeast strains possessed varying preferences for metabolizing the specific fermentable sugar or free amino acid involved in the wort medium. The lager strain had a strong capacity to synthesize the extracellular invertase required for hydrolyzing sucrose as well as a strong capability to metabolize glucose and fructose. Furthermore, the lager strain had an advantage in consuming Lys, Arg, Val, and Phe, whereas the ale strain had a higher assimilation rate in consuming Tyr. These findings provide valuable insights into selecting the appropriate brewer's yeast strain based on the wort components for the industrial fermentation process. KEY POINTS: • The lager strain is more tolerant to the stressful environment. • The lager strain has the great capability to synthesize the extracellular invertase. • The assimilation efficiency of free amino acid varies between ale and lager.

Kveik Brewing Yeasts Demonstrate Wide Flexibility in Beer Fermentation Temperature Tolerance and Exhibit Enhanced Trehalose Accumulation

Traditional Norwegian Farmhouse ale yeasts, also known as kveik, have captured the attention of the brewing community in recent years. Kveik were recently reported as fast fermenting thermo- and ethanol tolerant yeasts with the capacity to produce a variety of interesting flavor metabolites. They are a genetically distinct group of domesticated beer yeasts of admixed origin with one parent from the “Beer 1” clade and the other unknown. While kveik are known to ferment wort efficiently at warmer temperatures, their range of fermentation temperatures and corresponding fermentation efficiencies, remain uncharacterized. In addition, the characteristics responsible for their increased thermotolerance remain largely unknown. Here we demonstrate variation in kveik strains at a wide range of fermentation temperatures and show not all kveik strains are equal in fermentation performance and stress tolerance. Furthermore, we uncovered an increased capacity of kveik strains to accumulate intracellular trehalose, which likely contributes to their increased thermo- and ethanol tolerances. Taken together our results present a clearer picture of the future opportunities presented by Norwegian kveik yeasts and offer further insight into their applications in brewing.

Mechanisms of Metabolic Adaptation in Wine Yeasts: Role of Gln3 Transcription Factor

Wine strains of Saccharomyces cerevisiae have to adapt their metabolism to the changing conditions during their biotechnological use, from the aerobic growth in sucrose-rich molasses for biomass propagation to the anaerobic fermentation of monosaccharides of grape juice during winemaking. Yeast have molecular mechanisms that favor the use of preferred carbon and nitrogen sources to achieve such adaptation. By using specific inhibitors, it was determined that commercial strains offer a wide variety of glucose repression profiles. Transcription factor Gln3 has been involved in glucose and nitrogen repression. Deletion of GLN3 in two commercial wine strains produced different mutant phenotypes and only one of them displayed higher glucose repression and was unable to grow using a respiratory carbon source. Therefore, the role of this transcription factor contributes to the variety of phenotypic behaviors seen in wine strains. This variability is also reflected in the impact of GLN3 deletion in fermentation, although the mutants are always more tolerant to inhibition of the nutrient signaling complex TORC1 by rapamycin, both in laboratory medium and in grape juice fermentation. Therefore, most aspects of nitrogen catabolite repression controlled by TORC1 are conserved in winemaking conditions.

Saccharomyces cerevisiae nutrient signaling pathways show an unexpected early activation pattern during winemaking

Background

Saccharomyces cerevisiae wine strains can develop stuck or sluggish fermentations when nutrients are scarce or suboptimal. Nutrient sensing and signaling pathways, such as PKA, TORC1 and Snf1, work coordinately to adapt growth and metabolism to the amount and balance of the different nutrients in the medium. This has been exhaustively studied in laboratory strains of S. cerevisiae and laboratory media, but much less under industrial conditions.

Results

Inhibitors of such pathways, like rapamycin or 2-deoxyglucose, failed to discriminate between commercial wine yeast strains with different nutritional requirements, but evidenced genetic variability among industrial isolates, and between laboratory and commercial strains. Most signaling pathways involve events of protein phosphorylation that can be followed as markers of their activity. The main pathway to promote growth in the presence of nitrogen, the TORC1 pathway, measured by the phosphorylation of Rps6 and Par32, proved active at the very start of fermentation, mainly on day 1, and ceased soon afterward, even before cellular growth stopped. Transcription factor Gln3, which activates genes subject to nitrogen catabolite repression, was also active for the first hours, even when ammonium and amino acids were still present in media. Snf1 kinase was activated only when glucose was exhausted under laboratory conditions, but was active from early fermentation stages. The same results were generally obtained when nitrogen was limiting, which indicates a unique pathway activation pattern in winemaking. As PKA remained active throughout fermentation, it could be the central pathway that controls others, provided sugars are present.

