88 R. Peinado and J. Mauricio
acetaldehyde, oxidative stress resulting from the metabolization of unfermentable
carbon sources, and nitrogen sources of assimilation difficult as L-proline. The
adaptive mechanism involves changes in cell size, shape and hydrophobicity, all
of which can reduce the density of a flor yeast population below that of wine and
float in it.
Some authors have found the transitionfrom a fermentative status to a film
forming status in the yeasts to occur simultaneously with an increased saturation
in long-chained fatty acids and, especially, an increased C18:1/C18:0 ratio (Aguil-
era et al. 1997; Farris et al. 1993; Valero et al. 2002); this probably increases the
tolerance to ethanol of the yeasts (Aguilera et al. 2006) and their hydrophobicity,
thereby facilitating flotation of cells and the formation of a stable film.
The hydrophobicity of yeasts is a result of their producing hydrophobic proteins;
in fact, treating yeast cells with proteinases K, I and VI inhibits formation of the
biofilm (Mart ́ınez et al. 1997b).
The molecular base of the film formation has recently been examined by some
authors (Fidalgo et al. 2006; Ishigami et al. 2004, 2006; Purevdorj-Gage et al. 2007;
Verstrepen and Klis 2006). The molecular mechanism behind it remains obscure
owing to its extreme complexity. The film is formed by effect of flor yeasts grow-
ing on ethanol as their aerobic carbon source and its development is repressed by
glucose. The primary factor for the biofilm formation isFLO11, which encodes a
hydrophobic glycoprotein in cell walls.FLO11is required for the air-liquid interfa-
cial biofilm to form and also for the biofilm cells to reach a buoyant density greater
than that of the suspending medium (Zara et al. 2005). Ishigami et al. (2006) have
shownNRG1to be closely related withFLO11expression in flor yeasts and to
facilitate formation of the biofilm.NRG1expression makes cell surfaces highly
hydrophobic. There is thus abundant evidence that the formation of a biofilm by flor
yeasts relies heavily on an increased hydrophobicity and that such hydrophobicity
allows the yeasts to float by effect of surfacetension; this provides an adaptive mech-
anism for gaining direct access to oxygen in oxygen-deficient liquid environments
(Alexander et al. 1998; Fidalgo et al. 2006; Mart ́ınez et al. 1997c).
3B.2.4 Genetic Characteristics of Flor Yeasts
Flor yeasts exhibit high variability in terms of nuclear and mitochondrial genome
(Ibeas and Jim ́enez 1996; Infante et al. 2003; Mart ́ınez et al. 1995; Mesa et al. 1999).
Genetic studies have exposed many aneuploidies and a surprisingly large number of
additional copies of some chromosomes, particular chromosome XIII, which con-
tains the genes that code alcohol and aldehyde dehydrogenases, which are related
with the metabolism of ethanol, acetaldehyde and acetic acid (Blandino et al. 1997;
Fern ́andez et al. 1972; Guijo et al. 1997; Mauricio et al. 1997).
Aneuploidies and chromosomal rearrangements allow flor yeasts to withstand
the conditions prevailing in aging wines and play a central role in sexual isola-
tion (Jim ́enez and Ben ́ıtez 1987; Sancho et al. 1986;). Sexual isolation prevents the
random distribution of favourable features; hence most flor strains sporulate very