Aging in yeast

Sinclair, D.A. and Guarente, L. (1997) Extrachromosomal rDNA circles a cause-of aging in yeast. Cell, 91, 1033-1042. 

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. 

In the fermentations under the cellar conditions, higher ethanol concentrations were reached compared with those under the vinsantaia temperature conditions. This could be due to the combined stress effects on the yeasts of alcohol and high temperatures during the summer period in the vinsantaia aging. In the Vin Santo obtained without inoculation with the commercial starter, satisfactory ethanol levels were however reached (13.7-15.3%, v/v). 

Indeed, after a maturation period of 18 months, no significant differences were seen between the wines obtained without the yeast inoculums, either if they were aged in vinsantaia or at a constant temperature of 18 °C, except for the honey character and the color intensity. Similarly, also inside the cluster of wines obtained with both the yeast inoculum and the addition of madre there were no differences found in the perception of individual sensory attributes. However, the wines from the yeast-inoculated fermentation with the addition of madre were perceived to be more acidic, astringent, bitter, and viscous than the non-inoculated wines when they were aged in the cellar at constant temperature (18 °C). In contrast, there were no significant differences among the wines aged under the traditional conditions (vinsantaia). 

On the other hand, several authors reported a relation between Sotolon levels and the presence of sugar, which are present in great quantities in Madeira wines (Camara et al, 2006c). It is important to note that the fortified Madeira wines were always aged in thermal/oxidizing environment without yeast "flor."... 

Little or no cooling was used during fermentation. Sulfur dioxide began to be employed at the end of the nineteenth century, as were pure yeast cultures. Wines were aged in tanks and barrels. Eastern wines are usually, but not always, easier to clarify than California wines. While some wine was bottled for sale, a great deal was sold in bulk to local bottlers or was sold directly to the consumer in barrels (50 gallons) or in demijohns (3-5 gallons). 

Bulk produced California sparkling wines are made for mass market customers. They are not connoisseurs and do not require the aromas and tastes associated with sparkling wines aged on the yeast. For this reason, bulk processors in California are not at present interested in aging on yeast in tanks for a period of months as do the Germans, or in the yeast-cell-and-wine contact method used in the Soviet Union (6) in their continuous system. 

All finally differentiated eukaryotic cells lack telomerase activity. Thus, when cells divide, the ends of the chromosomes can not be duplicated and the telomeres get shorter and shorter with every cell division, until the cell finally dies. Thus, the lack of telomerase activity may determine the number of divisions a cell can go through. And indeed, disruption of the telomerase gene in yeast leads to senescence and death (see Fig. 12.12). The ageing process in the yeast cells was reversed when human telomerase was expressed. Expression of the catalytic subunit of the human telomerase holoenzyme (hTERT) also enabled human cells to avoid senesce and to multiply indefinitely. This raised hopes to immortalize human cells, without interfering with their differentiation. (For more information, see ref. 21 and Chapter 17). 

However, a smaller amount of cytoplasmic contents were solubilized after 12 months of yeast aging in sparkling wines under autolysis in natural conditions (Fig. 3A.2). This shows that the autolysis conditions during aging of sparkling wines elaborated by the traditional method are not optimal and it may take several months or years before this is finished. 

Martinez-Rodriguez, A.J., Gonzalez, R., and Carrascosa, A.V. (2004). Morphological changes in autolytic wine yeast during aging in two model systems. /. Food Sci., 69, 233-239. 

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... 

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