3B Biologically Aged Wines 89
little, if any, and the resulting meiotic products are frequently inviable (Mart ́ınez
et al. 1995). Intergenic region restriction analyses (ITS internal transcribed spacers)
of 5.8S rRNA gene have revealed the presence of a 24 bp deletion inS. cerevisiae
flor strains that may be useful with a view to their identification and authentication
(Fern ́andez-Espinar et al. 2000; Esteve-Zarzoso et al. 2004).
Aranda et al. (2002) examined the stress resistance of flor yeasts and found races
isolated from wine in the solera row to be more resistant to acetaldehyde and ethanol
than those in the other scales. Also, they found the resistance to stress from these
compounds to be correlated with high expression levels forHSPgenes.
3B.3 Chemistry and Biochemistry of Biological Aging
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 crys-
tal precipitation, chemical reactions between wine components and extraction of
substances from cask wood.
The formation of a yeast biofilm is largely associated with the presence of the
nutrients required for the yeasts to grow. The major sources of carbon for this pro-
cess are ethanol, glycerol and acetic acid, and those of nitrogen ammonium ion and
amino acids (particularly L-proline). Oxygen is required for the synthesis of unsat-
urated fatty acids and sterols (Mauricio et al. 1991), and also for the assimilation of
L-proline (Mauricio et al. 2001) (see Fig. 3B.4).
Flor yeasts can undergo autolysis during biological aging of wine (Charpentier
et al. 2004). This biochemical process occurs under special conditions such as those
in wines stored and aged in contact with yeasts for months or even years (e.g. in
the aging of sparkling wines, white wines on lees and biological aging of wines).
Yeast autolysis releases a variety of compounds into the wine (amino acids, peptides,
nucleotides, manoproteins, esters, alcohols, aldehydes, acids and lactones) all of
which influence its sensory properties (Charpentier and Feuillat 1993; Mart ́ınez-
Rodr ́ıguez and Polo 2000).
Metabolite concentrations in wine depend on the particular aging conditions,
number of rows in thecriaderas and solerasystem, number ofrociosand 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 concen-
tration, 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.
3B.3.1 Ethanol
Yeasts use ethanol as a source of carbon and energy. A fraction of the alcohol is
converted into other compounds includingacetaldehyde, acetic acid, butanediol,