Fuel ethanol production from sweet sorghum using repeated-batch fermentation

Ethanol was efficiently produced from three varieties of sweet sorghum using repeated-batch fermentation without pasteurization or acidification. Saccharomyces cerevisiae cells could be recycled in 16 cycles of the fermentation process with good ethanol yields. This technique would make it possible to use a broader range of sweet sorghum varieties for ethanol production.     

Disruption of ubiquitin-related genes in laboratory yeast strains enhances ethanol production during sake brewing

Sake yeast can produce high levels of ethanol in concentrated rice mash. While both sake and laboratory yeast strains belong to the species Saccharomyces cerevisiae, the laboratory strains produce much less ethanol. This disparity in fermentation activity may be due to the strains' different responses to environmental stresses, including ethanol accumulation. To obtain more insight into the stress response of yeast cells under sake brewing conditions, we carried out small-scale sake brewing tests using laboratory yeast strains disrupted in specific stress-related genes. Surprisingly, yeast strains with disrupted ubiquitin-related genes produced more ethanol than the parental strain during sake brewing. The elevated fermentation ability conferred by disruption of the ubiquitin-coding gene UBI4 was confined to laboratory strains, and the ubi4 disruptant of a sake yeast strain did not demonstrate a comparable increase in ethanol production. These findings suggest different roles for ubiquitin in sake and laboratory yeast strains.     

Effect of inoculation on strawberry fermentation and acetification processes using native strains of yeast and acetic acid bacteria

The aim of this work was to analyze the microbiota involved in the traditional vinegar elaboration of strawberry fruit during a spontaneous and inoculated process. In the spontaneous processes, low biodiversity was detected in both alcoholic fermentation (AF) and acetification. Nevertheless, a strain of Saccharomyces cerevisiae and of Acetobacter malorum were selected and tested as starter cultures in the inoculation study. The inoculated processes with these strains were compared with another spontaneous process, yielding a significant reduction in time for AF with a total imposition of the S. cerevisiae strain. The resulting strawberry wine was acetified in different containers (glass and wood) yielding an initial imposition of the A. malorum inoculated strain, although displacement by Gluconacetobacter species was observed in the wood barrels.     

Optimization of honey-must preparation and alcoholic fermentation by Saccharomyces cerevisiae for mead production

Mead fermentation is a time-consuming process, often taking several months to complete. Despite of the use of starter cultures several problems still persist such as lack of uniformity of the final products, slow or premature fermentation arrest and the production of off-flavors by yeast. Thus the aim of this study was to optimize mead production through the use of an appropriate honey-must formulation to improve yeast performance alcoholic fermentation and thereby obtain a high quality product. Honey-must was centrifuged to reduce insoluble solids, pasteurized at 65 °C for 10 min, and then subjected to different conditions: nitrogen supplementation and addition of organic acids. Although the addition of diammonium phosphate (DAP) reduced fermentation length, it did not guarantee the completeness of the fermentation process, suggesting that other factors could account for the reduced yeast activity in honey-must fermentations. Sixteen yeast-derived aroma compounds which contribute to the sensorial quality of mead were identified and quantified. Global analysis of aromatic profiles revealed that the total concentration of aroma compounds in meads was higher in those fermentations where DAP was added. A positive correlation between nitrogen availability and the levels of ethyl and acetate esters, associated to the fruity character of fermented beverages, was observed whereas the presence of potassium tartrate and malic acid decreased, in general, their concentration.This study provides very useful information that can be used for improving mead quality.     

Oleic acid and ergosterol supplementation mitigates oxidative stress in wine strains of Saccharomyces cerevisiae

During fermentation of high-sugar-containing medium lacking lipid nutrients, wine yeasts undergo oxidative stress and oxidative damage to cell membranes and proteins. Considering that cell membranes are important stress sensors, and that under hypoxic conditions wine yeasts modulate cell membranes composition by incorporating lipids available in the growth medium, in the present work, the effects of lipid nutrition on wine yeast oxidative stress response were evaluated on two strains of Saccharomyces cerevisiae. Biomarkers of oxidative stress, oxidative damage and antioxidant response were evaluated together with viability and acetic acid production during fermentation of a synthetic must lacking lipid nutrients as compared to added oleic acid and ergosterol. The results show that the availability of lipid nutrients causes a significant reduction in the intracellular content of reactive oxygen species and in the oxidative damage to membranes and proteins, as indicated by flow cytometry of cells stained with dihydroethidum (DHE) and propidium iodide (PI) and by Western blot of protein carbonyls. Accordingly, lipid nutrients feeding results in the increase in cell viability and superoxide activity, and the reduction in trehalose accumulation, proteinase A activity and production of acetic acid. In summary, these results are compatible with the hypothesis that the supplementation of lipid nutrients mitigates oxidative stress and oxidative damage in wine strains of S. cerevisiae during growth under unfavourable conditions.     

