9B Flavanols, Flavonols and Dihydroflavonols 495
Factors Affecting the Interaction
Phenolic acids and flavanol monomers do not absorb on plant cell walls while
adsorption of procyanidins increases with their degree of polymerisation (Renard
et al. 2001). Selective adsorption of molecules showing higher proportions of gal-
loylated units and of catechin was also demonstrated (Le Bourvellec et al. 2004).
Similarly, higher molecular weight proanthocyanidins were preferentially adsorbed
on yeast lees (Mazauric and Salmon 2006). Adsorption of procyanidins on plant cell
walls was not affected by pH. It increased with ionic strength and decreased as the
temperature increased and in the presence of ethanol (Le Bourvellec et al. 2004),
like procyanidin self-aggregation (Poncet-Legrand et al. 2003). Partial desorption
from isolated plant cell wall material could be achieved by rinsing with buffer and
was more efficient in the presence of dissolved polysaccharides (Renard et al. 2001).
In contrast, desorption from yeast lees was extremely difficult, indicating stronger
adsorption (Mazauric and Salmon 2006). This is possibly due to the presence of
larger amounts of non polysaccharidic components such as proteins. Indeed proan-
thocyanidins were also show to be difficult to release from the pellets recovered
after protein fining (Maury et al. 2003).
9B.4.4.2 Adsorption on Winery Equipment
Actors
Formation of tartrate crystals is a major source of instability in wines and treatments
such as cold stabilisation, electrodialysis and ion-exchange are usually performed
to prevent it from occurring in the bottled wine (Vernhet et al. 1999a). Polysac-
charides and polyphenols, as well as yeast cells, were shown to be associated to
tartrate crystals in the deposits formed after cold stabilisation both in white wines
(Vernhet et al. 1999a) and in red wines (Vernhet et al. 1999b). Wine polysaccha-
rides and polyphenols are also involved in the fouling of organic membranes during
microfiltration of red wine (Vernhet and Moutounet 2002).
Mechanisms
Yeast cells represented at least 20% of the tartrate deposits in red wines. Scan-
ning electron microscopy observations revealed that they adhere first on the stain-
less steel and suggest that they act as primary nucleation germs for crystalliza-
tion (Vernhet and Moutounet 2002). The differences in shapes and composition
of crystals formed in white and red wines may be attributed to higher adsorption
of organic material in the latter (Vernhet and Moutounet 2002) as this is known to
block the crystal growth (Rodriguez-Clemente and Correa-Gorospe 1988). Arabino-
galactan protein andmannoproteins are the major polysaccharides in crystals and
seem to take part in the reduced growth rate (Vernhet and Moutounet 2002). Since
they had almost no effect on tartrate crystallization in model ethanolic solutions
(Gerbaud and Gabas 1997), this may be related to their interaction with procyanidins
(Riou et al. 2002).