Yeasts metabolism

Fungi are microscopic nonphotosynthetic plants which include in their classification yeast and molds. Yeasts have a commercial value as they are used for fermentation operations in distilling and brewing. When anaerobic conditions exist, yeasts metabolize sugar, manufacturing alcohol from the synthesis of new cells. Alcohol... 

Eukaryotic Microorganisms Fungi and Yeasts Metabolism by Fungi... 

Reactions of anthocyanins and flavanols take place much faster in the presence of acetaldehyde that is present in wine as a result of yeast metabolism and can also be produced through ethanol oxidation, especially in the presence of phenolic compounds, or introduced by addition of spirit in Port wine technology. The third mechanism proposed involves nucleophilic addition of the flavanol onto protonated acetaldehyde, followed by protonation and dehydration of the resulting adduct and nucleophilic addition of a second flavonoid onto the carbocation thus formed. The resulting products are anthocyanin flavanol adducts in which the flavonoid units are linked in C6 or C8 position through a methyl-methine bond, often incorrectly called ethyl-link in the literature. 

Sulfur compounds produced by yeast metabolism (and residues of pesticide treatment)... 

If the concentration of inhibitors increases to such levels that the yeast metabolism is affected, the numerator in Eq. 6 will decrease. In case the ratio on the right-hand side of Eq. 6 decreases below a threshold value, no further increase in feed rate will be made. 

The major change in yeast metabolism induced by casamino acids supplementation was the marked increase in xylitol production, being the major metabolic product by D. hansenii grown in supplemented concentrated medium (Fig. 2B). Both arabitol and xylitol production in yeast are described to be augmented under stress conditions. Arabitol is usually found as a product of arabinose metabolism in oxygen-limited conditions (8,22), but arabitol production is not restricted to arabinose metabolism,... 

Yeasts metabolize sugars in honey, producing acids, gas, and other products that make honey unfit for consumption. Unlike yeasts and molds, bacteria can survive in honey but are unlikely to grow (Snowdon and Cliver, 1996). Growth of pathogenic bacteria has not been detected in honey. Thus, a high bacterial count is indicative of recent contamination... 

The yeasts used in the production of mead are usually strains of Saccharo-myces cerevisiae, similar to that used in wine, beer, and champagne productions. These yeasts metabolize sugars, such as glucose and fructose, resulting in the formation of ethanol and carbon dioxide. Nevertheless, the yeast Hansenula anomala had also given good results (Qureshi and Tamhane, 1987). 

After sterilization, yeast is added to initiate fermentation. McConnell and Schramm (1995) recommend inoculation with no less than 10% by volume. Moreover, as the pH of honey is naturally low and because it is poorly buffered, the pH of must may drop during fermentation to a point limiting yeast efficiency. pH reduction can result from the synthesis of acetic and succinic acids by the yeast cells (Sroka and Tuszynski, 2007). While a rapid decline in pH inhibits undesirable microbial activity (Sroka and Tuszynski, 2007), it also reduces the dissociation of fatty acids in the wort, potentially slowing yeast metabolic action. For this, addition of a buffer is important to maintain the pH within a range of 3.7-4.0 throughout fermentation (McConnell and Schramm, 1995). Calcium carbonate, potassium carbonate, potassium bicarbonate, and tartaric acid are potential candidates. However, as some of these salts can add a bitter-salty... 

Casellas, G. B. (2005). Effect of low temperature fermentation and nitrogen content on wine yeast metabolism. Universitat Rovira i Virgili, Tese de Doutoramento. 

Together with fermentation, both the prefermentative and simultaneous maceration influences the supply of essential yeast nutrients and substrates for their enzymatic transformation. The release of nutrients from the pomace is also under the influence of heat and alcohol, generated by yeast metabolism. 

In fermentation practice, the yields of ethanol and C02 have been reported to vary from 92 to 98 percent of theory. This is attributable to the formation of small amounts of aldehydes, volatile and fixed acids, glycerol, and other substances, to use of sugar in the yeasts metabolism, and to small losses of ethanol during the fermentation. 

Redox potential is measured potentiometrically with electrodes made of noble metals (Pt, Au) (Fig. 12). The mechanical construction is similar to that of pH electrodes. Accordingly, the reference electrode must meet the same requirements. The use and control of redox potential has been reviewed by Kjaergaard [218]. Considerations of redox couples, e.g. in yeast metabolism [47], are often restricted to theoretical investigations because the measurement is too unspecific and experimental evidence for cause-effect chains cannot be given. Reports on the successful application of redox sensors, e.g. [26,191], are confined to a detailed description of observed phenomena rather than their interpretation. 

The transformation of pyruvate into ethanal or acetyl-coA is therefore a key point for regulating yeast metabolism (Rib6reau-Gayon et al. 2000c). 

Hensche, P.A., Jiranek, V. (1993) Yeast - metabolism of nitrogen compounds. In G.H. Fleet (Ed.), Wine Microbiology and Biotechnology (pp. 77-164). Reading Hrawood Academic. 

Fig. 3B.2 Effect of the rocto on the compounds highly related with yeast metabolism during the biological aging, a Just after rocto. b Seven days after rocto...

Metabolite concentrations in wine depend on the particular aging conditions, number of rows in the criaderas and solera system, number of rocios and volume extracted for bottles per year, ratio area of the flor film to volume of wine, climatic conditions of the cellar (temperature and relative humidity) and alcoholic concentration, in addition to the particular flor yeasts present. Below are described the most common changes observed during the biological aging of wine, whether related to yeast metabolism or otherwise. 

Acetaldehyde is also involved in the formation of acetoin and 2,3-butanediol. Although some authors have stressed the significance of a chemical pathway, these compounds are more likely to originate from yeast metabolism (Romano and Suzzi 1996). 

Dos Santos, A.M., Feuillat, M., Charpentier, C. (2000). Flor yeast metabolism in a model system similar to cellar ageing of the french Evolution of some by-products, nitrogen compounds and polysaccharides. Vitis, 39, 129-134. 

Compound(s) Origin Sensory attribute Effect of yeast Effect of yeast metabolism nutrients ... 

Table 8D.2 Summary of yeast metabolic interactions with grape compounds...

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