Ultrasound-assisted production of bioethanol by simultaneous saccharification and fermentation of corn meal

An ultrasound-assisted liquefaction as a pretreatment for bioethanol production by simultaneous saccharification and fermentation (SSF) of corn meal using Saccharomyces cerevisiae var. ellipsoideus yeast in a batch system was studied. Ultrasound pretreatment (at a frequency of 40 kHz) was performed at different sonication times and temperatures, before addition of liquefying enzyme. An optimal duration of the treatment of 5 min and sonication temperature of 60 °C were selected, taking into account glucose concentration after the liquefaction step. Under the optimum conditions an increase of glucose concentration of 6.82% over untreated control sample was achieved. Furthermore, the SSF process kinetics was assessed and determined, and the effect of ultrasound pretreatment on an increase of ethanol productivity was investigated. The obtained results indicated that the ultrasound pretreatment could increase the ethanol concentration by 11.15% (compared to the control sample) as well as other significant process parameters. In this case, the maximum ethanol concentration of 9.67% w/w (which corresponded to percentage of the theoretical ethanol yield of 88.96%) was achieved after 32 h of the SSF process. A comparison of scanning electron micrographs of the ultrasound-pretreated and untreated samples of corn meal suspensions showed that the ultrasound stimulated degradation of starch granules and release of glucose, and thereby accelerated the starch hydrolysis due to the cavitation and acoustic streaming caused by the ultrasonic action.     

Characteristics of the high malic acid production mechanism in Saccharomyces cerevisiae sake yeast strain No. 28

We characterized a high malic acid production mechanism in sake yeast strain No. 28. No considerable differences in the activity of the enzymes that were involved in malic acid synthesis were observed between strain No. 28 and its parent strain, K1001. However, compared with strain K1001, which actively took up rhodamine 123 during staining, the cells of strain No. 28 were only lightly stained, even when cultured in high glucose concentrations. In addition, malic acid production by the respiratory-deficient strain of K1001 was 2.5-fold higher than that of the wild-type K1001 and wild-type No. 28. The findings of this study demonstrated that the high malic acid production by strain No. 28 is attributed to the suppression of mitochondrial activity.     

Stable disruption of ethanol production by deletion of the genes encoding alcohol dehydrogenase isozymes in Saccharomyces cerevisiae

We analyzed the effects of the deletions of genes encoding alcohol dehydrogenase (ADH) isozymes of Saccharomyces cerevisiae. The decrease in ethanol production by ADH1 deletion alone could be partially compensated by the upregulation of other isozyme genes, while the deletion of all known ADH isozyme genes stably disrupted ethanol production.     

High-cell-density fermentation of Saccharomyces cerevisiae for the optimisation of mead production

Mead is a traditional drink that contains 8%–18% (v/v) of ethanol, resulting from the alcoholic fermentation of diluted honey by yeasts. Mead fermentation is a time-consuming process and the quality of the final product is highly variable. Therefore, the present investigation had two main objectives: first, to determine the adequate inoculum size of two commercial wine-making strains of Saccharomyces cerevisiae for the optimisation of mead fermentation; and second, to determine if an increase in yeast pitching rates in batch fermentations altered the resulting aroma profiles. Minor differences were detected in the growth kinetics between the two strains at the lowest pitching rate. With increasing pitching rates net growth of the strain ICV D47 progressively decreased, whereas for the QA23 the increasing inoculum size had no influence on its net growth. The time required to reach the same stage of fermentation ranged from 24 to 96 h depending on the inoculum size. The final aroma composition was dependent on the yeast strain and inoculum size. Fourteen of the twenty-seven volatile compounds quantified could contribute to mead aroma and flavour because their concentrations rose above their respective thresholds. The formation of these compounds was particularly pronounced at low pitching rates, except in mead fermented by strain ICV D47, at 10⁶ CFUs/mL. The esters isoamyl acetate, ethyl octanoate and ethyl hexanoate were the major powerful odourants found in the meads. The results obtained in this study demonstrate that yeast strain and inoculum size can favourably impact mead's flavour and aroma profiles.     

