Growth and fermentation patterns of Saccharomyces cerevisiae under different ammonium concentrations and its implications in winemaking industry

To study the effects of assimilable nitrogen concentration on growth profile and on fermentation kinetics of Saccharomyces cerevisiae. Aims: To study the effects of assimilable nitrogen concentration on growth profile and on fermentation kinetics of Saccharomyces cerevisiae. Methods and Results: Saccharomyces cerevisiae was grown in batch in a defined medium with glucose (200 g l−1) as the only carbon and energy source, and nitrogen supplied as ammonium sulphate or phosphate forms under different concentrations. The initial nitrogen concentration in the media had no effect on specific growth rates of the yeast strain PYCC 4072. However, fermentation rate and the time required for completion of the alcoholic fermentation were strongly dependent on nitrogen availability. At the stationary phase, the addition of ammonium was effective in increasing cell population, fermentation rate and ethanol. Conclusions: The yeast strain required a minimum of 267 mg N l−1 to attain complete dryness of media, within the time considered for the experiments. Lower levels were enough to support growth, although leading to sluggish or stuck fermentation. Significance and Impact of the Study: The findings reported here contribute to elucidate the role of nitrogen on growth and fermentation performance of wine yeast. This information might be useful to the wine industry where excessive addition of nitrogen to prevent sluggish or stuck fermentation might have a negative impact on wine stability and quality

Discovering a chemical basis for differentiating wines made by fermentation with 'wild' indigenous and inoculated yeasts: role of yeast volatile compounds

Background and Aims:  Winemakers are constantly searching for new techniques to modulate wine style. Exploiting indigenous yeasts present in grape must is re-emerging as a commercial option in New World wine regions. Wines made with indigenous or ‘wild’ yeasts are perceived to be more complex by showing a greater diversity of flavours; however, the chemical basis for the flavour characteristics is not yet defined. In order to evaluate techniques for making wine with the ‘wild yeast fermentation’ character more reliably, it is necessary to define the salient chemical characteristics of such wines. Methods and Results:  Pairs of Chardonnay wines were prepared from the same must and subjected to similar fermentation conditions in the wineries of origin, except for the mode of inoculation. Reference wines were made by inoculation with a Saccharomyces cerevisiae starter culture, whereas companion wines were allowed to undergo fermentation with the indigenous microflora. Of all wine chemicals analysed, only yeast-derived volatile fermentation products showed significant differences between the yeast treatments. Conclusions:  Inoculated wines were associated with the esters ethyl hexanoate and 3-methylbutyl acetate and formed a clear cluster by principal component analysis. By comparison with inoculated wines, ‘wild’ yeast fermented wines showed high variability in volatile compounds that contribute to wine aroma, with higher concentrations of 2-methylpropanol, 2-methylbutanoic acid, ethyl 2-methylpropanoate, ethyl decanoate and ethyl dodecanoate potentially being sensorially important. Significance of the Study:  This study shows that yeast-derived volatile fermentation products are a key difference between inoculated and uninoculated ferments and provides a chemical basis for the ‘wild yeast fermentation’ character.C. Varela, T. Siebert, D. Cozzolino, L. Rose, H. McLean, and P.A. Henschk

Evaluation of non-Saccharomyces yeast for the reduction of alcohol content in wine

