Yeasts metabolic products

Yeast (qv) metabolize maltose and glucose sugars via the Embden-Meyerhof pathway to pymvate, and via acetaldehyde to ethanol. AH distiUers yeast strains can be expected to produce 6% (v/v) ethanol from a mash containing 11% (w/v) starch. Ethanol concentration up to 18% can be tolerated by some yeasts. Secondary products (congeners) arise during fermentation and are retained in the distiUation of whiskey. These include aldehydes, esters, and higher alcohols (fusel oHs). NaturaHy occurring lactic acid bacteria may simultaneously ferment within the mash and contribute to the whiskey flavor profile. 

Beckh, G., Wessel, P., and Lullmarm, C. (2005). Contribution to yeasts and their metabolisms products as natural components of hone—Part 3 Contents of ethanol and glycerol as quality parameters. Dtsch. Lebensm. Rundsch. 101,1-6. 

Although polysaccharide metabolic products of molds and yeasts are not strictly bacterial polysaccharides, they are considered briefly here because of similarities in chemical structure (see also page 191). 

D-erj/photosynthesis metabolic product of yeasts and many other organisms... 

Biochemical industries are based on the growth of microbes such as bacteria, fungi, molds, yeasts and others. Although some microbes are grown as food, interest here is in the production of chemicals with their aid. A distinction is drawn between steps that involve cells and those that employ isolated catalytic enzymes which are metabolic products of cells. Major characteristics of microbial processes that may be contrasted with those of ordinary chemical processing include the following ... 

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. 

Microorganisms (microbes) are living cells, single or in multiples of the same kind, including bacteria, yeasts, fungi, molds, algae and protozoa. Their metabolic products may be of simple or complex structure... 

Another important carbohydrate group of cane final molasses, and one which has been little studied, consists of the products formed by the action of heat, alkali and amino acids upon the reducing sugars. This is sometimes termed the non-fermentable fraction as it remains after yeast fermentation although it is thereby badly contaminated with metabolic products from the yeast. 

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). 

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. 

Various aldehydes are encountered in wine. The most abundant is acetaldehyde which is both a product of yeast metabolism and an oxidation product of ethanol. Glyoxylic acid, resulting from oxidation of tartaric acid, especially catalyzed by metal ions (Fe, Cu) or ascorbic acid, can also be present. Other aldehydes reported to participate in these reactions include furfural and 5-hydroxymethylfurfural that are degradation products of sugar and can be extracted from barrels (Es-Safi et al. 2000), vanillin which also results from oak toasting, isovaleraldehyde, benzaldehyde, pro-pionaldehyde, isobutyraldehyde, formaldehyde and 2-methylbutyraldehyde which are present in the spirits used to produce fortified wines (Pissara et al. 2003). 

Lonvaud-Funel, A., Joyeux, A., and Desens, C. 1988. Inhibition of malolactic fermentation of wines by products of yeast metabolism. J. Sci. Food Agric. 44, 183-191. 

In experiment 2, higher alcohols originating from amino acid metabolism, such as 2-methy1-1-propanol, 3-methyl-l-butanol and 2-phenylethanol were found as metabolization products of B cinerea. Figure 3 shows distinct quantitative differences depending on the strain used. Control experiments demonstrated that these alcohols were exclusively formed by B cinerea and not by contamination by yeasts. This fact should be stressed since contradictory results have been published (2,3). 

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