Flor yeasts metabolism

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. 

During the biological aging of sherry, the concentration of ethanol decreases because of its consumption by flor yeast. Its respiration via the tricarboxylic acid pathway (Suarez-Lepez and Inigo-Leal, 2004) provides the main source of carbon and energy. Acetaldehyde is the main organic byproduct of ethanol metabolism, but other volatile compounds, notably acetic acid, butanediol, diacetyl, and acetoin, can also be formed. In addition,... 

Martinez, P., Valcarcel, M. J., Perez, L., and Benitez, T. (1998). Metabolism of Saccharomyces cerevisiae flor yeast during fermentation and biological aging of fino sherry By-products and aroma compounds. Am. J. Enol. Vitic. 49, 240-250. 

The biological aging of wines has aroused increasing interest in recent years, as reflected in the large number of papers on this topic over the last decade. Biological aging in wine is carried out by flor yeasts. Once alcoholic fermentation has finished, some Saccharomyces cerevisiae yeast races present in wine switch from a fermentative metabolism to an oxidative (respiratory metabolism) and spontaneously form a biofllm called flor on the wine surface. Wine under flor is subject to special conditions by effect of oxidative metabolism by yeasts and of the reductive medium established as they consume oxygen present in the wine. These conditions facilitate... 

Although flor yeasts possess good fermentation ability, they differ from typically fermentative yeasts in metabolic, physiological and genetic respects (Esteve-Zarzoso et al. 2001, 2004 Budroni et al. 2005). 

The nitrogen requirements of flor yeasts are supplied largely by L-proline, which they metabolize under aerobic conditions as the biological aging of wines. L-Proline is converted into glutamic acid, which favours growth and persistence of the flor fllm (Botella et al. 1990). 

The biological aging process involves various changes in wine composition. Such changes are essentially the result of the metabolism of flor yeasts and, to a lesser extent, of other phenomena common to all types of aging processes including crystal precipitation, chemical reactions between wine components and extraction of substances from cask wood. 

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. 

Acetic acid is produced at concentrations rarely exceeding 0.7 g/L by yeasts during fermentation. The acid is metabolized by flor yeasts during the biological aging process, its concentration being reduced through consumption via acetyl-CoA for incorporation into the Krebs cycle or fatty acid synthesis. 

Fino wines are only produced by biological aging. The aerobic metabolism of flor yeasts causes changes in the aroma fraction that endows them wine with their typical flavour. In addition, they protect these wines against browning and allow them to retain their pale colour for years. 

Sherry wines are obtained from young wines, carefully selected soon after completing fermentation. These are typically fortified by adding vinous alcohol until they reach an alcohol content of 15-15.5°. They are subsequently transferred to oak barrels before being aged. In most sherries, wine aging occurs in the so-called solera and criaderas system under the flor film of yeast. Once alcoholic fermentation is finished, races of Saccharomyces cerevisiae that can grow on the surface of the wine switch from fermentative to oxidative (respiratory) metabolism. They spontaneously form a biofilm called flor on the wine surface. 

Recently Webb (16) explained the aromas of film sherries as by-products of an incomplete metabolic sequence where ethanol is being metabolized by film yeasts to carbon dioxide and water. Webb also refers to Pasteur s Etudes sur le Vin, published in 1866, as the basis of the specialization of zymology, which recognizes different types of film formers that yield the wines of the Jerez district of Spain, Vinsjaunes of the Jura region of France, and the flor wines of Australia, South Africa, the Soviet Union, and California. 

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