10.1128/AEM.03780-13Over recent decades the average ethanol concentration of wine has increased, due largely to consumer preference for wine styles associated with increased grape maturity; sugar content increases with grape maturity and this translates into increased alcohol content in wine. However, high ethanol content impacts on wine sensory properties, reducing the perceived complexity of flavours and aromas. In addition, for health and economic reasons, the wine sector is actively seeking technologies to facilitate the production of wines with lower ethanol content. Non-conventional yeast, in particular non-Saccharomyces yeast, have shown potential for producing wines with lower alcohol content. These yeast species, which are largely associated with grapes pre-harvest, are present in the early stages of fermentation, but in general are not capable of completing alcoholic fermentation. We have evaluated 50 different non-Saccharomyces isolates, belonging to 24 different genera, for their capacity to produce wine with lower ethanol concentration when used in sequential inoculation regimes with a S. cerevisiae wine strain. A sequential inoculation of Metschnikowia pulcherrima AWRI1149 followed by a S. cerevisiae wine strain was the best combination able to produce wine with a lower ethanol concentration than the single-inoculum, wine yeast, control. Sequential fermentations utilising AWRI1149 produced wines with 0.9% (v/v) and 1.6% (v/v) (corresponding to 7.1 g/L and 12.6 g/L) less ethanol concentration in Chardonnay and Shiraz, respectively. In Chardonnay, the total concentration of esters and higher alcohols was higher for wines generated from sequential inoculations, whereas the total concentration of volatile acids was significantly lower. In sequentially inoculated Shiraz wines the

The effect of powdery mildew infection of grape berries on juice and wine composition and on sensory properties of Chardonnay wines

The composition of juice and wine made from Chardonnay, Shiraz and Cabernet Sauvignon grapes with increasing levels of powdery mildew infection was assessed over two vintages (2000 and 2001). Chardonnay juice and wine made from grape lots with 0 %, 1-5 %, 10-30 % and 31-100 % of bunches visually infected with powdery mildew showed an increase in titratable acidity, total phenolics, hydroxycinnamates and flavonoids with increasing level of infection. In vintage 2001, Chardonnay bunches with greater than 30% powdery mildew infection had lower total soluble solids (TSS) than lower infection categories and healthy grapes. Powdery mildew infection of Cabernet Sauvignon bunches (1-20%) also resulted in lower TSS, and in lower wine phenolic concentration and spectral colour values compared with healthy grapes. The total microbial population was higher on powdery mildew affected Chardonnay, Shiraz and Cabernet Sauvignon grapes than on healthy grapes. Sensory descriptive analysis of Chardonnay wines from the 2001 vintage showed that wines made from grapes with powdery mildew infection were rated higher in 'oily' and 'viscosity' attributes than wines made from disease-free grapes. Other sensory attribute differences were also apparent, but these were likely to be related to differences in fermentation rate or secondary compositional effects of the powdery mildew infection, notably differing TSS and acidity in the juice. No mouldy or 'off aromas' were associated with wine made from infected grapes. Nevertheless, the study indicated that even very low levels of infection appear to be detrimental to wine sensory characteristics.Belinda E. Stummer, I. Leigh Francis, Andrew J. Markides and Eileen S. Scot

Subunit and cofactor binding of Saccharomyces cerevisiae sulfite reductase - towards developing wine yeast with lowered ability to produce hydrogen sulfide

Copyright © 2008 Australian Society of Viticulture and Oenology Inc.Wine yeast, in common with other industrial Saccharomyces cerevisiae yeast, can produce hydrogen sulfide during alcoholic fermentation. The aim of this work was to evaluate a genetic strategy for reducing hydrogen sulfide production by lowering the activity of NADPH-dependent sulfite reductase, a key enzyme in the biosynthesis of the sulfur-containing amino acids. The role of amino acids predicted to be necessary for cofactor binding was investigated by comparisons with other proteins in the ferredoxin family. Substitutions in the glycine-rich loop domain (glycines 891 and 893) and at serine 953 of the alpha subunit, Met10p, appear to strongly affect NADPH binding. However, mutations in two amino acids that are completely conserved within the NADPH binding domain of the ferredoxin reductase family, lysine 959 and cysteine 987, have minimal effect on enzyme activity. Serine 820 is indicated to be essential for efficient FAD binding. The two-hybrid system was used to confirm that Met10p interacts with Met5p, the predicted beta subunit of sulfite reductase. Binding of the subunits could only be demonstrated in the absence of methionine, indicating that an additional factor, possibly siroheme, is required to mediate this interaction. Strategies to decrease, but not eliminate, sulfite reductase activity in the cell using identified mutants were tested, and the findings are discussed with the a view to producing improved wine strains.Catherine M. Sutherland, Paul A. Henschke, Peter Langridge and Miguel de Barros Lope