Bioethanol production from rice straw by a sequential use of Saccharomyces cerevisiae and Pichia stipitis with heat inactivation of Saccharomyces cerevisiae cells prior to xylose fermentation

In order to establish an efficient bioethanol production system from rice straw, a new strategy to ferment the mixture of glucose and xylose by a sequential application of Saccharomyces cerevisiae and Pichia stipitis was developed, in which heat inactivation of S. cerevisiae cells before addition of P. stipitis was employed. The results showed that heating at 50°C for 6h was sufficient to give high xylose fermentation efficiency. By application of the inactivation process, 85% of the theoretical yield was achieved in the fermentation of the synthetic medium. At the same time, the xylitol production was reduced by 42.4% of the control process. In the simultaneous saccharification and fermentation of the lime-pretreated and CO(2)-neutralized rice straw, the inactivation of S. cerevisiae cells enabled the full conversion of glucose and xylose within 80 h. Finally, 21.1g/l of ethanol was produced from 10% (w/w) of pretreated rice straw and the ethanol yield of rice straw reached 72.5% of the theoretical yield. This process is expected to be useful for the ethanol production from lignocellulosic materials in the regions where large-scale application of recombinant microorganisms was restricted.     

Meta-analysis of the influence of Saccharomyces cerevisiae supplementation on ruminal parameters and milk production of ruminants

The effects of yeast supplementation on intake, production, and rumen fermentation characteristics have been widely studied, but results are inconsistent between different studies. A quantitative meta-analysis was applied to 110 papers, 157 experiments, and 376 treatments dealing with yeast supplementation in ruminants. The objective was first to highlight the major quantitative effects of live yeast supplementation on intake, rumen fermentation, and milk production, and second, to identify major differences in experimental conditions between studies that can affect the response to treatment. Some of these experimental conditions are referred to as interfering factors. Yeast supplementation increased rumen pH (+0.03 on average) and rumen volatile fatty acid concentration (+2.17 mM on average), tended to decrease rumen lactic acid concentration (−0.9 mM on average), and had no influence on acetate-to-propionate ratio. Total-tract organic matter digestibility was also increased by yeast supplementation (+0.8% on average). Yeast supplementation increased dry matter intake (DMI; +0.44 g/kg of body weight; BW), milk yield (+1.2 g/kg of BW), and tended to increase milk fat content (+0.05%), but had no influence on milk protein content. Dose effects of yeast supplementation, expressed as log₁₀ [1+(cfu per 100 kg of BW)], globally confirmed the qualitative effects observed in the first analysis. The positive effect of yeast supplementation on rumen pH increased with the percentage of concentrate in the diet and with the DMI level. It was negatively correlated with the level of dietary neutral detergent fiber (NDF). The positive effect of yeast supplementation on rumen volatile fatty acid concentration increased with DMI and crude protein levels. The positive effect of yeast supplementation on organic matter digestibility increased with the percentage of concentrate and NDF in the diet. The negative effect of yeast supplementation on lactic acid concentration tended to decrease when the DMI level and the percentage of concentrate in the diet increased. The effects of interfering factors were globally similar when either dose effect or qualitative effect of yeast was taken into account. Although rumen fermentation efficiency per se was not measured, these results suggest an improvement in rumen fermentation by yeast supplementation. This effect could, however, be modulated by several different factors such as DMI, percentage of concentrate or NDF in the diet, or species.     

Understanding the mechanism of heat stress tolerance caused by high trehalose accumulation in Saccharomyces cerevisiae using DNA microarray

DNA microarray analysis was performed to examine the stress tolerance mechanism of a Saccharomyces cerevisiae recombinant strain exhibiting high trehalose accumulation and heat stress tolerance. Results suggest that the upregulation of sugar transporter genes is one of the key events for heat stress tolerance of the recombinant strain.