Conclusions

Wine fermentation is a distinct environmental situation from growth in laboratory media in molecular terms. The mechanisms involved in glucose and nitrogen repression respond differently under winemaking conditions.

Yeast bioprospecting versus synthetic biology-which is better for innovative beverage fermentation?

Producers often utilise some of the many available yeast species and strains in the making of fermented alcoholic beverages in order to augment flavours, aromas, acids and textural properties. But still, the demand remains for more yeasts with novel phenotypes that not only impact sensory characteristics but also offer process and engineering advantages. Two strategies for finding such yeasts are (i) bioprospecting for novel strains and species and (ii) genetic modification of known yeasts. The latter enjoys the promise of the emerging field of synthetic biology, which, in principle, would enable scientists to create yeasts with the exact phenotype desired for a given fermentation. In this mini review, we compare and contrast advances in bioprospecting and in synthetic biology as they relate to alcoholic fermentation in brewing and wine making. We explore recent advances in fermentation-relevant recombinant technologies and synthetic biology including the Yeast 2.0 Consortium, use of environmental yeasts, challenges, constraints of law and consumer acceptance.

Autophagy gene overexpression in Saccharomyces cerevisiae perturbs subcellular organellar function and accumulates ROS to accelerate cell death with relevance to sparkling wine production

Traditional sparkling wines are produced by the refermentation of a base wine with yeast in the bottle followed by a critical period of aging. During the often lengthy aging process, yeast undergoes cell death and autolysis to release cellular compounds that over time ultimately contribute to the flavor and appearance of the product. While accelerating yeast autolysis for sparkling wine production has been the focus of several studies, employing overexpressed native yeast alleles for this purpose remains poorly explored. Here, we show that the overexpression of native yeast genes, specifically selected autophagic genes, results in accelerated cell death in nitrogen starvation and base wine refermentation. We show ATG3 or ATG4 overexpression has pleiotropic intracellular ramifications including reduced turnover of autophagic cargo, vacuolar fragmentation, abnormal accumulation of lipids, and accelerated accumulation of reactive oxygen species (ROS), all of which precede accelerated cell death. Our findings suggest that the increased expression of autophagy-related genes, such as ATG3 and ATG4, in industrial wine yeast can serve as a suitable marker or breeding strategy to accelerate the cell death and autolysis of wine yeast during sparkling wine production.

Establishing the relative importance of damaged starch and fructan as sources of fermentable sugars in wheat flour and whole meal bread dough fermentations

It is generally believed that maltose drives yeast-mediated bread dough fermentation. The relative importance of fructose and glucose, released from wheat fructan and sucrose by invertase, compared to maltose is, however, not documented. This is surprising given the preference of yeast for glucose and fructose over maltose. This study revealed that, after 2h fermentation of wheat flour dough, about 44% of the sugars consumed were generated by invertase-mediated degradation of fructan, raffinose and sucrose. The other 56% were generated by amylases. In whole meal dough, 70% of the sugars consumed were released by invertase activity. Invertase-mediated sugar release seems to be crucial during the first hour of fermentation, while amylase-mediated sugar release was predominant in the later stages of fermentation, which explains why higher amylolytic activity prolonged the productive fermentation time only. These results illustrate the importance of wheat fructan and sucrose content and their degradation for dough fermentations.

Engineering high α-amylase levels in wheat grain lowers Falling Number but improves baking properties

Late maturity α-amylase (LMA) and preharvest sprouting (PHS) are genetic defects in wheat. They are both characterized by the expression of specific isoforms of α-amylase in particular genotypes in the grain prior to harvest. The enhanced expression of α-amylase in both LMA and PHS results in a reduction in Falling Number (FN), a test of gel viscosity, and subsequent downgrading of the grain, along with a reduced price for growers. The FN test is unable to distinguish between LMA and PHS; thus, both defects are treated similarly when grain is traded. However, in PHS-affected grains, proteases and other degradative process are activated, and this has been shown to have a negative impact on end product quality. No studies have been conducted to determine whether LMA is detrimental to end product quality. This work demonstrated that wheat in which an isoform α-amylase (TaAmy3) was overexpressed in the endosperm of developing grain to levels of up to 100-fold higher than the wild-type resulted in low FN similar to those seen in LMA- or PHS-affected grains. This increase had no detrimental effect on starch structure, flour composition and enhanced baking quality, in small-scale 10-g baking tests. In these small-scale tests, overexpression of TaAmy3 led to increased loaf volume and Maillard-related browning to levels higher than those in control flours when baking improver was added. These findings raise questions as to the validity of the assumption that (i) LMA is detrimental to end product quality and (ii) a low FN is always indicative of a reduction in quality. This work suggests the need for a better understanding of the impact of elevated expression of specific α-amylase on end product quality.

