3B Biologically Aged Wines 87
across scales. One type prevails throughout the system and coexists with other
yeast types in the younger scales–with which it might be closely related judging
by the similarity of their chromosomal and mitochondrial DNA patterns. As aging
progresses, yeast variabilitydecreases through selection of the specific strains best
responding to the increasingly stringent conditions in the aged wine. The solera
casks are those most clearly reflecting the evolution towards the best adapted strains
(Infante 2002). This trend has also been observed in Jura yellow wines, where only
one of the six karyotypes identified at the beginning of the aging process remained
after five years (Charpentier et al. 2000).
3B.2.2 Factors Influencing the Formation of Flor Films
The way the flor film forms, and its thickness, appearance and colour, depend on
a variety of factors, mainly for the raceS. cerevisiae. Especially influential among
the cellar environmental conditions are temperature and moisture. The acceptable
temperature range for this yeast race is 15–20◦C (Marcilla et al. 1936) and the
optimum value 15–17◦C. Ibeas et al. (1997) found a temperature of 22.5◦Cto
be the threshold above which the frequency of respiratory mutants (rho−), which
are unable to form a flor film, rises to a substantial extent. The relative humidity
should be higher than 70%.
One other major influence on film development is that of the oxygen concentra-
tion, which affects the rate with which the film is formed and its thickness (Mauricio
et al. 2001).
Formation of the flor film is also affected by the ethanol content of the wine, the
optimum value for which is 14.5–15.5 vol.%; in fact, the film rarely forms above a
16.5 vol.% content.
The influence of residual sugars on film formation is unclear. Thus, some authors
have deemed their presence indispensable for the film to form and calculated that
aging wine under flor yeasts should contain at least a 1–1.6 g/L concentration. Other
authors, however, claim that these concentration levels have no effect on film for-
mation (Su ́arez-Lepe and I ̃nigo-Leal 2004).
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 film
(Botella et al. 1990).
Other factors seemingly influencing the formation of a flor film include the pres-
ence of phenol compounds (Budroni et al. 2005), biotin (Bravo-Abad 1986) and
pantothenate (Su ́arez-Lepe and I ̃nigo-Leal 2004).
3B.2.3 Formation of the Flor Film
The formation of a flor film is the adaptive response of the yeasts to the extreme con-
ditions prevailing in the medium, which include high concentrations of ethanol and