Allelic variants of hexose transporter Hxt3p and hexokinases Hxk1p/Hxk2p in strains of Saccharomyces cerevisiae and interspecies hybrids

The transport of sugars across the plasma membrane is a critical step in the utilization of glucose and fructose by Saccharomyces cerevisiae during must fermentations. Variations in the molecular structure of hexose transporters and kinases may affect the ability of wine yeast strains to finish sugar fermentation, even under stressful wine conditions. In this context, we sequenced and compared genes encoding the hexose transporter Hxt3p and the kinases Hxk1p/Hxk2p of Saccharomyces strains and interspecies hybrids with different industrial usages and regional backgrounds. The Hxt3p primary structure varied in a small set of amino acids, which characterized robust yeast strains used for the production of sparkling wine or to restart stuck fermentations. In addition, interspecies hybrid strains, previously isolated at the end of spontaneous fermentations, revealed a common amino acid signature. The location and potential influence of the amino acids exchanges is discussed by means of a first modelled Hxt3p structure. In comparison, hexokinase genes were more conserved in different Saccharomyces strains and hybrids. Thus, molecular variants of the hexose carrier Hxt3p, but not of kinases, correlate with different fermentation performances of yeast.

Prospecting for Energy-Rich Renewable Raw Materials: Agave Leaf Case Study

Plant biomass from different species is heterogeneous, and this diversity in composition can be mined to identify materials of value to fuel and chemical industries. Agave produces high yields of energy-rich biomass, and the sugar-rich stem tissue has traditionally been used to make alcoholic beverages. Here, the compositions of Agave americana and Agave tequilana leaves are determined, particularly in the context of bioethanol production. Agave leaf cell wall polysaccharide content was characterized by linkage analysis, non-cellulosic polysaccharides such as pectins were observed by immuno-microscopy, and leaf juice composition was determined by liquid chromatography. Agave leaves are fruit-like--rich in moisture, soluble sugars and pectin. The dry leaf fiber was composed of crystalline cellulose (47-50% w/w) and non-cellulosic polysaccharides (16-22% w/w), and whole leaves were low in lignin (9-13% w/w). Of the dry mass of whole Agave leaves, 85-95% consisted of soluble sugars, cellulose, non-cellulosic polysaccharides, lignin, acetate, protein and minerals. Juice pressed from the Agave leaves accounted for 69% of the fresh weight and was rich in glucose and fructose. Hydrolysis of the fructan oligosaccharides doubled the amount of fermentable fructose in A. tequilana leaf juice samples and the concentration of fermentable hexose sugars was 41-48 g/L. In agricultural production systems such as the tequila making, Agave leaves are discarded as waste. Theoretically, up to 4000 L/ha/yr of bioethanol could be produced from juice extracted from waste Agave leaves. Using standard Saccharomyces cerevisiae strains to ferment Agave juice, we observed ethanol yields that were 66% of the theoretical yields. These data indicate that Agave could rival currently used bioethanol feedstocks, particularly if the fermentation organisms and conditions were adapted to suit Agave leaf composition.

Fermentation performance and physiology of two strains of Saccharomyces cerevisiae during growth in high gravity spruce hydrolysate and spent sulphite liquor

BACKGROUND

Lignocellulosic materials are a diverse group of substrates that are generally scarce in nutrients, which compromises the tolerance and fermentation performance of the fermenting organism. The problem is exacerbated by harsh pre-treatment, which introduces sugars and substances inhibitory to yeast metabolism. This study compares the fermentation behaviours of two yeast strains using different types of lignocellulosic substrates; high gravity dilute acid spruce hydrolysate (SH) and spent sulphite liquor (SSL), in the absence and presence of yeast extract. To this end, the fermentation performance, energy status and fermentation capacity of the strains were measured under different growth conditions.

RESULTS

Nutrient supplementation with yeast extract increased sugar uptake, cell growth and ethanol production in all tested fermentation conditions, but had little or no effect on the energy status, irrespective of media. Nutrient-supplemented medium enhanced the fermentation capacity of harvested cells, indicating that cell viability and reusability was increased by nutrient addition.

CONCLUSIONS

Although both substrates belong to the lignocellulosic spruce hydrolysates, their differences offer specific challenges and the overall yields and productivities largely depend on choice of fermenting strain.

A novel methodology independent of fermentation rate for assessment of the fructophilic character of wine yeast strains

The yeast Saccharomyces cerevisiae has a fundamental role in fermenting grape juice to wine. During alcoholic fermentation its catabolic activity converts sugars (which in grape juice are a near equal ratio of glucose and fructose) and other grape compounds into ethanol, carbon dioxide and sensorily important metabolites. However, S. cerevisiae typically utilises glucose and fructose with different efficiency: glucose is preferred and is consumed at a higher rate than fructose. This results in an increasing difference between the concentrations of glucose and fructose during fermentation. In this study 20 commercially available strains were investigated to determine their relative abilities to utilise glucose and fructose. Parameters measured included fermentation duration and the kinetics of utilisation of fructose when supplied as sole carbon source or in an equimolar mix with glucose. The data were then analysed using mathematical calculations in an effort to identify fermentation attributes which were indicative of overall fructose utilisation and fermentation performance. Fermentation durations ranged from 74.6 to over 150 h, with clear differences in the degree to which glucose utilisation was preferential. Given this variability we sought to gain a more holistic indication of strain performance that was independent of fermentation rate and therefore utilized the area under the curve (AUC) of fermentation of individual or combined sugars. In this way it was possible to rank the 20 strains for their ability to consume fructose relative to glucose. Moreover, it was shown that fermentations performed in media containing fructose as sole carbon source did not predict the fructophilicity of strains in wine-like conditions (equimolar mixture of glucose and fructose). This work provides important information for programs which seek to generate strains that are faster or more reliable fermenters.

Microarray karyotyping of maltose-fermenting Saccharomyces yeasts with differing maltotriose utilization profiles reveals copy number variation in genes involved in maltose and maltotriose utilization

AIMS

We performed an analysis of maltotriose utilization by 52 Saccharomyces yeast strains able to ferment maltose efficiently and correlated the observed phenotypes with differences in the copy number of genes possibly involved in maltotriose utilization by yeast cells.

METHODS AND RESULTS

The analysis of maltose and maltotriose utilization by laboratory and industrial strains of the species Saccharomyces cerevisiae and Saccharomyces pastorianus (a natural S. cerevisiae/Saccharomyces bayanus hybrid) was carried out using microscale liquid cultivation, as well as in aerobic batch cultures. All strains utilize maltose efficiently as a carbon source, but three different phenotypes were observed for maltotriose utilization: efficient growth, slow/delayed growth and no growth. Through microarray karyotyping and pulsed-field gel electrophoresis blots, we analysed the copy number and localization of several maltose-related genes in selected S. cerevisiae strains. While most strains lacked the MPH2 and MPH3 transporter genes, almost all strains analysed had the AGT1 gene and increased copy number of MALx1 permeases.

CONCLUSIONS

Our results showed that S. pastorianus yeast strains utilized maltotriose more efficiently than S. cerevisiae strains and highlighted the importance of the AGT1 gene for efficient maltotriose utilization by S. cerevisiae yeasts.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our results revealed new maltotriose utilization phenotypes, contributing to a better understanding of the metabolism of this carbon source for improved fermentation by Saccharomyces yeasts.

Glucose and sucrose: hazardous fast-food for industrial yeast?

Yeast cells often encounter a mixture of different carbohydrates in industrial processes. However, glucose and sucrose are always consumed first. The presence of these sugars causes repression of gluconeogenesis, the glyoxylate cycle, respiration and the uptake of less-preferred carbohydrates. Glucose and sucrose also trigger unexpected, hormone-like effects, including the activation of cellular growth, the mobilization of storage compounds and the diminution of cellular stress resistance. In an industrial context, these effects lead to several yeast-related problems, such as slow or incomplete fermentation, 'off flavors' and poor maintenance of yeast vitality. Recent studies indicate that the use of mutants with altered responses to carbohydrates can significantly increase productivity. Alternatively, avoiding unnecessary exposure to glucose and sucrose could also improve the performance of industrial yeasts.